Apparatus and method for depositing particulate material in additive manufacturing

Abstract

An apparatus for depositing a layer of granular material on a deposition surface, the apparatus comprising: • - a discharge hopper; • - a scraper; • - means for moving the discharge hopper and the scraper relative to the deposition surface; characterized in that the discharge hopper comprises: • - a plurality of discharge holes (331…335) arranged in a transverse direction (y); • - a top (310) and a bottom conical discharge section (320), the bottom conical discharge section (320) comprising two transverse walls (321, 322) inclined towards the discharge holes; • - a transverse compartment of the discharge section, each compartment comprising a transverse wall perpendicular to the bottom conical discharge section and two compartment walls (341, 342, 343) inclined towards the discharge holes so as to form a pyramidal conduit in the bottom conical discharge section facing each discharge hole, and the continuous compartment walls (342, 343) of the two compartments being joined at a corner where the continuous compartment walls of the two compartments converge in the bottom conical discharge section.

Description

FIELD OF THE INVENTION

[0001] The present invention relates to an apparatus and a method for depositing particulate material for additive manufacturing. The present invention is applicable to additive manufacturing methods employing selective sintering, selective melting or selective jetting of a binder on a bed of particulate material, the particulate material being polymer powder, metal powder, sand or ceramic powder.

[0002] The principle of these additive manufacturing methods lies in selectively coalescing the particles of the particulate material in a layer of defined thickness by melting, sintering or jetting a binder, the layer being deposited on a layer of particulate material that has previously undergone the same selective coalescence process. The term "coalescence" is considered here in a broad sense, and the different methods considered produce very different binding properties between the particles subjected to the selective effect.

[0003] The selectively coalesced portion of the layer is bonded to the selectively coalesced portion of the previous layer, and the part that is the subject of the manufacturing method is thus manufactured by the layering of successive layers. The finished part is then removed from the bed of particulate material.

[0004] These methods require depositing a layer of particulate material of controlled thickness on the material bed before each selective melting, sintering or jetting of a binder. BACKGROUND OF THE INVENTION

[0005] Relating to the prior art Figure 1 A schematic embodiment representing such a method is shown. The part 110 is made by layering successive layers by selectively coalescing particles in a bed of particulate material 120. For example, selective coalescence is accomplished by sintering, by exposing regions in the layer of the particulate material bed to laser radiation 130, the movement of the laser radiation over the particulate material bed 120 being controlled according to a defined path.

[0006] The bed of particulate material is placed on a plate 125 that is vertically movable by a controlled jack (e.g., a screw jack or a linear motor).

[0007] Initially, the plate 125 is at the same height as the workbench 142. A hopper 140 stores particulate material.

[0008] By a means not shown, a heap of particulate material 141 is dispensed from the hopper 140 onto the workbench 142.

[0009] The plate 125 descends by the thickness of the layer to be formed, and a doctor blade 145 moves parallel to the surface of the plate, pushing the heap of material 141 to distribute the heap of material on the surface of the plate with a uniform layer thickness.

[0010] The coalescing device 130 starts to operate and selectively coalesces the layer of the part in the deposited layer.

[0011] Then, the plate 125 descends again by one layer thickness. The hopper 140 conveys a new material pile, and the new material pile is distributed on the previous layer by the squeegee 145, and so on until the component 110 is fabricated.

[0012] According to this method and its variants, the operation of depositing a layer of granular material, especially but not exclusively when the surface of the material bed is important, does not easily result in a layer of uniform thickness.

[0013] The starting pushing and distribution of the material pile 141 are more difficult than at the end of the distribution, so the layer does not directly have a uniform thickness. The squeegee 145 equalizes the layer thickness during its return journey, during which a greater quantity of material than required must be deposited to obtain a uniform layer thickness.

[0014] Even if this quantity of surplus material is recyclable, devices for this recycling must be provided, which makes the machine complex and increases its cost.

[0015] This method of depositing a material layer is not suitable for additive manufacturing machines using cylindrical kinematics. Document US 2015 / 0306819 gives an example of such a machine.

[0016] Figure 2 , according to another embodiment of the prior art, the material layer is deposited by moving the hopper 240. The squeegee 245 is placed in front of or behind the hopper.

[0017] According to an exemplary embodiment, the hopper 240 includes a cylinder 250 with a notch near its lower opening, and the cylinder is driven to rotate by a stepper motor (not shown). Before depositing the layer, the hopper is filled with a necessary quantity of material, which is equal to or slightly greater than the quantity of material required for the deposited layer.

[0018] Thus, when the hopper moves above the material bed, by controlling the rotation of the cylinder 250 with the notch, the quantity of deposited material, or more specifically its flow rate, is controlled to obtain a uniform deposited layer.

[0019] However, if this device can control the quantity of material deposited in the direction of movement of the hopper (direction x in this figure), the uniformity of the layer in the direction y parallel to the plate 125 and perpendicular to the direction of movement of the hopper depends on the falling conditions of the granular material under the action of gravity. The equalizing effect of the squeegee 245 in this same direction (y) is limited, and the squeegee and the hopper move in the same direction.

[0020] In addition, when the produced component includes different materials along the z - axis (which is allowed by the additive manufacturing method), it is necessary to deposit successive layers of different materials.

[0021] Each time the material is changed, the hopper needs to be thoroughly cleaned to remove the material previously deposited in the hopper, especially the material remaining in the recesses around the metering feed cylinder 250. These cleaning operations are cumbersome and prolong the production time of the components.

[0022] The flow of granular material in the hopper depends on the following factors:

[0023] - The nature of the material and its particle size;

[0024] - The coefficient of friction between the inner wall of the hopper and the material;

[0025] - The surface of the opening at the bottom of the hopper;

[0026] - The angle of the cone at the bottom of the hopper.

[0027] All these parameters interact to define the discharge mode and the flow rate. In general, when the hopper is tall and narrow and the angle at the top of the discharge cone is small, the discharge conditions are more favorable. Additionally, the smaller the surface of the opening at the bottom of the hopper, i.e., an opening with a small diameter, the smaller the angle at the top of the cone should be to ensure a uniform flow of the material.

[0028] A hopper designed according to the latter principle, suitable for depositing a layer of granular material in an additive manufacturing method, exhibits significant vertical dimensions, thus affecting the machine design.

[0029] Additionally, the general design principles of discharge hoppers document the case of unloading large volumes of material, which in practice is in the order of tens of cubic meters (m 3 ), specifically as described, for example, in Jenike A.W., "Storage and flow of solids" Bulletin #123 of the Utah Engineering Experiment Station - March 1970.

[0030] However, for a hopper that unloads granular material to form an additive manufacturing layer, the volume of the deposited material layer to be unloaded is approximately 100 cm 3 .

[0031] Therefore, for the industrial discharge hopper for transporting powdered products, the target application of the present invention lies in a scale factor in the order of about 10 -5 to 10 -6 . Summary of the Invention

[0032] The present invention aims to address the drawbacks of the prior art and for this purpose relates to a device for depositing a layer of granular material on a deposition surface, the device comprising:

[0033] - a discharge hopper;

[0034] - a scraper;

[0035] - means for moving the discharge hopper and the scraper relative to the deposition surface;

[0036] wherein the discharge hopper comprises:

[0037] - a plurality of discharge holes, the plurality of discharge holes being arranged transversely;

[0038] - a top and a bottom conical discharge part, the bottom conical discharge part comprising two transverse walls inclined towards the discharge holes;

[0039] - a transverse compartment in the bottom conical discharge part, each compartment comprising a transverse wall perpendicular to the bottom conical discharge part and two compartment walls inclined towards the discharge holes to form a pyramidal conduit in the bottom conical discharge part, the pyramidal conduit facing each discharge hole, and the continuous compartment walls of the two compartments being connected in a wedge-like manner and converging in the bottom conical discharge part.

[0040] Thus, according to the flow rate defined by the characteristics of a specific discharge hopper, through this specific discharge hopper, through each discharge hole (this specific discharge hopper being specific to the discharge hole), the granular material initially loaded in the discharge hopper is discharged onto the deposition surface, forming a material pile extending in a strip along the movement direction of the discharge hopper, the height of the material pile being uniform and defined by the movement speed.

[0041] The thickness of the deposited material is scraped evenly with the scraper to form a material layer suitable for subsequent additive manufacturing operations.

[0042] The deposited material strips are close to each other and are easily evenly distributed on the deposition surface when the scraper moves. There is no mobile metering device in the hopper, and the wedge-like connection and inclination of the compartment walls ensure that there is no material residue after the discharge hopper is emptied, and different materials can be continuously deposited without cleaning the discharge hopper.

[0043] The present invention is advantageously implemented according to the embodiments and variants disclosed below, which should be considered separately or in any technically possible combination.

[0044] Advantageously, the inclination angles of the two transverse inclined walls of the bottom conical discharge part and the two compartment walls for the same discharge hole with respect to the vertical direction are less than or equal to 40°.

[0045] Under these conditions, the material is discharged according to the mass flow rate in the discharge hopper, and thus the material flow rate discharged from each discharge hole during the deposition process is substantially constant.

[0046] Advantageously, the inner side of the discharge hopper includes a coating having a coefficient of friction with the deposited granular material of less than 0.1.

[0047] Advantageously, the discharge hopper includes a vibration device mounted on the outer wall.

[0048] Using the two features alone or in combination can reduce the apparent coefficient of friction between the granular material and the hopper, so that a larger inclination angle (usually closer to 40°) can be used in the discharge part, the bottom wall of the hopper, and the partition wall dividing the compartments, while maintaining the mass flow of the material. Therefore, the vertical volume of the hopper is reduced.

[0049] According to one embodiment, the discharge holes of the discharge hopper are distributed at regular intervals in the transverse direction.

[0050] According to another embodiment, the hopper includes three or more discharge holes, and the discharge holes are distributed at irregular intervals in the transverse direction.

[0051] Furthermore, according to another embodiment, which is compatible with the first two embodiments, the discharge hopper includes discharge holes having an outlet cross-section different from that of another discharge hole.

[0052] These characteristics can adjust the amount of material deposited by the discharge hopper in the transverse direction to adapt to the deposition of the material, for example, when the path of the discharge hopper on the deposition surface is circular, such as in the case of using a cylindrical kinematic machine.

[0053] Advantageously, the scraper of the device of the present invention includes a convex portion opposite to each discharge hole of the discharge hopper. Therefore, it is beneficial to equalize the deposited layer.

[0054] The present invention also relates to a method for depositing a layer of granular material on a deposition surface, the method implementing the device according to any one of the embodiments of the present invention according to the embodiments of the present invention, and including the following steps:

[0055] i. Filling the discharge hopper with a first granular material;

[0056] ii. Moving the discharge hopper above the deposition surface along a defined path at a defined speed;

[0057] iii. Depositing the continuous and parallel strip materials discharged in step ii) on the deposition surface;

[0058] iv. Spreading the material deposited in step iii) on the deposition surface by a scraper to obtain a layer of uniform thickness.

[0059] Advantageously, after step iv), the method of the present invention includes the following steps:

[0060] v. Fill the discharge hopper with a second particulate material different from the first particulate material;

[0061] vi. Repeat steps ii) to iv) with the second particulate material.

[0062] Thus, the method of the present invention can produce components containing different compositions in the lamination direction. Description of the Drawings

[0063] The present invention is disclosed below according to a preferred embodiment of the present invention (which is in no way restrictive), with reference to Figures 1 to 8 In the drawings:

[0064] Figure 1 , Figure 2

[0065] Figure 1 and Figure 2 show embodiments of an additive manufacturing method by selective coalescence in a material bed, as compared to the prior art;

[0066] Figure 3

[0067] [Figure 3] represents an exemplary embodiment of a discharge hopper for deposition in the device of the present invention, according to a perspective view Figure 3A and according to a sectional view AA Figure 3B (AA is defined in Figure 3A );

[0068] Figure 4

[0069] Figure 4 shows an example of depositing a material layer on a deposition surface by the device of the present invention, according to a perspective view;

[0070] Figure 5

[0071] [Figure 5] schematically shows an example of deposition using the device of the present invention, Figure 5A with a scraper placed behind the discharge hopper, while Figure 5B has a scraper placed in front of the discharge hopper;

[0072] Figure 6

[0073] Figure 6 shows an exemplary embodiment of a scraper suitable for the device of the present invention, according to a perspective view;

[0074] Figure 7

[0075] [Figure 7] shows according to a sectional view Figure 7A and Figure 7B ​​​, showing an exemplary embodiment of the compartments of the discharge hopper of the device of the present invention;

[0076] Figure 8

[0077] Figure 8 is a flowchart of an exemplary embodiment of the method of the present invention. Detailed Embodiment

[0078] Figure 3A , according to an exemplary embodiment, the discharge hopper 300 of the device of the present invention is generally wedge-shaped, including a top 310 and a bottom conical discharge part 320. The bottom conical discharge part is the so-called discharge part, and the discharge part includes two inclined lateral walls 321, 322 converging towards the bottom opening of the discharge hopper. The openings are distributed in the lateral direction (y).

[0079] The discharge hopper 300 includes means at its top for fixing the discharge hopper to the additive manufacturing machine. The discharge hopper is fixed to the additive manufacturing machine, in particular by a bolt connection, and in particular to a bracket, and the movement speed of the bracket is controlled.

[0080] According to this exemplary embodiment, the two lateral walls 321, 322 of the bottom conical discharge part 320 of the discharge hopper are symmetrically inclined at an angle θ1 = θ2 with respect to the lateral vertical plane (x, y).

[0081] Alternatively, the two lateral walls are inclined according to angles (θ1, θ2) that are different with respect to the lateral vertical plane. The inclination angles (θ1, θ2) of the lateral walls are calculated between 0° (for a vertical wall) and 90° (for a horizontal wall). In any case, the inclination angles (θ1, θ2) are less than 40°, and preferably less than 30°, such that the angle (θ1 + θ2) of the discharge cone between the two inclined lateral walls 321, 322 of the bottom conical discharge part is at most equal to 80°, and preferably less than 60°.

[0082] Figure 3B , the bottom conical discharge part 320 of the discharge hopper is partitioned into compartments.

[0083] Each compartment corresponds to a discharge hole 331, 332, 333, 334, 335 respectively. The discharge hopper includes a plurality of discharge holes arranged in the lateral direction (y). Each compartment forms a conical hopper, and the conical hopper forms a rectangular cross-section in the bottom conical discharge part 320 of the discharge hopper of the device of the present invention.

[0084] ​This "sub-hopper" facing each discharge orifice is delimited in the longitudinal direction (x) by the inclined transverse walls 321, 322 of the discharge hopper 300 and in the transverse direction (y) by the compartment walls 341, 342, which are inclined with respect to the longitudinal vertical plane (x, z) and converge towards the discharge orifice.

[0085] The angle of inclination (θ3, θ4) of each compartment wall is less than 40°, preferably less than 30°, such that the opening angle (θ3 + θ4) between the two compartment walls leading to the discharge orifice is at most equal to 80°, preferably less than 60°.

[0086] Thus, the conduit formed between the inclined transverse wall and the compartment wall, down to the outlet of the discharge orifice, does not include any surface inclined at an angle greater than 40° with respect to the vertical direction. These conditions ensure that the fluid flow rate (in the form of a mass flow rate) of the granular material contained in the discharge hopper 300 flows towards the discharge orifices 331, 332, 333, 334, 335 of the discharge hopper 300.

[0087] The geometry of the compartment that guides the material towards the discharge orifice is such that the actual flow cross-section through the discharge orifice is equal to or substantially equal to the cross-section of the discharge orifice for almost all of the time of emptying the discharge hopper 300.

[0088] The actual cross-section is the cross-section through which the material flows through the discharge orifice. This actual cross-section can only be less than or equal to the cross-section of the discharge orifice.

[0089] In the discharge hopper of the device according to the invention, from the filling of the discharge hopper 300 and the start of discharging the granular material, this actual cross-section is equal to the cross-section of the discharge orifice for each discharge orifice, and ensuring this condition only requires that the granular material contained in the compartment has a sufficient height.

[0090] In the final emptying phase, the actual discharge cross-section may be less than the cross-section of the discharge orifice, but this is a marginal effect.

[0091] These conditions result in an almost constant discharge flow rate through each of the said discharge orifices, this flow rate being determined by the nature of the discharged material (in particular its bulk density and its particle size) and the geometry of the discharge hopper, i.e. no specific device for controlling the discharge flow rate (such as a notched bucket) is required.

[0092] The adjacent compartment walls 342, 343 of two consecutive compartments are joined together in the bottom conical discharge part 320 of the discharge hopper by a sharp connection, and the compartment walls are thus joined in a toothed or wedge shape, where the cone angle is less than 60°, preferably less than 40°, depending on the respective inclination of the compartment walls 342, 343 thus connected.

[0093] This characteristic easily separates the fluids towards the multiple discharge orifices of the discharge hopper, and there is no material residue after the discharge hopper is completely emptied.

[0094] As an informative and non - limiting example, the discharge hopper of the device of the present invention includes square holes of 2 mm, the square holes are spaced apart at intervals of 15 mm to 20 mm, the total height of the discharge hopper is 40 mm, and the separation height h or tooth height relative to the bottom of the discharge hopper is between 10 mm and 20 mm.

[0095] Such a discharge hopper is suitable for depositing a layer of granular material with a uniform thickness between 0.3 mm and 0.8 mm and a stroke of 500 mm across the width of the discharge hopper. For granular materials (such as silica sand, aluminum, or stainless steel), the particle size D of the granular material is between 20 μm and 200 μm.

[0096] According to this example, the opening of the discharge hole is at least equal to 10 times the particle size of the deposited material. Smaller openings can be used while maintaining the mass flow rate, and the lower limit of the possible opening is greater than 2 times the particle size of the discharged material. These dimensions indicate that although the term "discharge hopper" is used, the technical field to which the discharge hopper of the device of the present invention belongs is different from that of industrial hoppers and is closer to that of porous nozzles for depositing granular materials.

[0097] Figure 4 The operation of depositing material on the deposition surface 400 through the discharge hopper of the device of the present invention is shown, where there is no scraper.

[0098] The final thickness of the deposited material layer varies with the component size, the additive manufacturing method implemented, and the targeted finishing.

[0099] As a non - limiting example, for components contained in a volume less than or equal to 500 cm 3 the thickness of the deposited layer is between 0.3 mm and 1 mm.

[0100] For components contained in a volume of approximately 1 m 3 or larger, the thickness of the deposited layer is greater than 2 mm.

[0101] For a discharge hopper with given characteristics, the thickness of the deposited layer depends on the relative movement speed of the discharge hopper with respect to the deposition surface. The lower the deposition speed, the thicker the deposited layer.

[0102] The amount of deposited material depends on the material flow rate conveyed by the discharge hopper. The larger the opening of the discharge hole, the greater the flow rate.

[0103] According to this exemplary embodiment, the discharge hopper 300 filled with the granular material to be deposited moves relative to the deposition surface at a constant speed 401.

[0104] According to various embodiments, this movement is achieved by moving the discharge hopper or by moving a support located on the material bed.

[0105] During this movement, each discharge hole deposits material piles 431, 432, 433, 434, 435 according to the deposition stripes on the deposition surface 400.

[0106] When the scraper is separated from the discharge hopper, this operation produces material piles in the form of stripes, as Figure 4 represented. When the scraper is fixed to the discharge hopper and moves with the discharge hopper, as the discharge hopper 300 moves, the material piles are flattened.

[0107] Due to the geometric characteristics of the discharge hopper 300 and the constant relative movement speed 401, the flow rate through each discharge hole is constant, the cross-section of each material pile is substantially constant in the longitudinal direction (x), and the inclination angle of the material pile is defined by the deposited material.

[0108] For the clarity of the drawings, the material piles 431, 432, 433, 434, 435 are represented here at a high inclination angle and are spaced apart from each other quite a bit. In practice, the inclination angle is lower (usually around 30°), and the angle at the top of the material pile is around 120°, and this value is not limited.

[0109] According to a variant, the deposition is completed in one pass: the discharge hopper is initially filled with a certain amount of material to be deposited or slightly more material and is emptied in a single path along the deposition surface.

[0110] Alternatively, the deposition is completed in two passes: during the back-and-forth movement above the deposition surface, the discharge hopper initially filled with the necessary amount of material or slightly more material is emptied. According to this variant embodiment, during the return journey, the discharge hopper is optionally laterally offset relative to the deposition surface, for example, by half the pitch of the holes, so that the deposition on the surface is distributed in a more uniform manner.

[0111] Furthermore, according to another variant, the material reservoir is positioned at both ends of the path. After being filled by the first material reservoir, the discharge hopper performs the first stroke, and then after being filled by the material reservoir at the other end of the material bed, it performs the second stroke in the opposite direction. The two strokes are completed continuously, or an additive manufacturing operation is inserted in the middle of the two strokes.

[0112] FIG. 5, a schematic embodiment of the device according to the present invention, the discharge hopper 300 is attached to a carriage (not shown) of the machine, and the carriage is movable relative to the deposition surface in at least one direction (x). The discharge hopper 300 is oriented such that the holes are aligned in a direction (y) perpendicular to this main movement direction.

[0113] The material bed 120 is placed on a plate 125 which can be vertically moved by suitable means.

[0114] Before depositing a layer of material, the plate 125 is lowered by a height corresponding to the thickness of the layer to be deposited.

[0115] The discharge hopper 300 is filled with a suitable quantity of material from the material reservoir 140. According to an exemplary embodiment, this allows different materials to be deposited on the material bed 120.

[0116] By way of example, these different material reservoirs include materials of the same nature but different particle sizes, or materials of different natures (such as different metals).

[0117] Figure 5A , according to this exemplary embodiment, the doctor blade 545 is connected to the discharge hopper 300 behind it in the direction of its movement during discharge.

[0118] The bottom part of the doctor blade is adjusted according to the thickness of the deposited layer.

[0119] The discharge hopper filled with material moves above the deposition surface at a constant speed, and when depositing the material, the doctor blade levels the deposited layer of material to the desired thickness.

[0120] Figure 5B , according to this exemplary embodiment, during discharge the doctor blade 546 is attached to the discharge hopper 300 at the front of the discharge hopper in the direction of movement of the discharge hopper. Thus, the discharge hopper filled with material moves above the deposition surface at a constant speed, and then during the return path of the discharge hopper, the layer is levelled by the doctor blade 546.

[0121] According to another embodiment (not shown), two doctor blades are fixed to the discharge hopper (one in front and the other behind), for example adjusted to different heights, to level the layer for the first time during the outward path and to finally level the layer during the return path of the discharge hopper.

[0122] According to an exemplary embodiment, the doctor blade is a simple straightedge (règle) with a parallelepiped cross-section.

[0123] Figure 6 , according to another exemplary embodiment, the doctor blade 645 is placed behind the discharge hopper. If the device includes two doctor blades, one or both of the two doctor blades include projections 646 opposite each discharge orifice of the discharge hopper to enable a uniform distribution of the deposited material.

[0124] According to another variant (not shown), the (multiple) doctor blades are not attached to the discharge hopper but include their own movement means.

[0125] In this case, during the deposition process, a scraper is used by following the forward and backward movement of the discharge hopper, or after the material has been completely deposited on the deposition surface.

[0126] The scrapers 645, 646 are shown here in their simplest embodiment or in the general shape of a straightedge. Alternatively, the scraper can be a roller, the axis of rotation of which is perpendicular to the direction of movement of the discharge hopper and which exerts a certain compaction pressure on the deposited layer of material. Thus, this type of scraper both evenly distributes the deposited layer of material and densifies the layer of material.

[0127] According to an exemplary embodiment, a roller scraper and a straightedge scraper are associated with the discharge hopper.

[0128] Figure 7, in all of the previous examples, shows the discharge holes of the discharge hopper, which are distributed in the transverse direction at regular intervals and have the same opening cross-section.

[0129] Figure 7A , according to an exemplary embodiment, the discharge hopper 701 of the device of the present invention includes discharge holes 731, 732, 733, 734, 735 with different openings, for example, the openings of these discharge holes increase in the transverse direction.

[0130] This configuration allows the adjustment of the material flow rate through each discharge hole according to the position of the discharge hole.

[0131] For example, as shown, the discharge hopper 701 is suitable for depositing material on the deposition surface along a circular path by a cylindrical robot.

[0132] Therefore, during the use of the discharge hopper, the discharge hole 731 with the smallest opening follows a circular path closer to the center of curvature and thus with a smaller radius, while the discharge hole 735 with the largest opening follows a path further from the center of curvature and with a larger radius.

[0133] The gradual increase in the opening in the transverse direction, as well as the geometric characteristics of the compartment walls, allows a greater material flow rate of the deposited material to be obtained along the path with a larger radius, while a smaller material flow rate of the deposited material is obtained along the path with a smaller radius.

[0134] The geometric characteristics of the compartment walls, more specifically the inclination angle of the compartment walls, are determined by calculation and improved through experimentation. The inclination of each compartment wall with respect to the vertical is always less than 40°. Thus, the opening angle between two compartment walls converging towards the discharge hole becomes smaller, for example, when the opening of the discharge hole decreases.

[0135] Figure 7B, according to an embodiment, compatible with the previous embodiments, the discharge holes 741, 742, 743, 744, 745 of the discharge hopper 702 are distributed at irregular intervals in the lateral direction.

[0136] According to this exemplary embodiment, the spacing between the discharge holes increases in the positive y direction, but the opposite situation can also be considered. The angle of the compartment wall is adjusted by keeping it less than 40° relative to the vertical direction to obtain a constant flow rate in all discharge holes or a gradually varying flow rate from one discharge hole to another.

[0137] In a separate embodiment or a combination of these embodiments, the material flow rate through each discharge hole is defined by:

[0138] - The properties of the deposited granular material, especially its internal friction (friction between particles);

[0139] - The opening cross-section of the discharge hole;

[0140] - For the compartment wall and the lateral wall, the inclination angle converging towards the discharge hole;

[0141] - The flow separation height at the connection of the compartment walls;

[0142] - The friction of the material deposited on the discharge hopper wall.

[0143] The effects of these parameters are related. Trends can be defined that allow the design of a discharge hopper suitable for a given situation in its profile, and then this design can be improved through experiments.

[0144] Therefore, the target flow rate is the mass flow rate, that is, all the materials contained in the discharge hopper or in the conduit between the two compartment walls flow through different discharge holes simultaneously.

[0145] The more the opening cross-section of the discharge hole decreases, the more the opening angle between the compartment walls should decrease. The greater the target flow rate difference between two juxtaposed discharge holes, the more flow separation occurs upstream of the flow. Finally, the angle of the compartment wall relative to the vertical must be less than 40°, and preferably less than 30°.

[0146] In addition, by coating the compartment walls with a material having a low coefficient of friction (such as ), the friction between the discharged material and the compartment walls of the discharge hopper can be reduced. In order to have a significant impact on the allowable inclination angle of the compartment walls, the coefficient of friction between the coating and the discharged material must be less than 0.1.

[0147] According to another embodiment, the apparent coefficient of friction is actively reduced by vibrating one wall of the discharge hopper externally. For this purpose, an electric micro-vibrator is fixed to the outer surface of the discharge hopper, for example, to an inclined lateral wall. This type of vibrator includes an electric motor that drives at least one flywheel to rotate. The flywheel includes flyweights that generate vibrations according to a frequency mainly included in the audible spectrum. Depending on the type of vibrator, this frequency is generally between 25 Hz and 500 Hz.

[0148] For small discharge hoppers, this function is implemented, for example, by one or more micro-vibrators, such as those used in mobile phones or video game controllers.

[0149] For large components suitable for production volumes of 1 m 3 or larger, the discharge hopper uses a vibrator distributed, for example, under the trademark OLI7 S.r.l., Via Canalazzo, 35I - 41036 Medolla (MO), Italy, model Micro MVE.

[0150] Reducing the apparent coefficient of friction between the product and the walls of the discharge hopper (whether by coating, vibration, or both) mainly has the effect of having greater flexibility in determining the angle of the wall facing the discharge opening by using an inclination angle close to 40° (however, in any case, this inclination angle must remain less than this value while maintaining the mass flow rate).

[0151] Due to the geometric characteristics of the discharge hopper of the device of the present invention, there is no residue in the discharge hopper when discharging the material onto the deposition surface.

[0152] Therefore, two different but compatible materials can be deposited in an additive manufacturing sequence without cleaning the discharge hopper between two deposition operations.

[0153] Figure 8 , according to an example of the method of the device implementing the present invention, the method includes a first step 8101 of filling the discharge hopper. This filling is advantageously carried out in an area close to, but outside, the material bed. Considering the amount discharged outside the material bed, the amount of material loaded into the discharge hopper is slightly greater than the amount of material required for the deposited layer on the material bed.

[0154] According to the deposition step 820, the discharge hopper moves relative to the deposition surface at a controlled speed, and discharging is carried out according to the deposition strips of each discharge hole.

[0155] According to the equalization step 830, these piles are leveled and distributed onto the deposition surface by means of a straight-edge or roll doctor blade.

[0156] According to a variant, the doctor blade is attached to the discharge hopper, the equalization step occurs simultaneously with the deposition, or the doctor blade is controlled separately and the equalization step takes place after the deposition.

[0157] As the layer is equalized, the deposited material is selectively aggregated 840 during the additive manufacturing process.

[0158] According to the first possibility, the cycle restarts with a new filling of the discharge hopper and the deposition of a new layer of the same material.

[0159] Alternatively, according to step 8102, after the additive manufacturing step, the discharge hopper is filled with another granular material. The subsequent operations then restart with this second granular material.

[0160] The above description and the example embodiments show that the present invention achieves the targeted objectives and allows for the deposition of granular materials on the deposition surface according to uniform layers and for easily changing the nature of the deposited material from one layer to another without the expensive operation of cleaning the deposition device. The device is simple and easily adaptable to a variety of existing machines.

Claims

1. An apparatus for depositing a layer of granular material on a deposition surface (400), the apparatus comprising: a discharge hopper (300, 701, 702); a doctor blade (545, 546, 645); and means for moving the discharge hopper and the doctor blade relative to the deposition surface; characterized in that the discharge hopper comprises: A plurality of discharge holes (331…335, 731…735, 741…745), the plurality of discharge holes being arranged in a lateral direction ( y ); a top portion (310) and a bottom conical discharge portion (320), the bottom conical discharge portion (320) including two lateral walls (321, 322) inclined towards the discharge holes; compartments lateral to the bottom conical discharge portion, each compartment comprising a lateral wall perpendicular to the bottom conical discharge portion and two compartment walls (341, 342, 343) inclined towards the discharge orifice so as to form a pyramidal conduit in the bottom conical discharge portion, the pyramidal conduit facing each discharge orifice, and the consecutive walls (342, 343) of two compartments being connected in a wedge-shaped connection, at which connection the consecutive walls converge in the bottom conical discharge portion; wherein the height of each wedge-shaped connection formed by adjacent said consecutive walls decreases as the outlet size of the plurality of discharge orifices (731…735) increases.

2. The device according to claim 1, wherein The two inclined lateral walls (321, 322) of the bottom conical discharge portion, and the two compartment walls (341, 342) for the same discharge hole, with respect to the vertical direction ( z ) the inclination angle ( θ 1 , θ 2 , θ 3 , θ 4 ) is less than or equal to 40°.

3. The device according to claim 1, wherein, The inner side of the discharge hopper comprises a coating, the coefficient of friction of which with the deposited granular material facing each discharge orifice is less than 0.

1.

4. The device according to claim 1, wherein, The discharge hopper comprises vibration means mounted on the outer wall.

5. The device according to claim 1, wherein The discharge orifices (731…735) of the discharge hopper are distributed at regular intervals along the lateral direction.

6. The device according to claim 1, wherein, The discharge hopper (702) comprises three discharge orifices or more (741, 742, 743, 744, 745), the discharge orifices being distributed at irregular intervals along the lateral direction.

7. The device according to claim 1, wherein, The doctor blade (545, 546, 645) is attached to the discharge hopper.

8. The apparatus according to claim 1, wherein The doctor blade (645) comprises a protrusion (646) opposite each discharge orifice of the discharge hopper (300, 701, 702).

9. A method for depositing a layer of particulate material on a deposition surface, the method implementing the apparatus according to claim 1, and the method comprising the following steps: (i) filling (8101) the discharge hopper (300, 701, 702) with a first granular material; (ii) moving (820) the discharge hopper above the deposition surface (400) along a defined path at a defined speed (401); (iii) depositing the continuous and parallel deposition strip material (431…435) formed on the deposition surface (400) during step ii); (iv) spreading (830) the material deposited in step iii) on the deposition surface by means of the doctor blade to obtain a layer of uniform thickness.

10. The method according to claim 9, after step iv), the method comprises the following steps: (v) filling (8102) the discharge hopper with a second granular material different from the first granular material; (vi) repeating steps ii) to iv) with the second granular material.

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