Mold for injection molding made by additive manufacturing

Abstract

A mold for injection molding, comprising a mold body (10) having a plurality of boundary surfaces (10a-10f), the plurality of boundary surfaces comprising at least one molding surface (10b) configured to bound a mold cavity, wherein the mold body is made by additive manufacturing. The mold body comprises a functional area portion (13) and an application area portion (14), the plurality of boundary surfaces and the molding surface being formed on the functional area portion, the functional area portion consisting of a solid and continuous material structure occupying a portion of the mold body (10), and the application area portion being a complement of the functional area portion (13) in the mold body (10), the application area portion consisting of a three-dimensional material lattice structure, the three-dimensional material lattice structure comprising an ordered repetition of unit cells, the unit cells comprising a periodic minimal surface, wherein at least one geometric parameter of the periodic minimal surface is locally adjusted to form unit cells having different material densities.

Description

[0001] The present invention relates generally to a mold for injection molding.

[0002] Plastic parts produced using injection molding require a long cooling time, which on average occupies two thirds of the total production cycle, and cause a high scrap rate due to the warping (deformation) of the parts caused by thermal stress. In fact, traditional mold production techniques present some problems.

[0003] First of all, the technology used, i.e. the use of CNC machine tools, does not allow to create cooling channels that conform to the geometry of the mold. In fact, the drilling tools used by these machine tools have limited mobility, so they can only drill straight channels in the metal. The channels resulting from this process require a very long cooling period, thus lengthening the production cycle, while increasing the costs for the user of the mold.

[0004] Another problem caused by the deficiencies of the cooling channels produced by CNC machine tools is the non-uniform cooling of the objects produced by the mold. Linear channels do not conform to the geometry of the objects, so the cooling speed of some parts of the object will be faster than that of other parts, thus creating conditions for an increase in deformation and, consequently, an increase in the scrap of the final product.

[0005] Finally, traditional production techniques require a long planning time, since they require the intervention of personnel with different specific skills, especially in the programming phase of the CNC machine tools. Therefore, there is a significant time difference between the demand for a mold by a customer and the actual production using traditional CNC machine tools.

[0006] Today, the problems mentioned earlier are solved by using additive manufacturing techniques and the design of a conformal cooling system that conforms to them.

[0007] However, this solution adds another big problem, which is that the cost of this mold is much higher compared to the molds produced with traditional techniques. This mold is designed in the same way as traditional techniques, so it requires a long production time and more material. This results in relatively high production costs.

[0008] From the point of view of the potential customer, i.e. the injection molder, the high cost is a deterrent, since it requires a significant increase in the necessary monetary investment to purchase the mold in exchange for uncertain subsequent savings.

[0009] The conformal cooling molds currently produced do not have a lattice structure. Some software companies are working to integrate a lattice structure automatically into the mold, as disclosed for example in US2019 / 0111590 A1.

[0010] The aim of the present invention is to provide an alternative solution to a mold for injection molding made by additive manufacturing.

[0011] It is another object of the present application to provide a method for producing such a mold.

[0012] According to the present application, a mold for injection molding is provided, the mold comprising a mold body having a plurality of boundary surfaces, the plurality of boundary surfaces comprising at least one molding surface configured to delimit a mold cavity, wherein the mold body is made by additive manufacturing, wherein the mold body comprises:

[0013] a functional area portion, the plurality of boundary surfaces and the at least one molding surface being formed on the functional area portion, the functional area portion consisting of a solid and continuous material structure occupying a portion of the mold body; and

[0014] an application area portion, the application area portion being complementary to the functional area portion in the mold body, the application area portion consisting of a three-dimensional material lattice structure, the three-dimensional material lattice structure comprising an ordered repetition of unit cells, the unit cells comprising periodic minimal surfaces, wherein at least one geometric parameter of the periodic minimal surfaces is locally adjusted to form unit cells having different material densities.

[0015] Further, according to the present application, a method for designing a mold according to any of the preceding claims is provided, comprising:

[0016] a step a) providing a three-dimensional geometric model of the mold to be designed;

[0017] a step b) determining a functional area portion and an application area portion in the mold body of the mold;

[0018] a step c) adjusting the at least one geometric parameter of the periodic minimal surfaces based on a physical model of the mold; and

[0019] a step d) creating an electronic file storing data representing a digital model for building the mold by additive manufacturing.

[0020] Since the optimized design of the mold allows for a significant saving of processing time and used material during the manufacturing process, the present application combines the advantages of making full use of additive manufacturing and reducing the associated costs.

[0021] The use of additive manufacturing technology allows to design and create cooling channels that conform to the geometry of the mold. This allows for faster and more uniform cooling, thereby reducing the production cycle and avoiding the occurrence of deformations and distortions that would lead to the rejection of the object.

[0022] Moreover, the nature of the additive technology does not cause the generation of material waste, thus reducing the environmental impact of the activity. Finally, the mould is designed and then produced with optimal periodic minimal surface structures, such as triple periodic minimal surface structures (hereinafter referred to as TPMS structures), thus reducing the quantity of material required and the use time of the machine, which are two fundamental cost factors in the additive manufacturing process.

[0023] The present application, using additive manufacturing technology, commonly known as 3D printing, reduces the cooling time by up to 75%, the distortion by up to 40% and the production costs by up to 60% compared to moulds produced using CNC technology.

[0024] The inventors have found that TPMS structures have higher performance than the strut-based lattice structures disclosed by US 2019 / 0111590 Al, in particular for the injection moulding industry and the die casting industry.

[0025] The present application provides an effective "green" solution, using quantities of material as low as about 10% of those required to produce a conventional mould.

[0026] In summary, the provision of the conformal cooling system improves the mould cooling cycle, reducing the cooling time, reducing the warping or distortion effect and the objects produced with this mould have a better microstructure.

[0027] On the other hand, the provision of the TPMS lattice structure makes the mould light in weight, with material provided only where necessary, with sufficient strength to withstand the local loads in the mould.

[0028] Other characteristics and advantages of the present application will be presented in the following detailed description, by referring to the attached drawings, provided by way of non-limiting example, in which:

[0029] - Figure 1 is a cross-sectional view of a mould according to the present application;

[0030] - Figure 2 shows a comparison between the net-like phase lattice and the matrix phase lattice obtained from a gyroid surface;

[0031] - Figure 3 a to c show a comparison between TPMS structures with different wall thicknesses;

[0032] - Figure 4 is a block diagram presenting the design method according to the present application;

[0033] - Figures 5 to 9 is a schematic view of a mould, showing different stages of the design method;

[0034] - Figure 10 isFigure 4 A block diagram providing further details of the method in FIG. 1 ; and

[0035] - Figures 11 to 19 are further diagrams illustrating different stages of the design method.

[0036] Figure 1 A mold for injection molding according to the present invention is shown. The mold includes a mold body 10 made by additive manufacturing of, for example, a metal material. The mold body 10 has a plurality of boundary surfaces denoted by 10a to 10f. In the example shown, reference numeral 10a denotes the top surface of the mold, and also includes at least one molding surface 10b configured to define the mold cavity MC. Reference numeral 10c denotes the bottom surface of the mold, reference numeral 10d denotes the side surface of the mold, reference numeral 10e denotes the surface of the injection channel 11 formed in the mold body 10, and reference numeral 10f denotes the surface of the conformal cooling channel 12 formed in the mold body 10. For simplicity, Figure 1 In FIG. 1 , only the surface of one conformal cooling channel among the plurality of conformal cooling channels 12 of the mold is indicated by reference numeral 10 f .

[0037] The mold body 10 comprises a functional area portion 13 on which the boundary surfaces 10a to 10f are formed. The functional area portion 13 consists of a solid and continuous material structure occupying a portion of the mold body 10. In other words, the functional area portion 11 is a portion of the mold body 10 without a lattice.

[0038] The mold body 10 further includes an application region portion 14, which is a supplement to the functional region portion 13 in the mold body 10. In other words, the application region portion 14 is the remaining portion of the mold body 10 excluding the functional region portion 13.

[0039] The application area portion 14 is composed of a three-dimensional material lattice structure, which includes orderly repeated unit cells, and the unit cells include periodic minimal surfaces, such as three-periodic minimal surfaces, especially spiral icosahedral surfaces. At least one geometric parameter of the periodic minimal surface is locally adjusted to form unit cells with different material densities. Figure 1 In the example shown, the geometric parameter is the wall thickness of the periodic minimum surface, which is thicker in the application area portion 14 near the injection channel and thinner in the application area portion 14 near the side surface 10d of the mold.

[0040] The functional area portion 13 surrounds the injection channel 11 , and the application area portion 14 surrounds the functional area portion 13 around the injection channel 11 .

[0041] The functional region portion 13 also encloses the conformal cooling channel 12. According to an alternative embodiment (not shown), the application region portion can also occupy the region where the conformal cooling channel is located. Incorporating a TPMS or other periodic minimal surface within the conformal cooling channel can induce turbulent flow, thereby increasing the efficiency of cooling transfer. According to another embodiment (not shown), the conformal cooling channel can be eliminated, and the voids inherent to the TPMS structure can be used to provide the cooling channel.

[0042] A method for designing a conformal channel 12 for a mold can be as follows.

[0043] An initial thermal simulation is performed on the mold to determine the thermal improvement space for the mold, which results in an improvement in the cooling cycle of the object produced by the mold. The object can be metal in a die casting process or plastic in an injection molding process. Subsequently, a conformal cooling system is designed using commercial CAD software.

[0044] The effect of introducing the conformal cooling system is then measured by another thermal simulation to see the improvement of the new designed conformal cooling relative to the initial thermal simulation.

[0045] Upon completion, a structural analysis simulation is performed on the mold to see if there is any deformation in the mold.

[0046] A method for designing a periodic minimal surface in a mold can be as follows, assuming for simplicity that the TPMS is a gyroid.

[0047] The gyroid is a triply periodic minimal surface discovered by Alan Schoen, a scientist at the National Aeronautics and Space Administration (NASA), in 1970. The gyroid divides space into two equidistant regions. The mathematical description of the gyroid surface can be approximated trigonometrically by the short equation:

[0048] U = (cos(k x x) sin(k y y) + cos(k y y) sin(k z z)

[0049] + cos(k z z) sin(k x x)) 2 – t 2 ,(1)

[0050] where k i is the TPMS function period, defined by (where I = x, y, z), n i is the number of times the unit cell repeats in x, y, and z, and L iis the absolute size of the structure in these dimensions. The matrix phase lattice consists of walls of solid material bounded by two unconnected void regions. These matrix phase lattices are different from the network phase structure which contains only one solid and one void region. This is shown in Figure 2

[0051] The TPMS equation describes a 3D surface which, for the purpose of additive manufacturing, can be taken as the boundary between voids and solid material. By finding the isosurface of U = 0 in equation (1), a matrix phase gyroid icosahedron structure with an arbitrary number of unit cells and an arbitrary volume fraction can be generated.

[0052] Filling one of the two separated regions generates a porous solid with a volume fraction of 0.5. Another way to obtain a solid from the gyroid icosahedron surface involves "offsetting" the original surface in two opposite directions (i.e. creating a surface that keeps a constant distance from the original surface at any point on it) and filling the space between the two surfaces. The resulting solid results in a lower volume fraction (<0.5). A third way is to combine the two previously described ways by "offsetting" the original surface to separate the space into two non-equal regions (one with a volume fraction greater than 0.5 and the other with a volume fraction less than 0.5) and filling one of the two regions to obtain a solid.

[0053] In equation (1), t effectively controls the thickness of the unit walls and, thus, also the volume fraction, p* of the resulting lattice structure. The relationship between t and p* is unique for each TPMS. Figure 3 Figures a-c of the drawings show a comparison between three lattice structures with different wall thicknesses, and thus different material densities. In particular, Figure 3 Figure a of the drawings shows a lattice structure with thin walls, and Figure 3 Figure c of the drawings shows a lattice structure with thick walls.

[0054] Furthermore, it is possible to extend the range of geometric designs by applying the three previously described basic concepts to other periodic minimal surfaces or surfaces similar to the gyroid icosahedron.

[0055] ​These periodic solids will be referred to as gyroidal tetrakaidecahedra and are expected to replace the classical truss-like lattice structures. One of the main drawbacks of using this kind of structure is the stress concentration caused by the sharp changes in the outer surface curvature. These stress concentrations will significantly reduce the resistance of the structure under load and the service life of the structure under cyclic loads. On the other hand, the gyroidal tetrakaidecahedra belong to the class of triply periodic minimal surfaces (TPMS), which is a subset of the larger class of constant mean curvature (CMC) surfaces. In particular, the classification of the TPMS according to the zero mean curvature of the TPMS at every point or with controlled variation solves the main drawback of introducing a standard strut-based lattice structure into the solid.

[0056] Furthermore, in strut-based lattice structures, there is the problem of overhangs, which require support structures to be successfully manufactured. In TPMS this is not needed, since every layer acts as a support for the subsequent layer.

[0057] Reference is now made to the accompanying drawings, which show Figure 4 The method according to the present application comprises providing a three-dimensional geometrical model of the mold to be designed (step 100). This model defines the geometrical features of the mold such as, for example, the boundary surfaces, the molding surfaces, the injection channels, the cooling channels, etc. Figure 5 and Figure 6 are a perspective view and a cross-sectional view of this 3D model, respectively.

[0058] Then, the lattice structure described above is implemented within the 3D model (step 110). This step is achieved by defining a so-called "application area". The initial 3D model is a closed solid bounded by a plurality of surface boundaries. These surface boundaries are divided into two categories: functional surface boundaries, which should remain unchanged, such as the mold cavity / core surfaces 10b, the cooling channel surfaces 10f, etc.; and non-functional surface boundaries, such as the basic planes of the mold 10c. After defining these two categories of surface boundaries, the lattice structure is included in the model by:

[0059] First, the functional boundaries are offset, i.e. a set of surfaces is obtained on which any point is located at a given distance from the original set of functional boundaries. This is achieved by filling the space between the offset set of surfaces (indicated by 13a' in Figure 7 ) and the original functional boundary surfaces, and finally adding a closing surface, we obtain a set of closed boundary solids, i.e. a so-called functional area 13' (as shown in Figure 7 ).

[0060] Second, we subtract the functional area from the initial 3D model. This can be achieved by a Boolean operation. The resulting solid is referred to as the "application area", indicated by 14' in Figure 8 . Figure 9A 3D model with a functional area 13' and an application area 14' is shown.

[0061] Third, the application area 14' is filled with a lattice structure. Finally, the resulting lattice is added to the functional area.

[0062] Reference Figures 10 to 18 The method of applying a lattice structure to the application area 14' is discussed below.

[0063] As Figure 11 shown, a mesh is generated corresponding to the entire mold (functional area and application area) and physical models (forces, pressures, thermal loads, support constraints, etc.) are applied to the nodes of the generated mesh (steps 200 and 210 in Figure 10 ). Figure 12 An example of applying boundary conditions to the mold surface (forces on the mesh nodes) is shown.

[0064] A topology optimization is then performed by placing material within the application area to achieve the required loads applied by the boundary conditions defined in step 210, thus providing an optimal solution.

[0065] The optimization problem can be divided into three main parts:

[0066] 1 - Model mesh definition: This relates to the mesh of the functional area and the application area. The properties of the model (e.g. the material used) are also defined in this stage. Figure 13 and Figure 14 The mechanical properties (elastic modulus) of one of the helical icosahedron structures are shown in gray scale.

[0067] 2 - Optimization objective: The optimization function is determined in this stage. The objective function used here is to identify the optimal distribution of material density to minimize the flexibility of the structure.

[0068] 3 - Optimization constraints: The optimization constraints are finally determined, for example: the total volume of material removed should not exceed 30% of the total amount of material available.

[0069] After several optimization iterations, the optimal density of the entire structure is obtained (step 220). Figure 15 The optimal volume fraction of each FE element is shown.

[0070] To optimize, the "application area" is mapped so that each node of the FEM model is assigned to a given cell of the lattice structure. This mapping from nodes to lattice cells is called "geometric mapping". As mentioned above, the helical icosahedron lattice properties can be locally adjusted by geometric parameters (e.g. wall thickness). These geometric features represent the local density of a given lattice cell. By adjusting the local polynomial functions representing the lattice properties accordingly, these geometric features can be included in the FEM calculation. Thus, we create an "properties map" of the lattice structure in the "application area"; the local polynomial functions used for the FEM calculation elements are the average properties resulting from the node properties based on the position of the nodes in the "geometric map" and the properties of the corresponding cell units in the "properties map".

[0071] After the FEM provides the solution domain, the "properties map" of the lattice can be adjusted to optimize the overall structure. All local properties are reset to achieve a more uniform solution and reduce stress or thermal loads in response to given criteria. The last approach can be repeated until the solution meets the technical specifications. This will ensure an increased service life and a reduction of potential early cycle failures of the parts.

[0072] Then, based on the optimized "properties map" at the end of the FEM homogenization process, the local geometry of each cell is set according to its density (step 120 in Figure 4 and step 230 in Figure 10 , thus obtaining the final lattice design. Figure 16 The application area before the optimization process is shown in Figure 17 and Figure 18 The application area after the optimization process is shown in Figure 18 is a 45° cross-section of the application area.

[0073] The electronic file (e.g. CAD file) is then stored on the cloud / server with the manufacturing parameters for the customer to access and start the additive manufacturing process (step 130 in Figure 4 ). Figure 19 is a cross-section of the sliced model incorporating the manufacturing parameters.

Claims

1. A mold for injection molding, comprising a mold body (10) having a plurality of boundary surfaces (10a-10f), the plurality of boundary surfaces including at least one molding surface (10b) configured to delimit a mold cavity, wherein The mold body is manufactured by additive manufacturing and is characterized in that the mold body comprises: a functional area portion (13) on which the plurality of boundary surfaces and the at least one molding surface are formed, the functional area portion consisting of a solid and continuous material structure occupying a portion of the mold body (10); and An application area portion (14), the application area portion is a supplement to the functional area portion (13) in the mold body (10), the application area portion is composed of a three-dimensional material lattice structure, the three-dimensional material lattice structure includes orderly repeated unit cells, the unit cells include periodic minimal surfaces, wherein at least one geometric parameter of the periodic minimal surface is locally adjusted to form the unit cells with different material densities.

2. The mold according to claim 1, wherein The periodic minimal surface is a triply periodic minimal surface.

3. The mold according to claim 2, wherein The periodic minimal surface is a helical icosahedron.

4. A mould according to any one of the preceding claims, wherein The mold body comprises at least one injection channel (11), wherein the functional area portion (13) surrounds the at least one injection channel (11), and wherein the application area portion (14) surrounds the functional area portion (13) around the at least one injection channel (11).

5. The mold according to any one of the preceding claims 1 to 3, wherein The mold body comprises at least one conformal cooling channel (12), and wherein the functional area portion (13) surrounds the at least one conformal cooling channel (12).

6. The mold according to any one of claims 1 to 3, wherein The mold body includes at least one conformal cooling channel, and wherein the application area partially surrounds the at least one conformal cooling channel.

7. The mold according to any one of claims 1 to 3, wherein Cooling channels are formed by the interstices inherent in the structure of the periodic minimal surface.

8. A method for designing a mould according to any one of the preceding claims, comprising: Step a) providing a three-dimensional geometric model of the mold to be designed; Step b) determining the functional area portion (13) and the application area portion (14) in the mold body (10) of the mold; Step c) adjusting the at least one geometric parameter of the periodic minimal surface based on the physical model of the mold; as well as Step d) creating an electronic file storing data representing a digital model for constructing the mold by additive manufacturing.

9. The method according to claim 8, wherein The step c) comprises: Step c1) generating meshes corresponding to both the functional area portion and the application area portion, and applying the physical model to a plurality of nodes of the generated meshes; Step c2) determining the material density values ​​at the nodes of the generated mesh based on the applied physical model; Step c3) determining the value of at least one geometric parameter of the periodic minimal surface that corresponds to the determined material density value.

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