Method and apparatus for producing a gypsum board, and a gypsum board produced according to the method

Abstract

A method and apparatus for manufacturing a gypsum board (10), comprising directing a first portion (76) of a higher density aqueous gypsum slurry towards a front cover sheet (90), directing a second portion (74) of a lower density aqueous gypsum slurry towards a back cover sheet (96), and passing the back cover sheet (96) over a roller (54) spaced above the front cover sheet (90) such that most of the second aqueous gypsum slurry (74) falls from the back cover sheet (96) onto the first portion (76) of the aqueous gypsum slurry on the front cover sheet (90). Also claimed is a gypsum board produced by the method.

Description

Technical Field

[0001] The present disclosure relates to a method for manufacturing a gypsum board, and more particularly, to a method and apparatus for dispensing an aqueous gypsum slurry for making a gypsum board.

Background Art

[0002] In building construction, one of the most common building elements is a gypsum board, also known as gypsum paneling, gypsum building panel, gypsum panel, or wall board, which is used in the construction of walls and / or ceilings. Walls made from gypsum wallboard are traditionally constructed by fixing the panels to wooden studs or metal frames and treating the joints between adjacent panels with a specially prepared adhesive called joint compound.

[0003] Gypsum boards are made primarily of gypsum, as opposed to cement boards, which are primarily cement such as Portland cement. In particular, gypsum boards are primarily composed of calcium sulfate dihydrate. Gypsum boards are made by reacting water with stucco (also known as calcined gypsum) such that the calcium sulfate hemihydrate hardens to form calcium sulfate dihydrate (gypsum). Stucco is made by calcining gypsum and typically consists primarily of calcium sulfate hemihydrate and may also contain calcium sulfate anhydrite. Calcium sulfate hemihydrate is produced by calcining calcium sulfate dihydrate to partially dehydrate the calcium sulfate dihydrate. When stucco is mixed with water, the calcium sulfate hemihydrate particles react and rehydrate to form hardened gypsum.

[0004] It is well known to produce gypsum boards by uniformly dispersing calcined gypsum (stucco) in water to form an aqueous gypsum slurry (e.g., a mixture containing stucco and water). For the purposes of this specification, an aqueous slurry that initially contains calcined gypsum and then hardens to form gypsum is referred to as a gypsum slurry. The aqueous gypsum slurry is typically produced in a continuous manner by inserting stucco, water, and any other additives into a mixer that includes means for agitating the contents to form a uniform gypsum slurry. Additives can include, for example, retarders, accelerators, foaming agents, wet strength enhancers, biocides, sag resistance components, cellulose fibers, glass fibers, flame retardant materials, binders, water repellent components, dust suppressants, starches, and other components or reinforcing materials known in the art.

[0005] The slurry is continuously directed towards, and through, the discharge outlet of the mixer and into a discharge conduit connected to the discharge outlet of the mixer. To reduce the total weight of the finished gypsum board, air can be incorporated into the aqueous gypsum slurry as air bubbles or air pockets to obtain a gypsum board having a foamed or cellular gypsum core with air voids (also referred to as air bubbles). For example, to provide air, an aqueous foam can be mixed with the aqueous gypsum slurry within the mixer and / or the discharge conduit. The flow of the slurry passes through the discharge conduit and from there is continuously deposited onto a moving web of cover sheet material such as a facer sheet of paper or a fiber mat supported by a forming table. The slurry is spread out onto the advancing web.

[0006] A second web of the cover sheet material, typically another facer sheet of paper or fiber mat, is applied to cover the slurry to form a multi-layer structure of a continuous wallboard preform such that the aqueous gypsum slurry forming the gypsum core is between the two facing materials. The wallboard preform is subjected to forming, such as at a conventional forming station, to obtain the desired thickness. The calcined gypsum reacts with the water in the wallboard preform and cures as the wallboard preform moves down the production line (e.g., forming an interlocking matrix of calcium sulfate dihydrate, referred to as hardened gypsum). The wallboard preform is cut into segments at points along a line where the wallboard preform is sufficiently cured, and the segments are turned over and dried (e.g., in a kiln) to drive out excess water and are processed to provide a final wallboard product of the desired dimensions. The produced gypsum board can be further processed as known in the art, then bundled and ready for shipment.

[0007] U.S. Patent No. 10,421,250 to Li et al. discloses a composite board including a board core and a concentrated layer of substantial thickness. The concentrated layer has a higher density than the board core.

[0008] U.S. Patent Application Publication No. 2012 / 0207989 to Xu et al. discloses a multi-layer core cementitious board with increased nail pull resistance. The board can include two or more layers of a cementitious composition, and each layer can have a different density.

[0009] Conventional devices and methods for addressing some of the operational problems associated with the production of gypsum wallboard are disclosed in U.S. Patent Nos. 5,683,635; 5,643,510; 6,494,609; 6,874,930; 7,007,914; 7,296,919; 9,999,989 (Rago et al.); 10,076,853 (Wittbold et al.); 9,909,718 (Wittbold et al.); 10,286,572 (Li et al.); 10,052,753 (Li et al.); U.S. Patent No. 10,239,230 (Li et al.); 9,616,591 (Li et al.). Other conventional devices and methods for addressing some of the operational problems associated with the production of gypsum wallboard are disclosed in International Publication Nos. 2015 / 185251-(A1) of Martin et al. and 2015 / 185143-(A1) of Martin et al.

[0010] Conventional manufacturing lines direct the core slurry (through a hose toward the densified layer on the face paper). This process can remove a portion of the densified layer on the face paper at the impact point. For the purposes of this specification, such removal is referred to as washout. SUMMARY OF THE INVENTION

[0011] The present invention provides a method and apparatus for manufacturing a gypsum board by directing a first portion of an aqueous gypsum slurry toward a front cover sheet, directing a second portion of the aqueous gypsum slurry toward a back cover sheet, and passing the back cover sheet over a roller spaced above the front cover sheet such that a majority of the second aqueous gypsum slurry falls from the back cover sheet onto the first portion of the aqueous gypsum slurry on the front cover sheet.

[0012] The present invention provides a method for manufacturing a gypsum board, the method comprising: A front cover sheet having surfaces on the first and second opposite sides on the forming surface, wherein the first surface of the front cover sheet is the lower surface facing the forming surface, and the second surface of the front cover sheet is the upper surface facing away from the forming surface, depositing the front cover sheet, and horizontally moving the front cover sheet along the forming surface in the machine direction, Depositing a first portion of the aqueous gypsum slurry on the front cover sheet to form a layer of the first portion of the aqueous gypsum slurry as a layer-like higher density region in contact with the upper surface of the front cover sheet, A back cover sheet having surfaces on the third and fourth opposite sides along a back cover sheet path including a first path segment and a second path segment downstream of the first path segment, wherein the back cover sheet moves along the first path segment above the higher density region on the front cover sheet with a movement including a first vertical movement component and / or a first horizontal movement component, the first vertical movement component being a downward movement towards the higher density region on the front cover sheet, and the first horizontal movement component being a movement in a direction opposite to the machine direction, moving the back cover sheet, A second portion of the aqueous gypsum slurry that is less dense than the first portion of the aqueous gypsum slurry for contacting the third surface of the back cover sheet in the first path segment, wherein the second portion of the aqueous gypsum slurry contacts the back cover sheet at an incident angle "B" of 0 to 90 degrees, for example, 0 to 20 degrees, preferably 5 to 90 degrees, more preferably 5 to 85 degrees or 5 to 60 degrees, with respect to the surface of the back cover sheet where the slurry contacts, The first part of the aqueous slurry and the second part of the aqueous slurry each contain a respective mixture of water and stucco, the stucco contains calcium sulfate hemihydrate, and the first part of the aqueous slurry and the second part of the aqueous slurry each contain, on a dry (anhydrous) basis, at least 60% by weight, typically 60 to 98% by weight, preferably at least 70% by weight, more preferably at least 80% by weight, typically at least 90% by weight, or typically at least 95% by weight of said calcium sulfate hemihydrate and water, respectively, in a weight ratio of water to calcium sulfate hemihydrate of 0.2:1 to 1.2:1, depositing the second part, Then, at the downstream end of the first path segment, while the fourth surface of the back cover sheet contacts the first transfer roller, passing the back cover sheet together with the second part of the aqueous gypsum slurry over the first transfer roller spaced a distance above the higher density region on the front cover sheet, depositing a majority by weight of the second part of the aqueous gypsum slurry that has fallen from the back cover sheet onto the higher density region on the front cover sheet, and forming a layer of the second part of the foamed aqueous gypsum slurry as a lower density layer in contact with the upper surface of the higher density region that is denser than the lower density region, The back cover sheet then passes around the first transfer roller to feed the back cover sheet into the second path segment, the second path segment having an upstream end and a downstream end, the upstream end being at the first transfer roller, and typically when the deposition of the second aqueous gypsum slurry occurs, the remaining portion of the second aqueous gypsum slurry remains in contact with the back cover sheet as the back cover sheet moves into the second path segment, At the downstream end of the second path segment, deposit a back cover sheet on the second portion of the aqueous gypsum slurry on the face cover sheet to form a multilayer assembly including the back cover sheet on the second portion of the aqueous gypsum slurry, typically, the multilayer assembly includes a first portion of the aqueous gypsum slurry layer on the front cover sheet, a second portion of the aqueous gypsum slurry layer on the first portion of the aqueous gypsum slurry layer, and a back cover sheet on the second portion of the aqueous gypsum slurry layer, to form a multilayer assembly. Pass the multilayer assembly through a forming station for forming the multilayer assembly, preferably, the multilayer assembly passes under the forming plate of the forming station. By reacting calcium sulfate hemihydrate with the water of each of the first portion of the aqueous slurry layer and the second portion of the aqueous gypsum slurry layer, cure the calcium sulfate hemihydrate of each of the first portion of the aqueous gypsum slurry layer and the second portion of the aqueous gypsum slurry layer to form respective first and second board layers containing calcium sulfate dihydrate between the front cover sheet and the back cover sheet, The first board layer includes a higher density region cured as a layer containing calcium sulfate dihydrate and has a first board layer density, the second board layer includes a lower density region cured as a layer containing calcium sulfate dihydrate and has a second board layer density lower than the first board layer density, and the cured higher density region is interposed between the cured lower density region and the front cover sheet, to form a panel including the gypsum cores of the respective first and second board layers. Dry the panel and cut the panel into gypsum boards.

[0013] Preferably, by simultaneously causing both the first vertical movement component and the first horizontal movement component to exist along all or the same part of the first segment, the back cover sheet moves along the downstream portion of the first path segment with a movement along an inclination defining an angle "A" of 0 to 90 degrees between the back cover sheet and the front cover sheet on the forming surface at the downstream portion of the first path segment. Typically, the angle "A" is 5 to 90 degrees or 5 to 85 degrees, and more typically, the angle "A" is 0 to 20 degrees or 20 to 60 degrees.

[0014] Typically, the discharge of the second portion of the aqueous gypsum slurry is in a countercurrent direction to the moving direction of the back cover sheet. Alternatively, typically, the discharge of the second portion of the aqueous gypsum slurry is in a co-current direction to the moving direction of the back cover sheet.

[0015] Preferably, the first transfer roller changes the moving direction of the back cover sheet to a movement along the first part of the second path segment having a second horizontal movement component and a second vertical movement component in the machine direction, and the second vertical movement component is upward, downward, or neutral to provide only movement along the second horizontal movement component. Typically, the second vertical movement component is an upward movement away from a higher density region on the front cover sheet.

[0016] Preferably, the first transfer roller is a freewheel roller. Typically, the second path segment has a first part and a second part, and at the downstream end of the first part of the second path segment, the back cover sheet passes over the second transfer roller to feed the back cover sheet to the second part of the second path segment and to change the moving direction of the back cover sheet.

[0017] Typically, at the downstream end of the second path segment, the method deposits a back cover sheet on a second portion of the aqueous gypsum slurry on the face cover sheet to form a multilayer assembly including the front cover sheet, the back cover sheet, and the second portion of the aqueous gypsum slurry. Most typically, the multilayer assembly includes a first portion of the aqueous gypsum slurry layer on the front cover sheet, a second portion of the aqueous gypsum slurry layer on the first portion of the aqueous gypsum slurry layer, and a back cover sheet on the second portion of the aqueous gypsum slurry layer, thereby forming the multilayer assembly.

[0018] The present invention provides a method for manufacturing a gypsum board, the method comprising: preparing an aqueous gypsum slurry containing a mixture of water and stucco, where the stucco contains calcium sulfate hemihydrate and the aqueous gypsum slurry contains at least 60% by weight, typically 60 - 98% by weight, preferably at least 70% by weight, more preferably at least 80% by weight, typically at least 90% by weight, or typically at least 95% by weight of said calcium sulfate hemihydrate on a dry (anhydrous) basis, and preparing a mixture with water in a weight ratio of water to calcium sulfate hemihydrate of 0.2:1 to 1.2:1; placing a front cover sheet on the forming surface and horizontally moving the front cover sheet along the forming surface in the machine direction; depositing a first portion of the aqueous gypsum slurry on the front cover sheet to form a layer of the first portion of the aqueous gypsum slurry as a layered higher density region in contact with the upper surface of the front cover sheet; moving a back cover sheet along a back cover sheet path including a first path segment and a second path segment, The back cover sheet moves along a first path segment above a region of higher density on the front cover sheet, with a movement including a first vertical movement component and / or a first horizontal movement component. The first vertical movement component is a downward movement toward the region of higher density on the front cover sheet, and the first horizontal movement component is a movement in a direction opposite to the machine direction. Preferably, by simultaneously having both the first vertical movement component and the first horizontal movement component along all or the same part of the first segment, the back cover sheet moves along the downstream part of the first path segment, along an inclination defining an angle "A" of 0 to 90 degrees between the back cover sheet in the downstream part of the first path segment and the front cover sheet on the forming surface. Typically, the angle "A" is from 0 to 90 degrees, more typically, the angle "A" is from 5 to 90 degrees or from 5 to 85 degrees, more typically, the angle "A" is from 0 to 20 degrees or from 20 to 60 degrees. The slurry mixer discharges a second part of the aqueous gypsum slurry that is less dense than a first part of the aqueous gypsum slurry for contacting the surface of the back cover sheet in the first path segment. The slurry contacts the back paper at an incident angle "B" of 0 to 90 degrees, for example, 0 to 20 degrees, preferably 5 to 90 degrees, more preferably 5 to 85 degrees, more preferably 5 to 60 degrees, with respect to the surface of the back cover sheet that the slurry contacts. Typically, the discharge of the calcined gypsum core slurry can be in a countercurrent direction to the movement direction of the back cover sheet, or typically, the discharge of the calcined gypsum core slurry can be in a co-current direction to the movement direction of the back cover sheet. At the downstream end of the first path segment, the back cover sheet then passes around a first transfer roller to feed the back cover sheet to a second path segment. The second path segment has an upstream end and a downstream end, and the upstream end is at the first transfer roller. Preferably, the first transfer roller changes the moving direction of the back cover sheet to a movement along a first portion of a second path segment having a second horizontal movement component and a second vertical movement component in the machine direction, and the second vertical movement component is upward, or downward, or neutral to provide only movement along the second horizontal movement component. Typically, the second vertical movement component is an upward movement away from a higher density region on the front cover sheet. moving the first transfer roller to be spaced at a distance above a higher density region on the front cover sheet; passing a second portion of the aqueous gypsum slurry over the first transfer roller to deposit a majority by weight of the second portion of the aqueous gypsum slurry over a higher density region on the front cover sheet. Typically, when this deposition occurs, the remaining portion of the second portion of the aqueous gypsum slurry remains in contact with the back cover sheet as the back cover sheet moves within the second path segment. Preferably, the first transfer roller is a freewheel roller. Typically, the second path segment has a first portion and a second portion. At the downstream end of the first portion of the second path segment, depositing the back cover sheet over a second portion of the aqueous gypsum slurry on the face cover sheet to form a multi-layer assembly including the back cover sheet over the second portion of the aqueous gypsum slurry, and typically, the multi-layer assembly includes a first portion of an aqueous gypsum slurry layer on the front cover sheet, a second portion of the aqueous gypsum slurry layer over the first portion of the aqueous gypsum slurry layer, and the back cover sheet over the second portion of the aqueous gypsum slurry layer, by passing the back cover sheet over a second transfer roller to feed the back cover sheet into the second portion of the second path segment and change the moving direction of the back cover sheet. Depositing the back cover sheet over a second portion of the aqueous gypsum slurry on the face cover sheet at the downstream end of the second path segment to form a multi-layer assembly including the back cover sheet over the second portion of the aqueous gypsum slurry, and typically, the multi-layer assembly includes a first portion of an aqueous gypsum slurry layer on the front cover sheet, a second portion of the aqueous gypsum slurry layer over the first portion of the aqueous gypsum slurry layer, and the back cover sheet over the second portion of the aqueous gypsum slurry layer. Passing the multi-layer assembly through a forming station for forming the multi-layer assembly, preferably passing the multi-layer assembly under a forming plate of the forming station. Curing calcium sulfate hemihydrate to form a panel comprising a gypsum core containing calcium sulfate dihydrate, drying the panel and cutting the panel into gypsum boards having one or more predetermined dimensions, wherein a first portion of the gypsum slurry forms a layer-like higher density region in contact with the front cover sheet, a second portion of the gypsum slurry is in a foamed state as a lower density region in contact with the higher density region, and the higher density region has a higher density than the lower density region, and the board core includes a cured lower density region containing calcium sulfate dihydrate and a cured higher density region containing calcium sulfate dihydrate, and cutting, wherein the cured higher density region is interposed as a layer between the cured lower density region and the front cover sheet.

[0019] The present invention also provides an apparatus for manufacturing a gypsum board, the apparatus comprising: a forming surface for depositing a front cover sheet thereon and horizontally moving the front cover sheet in a machine direction along the forming surface; a source of a first portion of an unfoamed aqueous gypsum slurry for depositing the first portion of the aqueous gypsum slurry on the front cover sheet to form a layer of the aqueous gypsum slurry as a layer-like higher density region in contact with the upper surface of the front cover sheet, wherein the first aqueous gypsum slurry layer has a first slurry density; a first transfer roller and typically a second transfer roller for moving a back cover sheet along a back cover sheet path comprising a first path segment and a second path segment; The first path segment is adapted and configured to move along the first path segment above a region of higher density on the front cover sheet with a movement involving a first vertical movement component and / or a first horizontal movement component, wherein the first vertical movement component is a downward movement towards the region of higher density on the front cover sheet, and the first horizontal movement component is a movement in a direction opposite to the machine direction, a back cover sheet drive unit, A second portion of the aqueous gypsum slurry that is less dense than the first portion of the aqueous gypsum slurry for contacting the surface of the back cover sheet in the first path segment, wherein the second portion of the aqueous gypsum slurry contacts the back cover sheet at an incident angle "B" of 0 to 90 degrees, such as 0 to 20 degrees, preferably 5 to 90 degrees, more preferably 5 to 85 degrees, even more preferably 5 to 60 degrees, with respect to the surface of the back cover sheet that the second portion of the aqueous gypsum slurry contacts, a source of the second portion of the aqueous gypsum slurry, A first transfer roller at the downstream end of the first path segment for passing the back cover sheet around the first transfer roller to feed the back cover sheet to the second path segment, wherein the second path segment has an upstream end and a downstream end, and the upstream end of the second path segment is at the first transfer roller, The first transfer roller is spaced a distance above the region of higher density on the front cover sheet to pass the second portion of the aqueous gypsum slurry over the first transfer roller and deposit a majority by weight of the second portion of the aqueous gypsum slurry that has fallen from the back cover sheet onto the region of higher density on the front cover sheet. Typically, when this deposition occurs, the remaining portion of the second portion of the aqueous gypsum slurry remains in contact with the back cover sheet as the back cover sheet moves within the second path segment, The first portion of the aqueous gypsum slurry and the second portion of the aqueous gypsum slurry each contain a respective mixture of water and stucco, the stucco contains calcium sulfate hemihydrate, and the first portion of the aqueous gypsum slurry and the second portion of the aqueous gypsum slurry each contain a mixture of at least 60% by weight, typically 60 - 98% by weight, preferably at least 70% by weight, more preferably at least 80% by weight, typically at least 90% by weight, or typically at least 95% by weight of said calcium sulfate hemihydrate and water, in a weight ratio of water to calcium sulfate hemihydrate of 0.2:1 to 1.2:1, a first transfer roller, Deposit a back cover sheet on the second portion of the aqueous gypsum slurry on the front cover sheet to form a multilayer assembly including the back cover sheet on the second portion of the aqueous gypsum slurry, preferably including the back cover sheet on the second portion of the aqueous gypsum slurry, preferably including a surface cover sheet, a first portion of the aqueous gypsum slurry layer on the front cover sheet, a second portion of the aqueous gypsum slurry layer on the first portion of the aqueous gypsum slurry layer, and a back cover sheet on the second portion of the aqueous gypsum slurry layer, the downstream end of the second path segment arranged and positioned to form a multilayer assembly, A forming station for forming a multilayer assembly, preferably the forming station comprises a forming plate, a forming station, Curing the calcium sulfate hemihydrate to form a panel comprising a gypsum core containing calcium sulfate dihydrate, The first portion of the gypsum slurry forms a layered higher density region in contact with the front cover sheet, The second portion of the gypsum slurry is in a foamed state as a lower density region in contact with the higher density region, and the higher density region has a higher density than the lower density region, The board core includes a cured lower density region containing calcium sulfate dihydrate and a cured higher density region containing calcium sulfate dihydrate, and forming the cured higher density region to be interposed as a layer between the cured lower density region and the front cover sheet.

[0020] Typically, the apparatus comprises a mixer for preparing an aqueous gypsum slurry for a first portion of an aqueous gypsum slurry containing a mixture of water and stucco, the stucco contains calcium sulfate hemihydrate, and the aqueous gypsum slurry contains at least 60 wt%, typically 60 - 98 wt%, preferably at least 70 wt%, more preferably at least 80 wt%, typically at least 90 wt%, or typically at least 95 wt% of said calcium sulfate hemihydrate and water in a weight ratio of water to calcium sulfate hemihydrate of 0.2:1 to 1.2:1.

[0021] Preferably, by causing both the first vertical movement component and the first horizontal movement component to be present simultaneously along all or the same part of the first segment, the back cover sheet moves along the downstream portion of the first path segment, along an inclination defining an angle "A" of 0 - 90 degrees between the back cover sheet at the downstream portion of the first path segment and the front cover sheet on the forming surface. Typically, the angle "A" is 0 - 90 degrees, more typically the angle "A" is 5 - 90 degrees or 5 - 85 degrees, and more typically the angle "A" is 20 - 60 degrees.

[0022] Typically, the discharge of the calcined gypsum core slurry is in a countercurrent direction to the direction of movement of the back cover sheet, or typically, the discharge of the calcined gypsum core slurry can be in a cocurrent direction to the direction of movement of the back cover sheet.

[0023] Preferably, the first transfer roller changes the direction of movement of the back cover sheet to movement along a first part of a second path segment having a second horizontal movement component and a second vertical movement component in the machine direction, and the second vertical movement component is upward, or downward, or neutral to provide only movement along the second horizontal movement component. Typically, the second vertical movement component is upward movement away from a higher density region on the front cover sheet.

[0024] Preferably, the first transfer roller is a freewheel roller.

[0025] Typically, the second path segment has a first portion and a second portion, and at the downstream end of the first portion of the second path segment, the back cover sheet passes over a second transfer roller to feed the back cover sheet to the second portion of the second path segment and to change the direction of movement of the back cover sheet.

[0026] The present invention also provides an apparatus for manufacturing a gypsum board, the apparatus comprising a mixer for preparing an aqueous gypsum slurry containing a mixture of water and stucco, the stucco containing calcium sulfate hemihydrate, the aqueous gypsum slurry containing at least 60% by weight, typically 60 - 98% by weight, preferably at least 70% by weight, more preferably at least 80% by weight, typically at least 90% by weight, or typically at least 95% by weight of said calcium sulfate hemihydrate on a dry (anhydrous) basis, and a mixer containing a mixture of water and the calcium sulfate hemihydrate in a weight ratio of water to calcium sulfate hemihydrate of 0.2:1 to 1.2:1, a forming surface for depositing a front cover sheet thereon and horizontally moving the front cover sheet in the machine direction along the forming surface, a source of a first portion of the aqueous gypsum slurry in an unfoamed state for depositing a first portion of the aqueous gypsum slurry on the front cover sheet to form a layer of the aqueous gypsum slurry as a layer of higher density in contact with the upper surface of the front cover sheet, a first transfer roller and a second transfer roller for moving a back cover sheet along a back cover sheet path comprising a first path segment and a second path segment, The first path segment is adapted and configured to move along the first path segment above a region of higher density on the front cover sheet with a movement involving a first vertical movement component and / or a first horizontal movement component, the first vertical movement component being a downward movement towards the region of higher density on the front cover sheet, and the first horizontal movement component being a movement in a direction opposite to the machine direction, a back cover sheet drive unit, Preferably, by simultaneously presenting both the first vertical movement component and the first horizontal movement component along all or the same portion of the first segment, the back cover sheet moves along an inclination defining an angle "A" of 0 to 90 degrees between the back cover sheet at the downstream portion of the first path segment and the front cover sheet on the forming surface along the downstream portion of the first path segment, typically the angle "A" is from 0 to 90 degrees, more typically the angle "A" is from 5 to 90 degrees or from 5 to 85 degrees, more typically the angle "A" is from 20 to 60 degrees, a back cover sheet drive unit, A second portion of the aqueous gypsum slurry that is less dense than the first portion of the aqueous gypsum slurry for contacting the surface of the back cover sheet in the first path segment, the slurry contacting the back paper at an incident angle "B" of 0 to 90 degrees, for example, 0 to 20 degrees, preferably 5 to 90 degrees, more preferably 5 to 85 degrees, more preferably 5 to 60 degrees, with respect to the surface of the back cover sheet that the slurry contacts, Typically, the discharge of the calcined gypsum core slurry can be in a countercurrent direction to the direction of movement of the back cover sheet, or typically, the discharge of the calcined gypsum core slurry can be in a co-current direction to the direction of movement of the back cover sheet, a second portion, A first transfer roller at the downstream end of the first path segment for passing the back cover sheet around a first transition to feed the back cover sheet to a second path segment, the second path segment having an upstream end and a downstream end, the upstream end being at the first transfer roller, Preferably, the first transfer roller changes the moving direction of the back cover sheet to a movement along a first portion of a second path segment having a second horizontal movement component and a second vertical movement component in the machine direction, and the second vertical movement component is upward, or downward, or neutral in order to provide only movement along the second horizontal movement component. Typically, the second vertical movement component is a movement in an upward direction away from a higher density region on the front cover sheet. The first transfer roller is spaced at a distance above a higher density region on the front cover sheet to pass a second portion of the aqueous gypsum slurry over the first transfer roller and deposit a majority by weight of the second portion of the aqueous gypsum slurry on the higher density region on the front cover sheet. Typically, when this deposition occurs, the remaining portion of the second portion of the aqueous gypsum slurry remains in contact with the back cover sheet as the back cover sheet moves within the second path segment. Preferably, the first transfer roller is a freewheel roller. Typically, the second path segment has a first portion and a second portion. At the downstream end of the first portion of the second path segment, the back cover sheet passes over a second transfer roller to feed the back cover sheet to the second portion of the second path segment and change the moving direction of the back cover sheet, and the first transfer roller. A downstream end of a second path segment arranged to deposit the back cover sheet on a second portion of the aqueous gypsum slurry on the front cover sheet to form a multi-layer assembly including the back cover sheet on the second portion of the aqueous gypsum slurry. Preferably, it includes the back cover sheet on the second portion of the aqueous gypsum slurry. Preferably, it includes a surface cover sheet, a first portion of an aqueous gypsum slurry layer on the front cover sheet, a second portion of the aqueous gypsum slurry layer on the first portion of the aqueous gypsum slurry layer, and the back cover sheet on the second portion of the aqueous gypsum slurry layer to form a multi-layer assembly. A forming station for forming a multi-layer assembly. Preferably, the forming station comprises a forming plate. Hardening calcium sulfate hemihydrate to form a panel comprising a gypsum core containing calcium sulfate dihydrate, A first portion of the gypsum slurry forms a layered higher density region that contacts the front cover sheet, A second portion of the gypsum slurry is in a foamed state as a lower density region that contacts the higher density region, and the higher density region has a higher density than the lower density region, The board core includes a cured lower density region containing calcium sulfate dihydrate and a cured higher density region containing calcium sulfate dihydrate, and the cured higher density region is interposed as a layer between the cured lower density region and the front cover sheet, including forming.

[0027] The present invention also provides a gypsum board produced according to the method of the present invention or by the apparatus of the present invention.

Brief Description of the Drawings

[0028]

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[0029] Like reference numerals in the figures indicate like elements unless otherwise indicated.

DETAILED DESCRIPTION OF THE INVENTION

[0030] The present invention includes a method and apparatus for orienting a back paper drive unit towards the deposition and distribution of core slurry onto the back paper, and then dropping it from the back paper onto a high-density layer on the face paper in a gypsum board manufacturing line to form a core layer, and then applying the back paper onto the core layer.

[0031] Rather than a higher density layer slurry on the face paper, the initial contact surface between the core slurry and the back paper avoids the significant contact forces and hydraulic jumps that can occur when the core slurry is applied directly from the mixer onto the higher density layer. Such significant contact forces and hydraulic jumps can move the higher density layer out of position and cause washout.

[0032] The proposed invention helps avoid washout of the denser layer on the face paper by not depositing the core layer slurry directly from the mixer onto the denser layer on the face paper. This helps avoid an otherwise non-uniform cross-sectional gypsum distribution and improves the overall conformance of the gypsum density distribution, nail pull test results, and board properties.

[0033] One skilled in the art could modify the methods described herein to produce interior wallboard having a gypsum core between paper facer sheets, exterior and roof gypsum panels, gypsum tile backer board, or other gypsum building panels. A typical gypsum panel produced in accordance with the present invention comprises, from front to back, a first fiber mat and a gypsum core layer having a front face and a back face, the gypsum core layer having a thickness of from about 0.25 inches to about 1.6 inches, typically from about 0.25 inches to about 1.25 inches, preferably from about 0.25 inches to about 1 inch, the first fiber mat being attached to the front face of the gypsum core layer as a facer cover sheet, and the second fiber mat being attached to the back face of the gypsum core layer as a backer cover sheet. The gypsum core layer comprises greater than about 50 weight percent, preferably at least about 75 weight percent, more preferably at least about 85 weight percent calcium sulfate dihydrate. The first fiber mat and the second fiber mat can comprise paper or a fibrous material (e.g., one or more of polymer fibers, glass fibers, and mineral fibers).

[0034] As used herein, all weight percent values are weight percent unless otherwise indicated. As used herein, "total dry weight" or "on a dry weight basis" refers to the weight of a mixture excluding any water components that may be present. "Water components" excludes water that may be present in the gypsum crystal structure. In contrast, "on a wet basis" includes water in weight percent calculations.

[0035] Gypsum board Gypsum board is a gypsum product having a board shape (i.e., in particular, at least substantially flat). Gypsum board typically has a rectangular shape.

[0036] Figure 1 shows an embodiment of the wall board panel 10 of the present invention. Figure 2 shows a top (axial direction) view of the wall board panel 10. The board has a core 28 that includes a lower density region (low density region) 12 cured as a layer containing calcium sulfate dihydrate and a higher density region (densified region, densified layer, or also known as a thin high density gypsum layer) 22 cured as a layer containing calcium sulfate dihydrate.

[0037] Figure 1 shows the wall board panel 10 of the present invention. The core 28 including a lower density region 12 of gypsum (e.g., 0.5 inches thick) is between a back cover sheet 14 (also known as a backing cover sheet) and a front cover sheet 16 (also known as a facer cover sheet), and each of these can be a fiber material such as a single-layer or multi-layer paper or a glass fiber mat. The inner surface of the back cover sheet 14 forms a bonding side 24 of the back cover sheet 14 facing the gypsum core 28. The gypsum core 28 has a thickness of about 0.25 inches to about 1.6 inches, typically about 0.25 inches to about 1.25 inches, preferably about 0.25 inches to about 1 inch. The inner surface of the front cover sheet 16 forms a bonding surface 26 of the front cover sheet 16 facing the lower density region 12 of gypsum. The higher density region (thin dense gypsum layer) 22 is located between the lower density region (low density region) 12 of gypsum and the front cover sheet 16 and is in contact with the lower density region 12 of gypsum and the front cover sheet 16. The outer surface of the back cover sheet 14 faces a room wall frame (not shown) after the wall board panel 10 is installed as an interior wall. The outer surface of the front cover sheet 16 faces the inside of the room after the wall board panel 10 is installed as an interior wall.

[0038] Generally, the relatively lower density region 12 and the relatively higher density region 22 have the same composition and are adjacent to each other. However, the lower density region can be formed from a foamed gypsum slurry, while the higher density region can be formed from a non-foamed gypsum slurry such that a higher density layer is formed. That is, the higher density region can have a lower porosity associated therewith than the lower density region.

[0039] The lower density region obtained from the slurry of the hardened gypsum, such as the lower density region 12 in FIG. 1, generally has a thickness of 0.25 inches to 1.5 inches, typically 0.3 inches to 1 inch, or 0.4 inches to 0.75 inches.

[0040] The higher density region 22 has a thickness of about 0.02 inches to about 0.75 inches, or about 0.02 inches to about 0.35 inches. More typically, the higher density region 22 has a thickness of about 0.02 inches to about 0.2 inches (about 0.05 to about 0.5 cm), for example, about 0.0625 inches to about 0.125 inches (about 0.16 to about 0.32 cm). Typically, the thickness of the lower density region layer 12 is greater than the thickness of the higher density region layer 22. Typically, the higher density region, such as the higher density region 22 in FIG. 1, generally has a thickness that is 2% to 60% of the thickness of the gypsum board 10, typically 2% to 50%, more typically 5% to 40%, or more typically 5% to 25%.

[0041] The density of the higher density region layer 22 is greater than the density of the lower density region layer 12. The lower density region 12 has a density of 15 to 55 pounds per cubic foot. Typically, the higher density region 22 has a density of 25 to 70 pounds per cubic foot, more typically 30 to 60, or even more typically 35 to 60 pounds per cubic foot.

[0042] When foamed, the lower density region layer of gypsum obtained from the cured foamed gypsum slurry has a total void volume (the sum of water voids and air voids) of 50 to 92 volume percent. The cured higher density region layer has a total void volume of less than 40 to 85 volume percent.

[0043] The combined density of the lower density region 12 and the higher density region 22 of the gypsum is from about 15 pounds per cubic foot to about 65 pounds per cubic foot, more typically from 20 pounds per cubic foot to about 65 pounds per cubic foot, or from 25 pounds per cubic foot to about 65 pounds per cubic foot, for example, from 25 pounds per cubic foot to 55 pounds per cubic foot.

[0044] Typically, the gypsum board has a width of 3 to 5 feet. The cured higher density region has an average thickness of 20 to 40 mils when measured from the number average of a series of 1-inch wide samples taken across the width of the cross-section of the board. The cured higher density region thickness of each sample has a difference of + / - 20 mils from the average thickness of the cured higher density region of the sample. The minimum thickness of the cured higher density region of the sample is at least 10 mils. The ratio of the minimum thickness to the average thickness of the cured higher density region of each sample, expressed as a percentage, is 40 to 100%, typically 40 to 90%, 40 to 80%, or 45 to 80%.

[0045] For example, the thickness of the cured higher density regions of each sample can be determined by cutting the board vertically across the width of the board into a plurality of samples, e.g., 48 one-inch (across the width) samples, slicing each sample in a direction parallel to the front and back surfaces of the board to form a plurality of horizontal slices of the sample, e.g., 10 to 100 horizontal slices. Then, the density of each slice is measured. Slices having a higher density represent the cured higher density regions. Slices having a relatively lower density represent the lower density regions. The total thickness of the slices of a sample having a higher density represents the thickness of the cured higher density region of that sample. Next, the thickness of the cured higher density regions of the samples across the width of the board and the position of each sample across the width of the board are tabulated and / or plotted to determine the lateral profile of the cured higher density regions across the board.

[0046] Method for the manufacture of gypsum boards Various methods can be employed to prepare the gypsum boards of the present invention from an aqueous gypsum slurry containing calcium sulfate hemihydrate.

[0047] The substrate on which gypsum wallboards and other gypsum products are manufactured is the hemihydrate form of calcium sulfate (CaSO4·1 / 2H2O), commonly referred to as "calcined gypsum" or "stucco", and is produced by the thermal conversion (calcination) of the dihydrate form of calcium sulfate (CaSO4).

[0048] The present invention encompasses a method of making a gypsum board, the method comprising preparing an aqueous gypsum slurry comprising a mixture of water and stucco, wherein the stucco comprises calcium sulfate hemihydrate and the aqueous gypsum slurry comprises at least 60 wt%, typically 60 - 98 wt%, preferably at least 70 wt%, more preferably at least 80 wt%, typically at least 90 wt%, or typically at least 95 wt% of said calcium sulfate hemihydrate on a dry (anhydrous) basis, and Preparing a mixture with water in a weight ratio of water to calcium sulfate hemihydrate of 0.2:1 to 1.2:1, Placing a front cover sheet (also known as the first cover sheet or face cover sheet or facer) on the forming surface and horizontally moving the front cover sheet along the machine direction along the forming surface, Depositing a first portion of the aqueous gypsum slurry on the front cover sheet to form a layer of the first portion of the aqueous gypsum slurry as a layered higher density region in contact with the upper surface of the front cover sheet, Moving a back cover sheet (also known as the back cover sheet or backer cover sheet or backer) along a back cover sheet path including a first path segment and a second path segment, The back cover sheet moves along the first path segment above the higher density region on the front cover sheet with a movement including a first vertical movement component and / or a first horizontal movement component, the first vertical movement component being a downward movement towards the higher density region on the front cover sheet, and the first horizontal movement component being a movement in a direction opposite to the machine direction, Preferably, by simultaneously presenting both the first vertical movement component and the first horizontal movement component along all or the same part of the first segment, the back cover sheet moves along the downstream part of the first path segment along an inclination defining an angle "A" of 0 to 90 degrees (see, for example, FIGS. 3, 5 and 6) between the back cover sheet within the first path segment and the front cover sheet on the forming surface, typically the angle "A" being from 0 to 90 degrees, more typically the angle "A" being from 5 to 90 degrees or 5 to 85 degrees, more typically the angle "A" being from 20 to 60 degrees, The slurry mixer discharges a second portion of the aqueous gypsum slurry that is less dense than a first portion of the aqueous gypsum slurry for contacting the surface of the back cover sheet in the first path segment, and the slurry contacts the back paper at an angle of incidence "B" of 0 to 90 degrees, for example, 0 to 20 degrees, preferably 5 to 90 degrees, more preferably 5 to 85 degrees, still more preferably 5 to 60 degrees, with respect to the surface of the back cover sheet with which the slurry contacts. Typically, the discharge of the calcined gypsum core slurry can be in a countercurrent direction to the direction of movement of the back cover sheet, or typically, the discharge of the calcined gypsum core slurry can be in a cocurrent direction to the direction of movement of the back cover sheet. At the downstream end of the first path segment, the back cover sheet then passes around a first transition to feed the back cover sheet into a second path segment. The second path segment has an upstream end and a downstream end, and the upstream end is at a first transition roller. Preferably, the first transition roller changes the direction of movement of the back cover sheet to movement along a first portion of the second path segment having a second horizontal movement component and a second vertical movement component in the machine direction, and the second vertical movement component is upward, or downward, or neutral to provide only movement along the second horizontal movement component. Typically, the second vertical movement component is upward movement away from a higher density region on the front cover sheet. Moving the first transition roller at a distance above a higher density region on the front cover sheet. Passing the second portion of the aqueous gypsum slurry over the first transition roller and depositing a majority by weight of the second portion of the aqueous gypsum slurry over a higher density region on the front cover sheet. Typically, when this deposition occurs, the remaining portion of the second portion of the aqueous gypsum slurry remains in contact with the back cover sheet as the back cover sheet moves within the second path segment. Preferably, the first transition roller is a freewheel roller. Typically, the second path segment has a first portion and a second portion, and at the downstream end of the first portion of the second path segment, the back cover sheet passes over the second transfer roller to feed the back cover sheet to the second portion of the second path segment and to change the direction of movement of the back cover sheet, depositing, At the downstream end of the second path segment, depositing the back cover sheet on the second portion of the aqueous gypsum slurry on the face cover sheet to form a multilayer assembly including the back cover sheet on the second portion of the aqueous gypsum slurry, typically, the multilayer assembly includes a first portion of the aqueous gypsum slurry layer on the front cover sheet, a second portion of the aqueous gypsum slurry layer on the first portion of the aqueous gypsum slurry layer, and the back cover sheet on the second portion of the aqueous gypsum slurry layer, forming a multilayer assembly, Passing the multilayer assembly through a forming station for forming the multilayer assembly, preferably, the multilayer assembly passes under the forming plate of the forming station, passing, Curing calcium sulfate hemihydrate to form a panel including a gypsum core containing calcium sulfate dihydrate, Drying the panel and cutting the panel into gypsum boards having one or more predetermined dimensions, The first portion of the gypsum slurry forms a layered higher density region in contact with the front cover sheet, The second portion of the gypsum slurry is in a foamed state as a lower density region in contact with the higher density region, and the higher density region has a higher density than the lower density region, Cutting, wherein the board core includes a cured lower density region containing calcium sulfate dihydrate and a cured higher density region containing calcium sulfate dihydrate, and the cured higher density region is interposed as a layer between the cured lower density region and the front cover sheet.

[0049] To produce gypsum board, stucco is mixed with water and optionally other additives to form an aqueous gypsum slurry, which is continuously fed between successive layers of paper on a board machine. One cover sheet is referred to as the front cover sheet, or the front cover sheet or facer. The other cover sheet is referred to as the back cover sheet orbacker. Since gypsum board is typically formed "face down", this front cover sheet typically corresponds to the facer (front cover sheet) at the completion of the manufacturing process. Typically, stucco is mixed with water and additives to form an aqueous slurry, which is continuously fed between successive layers of sheets, such as paper sheets, on a board machine. To form the aqueous gypsum slurry, the dry and / or wet components of the aqueous gypsum slurry are fed to a mixer (e.g., a pin or pinless mixer), where they are agitated. The aqueous gypsum slurry can be made at any suitable water / calcium sulfate hemihydrate ratio. As the board moves down the conveyor line to form the panel, the calcium sulfate hemihydrate recrystallizes or rehydrates back to the original rock-like calcium sulfate dihydrate. When the gypsum has cured, the cover sheets are bonded to the core. Next, the panel is cut to a predetermined length and conveyed through a dryer to remove free moisture.

[0050] The method deposits a first layer of aqueous gypsum slurry and a second layer of aqueous gypsum slurry. The first layer of aqueous gypsum slurry is relatively denser than the second layer of aqueous gypsum slurry. The method includes depositing a relatively high density layer on a first cover sheet, typically a front cover sheet (also known as a face cover sheet or face paper), that moves horizontally in the machine direction along the forming surface, directing the core slurry dispense from a core slurry mixer directly onto a second cover sheet, typically a back cover sheet (also known as a back cover sheet or back paper), and then dripping the core slurry from the second cover sheet (back paper) onto the relatively high density layer on the face paper.

[0051] Figure 3 shows a first embodiment of an apparatus for practicing the present method. In particular, Figure 3 illustrates an example of the wet end 80 of a manufacturing production line for producing a laminated gypsum board of the present invention having a gypsum layer between two cover sheets and provided with a densified layer. The cover sheets are made of paper such as, for example, manila paper or kraft paper.

[0052] The wet end 80 includes a gypsum slurry mixing and dispensing assembly 82 and a forming station 86 having a forming plate 86A. Figure 3 shows stacked particles sent to the gypsum slurry mixing and dispensing assembly 82 as a stream 74. The gypsum slurry mixing and dispensing assembly 82 has a mixer 81 in a fixed position within the wet end and a dispensing unit 83 from the mixer 81 for conveying the slurry stream 74. The dispensing unit 83 typically includes a hose and, optionally, a boot (not shown) at the downstream end of the hose. The boot is an "L"-shaped conduit.

[0053] A first moving web 90 of front cover sheet material (face sheet from face paper roll 89) is deposited on the forming surface 92 and moves in the longitudinal (machine) direction "T" of travel along the forming table 92. In particular, the method discharges the front cover sheet onto the moving conveyor 92. A lower density region slurry 94 of gypsum is mixed within the gypsum slurry mixing and dispensing assembly 82, where an additive is added and foaming of the slurry for the lower density region layer (e.g., layer 12 of FIG. 1) occurs. The gypsum slurry mixing and dispensing assembly 82 is illustrated as a single component of the wet end 80, but there may be a plurality of components that make up the gypsum slurry mixing and dispensing assembly 82.

[0054] The first portion of the aqueous gypsum slurry (densified layer slurry) 70 is applied onto the front cover sheet material 90 (e.g., face paper or glass mat) to form a densified layer (e.g., the higher density region 22 in FIG. 1) on the front cover sheet material 90. Next, the front cover sheet (face paper) 90 passes under the densified layer roller 72 to spread the first portion of the aqueous gypsum slurry 70, providing a first layer of the aqueous gypsum slurry (relatively high density layer slurry) 76. Next, the front cover sheet 90 having the layer 76 of the high density slurry 70 moves horizontally in the machine direction "T" along the forming surface 92.

[0055] The densified layer slurry 70 can be obtained from the gypsum slurry mixing and dispensing assembly 82 or a separate slurry mixer (not shown). The densified layer slurry 76 is applied to a second cover sheet material (back cover sheet material, e.g., paper or glass mat) 96 to form a second higher density region layer (e.g., the layer 20 in FIG. 3). Typically, the first portion 70 of the aqueous gypsum slurry does not foam.

[0056] The back cover sheet 96 moves from the back cover sheet roll 95 along the rollers into a back cover sheet (back paper) path having a first path segment 50 and a second path segment 55. Thus, the apparatus includes a back cover sheet drive that defines a back cover sheet path having the first path segment 50 and the second path segment 55. In FIG. 3, the second path segment 55 has an upstream portion 60A and a downstream portion 60B. As seen in FIG. 3, the back cover sheet 96 travels alone (except as it travels with the slurry it conveys) along the first path segment 50 and the second path segment 55 so as not to travel on the moving belt. This avoids problems arising from having a moving belt for conveying the slurry.

[0057] The back cover sheet 96 moves in the first direction 52 while moving along a first path segment above a higher density region on the front cover sheet, and the movement of the back cover sheet 96 along the first path segment has a first vertical movement component and / or a first horizontal movement component. The first vertical movement component is a downward movement toward the higher density region 76 on the front cover sheet 90. The first horizontal movement component is a movement in a direction opposite to the horizontal machine direction "T".

[0058] The first path segment 50 has an upstream portion 50A and a downstream portion 50B. Typically, by simultaneously having both the first vertical movement component and the first horizontal movement component along all or the same part of the first segment, the back cover sheet moves along the downstream portion 50B of the first path segment 50, along an inclination defining an angle "A" of 0 to 90 degrees between the back cover sheet 96 and the front cover sheet 90 on the forming surface at the downstream portion 50B of the first path segment 50. Typically, the angle "A" is from 0 to 90 degrees, more typically, the angle "A" is from 5 to 90 degrees or from 5 to 85 degrees, and more typically, the angle "A" is from 20 to 60 degrees or from 0 to 20 degrees.

[0059] More preferably, both the first vertical movement component and the first horizontal movement component are present simultaneously along all or the same part of the first path segment 50. As a result, the back cover sheet 96 moves in the first direction 52 of the back paper progression, along at least the downstream portion 50B of the first path segment 50, with a gradient inclined downward toward the high density layer (higher density region) on the front cover sheet 90, and the angle "A" is typically from 5 to 85 degrees, for example, from 20 to 60 degrees.

[0060] However, the first vertical movement component and the first horizontal movement component can be continuous along different portions of the first path segment 50, as shown, for example, in the catenary support systems of FIGS. 7A and 7B having a vertical upstream portion 50B for the first vertical movement and a horizontal downstream portion 50B for the subsequent first horizontal movement. This will be considered in more detail elsewhere in this specification.

[0061] The cover sheet material may not be coated, or may be coated, for example, with a pre-applied outer polymer coating and a hydrophobic finish. Typically, the outer surfaces of the applied moving webs 90 and 96, and the resulting outer surfaces of the front and back cover sheets of the gypsum board, are not coated and are not in contact with additional layers.

[0062] The slurry outlet of the gypsum slurry mixing and dispensing assembly 82 discharges a second portion of the aqueous gypsum slurry 74 and contacts the surface of the back cover sheet 96 within the first path segment 50. Typically, the second portion 74 of the aqueous gypsum slurry is of lower density than the first portion 70 of the aqueous gypsum slurry. Typically, the slurry outlet of the gypsum slurry mixing and dispensing assembly 82 discharges the second portion of the aqueous gypsum slurry 74 onto a portion of the back cover sheet 96 supported by the back plate 79. This discharge of the second portion 74 of the aqueous gypsum slurry can be in a direction of flow opposite to the first direction of travel of the back paper, as shown in FIG. 3.

[0063] Like reference numerals in FIGS. 3, 5, 6, 6A-6E, 7A, 7B and 8 indicate like elements unless otherwise indicated.

[0064] Typically, the apparatus of FIGS. 5 and 6B deposits the discharge of the second portion 74 of the aqueous gypsum slurry in a direction parallel and / or aligned with the longitudinal axis "LA" (see FIG. 6B) of the backsheet travel path along the first path segment 50 in a co-current direction with respect to the first direction 52 of backsheet travel along the first path segment 50. FIG. 5 shows the distance "L1" from the location where the aqueous gypsum slurry from the slurry mixer 81 is deposited on the back cover sheet 96 to the location where the aqueous gypsum slurry falls onto the first deposited portion of the aqueous gypsum slurry on the front cover sheet 90. The distance "L1" is typically 10 to 30 feet or 10 to 20 feet. FIG. 5 also shows the distance "L3" from the location where the discharge of the second portion of the aqueous gypsum slurry from the slurry mixer 81 falls onto the first deposited portion of the aqueous gypsum slurry via the roller 54 to the forming plate 86A. The distance "L3" is typically 10 to 30 feet or 10 to 20 feet.

[0065] Typically, the line speed in the direction "T" is 1 to 20 feet per second, typically 2 to 10 feet per second.

[0066] In FIG. 6B, this discharge of the second portion 74 of the aqueous gypsum slurry is in a counter-current direction of deposition inclined with respect to the first direction 52 of backcover sheet travel along the first path segment 50. This deposits the discharge of the second portion of the aqueous gypsum slurry 74 in a co-current direction with respect to the first direction 52 of backsheet travel along the first path segment 50, but at a transverse angle θ with respect to the longitudinal axis "LA" of the backsheet travel path along the first path segment 50 at the point of contact with the aqueous gypsum slurry 74. Typically, the angle θ is + / - 20 degrees with respect to the longitudinal axis "LA".

[0067] In contrast, in an alternative form as shown in FIGS. 6 and 6A, this discharge of the second portion 74 of the aqueous gypsum slurry can be in a direction parallel to the first direction 52 of the backsheet progression along the first path segment 50, and the discharge direction is parallel and / or aligned with the longitudinal axis "LA" (see FIG. 6A) of the backsheet progression path along the first path segment 50 at the point of contact with the aqueous gypsum slurry 74.

[0068] FIG. 6 shows the backsheet (backsheet 96) moving on roller 97 and then proceeding along the first path segment 50. Typically, the gypsum slurry mixing and dispensing assembly 82 discharges the second portion of the aqueous gypsum slurry 74 at an acute angle of incidence "B" between the discharging second portion of the aqueous gypsum slurry 74 and the surface of the backsheet 96 to bring the second portion of the aqueous gypsum slurry 74 into contact with the backsheet 96. For example, the acute angle of incidence "B" can be from 0 to 60 degrees or from 0 to 5 degrees or from 5 to 60 degrees. This acute contact helps to dissipate the flow energy of the second portion 74 of the aqueous gypsum slurry and serves to spread the perpendicular flow (spread) of the second portion 74 of the aqueous gypsum slurry. The spread of the second portion 74 of the aqueous gypsum slurry helps to increase the subsequent contact area when the second portion 74 of the aqueous gypsum slurry deposits on the densified layer 76 as flow 94, effectively reducing the contact force.

[0069] FIG. 6 shows the horizontal distance "L1" from the discharge opening of the hose or boot 83 that applies the second portion 74 of the gypsum slurry to the backsheet 96, where the second portion 74 of the gypsum slurry falls from the transfer roller 54 to the first deposited portion of the aqueous gypsum slurry on the front cover sheet 90. Typically, the horizontal distance "L1" is from 10 to 30 feet or from 10 to 20 feet. FIG. 6 also shows the horizontal distance "L3" from the location where the discharge of the second portion of the aqueous gypsum slurry from the slurry mixer 81 falls onto the first deposited portion of the aqueous gypsum slurry on the front cover sheet 90 via the roller 54 to the forming plate 86A. The distance "L3" is from 10 to 30 feet or from 10 to 20 feet.

[0070] In another alternative shown in FIG. 6C, this discharge of the second portion 74 of the aqueous gypsum slurry can be in a direction of flow deposition perpendicular to the first direction 52 of the back cover sheet advancement along the first path segment 50. This is the direction of flow “F” perpendicular to the first direction 52 of the back paper advancement along the first path segment 50 for the discharge of the second portion of the aqueous gypsum slurry 74, but at the point of contact with the aqueous gypsum slurry 74, it is 90 degrees + / - 10 degrees, preferably 90 degrees + / - 5 degrees, more preferably 90 degrees + / - 2 degrees with respect to the longitudinal axis “LA” of the back paper advancement path along the first path segment 50 at an incident angle “C”. Typically, the incident angle “C” is 90 degrees with respect to the longitudinal axis “LA”.

[0071] The extended spread in the present invention can be achieved by incorporating a vibration support (see optional diaphragm 79A in FIG. 60) if desired.

[0072] FIG. 6E shows an additional roller 57A that can optionally be used in the method such that after the back cover sheet 96 moves from roller 54 to the additional roller 57A, it continues to travel downward, and the additional roller then redirects the back cover sheet 96 upward to roller 57.

[0073] Figures 7A and 7B show a caterpillar roller system 110 that can be used for a first path segment 50 of the back cover sheet advancement that can optionally be used in the present method. Figure 7A shows that this caterpillar roller system provides a back cover sheet 96 within a first path segment 50 having a first portion 50A for a first vertical movement component and a second portion 50B for a first horizontal movement component. The back cover sheet rolls on the caterpillar roller system 110 as shown in Figure 7A. The caterpillar system generally has a straight roller along the center and inclined rollers along each side, forming a path that is curved in both the advancing direction of the cover sheet and its transverse direction for the back cover sheet. Thus, the first vertical movement component and the first horizontal movement component are continuous along these different portions 50A, 50B of the first path segment 50. The slurry can first be directed from the hose 83 to contact the vertical portion 50A, and then the slurry continues all the way onto the horizontal portion 50B up to the roller 54 (see Figure 6).

[0074] Figure 7B shows a side view of the caterpillar roller system 110 having a base 122 and a roller 120, with the back cover sheet 96 advancing on the roller 120.

[0075] The gypsum slurry mixing and dispensing assembly 82 includes a body and a discharge conduit (e.g., having a gate-canister-boot configuration known in the art or an alternative configuration as described in U.S. Pat. Nos. 6,494,609 and 6,874,930). For example, the gypsum slurry mixing and dispensing assembly 82 includes a mixer 81 and two slurry discharge conduits. One slurry discharge conduit (not shown) connected to the mixer 81 provides a first portion 70 of an aqueous gypsum slurry (the more dense slurry) to the front cover sheet 90. Another conduit 83 discharges a second portion 74 of the aqueous gypsum slurry onto the back cover sheet 96. However, these devices are modified in accordance with the present invention so as not to direct their slurry discharge conduits directly from the mixer to a more dense layer on the front cover sheet. The present invention directs their discharge to the back cover sheet and then allows the discharged slurry to fall from the back cover sheet onto the underlying front cover sheet.

[0076] Embodiments of the present invention may include adding a foaming agent to the second portion 74 of the aqueous gypsum slurry to create a low density layer in the core layer of the gypsum board. The foaming agent may or may not include an α-sulfo fatty acid disalt.

[0077] Typically, the second portion of the gypsum slurry is disposed in a foamed state as a lower density region that contacts a higher density region, and the higher density region has a higher density than the lower density region. Thus, after mixing, the aqueous gypsum slurry optionally has bubbles added to reduce the product density. The bubbles are produced by combining soap and water. The bubbles can then be injected into the aqueous gypsum slurry after the aqueous gypsum slurry exits the mixer through a hose or chute. The bubbles are typically added to the second portion of the aqueous gypsum slurry for the low density layer of the core, but not to the first portion of the slurry for the densified layer (the relatively high density layer of the core).

[0078] If desired, a blowing agent (typically soap) can also be added to the discharge conduit of the mixer (e.g., a gate as described in U.S. Patent Nos. 5,683,635 and 6,494,609, which are incorporated herein by reference) or to the body. The slurry discharged from the discharge conduit after all components including the blowing agent are added is a primary gypsum slurry and is used to form a lower density region layer. However, these devices are modified in accordance with the present invention so as not to directly direct their slurry discharge conduits from the mixer to a higher density layer on the front cover sheet. The present invention directs their discharge towards the back cover sheet.

[0079] Particularly, at the downstream end of the first path segment 50, the back cover sheet 96 passes around the first transfer roller 54, feeding the back cover sheet 96 to the second path segment 55 and changing the direction of movement of the back cover sheet 96 to a second direction of back paper travel 56.

[0080] The second path segment 55 picks up the back cover sheet 96 and pivots it to be above the front cover sheet 90. The second path segment 55 has an upstream end and a downstream end, and the upstream end is at the first transfer roller 54. Preferably, the first transfer roller 54 changes the direction of movement of the back cover sheet 96 to movement along a first portion 60A of the second path segment 55 having a second horizontal movement component and a second vertical movement component in the machine direction, and the second vertical movement component is upward or downward or neutral to provide only movement along the second horizontal movement component, and typically, the second vertical movement component is upward movement away from a higher density region on the front cover sheet.

[0081] However, as shown in FIG. 3, in the first portion 60A of the second path segment 55, the movement in the second direction 56 of the back paper travel has a second vertical movement component and a second horizontal movement component. The second vertical movement component is upward movement away from a higher density layer 76 on the front cover sheet 90. The second horizontal movement component is movement in the machine direction.

[0082] The second portion 74 of the gypsum slurry passes through the first transfer roller 54 and deposits most of the second portion 94 of the gypsum slurry onto the higher density layer 76 on the front cover sheet 90. The first transfer roller 54 is spaced a distance "H" above the higher density area on the front cover sheet to pass the core slurry over the first transfer roller and deposit most of the core slurry onto the higher density area on the front cover sheet. Most of the second portion 74 of the gypsum slurry falls from the back cover sheet 74 as a stream 94 and travels over the distance "H". The distance "H" in the present invention is typically 2 to 18 inches, more typically 2 to 12 inches, or 4 to 8 inches, for example, 2, 3, 4, 5, or 6 inches. When this deposition occurs, the remaining portion of the second portion 74 of the gypsum slurry remains in contact with the back cover sheet 96 as the back cover sheet 96 moves within the second path segment 55. In other words, the second portion 74 of the gypsum slurry is conveyed or "rained down" from the back cover sheet 96 at the downstream end of the first path segment 50 onto the higher density layer 76 on the front cover sheet 90, and a portion of the second portion of the gypsum slurry remains in contact with the back cover sheet 96 as the back cover sheet 96 moves along the second path segment 55. The first transfer roller 54 is preferably a freewheel roller.

[0083] The velocity of the second portion 74 of the gypsum slurry is decelerated by the time it reaches the downstream end of the first path segment relative to the velocity when it first contacts the back paper. Thus, the second portion 74 of the gypsum slurry deposits onto the relatively high density layer (also known as the densified layer) 76 from the back cover sheet 96 at a velocity slower than the velocity the second portion 74 of the gypsum slurry had when it first contacted the back cover sheet 96 in the first path segment 50.

[0084] Next, at the downstream end of the first portion 60A of the second path segment 55, the back cover sheet optionally passes through the second transfer roller 57 to feed the back cover sheet 96 to the second portion 60B of the second path segment 55, changing the direction of movement of the back cover sheet 96 to the third direction 62 of the back paper advancement.

[0085] At the downstream end of the second path segment 55, the back cover sheet 96 is deposited on the second portion of the gypsum slurry 94 on the face paper 90 to form a multi-layer assembly (sandwich assembly) having slurry between two facing materials. The resulting multi-layer assembly is in the form of a wet assembly which is a precursor of the final gypsum board product. The multi-layer assembly is then sent to the forming station 86 and typically passes under the forming plate 86A of the forming station 86 to compress the layers to the desired total thickness. The resulting structure is the gypsum board preform 98. The second transfer roller 57 at the start of the third path segment 60 is preferably at a sufficiently elevated position of the distance "H" so that the head 93 of the slurry (see FIG. 5) at the end of the third path segment is visible and it is easy for the operator to manage and access the forming plate area.

[0086] The second transfer roller 57 can be a freewheel roller or a drive roller. Optionally, since the second transfer roller 57 is on the same side as the slurry-exposed side of the back paper, it can be a center-projecting shaft roller to reduce the cleaning load on this roller. FIG. 4 shows a center-projecting shaft roller 100 having a central shaft 102 of a first diameter "D1" and opposing cylindrical end portions 104 of a second diameter "D2", the second diameter "D2" being larger than the first diameter "D1". The center-projecting shaft roller avoids cleaning at the second transfer roller 57 which is exposed to the gypsum slurry on the back cover sheet 96. The opposing cylindrical end portions 104 are separated by a length "L" which is greater than the spread on the relatively dense slurry 70 on the front cover sheet 90.

[0087] The height of the forming plate or other forming device on the front cover sheet 16 determines the thickness of the board. Next, the continuous multilayer assembly is cut to an appropriate length, usually 8 feet to 12 feet, with a cutting knife at the cutting station 200. As the board moves down the conveyor line to form the panel, the slurry sets (hardens). The calcium sulfate recrystallizes or rehydrates to return to its original rock-like state, forming a board core that includes an interlocking crystalline matrix of hardened gypsum. When the gypsum hardens, the cover sheet is bonded to the core. The panel is then cut to a predetermined length and conveyed through a kiln or dryer 300 to remove free moisture and obtain the board 10. The temperature in the kiln typically ranges from 450°F to 500°F.

[0088] Additional components can be included at the wet end 80 of the manufacturing line.

[0089] FIG. 8 shows a detailed aspect of a diagram of another alternative version of the present method, which is a modified version of the embodiment of FIG. 6. The second portion 74 of the aqueous gypsum slurry is applied by a hose or boot 83 to the upstream portion of the first advancing segment of the back cover sheet 96 that travels horizontally in a direction opposite to the machine direction "T" with respect to the roller 54A. FIG. 8 shows the horizontal distance "L1" from the discharge opening of the hose or boot 83 that applies the second portion 74 of the gypsum slurry to the back cover sheet 96, where the second portion 74 of the gypsum slurry falls from the transfer roller 54 to the deposited first portion of the aqueous gypsum slurry on the front cover sheet 90. Typically, the horizontal distance "L1" is 10 to 30 feet or 10 to 20 feet. The back cover sheet 96 travels in a downwardly inclined direction at the downstream end of the first segment of the travel from the roller 54A to the transfer roller 54. Typically, the horizontal component of the inclined downstream end of the first advancing segment has a distance "L2". The distance "L2" is about 2 to 8 feet, typically about 2 to 6 feet or 2 to 4 feet. Typically, the horizontal component of the inclined downstream end of the first advancing segment has an inclination angle such as the angle "A" in FIG. 3. FIG. 8 also shows the horizontal distance "L3" from the location where the discharge of the second portion of the aqueous gypsum slurry from the slurry mixer 81 falls onto the first portion of the aqueous gypsum slurry deposited on the front cover sheet 90 via the roller 54 to the forming plate 86A. The distance "L3" is 10 to 30 feet or 10 to 20 feet.

[0090] As described above, the front cover sheet is in interfacial contact with a region of higher density, also known as the densified layer. The densified layer is typically adjacent to a region layer of lower density after curing. When the foam is inserted into the discharge conduit, the flow of the secondary gypsum slurry can be removed from the mixer body before foaming to provide the slurry for forming the densified layer. The densified layer is deposited on the moving front cover sheet before the major portion of the gypsum slurry is deposited to form the region layer of lower density. The gypsum slurry for the region of lower density is spread over the front cover sheet and the densified layer as required after being discharged from the discharge conduit. The densified layer can be formed from the same or a different gypsum slurry as the region layer of lower density.

[0091] The gypsum densified layer is thinner and of higher density than the region layer of lower density. Thus, the gypsum densified layer slurry 70 is relatively higher in density than the region layer slurry 74 of lower density of gypsum, which can be the foamed gypsum slurry. Typically, the calcined gypsum (calcium sulfate hemihydrate) slurry 94 for the region layer of lower density is foamed to be lower in density than the slurry 70 of the densified layer. Thus, if required, the region layer slurry flow 74 of lower density of calcined gypsum can pass through a foaming device (not shown) that mixes the region layer slurry flow 74 of lower density of calcined gypsum with foam and / or air, for example, before deposition on the front cover sheet material 90.

[0092] In another method, the densified layer can be achieved by directing a portion of the gypsum slurry from the mixer towards the densified layer mixer before introducing foam into the gypsum slurry, or by knocking the foam out of the gypsum densified layer slurry.

[0093] Accordingly, the lower density region layer slurry 94 of gypsum for the lower density region layer of the board is deposited on the gypsum high density layer slurry 70. Typically, the lower density region layer slurry stream 94 of gypsum and the slurry stream 70 of the gypsum high density layer have the same composition. However, if desired, the lower density region layer slurry stream 94 of gypsum and the slurry stream of the gypsum high density layer 70 can have different compositions and / or densities. All the gypsum slurries 70, 94 can be obtained from the same gypsum slurry mixing and dispensing assembly 82. However, the calcined gypsum slurries 70, 94 can be obtained from different mixing and dispensing assemblies so as to have different properties such as different densities.

[0094] The gypsum high density layer roller 72, the forming table (conveyor belt) 92, and the forming plate 86A can all be equipped with conventional equipment suitable for their intended purposes, as is known in the art. The wet end 80 can be equipped with other conventional equipment, as is known in the art.

[0095] The calcined gypsum in the gypsum slurries 70, 94 reacts with water and hardens as the conveyor moves the gypsum board preform 98 downward along the production line. The gypsum board preform 98 is dried and cut into segments of a predetermined dimension at a point along the line where the gypsum board preform 98 is sufficiently hardened. The segments can be dried (e.g., in a kiln) to drive out excess water and processed to provide a final laminated wall board of the desired dimensions.

[0096] The forming station is a location within the board line where the wet board precursor is sized to a predetermined width and thickness, and optionally, length. Thus, the forming station can include, or can be, a forming plate 86A, or any device capable of effecting the final mechanical spreading and / or shaping of the slurry across the width of the backer layer, many of which are known in the art. The forming station adapts the thickness and width of the slurry to the final desired thickness and width of the wet board precursor that, when cured, produces the cementitious board product. The final desired thickness and width of the slurry produced at the forming station may, of course, differ from the final thickness and width of the finished board product. For example, the thickness and / or width of the slurry can expand and / or contract during crystallization (i.e., curing) and drying of the slurry. Typically, the desired slurry thickness is substantially equal to the desired board thickness (e.g., about 0.375 inches (about 0.95 cm), about 0.5 inches (about 1.27 cm), about 0.625 inches (about 1.59 cm), about 0.75 inches (about 1.90 cm), or about 1 inch (about 2.54 cm)). By way of mere illustration, the final board thickness is typically within a range of about plus or minus 1 / 8 inch (about 0.32 cm) of the final slurry thickness.

[0097] Accordingly, the forming station includes a forming plate 86A or any other device capable of creating the desired slurry thickness and width of the wet board precursor. Suitable devices include, for example, a forming plate 86A, forming rollers, a forming press, a screed, and the like. The particular device used depends in part on the type of cementitious board being produced. In a preferred embodiment, for example, where the board forming system is a gypsum board or acoustic panel forming system, the board forming station comprises a forming plate as known in the art. Any of the board forming systems of the above embodiments optionally further comprises a blade for cutting the wet board precursor or dry cementitious board product to a desired length, and / or a drying zone capable of removing water from the cured cementitious board.

[0098] Gypsum and stucco (calcined gypsum) The calcium sulfate hemihydrate component used to form the crystalline matrix of the gypsum panel core typically includes beta calcium sulfate hemihydrate, water-soluble calcium sulfate anhydrite, alpha calcium sulfate hemihydrate, or any combination or all of these, and is obtained from natural or synthetic sources. Calcium sulfate hemihydrate is typically provided in a raw material known as stucco or calcined gypsum. In some embodiments, the stucco may include non-gypsum minerals such as a small amount of clay or other components associated with the gypsum source or added during the calcination, processing, and / or delivery of the stucco to the mixer. The stucco can be fibrous or non-fibrous. Typically, the raw stucco has at least 70 wt% calcium sulfate hemihydrate, preferably at least 80 wt% calcium sulfate hemihydrate, more preferably at least 85 wt% calcium sulfate hemihydrate, and even more preferably at least 90 wt% calcium sulfate hemihydrate.

[0099] Additives In addition to the curing stabilizer particles of the present invention, other additives may be present in the gypsum slurry used to form the board core. Such additives include, but are not limited to, reinforcing agents, foams (prepared from suitable foaming agents), dispersants, polyphosphates (e.g., sodium trimetaphosphate), starches, retarders, accelerators, re-firing inhibitors, binders, adhesives, secondary dispersion aids, leveling agents or non-leveling agents, thickeners, bactericides, fungicides, pH adjusters, buffers, colorants, strengthening materials, flame retardants, water repellents (e.g., siloxane), fillers, and mixtures thereof.

[0100] The additives and other components of the gypsum slurry can be added to the mixer in various ways. For example, various combinations of the components can be pre-mixed before entering the mixer as either one or more dry components and / or one or more wet components. Similarly, a single component can be introduced into the mixer in wet or dry form. When introduced in wet form, the component can be included in a carrier fluid such as water at any suitable concentration.

[0101] Fibers can be optionally used in the methods and compositions of the present invention. Fibers can include mineral fibers (also known as mineral wool), glass fibers, carbon fibers, and mixtures of such fibers, as well as other equivalent fibers that provide equivalent benefits to wallboard. For example, glass fibers can be incorporated into the lower density region slurries and / or the higher density region layer slurries of gypsum and the resulting crystalline core structure. Glass fibers in such embodiments can have an average length of about 0.5 to about 0.75 inches and a diameter of about 11 to about 17 microns. In other embodiments, such glass fibers can have an average length of about 0.5 to about 0.675 inches and a diameter of about 13 to about 16 microns. In still other embodiments, E-glass fibers having a softening point above about 800 °C or at least above about 900 °C are utilized. Mineral wool or carbon fibers as known to those skilled in the art can be used instead of or in combination with glass fibers.

[0102] When included, the fibers can be present in the gypsum low density layer slurry and / or the gypsum high density layer slurry in an amount of about 0.5 to about 10 pbw, preferably about 1 to about 8 pbw, more preferably about 2 to about 7 pbw, and most preferably about 3 to about 6 pbw on a dry basis per 100 pbw of calcium sulfate hemihydrate. The fibers may also be absent.

[0103] Optionally, one or more phosphate-containing compounds can also be included in the slurry if desired. For example, these phosphate-containing components can include water-soluble components and can be in the form of ions, salts, or acids, i.e., condensed phosphoric acids, each of which contains two or more phosphate units, salts or ions of condensed phosphates, each of which contains two or more phosphate units, and monobasic salts or monovalent ions of orthophosphates and water-soluble acyclic polyphosphates. Illustrative examples are described in U.S. Patent Nos. 6,342,284, 6,632,550, 6,815,049, and 6,822,033, which are hereby incorporated by reference in their entirety.

[0104] Phosphate-containing components can enhance green strength, resistance to permanent deformation (e.g., deflection), dimensional stability, etc. For example, trimetaphosphate compounds including sodium trimetaphosphate, potassium trimetaphosphate, lithium trimetaphosphate, and ammonium trimetaphosphate can be used. Sodium trimetaphosphate (STMP) is commonly used. However, for example, sodium tetrametaphosphate, sodium hexametaphosphate having about 6 to about 27 repeating phosphate units and the molecular formula Na n+2 P n O 3n+1 (where n = 6 - 27), tetrapotassium pyrophosphate having the molecular formula K4P2O7, trisodium tripolyphosphate having the molecular formula Na3K2P3O 10 , sodium tripolyphosphate having the molecular formula Na5P3O 10 , tetrasodium pyrophosphate having the molecular formula Na4P2O7, aluminum trimetaphosphate having the molecular formula Al(PO3)3, sodium pyrophosphate having the molecular formula Na2H2P2O7, ammonium polyphosphate having 1000 - 3000 repeating phosphate units and the molecular formula (NH4) n+2 P n O 3n+1 (where n = 1000 - 3000), or other phosphates including polyphosphoric acid having two or more repeating phosphate units and the molecular formula H n+2 P n O 3n+1 (where n is 2 or more) may also be suitable.

[0105] Phosphates are usually added in dry form and / or aqueous liquid form. The dry components are added to a slurry mixer, and the liquid components are added to the mixer, or at other stages or procedures.

[0106] If present, the phosphate can be included in the gypsum slurry in dry form or in aqueous form (e.g., a phosphate solution of about 5% to about 20%, such as about 10% solution). If included, the phosphate can be present in any suitable amount (on a solid / solid basis), such as from about 0.01 wt% to about 0.5 wt% of the stucco (e.g., from about 0.03 wt% to about 0.4 wt%, from about 0.1 wt% to about 0.3 wt%, or from about 0.12 wt% to about 0.4 wt% of the stucco). There may also be cases where no phosphate is present.

[0107] The gypsum slurry can optionally contain at least one dispersant to enhance fluidity. The dispersant can be introduced into the gypsum slurry in dry form, optionally together with other additives, and / or in liquid form, optionally together with other liquid components. Examples of suitable dispersants include naphthalenesulfonates such as polynaphthalenesulfonic acid and its salts (polynaphthalenesulfonates), and derivatives that are condensation products of naphthalenesulfonic acid and formaldehyde, as well as polycarboxylate dispersants such as polycarboxylic acid ethers. Other examples of suitable dispersants include lignosulfonates or sulfonated lignin. Lignosulfonates are water-soluble anionic polyelectrolyte polymers and are by-products from the production of wood pulp using sulfite pulp.

[0108] Lower molecular weight dispersants may be desirable. Lower molecular weight naphthalenesulfonate dispersants may be preferred as they tend to have a lower water requirement than higher viscosity, higher molecular weight dispersants. Thus, a molecular weight of about 3000 to about 10,000 (e.g., about 8000 to about 10,000) may be a desirable molecular weight for the dispersant. If desired, the molecular weight of the polycarboxylate dispersant can be from about 20,000 to about 60,000, which may exhibit less retardation than dispersants having a molecular weight greater than about 60,000.

[0109] Typical naphthalene sulfonates are aqueous naphthalene sulfonate solutions having a naphthalene sulfonate solids content in the range of about 35 wt% to about 55 wt%. However, if desired, the naphthalene sulfonate can be used in dry solid or powder form.

[0110] When present, the dispersant can be included in the gypsum slurry in any suitable (solid / solid) amount, for example, in the range of about 0.1 wt% to about 5 wt%, such as about 0.1% to about 4%, about 0.1% to about 3%, about 0.2% to about 3%, about 0.5% to about 3%, about 0.5% to about 2.5%, about 0.5% to about 2%, about 0.5% to about 1.5%, etc. in the gypsum slurry. There may also be cases where none of the polynaphthalene sulfonates, polycarboxylic acid ethers or lignosulfonates are present.

[0111] Accelerators and / or retarders can be added to the gypsum low-density layer slurry and / or the high-density layer slurry to change the rate at which the hydration reaction of calcium sulfate hemihydrate occurs. Suitable accelerators can include, for example, wet gypsum accelerators, heat resistant accelerators (HRA), or climate stabilized accelerators (CSA). "CSA" is a setting accelerator containing 95% calcium sulfate dihydrate that is co-ground with 5% sugar and heated to 250°F (121°C) to caramelize the sugar. CSA is prepared according to U.S. Patent Nos. 3,573,947 and 6,409,825, which are incorporated herein by reference. Potassium sulfate is another potential accelerator. The accelerator HRA (Heat Resistant Accelerator) is calcium sulfate dihydrate just ground with sugar at a ratio of about 5 to 25 pounds of sugar per 100 pounds of calcium sulfate dihydrate. This is further described in U.S. Patent No. 2,078,199, which is incorporated herein by reference. When present, the accelerators and / or retarders can each be incorporated into the gypsum slurry in an amount of about 0 wt% to about 10 wt% (e.g., about 0.1 wt% to about 10 wt%) of the stucco, on a solids basis, such as about 0 wt% to about 5 wt% (e.g., about 0.1 wt% to about 5 wt%) of the stucco. Suitable accelerators can include, for example, calcium sulfate dihydrate, carbohydrate-coated calcium sulfate, calcium sulfate dihydrate / organic phosphonate, and calcium sulfate dihydrate / organic phosphate. There may also be cases where no accelerators and / or retarders are present.

[0112] Bubbles (also known as water bubbles) can be optionally introduced into the lower density region slurry and / or the higher density region slurry of gypsum (preferably the lower density region slurry of gypsum) in an amount that provides the reduced lower density region density and panel weight described above. The foaming agent for generating bubbles is typically soap or other suitable surfactant. By introducing bubbles into the lower density region slurry of gypsum in an appropriate amount, formulation, and process, the desired network structure and void distribution will be generated within the lower density region of the final drywall panel. This void structure enables the reduction of the density and weight of the lower density region components of gypsum and other components while maintaining the desired panel structure and strength characteristics. When present, the foaming agent can include a major weight portion of an unstable component and a minor weight portion of a stable component (e.g., when a blend of unstable and stable / unstable is combined). The weight ratio of the unstable component to the stable component is effective for forming a void distribution within the lower density region of the cured gypsum, as described in U.S. Pat. Nos. 5,643,510, 6,342,284, and 6,632,550, which are hereby incorporated by reference in their entirety. Approaches for adding bubbles to the lower density region slurry of gypsum are known in the art, and an example of such an approach is discussed in U.S. Pat. No. 5,683,635, the disclosure of which is hereby incorporated by reference. Evaporated water voids having a diameter generally of about 5 μm or less also contribute to the total void distribution along with the aforementioned air (bubble) voids. The volume ratio of voids having a pore diameter greater than about 5 microns to voids having a pore diameter of about 5 microns or less is from about 0.5:1 to about 9:1, for example, from about 0.7:1 to about 9:1, from about 1.8:1 to about 2.3:1, and so on. The foaming agent is present in the gypsum slurry in an amount of, for example, less than about 0.5% by weight of Stacco, for example, about 0.01% to about 0.5%, about 0.01% to about 0.2%, about 0.02% to about 0.4%, about 0.02% to about 0.2%, about 0.01% to about 0.1%, and so on. There may also be cases where no foaming agent is present.

[0113] Components for fire resistance and / or water resistance may also be included in the gypsum slurry. Examples include, for example, siloxanes (water resistance), fibers, heat sink additives such as aluminum trihydrite (ATH), magnesium hydroxide, and / or highly expandable particles (e.g., expandable to about 300% or more of the original volume when heated at 1560°F for about 1 hour). Further disclosure of such additives can be found in U.S. Patent No. 8,323,785, which is incorporated herein by reference in its entirety. High-expansion vermiculite may be included, but other fire-resistant materials may be included. When present, the fire-resistant or water-resistant additives may be included in any suitable amount desired, depending, for example, on the fire-resistance rating and similar performance parameters. For example, when included, the fire-resistant or water-resistant additives may be present individually in amounts of about 0.5 wt% to about 10 wt% of the stack, such as about 1 wt% to about 10 wt%, about 1 wt% to about 8 wt%, about 2 wt% to about 10 wt%, about 2 wt% to about 8 wt%, etc.

[0114] When included, the siloxane may desirably be introduced in the form of an emulsion. The slurry can then be formed and dried under conditions that promote the polymerization of the siloxane to form a highly cross-linked silicone resin. A catalyst that promotes the polymerization of the siloxane to form a highly cross-linked silicone resin can be added to the gypsum slurry. A solvent-free methylhydrogen siloxane fluid can be used as the siloxane. This product is a siloxane fluid that does not contain water or solvent. It is contemplated that, if desired, about 0.3% to about 1.0% of siloxane may be used based on the weight of the dry components. For example, if desired, about 0.4% to about 0.8% of siloxane may be present in the gypsum slurry based on the dry stack weight.

[0115] For fire resistance and / or water resistance, one or more of these components may not be present. For example, there may be no siloxane present.

[0116] Water Water is added to the slurry in any amount that produces a fluid slurry. The amount of water used varies widely depending on the application for which it is used, the exact dispersant used, the properties of the calcium sulfate hemihydrate, and the additives used.

[0117] The water used to make the slurry should be as pure as possible for best control of the properties of both the slurry and the hardened gypsum. Salts and organic compounds are well known to change the setting time of the slurry, varying widely from accelerators to setting inhibitors. Some impurities cause irregularities in the structure when the interlocking matrix of dihydrate crystals is formed, reducing the strength of the hardened product. Thus, the strength and consistency of the product are enhanced by using water that is free of contaminants to a practical degree.

[0118] Water may be present in the lower density region slurry and / or the higher density region layer slurry of the gypsum of the present invention in a weight ratio of water to calcium sulfate hemihydrate of from about 0.2:1 to about 1.2:1, preferably from about 0.3:1 to about 1.1:1, more preferably from about 0.6:1 to about 1:1, most preferably from 0.7:1 to 0.95:1, and typically about 0.85:1.

[0119] Back cover sheet and front cover sheet The front cover sheet and the back cover sheet can be made of other fibrous materials such as paper or a mat of glass fibers. The back paper cover sheet and the front cover sheet can be made from any suitable paper material having any suitable basis weight.

[0120] When the front and back cover sheets are made of paper, the paper materials for each cover sheet may be the same or different.

[0121] The gypsum panel may use various grades of paper, including smooth calendered manila grade paper often used as a facer paper cover sheet, and coarser finished newsprint paper often used as a backer paper cover sheet. Typically, both paper grades are multi-ply with at least one liner ply and several filler plies. However, if desired, at least one paper cover sheet or both paper cover sheets may be made of single-ply paper.

[0122] Typically, the back cover sheet covers only the back surface. In contrast, the front cover sheet covers the front of the board and also wraps around the board edge to contact the back cover sheet.

[0123] If desired, to increase the strength (e.g., nail pull strength) of the gypsum board, especially of lower density, one or both of the cover sheets may be formed from paper having a basis weight of, for example, at least about 45 lbs / MSF (e.g., about 45 lbs / MSF to about 65 lbs / MSF, about 45 lbs / MSF to about 60 lbs / MSF, about 45 lbs / MSF to about 55 lbs / MSF, about 50 lbs / MSF to about 65 lbs / MSF, about 50 lbs / MSF to about 60 lbs / MSF, etc.). If desired, the front paper cover sheet may have a higher basis weight than the back cover sheet, thereby providing improved nail pull resistance and handling. The back paper cover sheet may have a somewhat lower basis weight (e.g., less than 45 lbs / MSF, e.g., a basis weight of about 33 lbs / MSF to 45 lbs / MSF (e.g., about 33 lbs / MSF to about 40 lbs / MSF)) if desired.

[0124] Claims of the Invention The following claims disclose various aspects of the present invention.

[0125] Claim 1. A method for manufacturing a gypsum board, the method comprising A front cover sheet having surfaces on the first and second opposite sides on the forming surface, wherein the first surface of the front cover sheet is the lower surface facing the forming surface, and the second surface of the front cover sheet is the upper surface facing away from the forming surface, depositing the front cover sheet, and horizontally moving the front cover sheet in the machine direction along the forming surface, Depositing a first portion of the aqueous gypsum slurry on the front cover sheet to form a layer of the first portion of the aqueous gypsum slurry as a layered higher density region in contact with the upper surface of the front cover sheet, A back cover sheet having surfaces on the third and fourth opposite sides along a back cover sheet path including a first path segment and a second path segment downstream of the first path segment, wherein the back cover sheet moves along the first path segment above the higher density region on the front cover sheet with a movement including a first vertical movement component and / or a first horizontal movement component, the first vertical movement component is a downward movement towards the higher density region on the front cover sheet, and the first horizontal movement component is a movement in a direction opposite to the machine direction, moving the back cover sheet, A second portion of the aqueous gypsum slurry that is less dense than the first portion of the aqueous gypsum slurry for contacting the third surface of the back cover sheet in the first path segment, the second portion of the aqueous gypsum slurry contacts the back cover sheet at an incident angle "B" of 0 to 90 degrees, for example, 0 to 20 degrees, preferably 5 to 90 degrees, more preferably 5 to 85 degrees or 5 to 60 degrees, with respect to the surface of the back cover sheet where the slurry contacts, The first portion of the aqueous slurry and the second portion of the aqueous slurry each contain a respective mixture of water and stucco, the stucco contains calcium sulfate hemihydrate, and the first portion of the aqueous slurry and the second portion of the aqueous slurry each contain, on a dry (anhydrous) basis, at least 60% by weight, typically 60 to 98% by weight, preferably at least 70% by weight, more preferably at least 80% by weight, typically at least 90% by weight, or typically at least 95% by weight of said calcium sulfate hemihydrate and water, each in a weight ratio of water to calcium sulfate hemihydrate of 0.2:1 to 1.2:1, depositing the second portion, Then, at the downstream end of the first path segment, while the fourth surface of the back cover sheet contacts the first transfer roller, the back cover sheet is passed over the first transfer roller spaced a distance above the higher density region on the front cover sheet together with the second portion of the aqueous gypsum slurry, and most of the weight of the second portion of the aqueous gypsum slurry that has fallen from the back cover sheet is deposited in the higher density region on the front cover sheet to form a layer of the second portion of the foamed aqueous gypsum slurry as a lower density region in contact with the upper surface of the higher density region having a higher density than the lower density region, The back cover sheet then passes around the first transfer roller to feed the back cover sheet to the second path segment, the second path segment has an upstream end and a downstream end, the upstream end is at the first transfer roller, and typically, when the deposition of the second aqueous gypsum slurry occurs, the remaining portion of the second aqueous gypsum slurry remains in contact with the back cover sheet as the back cover sheet moves to the second path segment, At the downstream end of the second path segment, deposit a back cover sheet on the second portion of the aqueous gypsum slurry on the face cover sheet to form a multi-layer assembly including the back cover sheet on the second portion of the aqueous gypsum slurry, typically, the multi-layer assembly includes a first portion of an aqueous gypsum slurry layer on the front cover sheet, a second portion of the aqueous gypsum slurry layer on the first portion of the aqueous gypsum slurry layer, and a back cover sheet on the second portion of the aqueous gypsum slurry layer, to form a multi-layer assembly. Passing the multi-layer assembly through a forming station for forming the multi-layer assembly, preferably, the multi-layer assembly passes under the forming plate of the forming station. By reacting calcium sulfate hemihydrate with the water of each of the first portion and the second portion of the aqueous gypsum slurry layer, harden the calcium sulfate hemihydrate of each of the first portion and the second portion of the aqueous gypsum slurry layer to form respective first and second board layers containing calcium sulfate dihydrate between the front cover sheet and the back cover sheet. The first board layer includes a higher density region hardened as a layer containing calcium sulfate dihydrate and has a first board layer density. The second board layer includes a lower density region hardened as a layer containing calcium sulfate dihydrate and has a second board layer density lower than the first board layer density. Form a panel including the gypsum cores of the respective first and second board layers, wherein the hardened higher density region is interposed between the hardened lower density region and the front cover sheet. A method including drying the panel and cutting the panel into gypsum boards.

[0126] Clause 2. By simultaneously causing both the first vertical movement component and the first horizontal movement component to exist along all or the same part of the first segment, the back cover sheet moves along the downstream part of the first path segment with a movement along an inclination defining an angle "A" of 0 to 90 degrees between the back cover sheet at the downstream part of the first path segment and the front cover sheet on the forming surface, typically where the angle "A" is 5 to 90 degrees or 5 to 85 degrees or 0 to 20 degrees or 5 to 20 degrees, and more typically where the angle "A" is 20 to 60 degrees, the method according to Clause 1.

[0127] Clause 3. The method according to Clause 1 or 2, wherein the discharge of the calcined gypsum core slurry is in a countercurrent direction to the movement direction of the back cover sheet, or typically, the discharge of the calcined gypsum core slurry can be in a cocurrent direction to the movement direction of the back cover sheet.

[0128] Clause 4. The first transfer roller changes the movement direction of the back cover sheet to a movement along a first part of a second path segment having a second horizontal movement component and a second vertical movement component in the machine direction, and the second vertical movement component is upward or downward or neutral to provide only movement along the second horizontal movement component, typically where the second vertical movement component is a movement in an upward direction away from a higher density region on the front cover sheet, the method according to any one of Clauses 1 to 3.

[0129] Clause 5. The method according to any one of Clauses 1 to 4, wherein the first transfer roller is a freewheel roller.

[0130] Clause 6. The method according to any one of Clauses 1 to 5, wherein the second path segment has a first part and a second part, and at the downstream end of the first part of the second path segment, the back cover sheet passes over a second transfer roller to feed the back cover sheet to the second part of the second path segment and change the movement direction of the back cover sheet.

[0131] Clause 7. At the downstream end of the second path segment, the multilayer assembly comprises a first portion of the aqueous gypsum slurry layer on the front cover sheet, a second portion of the aqueous gypsum slurry layer on the first portion of the aqueous gypsum slurry layer, and a back cover sheet on the second portion of the aqueous gypsum slurry layer, the method according to any one of Clauses 1 to 6.

[0132] Clause 8. A method for manufacturing a gypsum board, the method comprising preparing an aqueous gypsum slurry comprising a mixture of water and stucco, the stucco comprising calcium sulfate hemihydrate, the aqueous gypsum slurry comprising, on a dry (anhydrous) basis, at least 60% by weight, typically 60 - 98% by weight, preferably at least 70% by weight, more preferably at least 80% by weight, typically at least 90% by weight, or typically at least 95% by weight of said calcium sulfate hemihydrate, and a mixture with water in a weight ratio of water to calcium sulfate hemihydrate of 0.2:1 to 1.2:1, preparing; placing a front cover sheet on the forming surface and horizontally moving the front cover sheet along the forming surface in the machine direction; depositing a first portion of the aqueous gypsum slurry on the front cover sheet to form a layer of the first portion of the aqueous gypsum slurry as a layered higher density region in contact with the upper surface of the front cover sheet; moving a back cover sheet along a back cover sheet path comprising a first path segment and a second path segment, wherein the back cover sheet moves along a first path segment above the higher density region on the front cover sheet with a movement comprising a first vertical movement component and / or a first horizontal movement component, the first vertical movement component being a downward movement towards the higher density region on the front cover sheet, and the first horizontal movement component being a movement in a direction opposite to the machine direction; Preferably, by simultaneously causing both the first vertical movement component and the first horizontal movement component to exist along all or the same part of the first segment, the back cover sheet moves along the downstream part of the first path segment, along an inclination defining an angle "A" of 0 to 90 degrees between the back cover sheet and the front cover sheet on the forming surface at the downstream part of the first path segment. Typically, the angle "A" is from 0 to 90 degrees, more typically the angle "A" is from 5 to 90 degrees or from 5 to 85 degrees, and more typically the angle "A" is from 20 to 60 degrees. The slurry mixer discharges a second portion of the aqueous gypsum slurry that is less dense than the first portion of the aqueous gypsum slurry for contacting the surface of the back cover sheet in the first path segment. The slurry contacts the back paper at an incident angle "B" of 0 to 90 degrees, for example, 0 to 20 degrees, preferably 5 to 90 degrees, more preferably 5 to 85 degrees, and even more preferably 5 to 60 degrees with respect to the surface of the back cover sheet that the slurry contacts. Typically, the discharge of the calcined gypsum core slurry can be in a countercurrent direction to the moving direction of the back cover sheet, or typically, the discharge of the calcined gypsum core slurry can be in a co-current direction to the moving direction of the back cover sheet. At the downstream end of the first path segment, the back cover sheet then passes around the first transfer roller to feed the back cover sheet into the second path segment. The second path segment has an upstream end and a downstream end, and the upstream end is at the first transfer roller. Preferably, the first transfer roller changes the moving direction of the back cover sheet to a movement along the first part of the second path segment having a second horizontal movement component and a second vertical movement component in the machine direction. The second vertical movement component is upward, or downward, or neutral to provide only movement along the second horizontal movement component. Typically, the second vertical movement component is an upward movement away from a higher density region on the front cover sheet. Moving the first transfer roller at a distance above a higher density region on the front cover sheet. Passing a second portion of the aqueous gypsum slurry over the first transfer roller to deposit a majority by weight of the second portion of the aqueous gypsum slurry onto a higher density region on the front cover sheet, typically when this deposition occurs, the remaining portion of the second portion of the aqueous gypsum slurry remains in contact with the back cover sheet as the back cover sheet moves within the second path segment, Preferably, the first transfer roller is a freewheel roller, Typically, the second path segment has a first portion and a second portion, and at the downstream end of the first portion of the second path segment, the back cover sheet passes over a second transfer roller to feed the back cover sheet into the second portion of the second path segment and to change the direction of movement of the back cover sheet, depositing, At the downstream end of the second path segment, depositing the back cover sheet onto the second portion of the aqueous gypsum slurry on the front cover sheet to form a multi-layer assembly including the back cover sheet on the second portion of the aqueous gypsum slurry, Passing the multi-layer assembly through a forming station for forming the multi-layer assembly, preferably passing the multi-layer assembly under a forming plate of the forming station, Curing the calcium sulfate hemihydrate to form a panel including a gypsum core containing calcium sulfate dihydrate, Drying the panel and cutting the panel into gypsum boards having one or more predetermined dimensions, The first portion of the gypsum slurry forms a layered higher density region in contact with the front cover sheet, The second portion of the gypsum slurry is in a foamed state as a lower density region in contact with the higher density region, and the higher density region has a higher density than the lower density region, A method, including cutting, wherein a board core includes a cured lower density region containing calcium sulfate dihydrate and a cured higher density region containing calcium sulfate dihydrate, and the cured higher density region is interposed as a layer between the cured lower density region and a front cover sheet.

[0133] Clause 9. The method according to Clause 1 or 8, wherein forming a layer of aqueous gypsum slurry as a layered higher density region in contact with the upper surface of the front cover sheet includes passing the front cover sheet under a slurry roller to spread the slurry of the higher density region.

[0134] Clause 10. The method according to Clause 1 or 8, wherein the discharge of the calcined gypsum core slurry can be in a co-current direction with the first direction of travel of the back cover sheet.

[0135] Clause 11. The method according to Clause 1 or 8, wherein the discharge of the calcined gypsum core slurry can be in a counter-current direction with the first direction of travel of the back cover sheet.

[0136] Clause 12. The method according to Clause 1 or 8, wherein a slurry mixer discharges the calcined gypsum core slurry onto a portion of the back cover sheet supported by a back plate.

[0137] Clause 13. The method according to Clause 6 or 8, wherein the second transfer roller is a free-wheel roller or a drive roller.

[0138] Clause 14. The method according to Clause 6 or 8, wherein the second transfer roller is a center-exposed shaft roller having a center shaft of a first diameter and opposing cylindrical end portions of a second diameter, and the second diameter is larger than the first diameter.

[0139] Clause 15. The method according to Clause 6 or 8, wherein the second transfer roller is at a position sufficiently high relative to the front cover sheet moving along the forming surface such that the head of the aqueous gypsum slurry at the downstream end of the third path segment is visible to the operator.

[0140] Clause 16. The method according to clause 1 or 8, wherein the front cover sheet comprises at least one of a glass mat facer sheet or a paper facer sheet.

[0141] Clause 17. The method according to clause 1 or 8, wherein the lower density region comprises air bubbles.

[0142] Clause 18. The method according to clause 1 or 8, wherein the hardened gypsum core comprises air bubbles having an average cross-sectional diameter of less than 1.5 mm, about 0.5 to about 0.8 mm, about 0.3 mm, or about 0.3 mm or less.

[0143] Clause 19. The method according to clause 1 or 8, further comprising adding air to the aqueous gypsum slurry before depositing the aqueous gypsum slurry.

[0144] Clause 20. The method according to clause 1 or 8, wherein the lower density region has a total air volume of about 30% to about 90% by volume.

[0145] Clause 21. The lower density region obtained from the lower density region slurry of the hardened gypsum has a thickness of 0.25 inches to 1.5 inches, typically 0.3 inches to 1 inch, or 0.4 inches to 0.75 inches, The higher density region has a thickness of about 0.02 inches to about 0.75 inches, about 0.02 inches to about 0.35 inches, or about 0.02 inches to about 0.2 inches (about 0.05 to about 0.5 cm), and typically the thickness of the lower density region layer is greater than the thickness of the higher density region layer, The lower density region has a density of 15 to 55 pounds per cubic foot, The higher density region has a density of 25 to 70 pounds per cubic foot, more typically 30 to 60 pounds per cubic foot, the method according to claim 1.

[0146] Clause 22. The method according to clause 21, wherein a lower density region layer of gypsum obtained from the hardened foamed gypsum slurry has a total void volume of 50 to 92 volume percent, the hardened higher density region layer has a total void volume of less than 40 to 85 volume percent, and the total void volume of the lower density region layer of gypsum is greater than the total void volume of the higher density region layer of gypsum.

[0147] Clause 23. An apparatus for manufacturing a gypsum board according to the method according to any one of clauses 1 to 22, the apparatus comprising A forming surface for depositing a front cover sheet thereon and horizontally moving the front cover sheet in the machine direction along the forming surface; A source of a first portion of an aqueous gypsum slurry for depositing the first portion of the aqueous gypsum slurry on the front cover sheet to form a layer of the aqueous gypsum slurry as a layered higher density region in contact with the upper surface of the front cover sheet, the first aqueous gypsum slurry layer having a first slurry density; A first transfer roller and typically a second transfer roller for moving a back cover sheet along a back cover sheet path comprising a first path segment and a second path segment; A back cover sheet drive adapted and configured to move along a first path segment above the higher density region on the front cover sheet with a movement including a first vertical movement component and / or a first horizontal movement component, the first vertical movement component being a downward movement towards the higher density region on the front cover sheet, and the first horizontal movement component being a movement in a direction opposite to the machine direction; A second portion of the aqueous gypsum slurry that is less dense than a first portion of the aqueous gypsum slurry for contacting a surface of the back cover sheet in the first path segment, wherein the second portion of the aqueous gypsum slurry contacts the back cover sheet at an angle of incidence "B" of 0 to 90 degrees, for example, 0 to 20 degrees, preferably 5 to 90 degrees, more preferably 5 to 85 degrees, even more preferably 5 to 60 degrees, with respect to the surface of the back cover sheet with which the second portion of the aqueous gypsum slurry contacts, and a source of the second portion of the aqueous gypsum slurry, A first transfer roller at the downstream end of the first path segment for passing the back cover sheet around the first transfer roller to feed the back cover sheet into the second path segment, wherein the second path segment has an upstream end and a downstream end, and the upstream end of the second path segment is at the first transfer roller, The first transfer roller is spaced a distance above a higher density region on the front cover sheet to pass a second portion of the aqueous gypsum slurry over the first transfer roller and deposit a majority by weight of the second portion of the aqueous gypsum slurry that has fallen from the back cover sheet onto the higher density region on the front cover sheet. Typically, when this deposition occurs, the remaining portion of the second portion of the aqueous gypsum slurry remains in contact with the back cover sheet as the back cover sheet moves within the second path segment. The first portion of the aqueous gypsum slurry and the second portion of the aqueous gypsum slurry each comprise a respective mixture of water and stucco, the stucco comprising calcium sulfate hemihydrate, and the first portion of the aqueous gypsum slurry and the second portion of the aqueous gypsum slurry each comprise a mixture of the calcium sulfate hemihydrate and water in a weight ratio of water to calcium sulfate hemihydrate of 0.2:1 to 1.2:1, at least 60% by weight, typically 60 to 98% by weight, preferably at least 70% by weight, more preferably at least 80% by weight, typically at least 90% by weight, or typically at least 95% by weight, on a dry (anhydrous) basis, and the first transfer roller, Positioned downstream of a second path segment for depositing a back cover sheet onto a second portion of the aqueous gypsum slurry on the front cover sheet to form a multi-layer assembly including the back cover sheet on the second portion of the aqueous gypsum slurry, A forming station for forming a multi-layer assembly, preferably, the forming station comprises a forming plate, the forming station, To cure calcium sulfate hemihydrate to form a panel including a gypsum core containing calcium sulfate dihydrate, The first portion of the gypsum slurry forms a layered higher density region in contact with the front cover sheet, The second portion of the gypsum slurry is in a foamed state as a lower density region in contact with the higher density region, and the higher density region has a higher density than the lower density region, The board core includes a cured lower density region containing calcium sulfate dihydrate and a cured higher density region containing calcium sulfate dihydrate, and forming the cured higher density region to be interposed as a layer between the cured lower density region and the front cover sheet. An apparatus comprising.

[0148] The apparatus according to clause 22, further comprising a mixer for preparing an aqueous gypsum slurry for a first portion of the aqueous gypsum slurry comprising a mixture of water and stacko, stacko comprising calcium sulfate hemihydrate, and the aqueous gypsum slurry being at least 60% by weight on a dry (anhydrous) basis, typically 60-98% by weight, preferably at least 70% by weight, more preferably at least 80% by weight, typically at least 90% by weight, or typically at least 95% by weight of said calcium sulfate hemihydrate and water in a weight ratio of water to calcium sulfate hemihydrate of 0.2:1 to 1.2:1.

[0149] Clause 25. By causing both the first vertical movement component and the first horizontal movement component to be present simultaneously along all or the same part of the first segment, the back cover sheet moves along the downstream part of the first path segment, with a movement along an inclination defining an angle "A" of 0 to 90 degrees between the back cover sheet at the downstream part of the first path segment and the front cover sheet on the forming surface. Typically, the angle "A" is from 0 to 90 degrees, more typically the angle "A" is from 5 to 90 degrees or from 5 to 85 degrees, and more typically the angle "A" is from 20 to 60 degrees, the apparatus according to Clause 23 or 24.

[0150] Clause 26. The apparatus according to any one of Clauses 23 to 25, wherein the discharge of the calcined gypsum core slurry is in a countercurrent direction to the movement direction of the back cover sheet, or typically, the discharge of the calcined gypsum core slurry can be in a cocurrent direction to the movement direction of the back cover sheet.

[0151] Clause 27. The first transfer roller changes the movement direction of the back cover sheet to a movement along the first part of the second path segment having a second horizontal movement component and a second vertical movement component in the machine direction, and the second vertical movement component is upward, or downward, or neutral to provide only movement along the second horizontal movement component. Typically, the second vertical movement component is a movement in an upward direction away from a region of higher density on the front cover sheet, the apparatus according to any one of Clauses 23 to 26.

[0152] Clause 28. The apparatus according to any one of Clauses 23 to 27, wherein the first transfer roller is a freewheel roller.

[0153] Clause 29. The second path segment has a first part and a second part, and at the downstream end of the first part of the second path segment, the back cover sheet passes over the second transfer roller to feed the back cover sheet to the second part of the second path segment and change the movement direction of the back cover sheet, the apparatus according to any one of Clauses 23 to 28.

[0154] Clause 30. The apparatus according to clause 29, wherein the downstream end of the second path segment is positioned to deposit the back cover sheet on the second portion of the aqueous gypsum slurry on the front cover sheet, thereby forming a multi-layer assembly including the front cover sheet, the first portion of the aqueous gypsum slurry layer on the front cover sheet, the second portion of the aqueous gypsum slurry layer on the first portion of the aqueous gypsum slurry layer, and the back cover sheet on the second portion of the aqueous gypsum slurry layer.

[0155] Clause 31. An apparatus for manufacturing a gypsum board according to the method according to any one of clauses 1 to 22, the apparatus comprising a mixer for preparing an aqueous gypsum slurry containing a mixture of water and stucco, the stucco containing calcium sulfate hemihydrate, the aqueous gypsum slurry containing, on a dry (anhydrous) basis, at least 60% by weight, typically 60 - 98% by weight, preferably at least 70% by weight, more preferably at least 80% by weight, typically at least 90% by weight, or typically at least 95% by weight of said calcium sulfate hemihydrate, and a mixer containing a mixture of water and said calcium sulfate hemihydrate in a weight ratio of water to calcium sulfate hemihydrate of 0.2:1 to 1.2:1; a forming surface for depositing the front cover sheet thereon and horizontally moving the front cover sheet in the machine direction along the forming surface; a source of the first portion of the unfoamed aqueous gypsum slurry for depositing the first portion of the aqueous gypsum slurry on the front cover sheet to form a layer of the aqueous gypsum slurry as a layered higher density region in contact with the upper surface of the front cover sheet; a first transfer roller and a second transfer roller for moving the back cover sheet along a back cover sheet path comprising a first path segment and a second path segment; The first path segment is adapted and configured to move along the first path segment above a region of higher density on the front cover sheet with a movement involving a first vertical movement component and / or a first horizontal movement component, the first vertical movement component being a movement in a downward direction towards the region of higher density on the front cover sheet, and the first horizontal movement component being a movement in a direction opposite to the machine direction. Preferably, by simultaneously presenting both the first vertical movement component and the first horizontal movement component along all or the same portion of the first segment, the back cover sheet moves along the downstream portion of the first path segment with a movement along an inclination defining an angle "A" of 0 to 90 degrees between the back cover sheet at the downstream portion of the first path segment and the front cover sheet on the forming surface. Typically, the angle "A" is from 0 to 90 degrees, more typically the angle "A" is from 5 to 90 degrees or from 5 to 85 degrees, and more typically the angle "A" is from 20 to 60 degrees, the back paper drive unit. A second portion of the aqueous gypsum slurry that is less dense than the first portion of the aqueous gypsum slurry for contacting the surface of the back cover sheet in the first path segment, the slurry contacting the back paper at an angle of incidence "B" of 0 to 90 degrees, for example 0 to 20 degrees, preferably 5 to 90 degrees, more preferably 5 to 85 degrees, more preferably 5 to 60 degrees, with respect to the surface of the back cover sheet that the slurry contacts. Typically, the discharge of the calcined gypsum core slurry can be in a countercurrent direction to the direction of movement of the back cover sheet, or typically, the discharge of the calcined gypsum core slurry can be in a co-current direction to the direction of movement of the back cover sheet, the second portion. A first transfer roller at the downstream end of the first path segment for passing the back cover sheet around a first transition to feed the back cover sheet to a second path segment, the second path segment having an upstream end and a downstream end, the upstream end being at the first transfer roller. Preferably, the first transfer roller changes the moving direction of the back cover sheet to a movement along a first portion of a second path segment having a second horizontal movement component and a second vertical movement component in the machine direction, and the second vertical movement component is upward, or downward, or neutral in order to provide only movement along the second horizontal movement component. Typically, the second vertical movement component is a movement in an upward direction away from a higher density region on the front cover sheet. The first transfer roller is spaced at a distance above a higher density region on the front cover sheet to pass a second portion of the aqueous gypsum slurry over the first transfer roller and deposit a majority by weight of the second portion of the aqueous gypsum slurry onto the higher density region on the front cover sheet. Typically, when this deposition occurs, the remaining portion of the second portion of the aqueous gypsum slurry remains in contact with the back cover sheet as the back cover sheet moves within the second path segment. Preferably, the first transfer roller is a freewheel roller. Typically, the second path segment has a first portion and a second portion. At the downstream end of the first portion of the second path segment, the back cover sheet passes over a second transfer roller to feed the back cover sheet into the second portion of the second path segment and change the moving direction of the back cover sheet. A downstream end of the second path segment located to deposit the back cover sheet onto the second portion of the aqueous gypsum slurry on the front cover sheet to form a multi-layer assembly including the back cover sheet on the second portion of the aqueous gypsum slurry. A forming station for forming a multi-layer assembly, preferably, the forming station comprises a forming plate. Curing calcium sulfate hemihydrate to form a panel containing a gypsum core containing calcium sulfate dihydrate. The first portion of the gypsum slurry forms a layered higher density region in contact with the front cover sheet. A second portion of the gypsum slurry is in a foamed state as a lower density region that contacts a higher density region, and the higher density region has a higher density than the lower density region. The board core includes a cured lower density region containing calcium sulfate dihydrate and a cured higher density region containing calcium sulfate dihydrate, and forming the cured higher density region is interposed as a layer between the cured lower density region and the front cover sheet. An apparatus, including.

[0156] A gypsum board produced according to the method according to any one of clauses 1 to 22 of clause 32.

[0157] Clause 33. Having a width of 3 to 5 feet, The cured higher density region has an average thickness of 20 to 40 mils measured from the average of a series of 1-inch wide samples taken across the width of the cross-section of the board. The cured higher density region thickness of each sample has a difference of + / - 20 mils from the average thickness. The minimum cured higher density region thickness of the sample is at least 10 mils thick. The minimum thickness / average thickness of the cured higher density region of the sample = 40 - 100%, typically 40 - 90 or 40 - 80%. The gypsum board according to clause 32.

[0158] The following examples further illustrate the present invention, but of course should in no way be construed as limiting the scope of the present invention.

[0159] Examples Example 1 In the comparative example, FIG. 9 shows a plot of nail pull versus distance from the edge for a 1 / 2-inch thick gypsum board showing the effect of high-speed slurry from a three-legged boot that displaces the densified layer slurry.

[0160] The lighter core slurry had a wet density of 30 - 100 pcf and a dry density of 20 - 60 pcf.

[0161] The higher density slurries had wet densities of 40 - 110 pcf and dry densities of 30 - 70 pcf.

[0162] The non - uniformity of the resulting densified layer can potentially affect board properties such as nail pull. A 0.5 - inch thick wall board sample formed was tested for nail pull across its width to obtain a nail pull profile for a 0.5 - inch thick and 48 - inch wide board as shown in FIG. 8.

[0163] FIG. 9 shows three dips 101 in the plots resulting respectively from the deposition of gypsum slurries in three - leg boots. The inventors theorize that the dips are from regions of relatively lower density slurries that move into the strip regions of the higher density densified layer and displace the densified layer slurry.

[0164] Example 2 This example measures the average density of the profile across a gypsum board obtained from a gypsum slurry deposited according to the present invention on a gypsum layer having a thickness of about 10 - about 40 mils to estimate the thickness of the high - density layer. The measured high density means the high - density layer. The measured low density means the low - density core layer. Thus, the density was measured, and the thickness of the high - density layer was estimated from where the measured value changed from high density to low density.

[0165] The gypsum board has a 4 - foot width, and the cured higher - density region has an average thickness of 20 - 40 mils measured from the average of a series of 1 - inch wide samples taken across the width of the cross - section of the board, the thickness of the cured higher - density region of each sample has a difference of + / - 20 mils from the average thickness, the minimum thickness of the cured higher - density region of the sample is at least 10 mils thick, The minimum thickness / average thickness of the more highly cured regions of the samples was 40 - 100%, typically 40 - 90 or 40 - 80%.

[0166] For example, the thickness of the more highly cured regions of each sample was determined by cutting the board vertically across the width of the board into a plurality of samples, e.g., 48 one-inch samples, and slicing each sample in a direction parallel to the front and back surfaces of the board to form a number of horizontal slices of the sample. Then, the density of each horizontal slice was measured. The horizontal slices having a higher density represent the more highly cured regions. The horizontal slices having a relatively lower density represent the lower density regions. The thickness of all the horizontal slices of the sample having a higher density represented the thickness of the more highly cured region of that sample. Next, the thickness of the more highly cured regions of the samples across the lateral direction of the board, and the position of each sample across the lateral direction of the board were tabulated and / or plotted to determine the lateral profile of the more highly cured regions across the board.

[0167] Results for gypsum boards having a higher density gypsum layer with a target thickness of 20 - 25 mils are shown in Table 1, except for Test 4 of the present invention which had a target thickness of 35 - 40 mils. Each test was on a separate board. The averages in Table 1 are the averages of the samples across the entire board. Table 2 shows the minimum and maximum thicknesses of the samples across the entire board.

[0168]

Table 1

[0169] Table 1 shows that for a target thickness of 20 - 25 mils, the average layer thickness of the present invention is approximately the same as, or relatively higher than, the average layer thickness of the control for outlier sample 3. Table 1 also shows that the minimum layer thickness of the present invention is approximately the same as, or relatively larger than, the minimum layer thickness of the control. Table 1 also shows that the average thickness, expressed as the minimum thickness / percentage of the higher density regions of the cured samples, exceeds 40% for the test samples of the present invention.

[0170] All documents described herein are incorporated herein by reference for the purposes of any jurisdiction in which such practices are permitted, including any priority documents and / or test procedures, to the extent they are not inconsistent with this specification. As will be apparent from the foregoing general description and the specific embodiments, the forms of the present disclosure have been illustrated and described, but various modifications can be made without departing from the spirit and scope of the present disclosure. Accordingly, it is not intended that the present disclosure be limited thereby. For example, the compositions described herein may not include any components or compositions not explicitly listed or disclosed herein. Any method may omit any steps not described or disclosed herein. Similarly, the term "comprising" is considered to be synonymous with the term "including". Whenever the transitional phrase "comprising" precedes a listing of a method, composition, element, or group of elements, it is understood that the same composition or group of elements having the transitional phrases "consisting essentially of", "consisting of", "selected from the group consisting of", or "being" preceding the listing of the composition, element, or elements is also contemplated, and vice versa.

[0171] Unless otherwise indicated, all numbers expressing quantities of ingredients, properties such as molecular weight, reaction conditions, and the like, used in the specification and the associated claims are to be understood as being modified in all instances by the term "about." Accordingly, unless indicated to the contrary, the numerical parameters set forth in the following specification and attached claims are approximations that may vary depending upon the desired properties sought to be obtained by the embodiments of the present invention. At the very least, and not as an attempt to limit the application of the doctrine of equivalents to the scope of the claims, each numerical parameter should be construed in light of the reported number of significant digits and by applying ordinary rounding techniques.

[0172] Whenever a numerical range having a lower limit and an upper limit is disclosed, any number and any included range falling within that range are specifically disclosed. In particular, any range of values (in the form of "about a to about b," or equivalently "approximately a to b," or equivalently "about a to b") disclosed herein is to be understood to set forth every number and range encompassed within the broader range of values. Also, the terms used in the claims have their plain ordinary meaning unless explicitly and clearly defined by the patentee. Further, the indefinite articles "a" or "an" used in the claims are defined herein to mean one or more of the elements that they introduce.

Claims

Claim 1 A method for manufacturing a gypsum board, the method comprising: depositing a front cover sheet having a first and a second opposite face on a forming surface, wherein the first face of the front cover sheet is a lower face facing the forming surface and the second face of the front cover sheet is an upper face facing away from the forming surface, and horizontally moving the front cover sheet along the forming surface in a machine direction; depositing a first portion of an aqueous gypsum slurry on the front cover sheet to form a layer of the first portion of the aqueous gypsum slurry as a layer-like higher density region in contact with the upper face of the front cover sheet; moving a back cover sheet having a third and a fourth opposite face along a back cover sheet path including a first path segment and a second path segment downstream of the first path segment, the back cover sheet moving along the first path segment above the higher density region on the front cover sheet with a movement including a first vertical movement component and / or a first horizontal movement component, the first vertical movement component being a downward movement towards the higher density region on the front cover sheet, and the first horizontal movement component being a movement in a direction opposite to the machine direction; contacting the back cover sheet with a second portion of the aqueous gypsum slurry that is less dense than the first portion of the aqueous gypsum slurry, for contacting the third face of the back cover sheet in the first path segment, the second portion of the aqueous gypsum slurry contacting the back cover sheet at an incident angle "B" of 0 to 90 degrees, for example, 0 to 20 degrees, preferably 5 to 90 degrees, more preferably 5 to 85 degrees or 5 to 60 degrees, with respect to the face of the back cover sheet with which the slurry contacts; The first portion of the aqueous slurry and the second portion of the aqueous slurry each contain a respective mixture of water and stucco, the stucco contains calcium sulfate hemihydrate, and the first portion of the aqueous slurry and the second portion of the aqueous slurry each contain, on a dry (anhydrous) basis, at least 60 wt%, typically 60 - 98 wt%, preferably at least 70 wt%, more preferably at least 80 wt%, typically at least 90 wt%, or typically at least 95 wt% of the respective mixture of the calcium sulfate hemihydrate and the water, with a weight ratio of water to the calcium sulfate hemihydrate of 0.2:1 to 1.2:1, depositing the second portion. Then, at the downstream end of the first path segment, while the fourth surface of the back cover sheet contacts the first transfer roller, passing the back cover sheet together with the second portion of the aqueous gypsum slurry over the first transfer roller spaced a distance above the higher density region on the front cover sheet, depositing a majority by weight of the second portion of the aqueous gypsum slurry that has fallen from the back cover sheet onto the higher density region on the front cover sheet, and forming a layer of the second portion of the aqueous gypsum slurry in a foamed state as a layered lower density region that contacts the upper surface of the higher density region having a higher density than the lower density region. The back cover sheet then passes around the first transfer roller to feed the back cover sheet to the second path segment, the second path segment has an upstream end and a downstream end, the upstream end is at the first transfer roller, and typically when deposition of the second aqueous gypsum slurry occurs, the remaining portion of the second aqueous gypsum slurry remains in contact with the back cover sheet as the back cover sheet moves to the second path segment. At the downstream end of the second path segment, depositing the back cover sheet on the second portion of the aqueous gypsum slurry on the face cover sheet to form a multilayer assembly including the back cover sheet on the second portion of the aqueous gypsum slurry, typically, the multilayer assembly includes a first portion of the aqueous gypsum slurry layer on the front cover sheet, a second portion of the aqueous gypsum slurry layer on the first portion of the aqueous gypsum slurry layer, and the back cover sheet on the second portion of the aqueous gypsum slurry layer, to form a multilayer assembly. Passing the multilayer assembly through a forming station for forming the multilayer assembly, preferably, the multilayer assembly passes under the forming plate of the forming station. Reacting the calcium sulfate hemihydrate with the water in the first portion of each of the aqueous slurry layers and the second portion of the aqueous gypsum slurry layer to cure the calcium sulfate hemihydrate in the first portion of each of the aqueous gypsum slurry layers and the second portion of the aqueous gypsum slurry layer, to form respective first and second board layers containing calcium sulfate dihydrate between the front cover sheet and the back cover sheet. The first board layer includes a higher density region cured as a layer containing calcium sulfate dihydrate and has a first board layer density. The second board layer includes a lower density region cured as a layer containing calcium sulfate dihydrate and has a second board layer density lower than the first board layer density, and the cured higher density region is interposed between the cured lower density region and the front cover sheet, to form a panel including a gypsum core of each of the first and second board layers. Drying the panel and cutting the panel into the gypsum board. A method comprising. **Claim 2** By causing both the first vertical movement component and the first horizontal movement component to be present simultaneously along all or the same portion of the first segment, the back cover sheet moves along the downstream portion of the first path segment, along an inclination defining an angle "A" of 0 to 90 degrees between the back cover sheet and the front cover sheet on the forming surface at the downstream portion of the first path segment. Typically, the angle "A" is 5 to 90 degrees or 5 to 85 degrees or 0 to 20 degrees, and more typically, the angle "A" is 20 to 60 degrees. The method according to claim 1.

3. The first transfer roller changes the moving direction of the back cover sheet to a movement along a first portion of the second path segment having a second horizontal movement component and a second vertical movement component in the machine direction. The second vertical movement component is upward or downward or neutral to provide only movement along the second horizontal movement component. Typically, the second vertical movement component is an upward movement away from the higher density region on the front cover sheet. The front cover sheet includes at least one of a glass mat facer sheet or a paper facer sheet. The discharge of the calcined gypsum core slurry is in a direction parallel to the first direction of the back cover sheet progression. The slurry mixer discharges the calcined gypsum core slurry onto a portion of the back cover sheet supported by the back plate. The method according to claim 1.

4. The second path segment has a first portion and a second portion. At the downstream end of the first portion of the second path segment, the back cover sheet passes over a second transfer roller to feed the back cover sheet to the second portion of the second path segment, changing the moving direction of the back cover sheet. At the downstream end of the second path segment, the multi-layer assembly includes a first portion of the aqueous gypsum slurry layer on the front cover sheet, a second portion of the aqueous gypsum slurry layer on the first portion of the aqueous gypsum slurry layer, and the back cover sheet on the second portion of the aqueous gypsum slurry layer. The method according to claim 1.

5. The method according to claim 4, wherein the second transfer roller is a center-exposed type shaft roller having a center shaft of a first diameter and opposing cylindrical end portions of a second diameter, and the second diameter is larger than the first diameter.

6. The lower density region obtained from the slurry of the lower density region of the hardened gypsum has a thickness of 0.25 inches to 1.5 inches, typically 0.3 inches to 1 inch, or 0.4 inches to 0.75 inches. The higher density region has a thickness of about 0.02 inches to about 0.75 inches, about 0.02 inches to about 0.35 inches, or about 0.02 inches to about 0.2 inches (about 0.05 to about 0.5 cm), and typically, the thickness of the lower density region layer is greater than the thickness of the higher density region layer. The method according to claim 1, wherein the lower density region has a density of 15 to 55 pounds per cubic foot, and the higher density region has a density of 25 to 70 pounds per cubic foot, more typically 30 to 60 pounds per cubic foot.

7. An apparatus for manufacturing a gypsum board according to the method according to any one of claims 1 to 6, the apparatus comprising: A forming surface for depositing a front cover sheet on the forming surface and horizontally moving the front cover sheet in the machine direction along the forming surface. A source for the first portion of the aqueous gypsum slurry for depositing the first portion of the aqueous gypsum slurry on the front cover sheet to form a layer of the aqueous gypsum slurry as a layered higher density region in contact with the upper surface of the front cover sheet, the first aqueous gypsum slurry layer having a first slurry density. A first transfer roller and typically a second transfer roller for moving a back cover sheet along a back cover sheet path comprising a first path segment and a second path segment. The first path segment is adapted and configured to move the back cover sheet along the first path segment above the higher density region on the front cover sheet with a movement including a first vertical movement component and / or a first horizontal movement component, the first vertical movement component being a downward movement towards the higher density region on the front cover sheet, and the first horizontal movement component being a movement in a direction opposite to the machine direction, a back cover sheet drive unit, A second portion of the aqueous gypsum slurry that is less dense than the first portion of the aqueous gypsum slurry for contacting the surface of the back cover sheet in the first path segment, the second portion of the aqueous gypsum slurry contacting the surface of the back cover sheet at an incident angle "B" of 0 to 90 degrees, for example, 0 to 20 degrees, preferably 5 to 90 degrees, more preferably 5 to 85 degrees, even more preferably 5 to 60 degrees, a supply source of the second portion, The first transfer roller is at the downstream end of the first path segment for passing the back cover sheet around the first transfer roller to send the back cover sheet to the second path segment, the second path segment having an upstream end and a downstream end, the upstream end of the second path segment being at the first transfer roller, The first transfer roller is spaced a distance above the higher density region on the front cover sheet to pass the second portion of the aqueous gypsum slurry over the first transfer roller and deposit a majority by weight of the second portion of the aqueous gypsum slurry that has fallen from the back cover sheet onto the higher density region on the front cover sheet. Typically, when this deposition occurs, the remaining portion of the second portion of the aqueous gypsum slurry remains in contact with the back cover sheet as the back cover sheet moves within the second path segment. The first portion of the aqueous gypsum slurry and the second portion of the aqueous gypsum slurry each contain a respective mixture of water and stucco, the stucco contains calcium sulfate hemihydrate, and the first portion of the aqueous gypsum slurry and the second portion of the aqueous gypsum slurry each contain, on a dry (anhydrous) basis, at least 60% by weight, typically 60 to 98% by weight, preferably at least 70% by weight, more preferably at least 80% by weight, typically at least 90% by weight, or typically at least 95% by weight of the calcium sulfate hemihydrate and the water in a weight ratio of water to the calcium sulfate hemihydrate of 0.2:1 to 1.2:

1. The downstream end of the second path segment is positioned to deposit the back cover sheet on the second portion of the aqueous gypsum slurry on the front cover sheet to form a multi-layer assembly including the back cover sheet on the second portion of the aqueous gypsum slurry. Comprising a forming station for forming the multi-layer assembly, preferably the forming station comprises a forming plate. Curing the calcium sulfate hemihydrate to form a panel comprising a gypsum core containing calcium sulfate dihydrate. The first portion of the gypsum slurry forms a layered higher density region in contact with the front cover sheet. The second portion of the gypsum slurry is in a foamed state as a lower density region in contact with the higher density region, and the higher density region has a higher density than the lower density region. An apparatus, wherein the board core includes a cured lower density region containing calcium sulfate dihydrate and a cured higher density region containing calcium sulfate dihydrate, and the cured higher density region is interposed as a layer between the cured lower density region and the front cover sheet. Claim 8 Further comprising a mixer for preparing the aqueous gypsum slurry comprising a mixture of water and stucco, wherein the stucco comprises calcium sulfate hemihydrate, and the aqueous gypsum slurry comprises, on a dry (anhydrous) basis, at least 60% by weight, typically 60 - 98% by weight, preferably at least 70% by weight, more preferably at least 80% by weight, typically at least 90% by weight, or typically at least 95% by weight of the calcium sulfate hemihydrate and the mixture of the water and the calcium sulfate hemihydrate in a weight ratio of water to calcium sulfate hemihydrate of 0.2:1 to 1.2:1, By simultaneously causing both the first vertical movement component and the first horizontal movement component to be present along all or the same part of the first segment, the back cover sheet moves along the downstream part of the first path segment along an inclination defining an angle "A" of 0 - 90 degrees between the back cover sheet and the front cover sheet on the forming surface at the downstream part of the first path segment, typically the angle "A" is 0 - 90 degrees, more typically the angle "A" is 5 - 90 degrees or 5 - 85 degrees, and more typically the angle "A" is 20 - 60 degrees, The discharge of the calcined gypsum core slurry is in a countercurrent direction to the moving direction of the back cover sheet, or the discharge of the calcined gypsum core slurry is in a co-current direction to the moving direction of the back cover sheet, The first transfer roller changes the moving direction of the back cover sheet to a movement along the first part of the second path segment having a second horizontal movement component and a second vertical movement component in the machine direction, and the second vertical movement component is upward, or downward, or neutral to provide only movement along the second horizontal movement component, typically the second vertical movement component is an upward movement away from the higher density region on the front cover sheet, The second path segment has a first part and a second part, and at the downstream end of the first part of the second path segment, the back cover sheet passes over a second transfer roller to feed the back cover sheet to the second part of the second path segment and change the moving direction of the back cover sheet. The apparatus according to claim 7.

9. A gypsum board produced according to the method according to any one of claims 1 to 6.

10. Having a width of 3 to 5 feet, wherein the cured higher density region has an average thickness of 20 to 40 mils measured from the average of a series of 1-inch wide samples taken across the width of the cross-section of the board, and the cured higher density region thickness of each sample has a difference of + / - 20 mils from the average thickness, the minimum cured higher density region thickness of the sample is at least 10 mils thick, The gypsum board according to claim 9, wherein the minimum thickness / average thickness of the cured higher density region of the sample is 40 to 100%, typically 40 to 90 or 40 to 80%.

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