Process for the construction of plastic components

Abstract

Method for constructing plastic components by layer-by-layer application of plastic powder and selective printing with a polymerizable liquid in a build space of a 3D printer, wherein the polymerization of the liquid is actively controlled and an initiator is used for the polymerization reaction, which is activated by heating, characterized in that the mixing ratio between plastic powder and polymerizable liquid is selected such that the air space in the powder is substantially filled.

Description

[0001] Several 3D printing processes are known for the additive manufacturing of objects. In this process, the virtual object, whose data is either obtained from a scanner or created using a CAD program, is divided into fine, plane-parallel layers in the computer. The data from these layers is then used by the 3D printer to build the real object layer by layer.

[0002] EP 0 431 924 B1 describes a process for a 3D printer based on the binder jetting process. This is a powder-based system that uses a powder as the base material and a liquid that bonds the powder.

[0003] The process generally takes place at room temperature. Precise temperature control, the temporal and spatial homogeneity of which would be technically challenging, is not necessary. The print head, which defines the areas to be solidified, can be scaled to any size. Print heads with several hundred to a thousand individually controllable nozzles are common. Furthermore, the powder represents the majority of the material, which does not need to be applied selectively. These technical advantages enable a high build speed, which is maintained even for large components. Accordingly, the process is usually used for large-volume components such as those required for metal casting. Either a positive is produced, which is then converted into a casting shell through further forming processes, or a negative is produced directly for the casting.

[0004] Fig. 1 schematically shows a conventional 3D printer for the binder jetting process as a Cartesian robot during the process step “layer application” and Fig. 2 during the "printing" process step. It comprises a movable platform 1, a powder coater 2, which is moved using machine axes, and a print head 3 with a dosing device for the liquid. The print head is moved with an axis system over the powder layer created by the powder coater 2. To support the building powder bed, walls 4 can be used that lie tightly against the platform 1.

[0005] The platform 1 with the walls 4 is often summarized as a removable build space, which is designed to be detached from the rest of the 3D printer after a completed build process and replaced by a new empty build space.

[0006] The powder coater 2 is movable across the width of the build space, serves as both a dosing and leveling device for the powder, and is regularly refilled from a powder reservoir (not shown). The leveling device is usually designed as a rotating roller or vibrating blade. The powder coater 2 either has a continuously vibrating dosing device in front of the leveling device, or it ejects powder from its initial position.

[0007] Printhead 3 consists of one or more print modules operating according to the dot-on-demand principle. Each module contains several hundred to a thousand nozzles that eject droplets in a position-controlled manner. The combination and positioning of the print modules within the printhead allows for the desired print resolution and width. Supply hoses direct the printing fluid to the printhead, which may also contain a reservoir for regulating the required fluid pressure.

[0008] Before the actual build process begins, platform 1 is covered with powder and leveled with powder coater 2. The build process itself recursively repeats the processes "lowering the platform," "applying a layer," and "printing": - Lower platform: Platform 1 is lowered by the layer thickness as previously defined when disassembling the virtual component. - Applying a layer: Powder coater 2 moves over the build area, filling the resulting depression with powder. At the same time, it levels the powder to create a flat powder surface. - Printing: The print head 3 moves across the build area in stripes and selectively prints liquid onto the powder layer where, according to calculations, the component intersects with the current build layer.

[0009] The build chamber is now filled with powder, partially moistened with liquid. After a solidification period, the loose powder and the component are removed from the build chamber. The component is then cleaned. The remaining powder can be reused depending on the process.

[0010] 3D printing generally produces porous components, as only a bulk powder is wetted and bonded. The components have a strength that is sufficient for subsequent processing steps, but is still low. The surface quality corresponds to the roughness of the powder grains. Warpage or shrinkage, and thus the dimensional accuracy, lie within a tolerance range required for the most common application: metal casting.

[0011] Rarely can a component be used directly as it is created in a 3D printer. The component usually requires drying. For use as a positive for metal casting, it is then additionally impregnated or coated with wax to create a waterproof surface. For use without a forming process, the porous component is impregnated with a self-hardening material to increase strength.

[0012] The main advantages of the binder jetting process—namely, component size, productivity, and cost-effectiveness—cannot be fully exploited compared to other additive manufacturing processes in the production of plastic models, as the components are not strong enough for direct use. This is because the solidification of the component is associated with shrinkage, which, due to the layered structure of the component, does not occur simultaneously, but rather at a later time. Although a high fluid input could increase strength, the increased shrinkage would compromise the dimensional accuracy of the components.

[0013] In addition, the component can warp so severely that the manufacturing process is disrupted. Therefore, an unsatisfactory compromise must be made between the required dimensional accuracy and the strength of the printed component.

[0014] Another disadvantage is that the components' strength is not yet fully developed when removed. To prevent bending or breaking, they must be carefully removed from the 3D printer, which prevents or at least complicates the possibility of automatic removal.

[0015] Although subsequent infiltration of the components leads to improved strength of the final product, it is associated with a high level of post-treatment effort, often manual, which limits the economic viability of the process.

[0016] The high porosity of the component itself is also disadvantageous, as it makes the component susceptible to dirt and allows liquids to easily penetrate unintentionally.

[0017] From the publication DE 10 2006 038 858 A1, a method for the layered construction of three-dimensional components using a 3D printer is known, which uses a self-curing material. This self-curing material system is a binder system that hardens without external influences, i.e., without heat, without the addition of energy, or any other external influence, within a period of time dependent on the self-curing material. The curing time and the solidification time are not affected, although in connection with the self-curing system, a solidification time is explicitly provided that is a multiple of the application time of a powder layer.

[0018] WO 2016 / 011098 A2 discloses a method for curing 3D-printed components containing a curable binder. Methods are specified for accelerating the curing of the (inherently self-curing) binder, for example, through heat.

[0019] DE 10 2013 021 091 A1 discloses a method for producing 3D-printed components using a binder fluid. A controlled air flow is intended to shorten the drying times of the binder fluid. A self-curing system is also disclosed.

[0020] DE 10 2006 038 858 A1 discloses a method for the layered construction of models using a self-curing material comprising a binder and a particulate material. The curing of the material is to be delayed to prevent warping. A self-curing system can be a binder system that can cure without external influences (e.g., temperature).

[0021] DE 10 2007 061 445 A1 discloses a liquid resin, particularly for use as a binder in the production of polymer or polymer composite bodies using a 3D printing process or a process for producing fiber composites. A process for producing a polymer body using the liquid resin is also disclosed.

[0022] The invention is based on the object of providing a method for constructing plastic components by layer-by-layer application of plastic powder and selective printing with a polymerizable liquid in a build chamber of a 3D printer. This method enables the creation of components that do not exhibit the aforementioned disadvantages of conventional methods and components. The components should exhibit high strength, particularly at the time of removal from the build chamber, and, in addition, low shrinkage, which only develops once the parts are fully assembled, thus keeping warpage to a minimum.

[0023] This object is achieved according to the invention by a method according to the independent claim. Advantageous embodiments are specified in the dependent claims.

[0024] Accordingly, a method is provided for the construction of plastic components by layer-by-layer application of plastic powder and selective printing with a polymerizable liquid in a build chamber of a 3D printer, wherein the polymerization of the liquid is actively controlled and an initiator is used for the polymerization reaction, which is activated by heating, wherein the mixing ratio between plastic powder and polymerizable liquid is selected such that the air space in the powder is substantially filled.

[0025] The polymerization of the liquid is thus actively controlled. Through positive, deliberate external control, with the option of deliberately selecting the timing of the polymerization start and / or its progression, it is possible to create components, particularly with the binder jetting process, that exhibit high strength and good dimensional stability even before removal from the plastic powder. A non-self-curing or at most slightly self-curing material is particularly advantageous for this.

[0026] Carefully choosing the timing for the start of polymerization makes it possible to avoid component distortion, i.e., shrinkage caused by polymerization during the build process. This results in good dimensional accuracy of the produced components.

[0027] It is particularly advantageous to actively control or trigger polymerization through thermal treatment or exposure. A further advantage associated with thermal treatment is that it increases the polymerization conversion rate. Residual polymer that cannot polymerize is expelled from the component. Both the increase in polymerization conversion rate and the expulsion of residual polymer from the component result in high component strength even before it is removed from the build space and external forces occur during removal. This also makes it possible to automate the removal of the component from the build space. The economic advantages of the binder jetting process can be fully exploited with the method according to the invention, particularly for large components.

[0028] Preferably, the thermal treatment is carried out using heating elements provided in the build space or the build space is pushed into an oven as a whole.

[0029] According to a further advantageous embodiment of the invention, the polymerization is controlled and activated by irradiation, in particular by thermal radiation, with the irradiation preferably being carried out with microwave radiation. Microwave irradiation has the advantage that it is absorbed more strongly by the components than by the dry plastic powder, thus enabling faster and more efficient polymerization and curing of the component. During irradiation, care should be taken to ensure the most homogeneous temperature distribution possible across the component. Overheating of the component must also be avoided, as this could cause the plastic to melt or burn.

[0030] According to a further embodiment of the invention, the active control of the polymerization of the liquid is achieved by exposing the build space to a heated gas, such as warm air. It is advantageous to use a gas with a high heat capacity in order to accelerate the polymerization process compared to air or nitrogen. The use of an inert gas, such as nitrogen, is advantageous when the inhibition of the polymerizable liquid by oxygen is to be reduced, thus achieving faster polymerization with higher conversion and less drying out. Yellowing of the components can also be essentially prevented in this way.

[0031] An advantageous embodiment of the invention involves actively controlling the polymerization process using a reactive gas, so that polymerization of the reactive gas begins upon contact with the components in the build space. A combination of the aforementioned gases is also possible.

[0032] It is advantageous to heat the build chamber during the build process. However, according to a preferred embodiment of the invention, the polymerization is controlled or activated after the build process has been completed and after the build chamber has been removed from the 3D printer. This is particularly advantageous when the build chamber and thus the component have large dimensions, for example a diameter of 1 m or more. Although the build chamber can also be equipped within the 3D printer, for example with heating elements installed therein for controlled polymerization of the liquid, it is also particularly advantageous to allow gas, be it warm air, an inert gas, or a reactive gas, to flow in through gas-permeable walls, in particular through a gas-permeable floor, such as a floor provided with holes, and to draw the gas through the interior of the build chamber.In this way, the reaction time of the gas with the polymerizable liquid can be reduced and the overall polymerization process can be intensified. However, if a heat gas is introduced into the build chamber, care must be taken to avoid excessive drying of the component before the actual polymerization when using a polymerizable liquid with a high vapor pressure.

[0033] The permeability of the walls of the build chamber, preferably the air-permeable floor, can also be subjected to cooling air, which is drawn through the build chamber and thereby accelerates the cooling of the build chamber and the component contained therein after completion of the polymerization process.

[0034] A particularly advantageous embodiment of the invention is one in which at least one edge region of the component is additionally printed with the polymerizable liquid. This prevents or at least limits evaporation of the polymeric liquid during the build process and during the heating process. The amount of liquid applied to the edge regions is too small for the liquid to polymerize; it largely evaporates into the air space beforehand, saturating it. This reduces the volatility of the liquid in the areas of the actual component, facilitating the production of delicate components because their surfaces dry out less.

[0035] A volume fraction of 0.03% of the liquid intended for the edge areas of the component is sufficient when using 2-hydroxyethyl methacrylate and polymethyl methacrylate powder.

[0036] According to a particularly advantageous embodiment of the process according to the invention, a methacrylate, in particular 2-hydroxyethyl methacrylate, is used as the polymerizable liquid, as it is particularly well suited for the binder jetting process. Many monomeric liquids from this chemical group exhibit suitable properties that can be processed with the print head.

[0037] A particularly advantageous feature is that the polymerization can take place in a typical room atmosphere. 2-Hydroxyethyl methacrylate is particularly well-suited for this purpose.

[0038] In conventional plastic-material systems, more liquid leads to greater and faster shrinkage. This can be explained by the solvent nature of the monomer: plastic particles soften and lose their original shape, they enter unfilled air spaces, and the packing of the powder increases. Upon subsequent evaporation into the porous environment, the component loses further volume.

[0039] Surprisingly, this relationship no longer holds for very large amounts of liquid. Shrinkage is minimal here. This can be explained by the fact that the deformation of the plastic particles merely represents a volume redistribution between powder and liquid, since there is hardly any air volume present. Furthermore, the majority of the monomer does not evaporate but polymerizes. The surface area available for evaporation is significantly smaller. This effect is particularly pronounced for monomers with low vapor pressure. Therefore, it is particularly advantageous to choose a polymerizable liquid that has a vapor pressure of less than 1 kPa at room temperature.

[0040] To obtain an object with the most homogeneous material possible, it is advantageous to use a chemical material similar to the polymerized liquid, such as polymethyl methacrylate powder, as the plastic powder. Due to its manufacturing form, such a material is particularly flowable and well-suited for 3D printing. As an alternative to a polymerizable liquid, whose polymerization can be triggered by heating, it is advantageous to use an initiator, i.e., a radical starter for polymerization reactions of vinyl monomers, which is activated or triggered by heating. Dibenzoyl peroxide is advantageous in this case, as it decomposes into radicals at the appropriate temperature and initiates the polymerization.

[0041] It is particularly advantageous if the initiator has already been mixed into the powder or is already present in the powder grains.

[0042] According to the invention, the mixing ratio between plastic powder and polymerizable liquid is selected such that the air space in the plastic powder is essentially filled. Overfilling should be avoided, as otherwise the component will not be able to maintain its shape even during the build process. When combining 2-hydroxyethyl methacrylate and polymethyl methacrylate powder, a volume ratio of 60% plastic powder and 20-25% liquid is particularly suitable. A peroxide content of 0.1% is sufficient.

[0043] The decomposition of the initiator dibenzoyl peroxide into radicals reaches a sufficiently high decomposition rate at a temperature of approximately 60°C. Therefore, the build chamber must be thoroughly heated to at least this temperature after the build process. Too high a temperature softens the powder, which can then no longer be reused. For the combination of 2-hydroxyethyl methacrylate and polymethyl methacrylate powder, approximately 80°C is suitable. After the build chamber has been thoroughly heated, maintaining this temperature for one day is ideal for complete polymerization and the elimination of residual monomer in the components. The build chamber is then cooled down again, as otherwise the temperature would still make the component flexible when removed.

[0044] The invention has been described above using preferred embodiments. However, further embodiments and modifications are possible without departing from the spirit of the invention.

Claims

[1] Method for constructing plastic components by layer-by-layer application of plastic powder and selective printing with a polymerizable liquid in a build chamber of a 3D printer, wherein the polymerization of the liquid is actively controlled and an initiator is used for the polymerization reaction, which is activated by heating, characterized by that the mixing ratio between plastic powder and polymerizable liquid is selected so that the air space in the powder is essentially filled. [2] Method according to claim 1, characterized by that the polymerization is controlled by thermal treatment. [3] Method according to one of claims 1 or 2, characterized by that the thermal treatment is controlled by heating elements provided in the build space. [4] Method according to one of the preceding claims, characterized by that polymerization is controlled by irradiation. [5] Method according to claim 4, characterized by that the irradiation is carried out with microwave radiation. [6] Method according to one of the preceding claims, characterized by that the polymerization is controlled with a gas. [7] Method according to claim 6, characterized by that the gas is air. [8] Method according to claim 6, characterized by that the gas has a high heat capacity. [9] Method according to claim 6, characterized by that the gas is an inert gas. [10] Method according to claim 6, characterized by that the gas is a reactive gas. [11] Method according to one of the preceding claims, characterized by that the polymerization is controlled during the build-up process. [12] Method according to one of the preceding claims, characterized by that the polymerization is controlled after the build space is removed from the 3D printer. [13] Method according to one of the preceding claims, characterized by that the construction space is pressurized with a gas via an air-permeable floor. [14] Method according to one of the preceding claims, characterized by that the installation space is supplied with cooling air to cool it down. [15] Method according to one of the preceding claims, characterized by that areas close to the component are provided with the polymerizable liquid during the build-up process. [16] Method according to one of the preceding claims, characterized by that a non- or only slightly self-curing material is used. [17] Method according to one of the preceding claims, characterized by that a methacrylate, in particular 2-hydroxyethyl methacrylate, is used as the polymerizable liquid. [18] Method according to one of the preceding claims, characterized by that a chemical material similar to the polymerized liquid is used as the plastic powder. [19] Method according to one of the preceding claims, characterized by that polymethyl methacrylate is used as plastic powder. [20] Method according to one of the preceding claims, characterized by that a combination of materials is used whose polymerization is triggered by heating. [21] Method according to one of the preceding claims, characterized by that dibenzoyl peroxide is used as initiator. [22] Method according to one of the preceding claims, characterized by that the plastic powder contains the initiator. [23] Method according to one of the preceding claims, characterized by that the plastic powder contains the chemical substance of the polymerized liquid. [24] Method according to one of the preceding claims, characterized by that 50 - 70% plastic powder and 15% to 30% polymerizable liquid is used. [25] Method according to one of the preceding claims, characterized by that the vapor pressure of the polymerizable liquid at room temperature is less than 1 hPa.

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