Panel for dry interior equipment

Description

[0001] The invention relates to a panel for dry interior construction, which has a sandwich-like structure, according to claim 1, and to a panel system in which these panels are used.

[0002] Standard drywall panels for constructing non-load-bearing partition walls are cut to size on-site and installed. Typically, gypsum plasterboard is used, consisting of a gypsum core covered on both main surfaces with a robust paper or paper-like material. The perimeter surfaces connecting the two main surfaces of the plasterboard panels are left uncovered. These panels are screwed onto steel studs, which are mounted to the floor, ceiling, and intermediate studs, ultimately forming a wall where the steel structure is concealed by the plasterboard. The space between the plasterboard and the frame is either empty or filled with insulation material.To increase fire resistance, it may be possible to mount two plates flush against each other on one or both sides of the steel structure.

[0003] The necessary work is time-consuming and therefore blocks the construction site for extended periods. It also generates waste that can potentially cause problems on the site, such as small pieces of steel or the dust produced when cutting the plasterboard. While the fire-retardant properties are satisfactory, the acoustic insulation is weak, and the space required is insufficient compared to the insulation effect.

[0004] CN209990012U discloses a system of fire-resistant gypsum boards in which the gypsum walls are bonded on one side to U-shaped metal plates. These gypsum composite boards have fire-resistant properties but require a relatively large amount of installation space and are susceptible to moisture penetration.

[0005] German patent DE202017104561U1 discloses a partition wall system, primarily for interior spaces in office buildings, exhibition halls, and residential buildings with a load-bearing frame. Partition walls according to this patent provide sound insulation but also require considerable installation space and are susceptible to moisture penetration.

[0006] Similarly, JP2020026674A and DE19960535A1 describe partition wall systems for improved sound absorption, which also require a relatively large amount of installation space and are susceptible to moisture ingress.

[0007] US 5 010 702 A discloses a plate with the features of the preamble of claim 1.

[0008] There is therefore a need for a panel for dry interior construction that has the most compact installation space possible and can be adapted to the intended use, so that it can be sealed against the penetration of moisture and still has good fire-retardant properties if required.

[0009] According to the invention, the desired objectives are achieved by a panel for dry interior construction which has the features specified in the characterizing part of claim 1. In other words, a conventional drywall panel, preferably made of gypsum fiber, is used as the core of the panel and is provided on each of its sides with a moisture-resistant, preferably metallic, cover panel, which may be coated, painted or laminated, and with an intermediate filling already bonded at the factory and preferably manufactured to precise dimensions, or alternatively cut to precise dimensions on site.

[0010] This dry construction panel has a predetermined underside. This underside features a groove to accommodate a floor profile that positions the panel. If the dry construction panel is intended to butt against another dry construction panel along a vertical joint, it also has a groove on this side surface, which may not necessarily have the same geometric dimensions as the groove on the underside. The panel is ultimately secured by a ceiling profile and, if required, a side profile.

[0011] If the dry interior panel is intended for installation in a wet room, or if there is a risk of moisture and / or water ingress, the panel must have a moisture-resistant filling, at least in the lower area (the area closest to the floor) where such ingress is feared (this can include the entire panel). Furthermore, the joints of the panel in the floor area, whether horizontal or vertical, are preferably sealed with a hydromechanical insulating compound to prevent rising damp. Additionally, a second sealing layer, using sealing tape and / or a sealing profile, is installed between the hydromechanical insulating compound and the panel core.

[0012] If a gypsum fiberboard is used as the core of the board, it is preferably glued to the cover boards, but it is also possible, for example, to use the cover boards as permanent formwork and to pour in the filling and allow it to harden, which also results in a permanent bond with the cover boards.

[0013] In this way, on the construction site it is only necessary to attach the floor profile, the ceiling profile explained below, and the profiles on the wall where the partition wall to be erected abuts the wall specified on the construction site, and then to insert the panels without further fasteners and thus without damaging the surface of the panels, which, as also explained below, is done by sliding or lifting them in.

[0014] The customer thus receives a finished partition wall with excellent fire-retardant, acoustically damping, and moisture-resistant properties, which can be designed according to the customer's wishes, and can also be finished with different surface coverings on one or both sides (tiles, etc.) and, if necessary, can also be dimensioned or constructed differently in terms of its thickness (of course only from partition wall to partition wall) and thus be provided with different insulating properties.

[0015] The individual wall panels can be made of different materials and combined with each other if they have the same thickness, so that glass in sandwich construction, i.e., made of laminated safety glass (VSG) or tempered safety glass (ESG) with a groove designed according to the application, i.e., on 3 sides or only on the bottom side, can be used in combination with panels according to the invention for dry interior construction.

[0016] The major advantage over the state of the art, even in the version without seals, lies in the fact that the double-sided formwork with cover plates allows for significant space savings compared to conventional systems, while still achieving surprisingly substantial improvements in sound insulation. In particular, the improved properties result in a noticeable reduction in the thickness of the partition wall compared to the state of the art, thereby considerably increasing the usable floor space of the apartment or office, or minimizing the area to be covered (the required floor sealing per apartment).

[0017] These walls, which are significantly thinner than those produced using conventional methods, can be easily dismantled and reused in a resource-efficient manner, as they always rest on the finished screed (or raised floor, etc., allowing the area of ​​several rooms to be covered with screed simultaneously) or on the finished floor. Disposal is minimal compared to conventional plasterboard stud walls.

[0018] Another major advantage over the state of the art for dry interior construction lies in the possibility of hydromechanically shielding the panel and thereby preventing, in particular, the penetration of water and, consequently, the swelling of classic drywall panels.

[0019] The invention is explained in more detail below with reference to the drawing. The drawing(s) show: the Fig. 1a and bpurely schematic vertical sections through preferred embodiments of the panels according to the invention for dry interior construction, which Fig. 2a and b purely schematic horizontal section through each pair of adjoining plates according to the invention for dry interior construction, which Fig. 3 a floor profile usable for a plate system according to the invention, in a longitudinal ( Fig. 3a ) and two cross-sections ( Fig. 3b & c ) and a perspective view ( Fig. 3d ), the Fig. 4 a purely schematic section through a ceiling profile usable for the panel system according to the invention, which Fig. 5 a purely schematic section through a T-joint profile usable for the plate system according to the invention, which Fig. 6 a purely schematic section through a 90° corner profile usable for the plate system according to the invention, which Fig. 7a and btwo purely schematic sections through two butt joint profiles usable for the plate system according to the invention for the application accordingly Fig. 2a or Fig.2b , the Fig. 8 a purely schematic section through a finishing profile usable for the plate system according to the invention, which Fig. 9 a purely schematic section through a 2-part wall connection profile usable for the plate system according to the invention, which Fig. 10 a purely schematic section through a frame profile usable for the panel system according to the invention and the Fig. 11 a floor plan section as an example, a purely schematic representation of the system used

[0020] The Fig. 1aFigure 1 shows, in a purely schematic manner, a vertical section through a panel 1 according to the invention for dry interior construction, which in the illustrated embodiment is equipped for use in a wet room. In principle, the panel 1 consists of a filling 2 (the panel core) and is provided on both sides with moisture-resistant cover plates 3, 4. The cover plates 3, 4 may, but need not, have the same mechanical and geometric properties. Furthermore, the cover plates 3, 4 can be made of a wide variety of moisture-resistant materials, such as plastic and metal. Preferably, the cover plates 3, 4 are made of metallic material in order to achieve fire-retardant properties in addition to moisture resistance. Steel or aluminum in varying thicknesses, preferably between 0.4 mm and 1 mm, is particularly preferred.In special cases where only imperceptible mechanical stresses are expected, thinner cover plates can be used; in cases of extreme stress, thicker ones are required. The surfaces can be painted, powder-coated, foil-wrapped, veneered, wallpapered, or left untreated, and can also be covered with ceramic wall coverings. In the example shown, the plate 1 has a groove 6 on its underside 5, which is suitable for receiving a web 7 of a floor profile 8.

[0021] These elements are provided in all panels 1 according to the invention. For installation in wet rooms, the following additional elements are required. Fig. 1The following features are evident: In the lower area, a moisture-resistant filling 2' is installed instead of the core plate 2. Preferably, at least in this area, between the filling 2' and the core plate 2, in the area between the cover plates 3, 4, a further, internal seal 9 is provided. In the illustrated embodiment, this internal seal 9 is shown and formed above the moisture-resistant filling 2'. Furthermore, the base profile 8 is not attached directly to the base 11, but rather via a moisture-repellent (hydrophobic) and moisture-insulating intermediate layer 10, which is preferably an easily applied hydrophobic swelling paste, in order to avoid direct contact with the base as much as possible. This also largely prevents moisture penetration through capillaries.

[0022] In Fig. 1The groove 6 is shown significantly deeper than the height of the web 7, but this overhang is not necessary. It is important to prevent the end of the web 7 from touching the bottom of the groove 6, as this can cause mechanical problems. Transferring the weight of the plate 1 via the metal cover plates 3, 4 to the base profile 8 is generally preferred. Therefore, the end of the web 7 and the bottom of the groove 6 should, at most, only touch when installed.

[0023] The inner seal 9 preferably consists of hydrophobic material such as rubber, silicone rubber, plastic, etc. and has a certain elasticity which, when installed, is used to ensure a force-fit connection and thus a sealing contact against the cover plates 3, 4, the filling 2 as well as the bottom profile 8 and / or the filling 2'.

[0024] In the example shown, the filling 2 is bonded to the cover plates 3, 4 over their entire surface using an adhesive. However, as already explained, it is also possible to obtain it by pouring it into the formwork. The moisture-resistant filling 2' can also be bonded to the cover plates 3, 4, or it can be foamed or cast in place, or held in place by elastic prestressing, which ensures the transfer of the weight of the plate 1 to the base profile 8 via the cover plates 3, 4.

[0025] The Fig. 1b shows a version of panel 1 for dry interior construction analogous to Fig. 1aThe inner seal 9 is replaced by the inner seal 9a. The inner seal 9a connects directly to the infill 2 inside the panel and forms two downward-running ribs, each enclosing infill 2' between itself and the cover panels 3, 4. No infill 2' is present between the ribs, so that the groove 6 extends vertically upwards from the base profile 8 to the lower edge of the inner seal 9a. The same applies to the top and side areas of the panel 1.

[0026] The Fig. 2a shows, by way of example, a horizontal section through a vertical joint between two panels 1 for dry interior construction in the area of ​​their side edges 24, analogous to Fig. 1aA butt joint profile 18 made of permanently elastic plastic or rubber is used to ensure a flat (aligned) connection between the panels 1. The horizontal length of the butt joint profile 18 is preferably slightly shorter than the sum L of the depths of the two grooves to reliably prevent longitudinal stresses. The vertical extension (normal to the plane of the image) of the butt joint profile 18 preferably extends from the base profile 8 (see Fig. 1 ) to the upper edge of the plates 1, whereby segmentation is of course also possible. Whether and in what form the drawn "barbs" are provided can be easily determined by a person skilled in the art with knowledge of the invention and the respective field of application. The use of the aforementioned materials ensures that the plates can be easily separated in the event of a modification or replacement.

[0027] It is in Fig. 2aEach also has a further, inner seal 9 provided to protect the neighboring panels in case of water ingress into one of the panels, so that only the panels damaged by the water need to be replaced.

[0028] The Fig. 2b shows an example of an alternative horizontal section through a vertical butt joint between two panels 1 for dry interior construction in the area of ​​their side edges 24, analogous to Fig. 1b The general statements that are in Fig. 2a The measures taken also have consequences for Fig. 2b Validity. It should be noted in particular that, due to the different shape of seal 9a in contrast to seal 9, Fig. 2a or Fig. 1a, another joint profile 18 can also be used. Here too, in order to reliably avoid longitudinal stresses, the length of the butt joint profile 18 in the horizontal direction should preferably be somewhat shorter than the sum L of the depths of the two grooves. Theoretically, a combination of the seal 9a made of Fig. 2b with the joint profile 18 made of Fig. 2a possible (as well as a combination of the joint profile 18 from Fig. 2b with the seal 9 from Fig. 2a ). The special one in Fig. 2b However, the combination shown is particularly advantageous for applications where the walls need to be separated again with minimal effort.

[0029] The Fig. 3a until d shows an embodiment of a floor profile 8 in a longitudinal ( Fig. 3a ) and two cross-sections ( Fig. 3b & c ) and a perspective view ( Fig. 3dThe respective sections are marked AA, BB, and CC. The height H of the web 7 is also indicated. Mounting to the subfloor or screed is achieved, for example, by screwing, gluing, or using nails. Compensation for construction tolerances can be made using shims made of various pressure-resistant materials. Sealing against the floor is preferably achieved using swelling paste 10 (see Fig. 1 ) ensured. The self-weight of the plates 1 is statically transferred via the cover plates 3, 4 to the base profile 8.

[0030] The width of the base profile 8 preferably corresponds to the thickness of the associated plate(s) 1. However, a slight overhang of the base profile 8 can be mechanically advantageous, allowing for a balance between aesthetics and safety.

[0031] In the illustrated embodiment, the holes for the fastening elements are shown without their own reference numerals; the height H of the web 7 above the upper, horizontal surface of the base profile 8 is, as already shown in Fig. 1 As described, the depth of the groove 6 of the plates 1 used must be coordinated to prevent vertical forces from being transferred via the web 7. The cross-sectional shape of the web 7 can vary within wide limits, as long as guidance and positioning in the horizontal direction perpendicular to the longitudinal axis of the base profile 8 are achieved. The slightly undercut shape shown represents a particularly advantageous embodiment, since a particularly favorable support for the plate 1 can be achieved through slight elastic deformation of the infill 2 or 2'.

[0032] The Fig. 4Figure 1 shows a ceiling profile 13 that is well suited for mounting the panel 1 according to the invention. In its mounted state, it is essentially inverted U-shaped, and the two legs 14, 15 of the profile 13 are significantly longer than the insertion of the panel 1, which is schematically shown with dotted lines and projects into the ceiling profile 13. The projection D is noticeably higher than the height H (see Figure 1). Fig. 3 ) of the web 7 of the base profile 8. This allows the panel 1 to be threaded into the ceiling profile 13 from below until its upper edge abuts the bottom of the ceiling profile 13 on the inside, then to pivot over the base profile 8 and allow its groove 6 to slide over the web 7 of the base profile. This projection D also accommodates structural tolerances, so that no statically effective forces are transferred vertically into the panel 1.

[0033] Furthermore, in the illustrated variant of the ceiling profile 13, various inwardly projecting webs (without reference numerals) are provided, which serve to accommodate rubber or plastic profiles 22 or permanently elastic sealants, indicated by faint lines. These can be identical on both sides, or as shown in Fig. 4 The design can vary in its specific characteristics. These rubber or plastic profiles 22, or permanently elastic sealants, hold the panel 1 in place without contact with the ceiling profile 13, thus preventing or significantly attenuating structure-borne sound transmission. To improve sound insulation, these ceiling profiles can be filled with foam strips (compression strips or other expanding filler materials) to varying degrees of density, depending on requirements. Furthermore, the ceiling profile can be connected to a two-part wall connection profile, as shown in Fig. 9 shown, to replace, i.e., and / or to use.

[0034] The ceiling profile 13 is attached either directly, or, like the floor profile 8 on the floor, via an insulating zone, for example by screwing or gluing or by means of nails to the ceiling.

[0035] The Figs. 5 and 6 The figures show horizontal sections through a connecting profile for a T-joint 16 or a corner 17, which is to be installed vertically between two (17) or three (16) panels 1. In each case, webs 7 are designed analogously to the web 7 of the base profile 8. Despite the different arrangement and potentially different dimensions, these profiles have been given the same reference numeral to clarify the analogy. These profiles 16, 17 can either be glued to the panels 1 or simply inserted. A design for the webs 7 like that of the butt joint element 18, i.e., with barbs, is also possible. Plastic with good mechanical properties or aluminum is preferred as the material.

[0036] The Fig. 7a or b The section perpendicular to the longitudinal extent shows examples of a typically band-shaped butt joint element 18, which can be inserted into the vertical grooves of adjacent plates 1 and holds the plates 1 butt-to-butt with the respective edges of the groove, see Fig. 2a or b. This shows Fig. 7a a in Fig. 2a butt joint element 18 used and Fig. 7b a in Fig. 2b butt joint element 18 used. It is advantageous to provide that the width L1, the element width of the butt joint element 18, is not greater than the combined depth of the two grooves of the adjacent plates 1. Shape and number of the in Fig. 7a The depicted barbs can be varied within wide limits and, for example, replaced by round shapes (similar to O-rings in grooves); all kinds of combinations are possible.

[0037] The Fig. 8Figure 19 shows an example of a wall termination profile. This can be glued or simply clipped into place. The web is designed similarly to the butt joint element 18. Fig. 7 With or without barbs, this is also possible. Plastic with good mechanical properties or aluminum is preferred as the material.

[0038] The Fig. 9This shows an example of a two-part wall connection profile 14, 21 with the free dimension L to accommodate structural tolerances. Seals 22 are again provided, which can of course be omitted (workshops, storage rooms, etc.) and which do not have to be identical. The leg 14 is first attached to the wall / ceiling, then the panel 1 is positioned, and finally the click profile 21 is clipped on. To increase sound insulation, these wall connection profiles can be filled more or less densely, depending on requirements, using foam strips (compression strips or other expanding filling materials). Here, too, it is possible to use a ceiling profile 13 instead of this wall connection profile ( Fig. 4 ) to use, which, due to its one-piece construction, has greater mechanical stability.

[0039] The Fig. 10Figure 1 shows an example of a door frame profile 23 for installation in a wall consisting of panels 1 in a horizontal section. The door can be floor-to-ceiling, or fitted with a transom, or constructed with panel(s) 1 above the lintel (in which case the door frame is built from such profiles, mitered at the joints). Here too, seals 22 are provided for enclosing the panel 1. The panel 1 is shown here as a typical panel for a dry area with only infill 2, thus without an internal seal 9, 9a, extending to the side edge 24 and without a groove 6 (which can, of course, be provided), in order to visually illustrate this variant, which can naturally be used anywhere and on all edges.

[0040] The Fig. 11 The schematic top view shows an example of a section of a floor plan, indicating where the parts and elements shown in the individual figures can be inserted.

[0041] Regarding the materials that can be used, the following should be noted, although a person skilled in the field of interior construction would not require such information if familiar with the invention: For example, the FERMACELL® board can be used as a gypsum fiberboard, with a density of 1150 ± 50 kg / m³, a water vapor diffusion resistance factor µ = 13, a thermal conductivity Δ = 0.32 W / mK, and a specific heat capacity c = 1.1 kJ / kgK. These are values ​​that other gypsum fiberboards also essentially exhibit and are therefore suitable.

[0042] For bonding the gypsum fiberboard to the cover boards, a double-sided adhesive tape can be used, for example, the S-4710 MF from ATP.

[0043] The cover plates 3, 4 can be made of moisture-resistant material, such as plastics or metals. Particularly preferred, due to their flame-retardant properties, are embodiments of the inventive plate 1 with cover plates 3, 4 made of aluminum, including various aluminum alloys, especially aluminum-manganese alloys, or cover plates 3, 4 made of steel. A person with expertise in dry interior construction can easily make their selection based on knowledge of the invention and the requirements for the finished plate 1 and the cover plates 3, 4.

[0044] For example, as an insulating intermediate layer 10, swelling pastes or sealants based on polyurethane with a low proportion of free monomeric diisocyanates can be used, such as Sikaflex® < 11FC Purform® < . Compounds based on silicones, polysulfides or hot melt can also be used.

[0045] For example, a two-component polyurethane sealant, such as that used in insulating glass, can be used as the moisture-resistant material for filling 2'. POLIVER GP-AC from FENZI is one such example.

[0046] Profiles 13, 14, 15, 19, 21 can be made independently of each other from aluminium, various aluminium alloys, in particular aluminium-manganese alloys, or plastic.

[0047] The impact element 18 can be made of, for example, aluminium, rubber, wood, or plastic.

[0048] The inner seal 9, 9a can consist, for example, of butyl-aluminum composite material or other silicones or other waterproof materials suitable as seal 9, 9a. The specialist is familiar with suitable sealants.

[0049] The term "classic drywall panel," for example in connection with filling 2, refers to a panel suitable for drywall construction. Experts typically understand this to mean gypsum fiberboards, wood fiberboards, clay building panels, or similar materials.

[0050] For the sake of completeness, it should be mentioned that these panels are of course also suitable for installing ceiling coverings.

[0051] In summary, the main advantage of the invention lies in providing panels 1 for dry interior construction that can be installed quickly, easily, and without causing unnecessary mess on construction sites. These panels 1 can be easily threaded into pre-installed floor profiles 8 and fixed, for example, using ceiling profiles 13. Due to the special sandwich construction and the possibility of additionally insulating the connecting profiles, good sound insulation properties can be achieved in a very compact installation space, which, according to the prior art, would require at least two parallel conventional drywall partitions with insulation material in between.

[0052] Furthermore, these properties of the panel 1 or the panel system can be improved by attaching fire-resistant cover panels 3, 4, preferably made of metal, thereby achieving a very good flame-retardant effect for the resulting wall. One reason for the improved flame-retardant effect is the extensive elimination of combustible materials such as insulation wool, which can be achieved through the special design.

[0053] Furthermore, the properties of the panel 1 can be improved even further by providing a moisture-resistant filling 2' and an internal seal 9, 9a. In this way, the sides provided with the filling 2' or the internal seal 9, 9a can also be sealed against the ingress of liquid. If this filling 2' or internal seal 9, 9a is provided on all circumferential sides (5, 24, top side without reference numeral) and the cover plates 3, 4 are made of metallic materials, a panel 1 is obtained that is optimally sealed against the ingress of liquid, is very well suited for use in wet rooms, and also exhibits flame-retardant properties. Reference symbol list:

[0054] 01 plate 15 leg 02 Filling (slab core) 16 T-profile 02' moisture-resistant filling 17 90° corner profile 03 Cover plate 18 Impact element 04 Cover plate 19 Graduation profile 05 bottom 20 glue 06 Nut 21 Click profile 07 web 22 seal 08 Ground profile 23 Door frame profile 09 Internal sealing 24 side surface 10 Intermediate layer (swelling paste) H Bridge height 11 Floor L Overhang 12 top edge of floor L1 Element length 13 Ceiling profile 14 leg

Claims

1. Panel (1) for the construction of non-load-bearing walls for dry interior finishing using a panel core (2) suitable for dry construction, wherein the panel core (2) is flat-connected on both sides to at least one moisture-resistant cover plate (3, 4) and has on at least one side, which extends between the cover plates (3, 4) and is designed to form the underside (5) when installed, a groove (6) designed to accommodate a web (7) of a floor profile (8), wherein at least in the lower region of the panel (1) a moisture-resistant filling (2') is attached, instead of the panel core (2), characterised in that an inner seal (9, 9a) is arranged between the panel core (2) and at least parts of the moisture-resistant filling (2').

2. Panel (1) according to claim 1, characterised in that the depth of the groove (6) is at most equal to the height (H) of the web (7) of an associated floor profile (8).

3. Panel (1) according to claim 1 or 2, characterised in that at least one of the cover plates (3, 4), preferably both cover plates (3, 4), is made of metal, preferably aluminium, or an aluminium alloy, in particular aluminium-manganese alloys.

4. Panel (1) according to one of claims 1 to 3, characterised in that it has a groove on at least one of its side surfaces (24), which is intended to accommodate either a butt element (18) or a web (7) of a connecting profile (16, 17, 19).

5. Panel (1) according to one of claims 1 to 4, characterised in that the panel (1) has a filling (2') made of moisture-resistant material in which the groove (6) is formed on several of its side surfaces (24), preferably on all of the surfaces extending between the cover panels (3, 4).

6. Panel (1) according to one of the preceding claims, characterised in that a two-component polyurethane sealant is used as the moisture-resistant material for the filling (2').

7. Panel (1) according to one of the preceding claims, characterised in that the inner seal (9, 9a) comprises butyl-aluminium composite material.

8. Panel (1) according to one of claims 1 to 6, characterised in that the inner seal (9, 9a) contains or consists of silicone.

9. Panel (1) according to one of the preceding claims, characterised in that the cover plates (3, 4) have a thickness of 0.4 mm to 1 mm independently of one another.

10. Panel (1) according to one of the preceding claims, characterised in that the cover plates (3, 4) are bonded to the panel core (2) by means of an adhesive (20).

11. Panel system, characterised in that it comprises a panel (1) according to one of the preceding claims and a floor profile (8).

12. Panel system according to claim 11, characterised in that a moisture-repellent and moisture-insulating intermediate layer (10) is arranged between the floor profile (8) and the floor (11).

13. Panel system according to one of claims 11 or 12, characterised in that the width of the floor profile (8) is at least as wide as the thickness of the associated panel (1).

14. Panel system according to one of claims 11 to 13, characterised in that the web (7) of the floor profile (8) has an undercut shape.