ES2595099B2 - Procedure for the design of the ejection rod system for a mold - Google Patents

Abstract

Procedure for the design of the ejection rod system for an injection mold comprising: creating a flat grid of nodes (Nk) on a plane perpendicular to the movement of the mold cavity; determine the points (Pij) of the piece (M ') corresponding to the intersection with a line (Rt) perpendicular to the plane that passes through each node (Nk); calculate the thickness (Eij) corresponding to each point (Pij); identify the points where they exist (Pijr) and where there are no (Pijv) thickness changes; determine as points (Pijexp) candidates those points (Pijv) whose first surrounding surface does not contain any points (Pijr); calculate the number of points (Pijr) contained in a second surface surrounding each point (Pijexp), where the second surface is larger than the first surface; and select as points (Pijexpo) of expulsion those points (Pijexp) whose number of points (Pijr) contained in the second surface constitutes a local maximum.

Description

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DESCRIPTION

Procedure for the design of the ejection rod system for a mold OBJECT OF THE INVENTION

The present invention belongs to the field of industrial injection molding processes.

The object of the present invention is a method that allows determining the optimal position for the ejection rods that allow the molded part to be separated from the corresponding mold.

BACKGROUND OF THE INVENTION

Injection molding is one of the most used technological processes in the transformation of thermoplastic polymers. The process begins with the injection of a precise amount of molten thermoplastic material into the mold cavities. Then the plastic piece begins to solidify, decreasing its volume and contracting on the nuclei of the cavities; This loss of volume is compensated by injecting a small amount of melt into the cavities and applying a maintenance pressure that prevents the reflux of the thermoplastic material. The high level of performance that is demanded today of the mold, motivates that the value of the cycle time should be reduced to the minimum possible, this is achieved by cooling the piece by means of cooling channels through which a fluid circulates at a lower temperature, accelerating in this way its solidification. When the piece becomes sufficiently rigid to be ejected from the mold, the ejection system is responsible for extracting the piece from the cavity to which it has been adhered, by means of a mechanical system that will release it. After the expulsion phase is finished, the mold will close and start the next cycle again.

Short cycle time requirements, force the extraction of the mold piece almost to the limit of its solidification. This can cause damage or deformation in the part if the design of the ejection system is not done properly. Although there are several procedures to expel the plastic part from the mold, many of them cause problems over time losing their functionality due to wear and gaining clearance

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with the use. For this reason in complex plastic parts that incorporate ribs, projections or areas with little output, the use of ejector rods is recommended for its extraction from the mold. These rods are characterized by their versatility in their localization in the piece, as well as by their ease of installation at a relatively economical cost.

The calculation and design of the distribution of the ejectors on the plastic part is not an easy task when the geometry of the piece is complex. Currently the distribution of the ejectors is projected based on the experience of the mold designer. In the specialized literature some basic empmco rules are suggested in relation to the design and selection of ejectors. In light of these problems, it is considered necessary a procedure that automates the design process of the mold ejection system being carried out in parallel to the design of the piece, preventing possible manufacturing failures already from the initial stages of the design.

Although CAD / CAM systems have been widely used in the design of injection molds, they do not incorporate geometric procedures to automate the design of the ejection system. Commercially specialized procedures for mold design require a lot of interaction and experience on the part of the designer as well as a specific knowledge about the manufacture of the plastic part. In the scope of the investigation there are also few works dedicated to the automation of the design of the expulsion stage, based mainly on the recognition of characteristics of the plastic part or on heunstic methods of optimization of the result.

DESCRIPTION OF THE INVENTION

Based on the problems raised, the present patent application describes a new procedure for the automated obtaining of the position of the ejector rods required to eject a piece of plastic to be designed. The proposed procedure will help the injection mold designers systematically obtain the optimum position of the ejector rods, substantially reducing the interaction time between the product designers and the mold designers when validating the design before proceeding to its design. manufacturing.

Essentially, the proposed procedure is designed to automatically locate those points of the piece that are not located in areas that are not optimal for the location of a ejector rod, for example in areas where there are edges,

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nerves, and other structures of the piece, but at the same time are close enough to such structures to present a suitable rigidity for expulsion. This procedure can be carried out in an automated way using a computer simply from a three-dimensional model of the piece in question and of the knowledge of the moldable surface by the mold cavity. A third configurable parameter allows to determine the accuracy of the analysis.

Preferably, the three-dimensional model of the piece can be obtained by digital design tools such as computer drawing programs, such as Autocad or Catia. Alternatively, in another preferred embodiment of the invention the three-dimensional model of the part can be obtained through a digital scanning process of a real part manufactured by any suitable method, such as for example machining or injection.

The main advantages of the process of the invention in relation to conventionally used procedures are the following:

- Validity of the procedure regardless of the modeler, avoiding problems of dependence on the modeler and the analysis of complex characteristics.

- Validity for parts in discrete or continuous format analyzing the procedure described in the document geometry externally, obtaining as an advantage the possibility of implementing the procedure in any CAD modeler.

- Validity for any geometry since it works independently of the procedure according to which the piece has been designed.

- Representation CAE on the plastic piece, since the solution of the procedure is superimposed on the piece in the form of an ejection map, resulting in a layout with the location of the ejectors on the piece.

- The results of the procedure allow its exportation and storage for other applications, providing the possibility of its use by a parametric system of CAD design of the injection mold.

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- The points obtained for expulsion are real points that can be placed on the real piece with a measuring device.

The present invention describes a method for designing the extraction system of a part of an injection mold from a three-dimensional model of the part and of the moldable surface by a mold cavity, which allows to select optimum ejection points for location of some ejector rods of the piece. As mentioned, the three-dimensional model of the part can be obtained by digital design tools or by a digital scanning process of a real part manufactured by any suitable method.

This procedure basically includes the following steps:

1) Create a flat grid of nodes located on a plane perpendicular to the direction of movement of the mold cavity.

The distance between the nodes of the grid, or precision of the two-dimensional nodal mesh, will coincide with the precision of the analysis to be performed. Preferably, this distance between nodes is chosen so that it is approximately half the size of the smallest geometric detail of the piece in question.

2) Determine the points of the piece corresponding to the intersection with a line perpendicular to said plane that passes through each node of the grid.

To carry out this operation, a beam of straight lines is drawn perpendicular to the plane mentioned above, starting at each node of the grid. Then, all the cut points of said lines are calculated with the surface of the moldable part by the mold cavity.

3) Calculate the thickness of the piece corresponding to each point of intersection.

Since a single straight can cut the surface of the piece in more than two points, in a preferred embodiment of the invention the thickness of the piece is calculated as the distance in the direction of the line between the point of intersection furthest from the plane and the point of intersection located on the moldable surface by

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The mold cavity.

4) Identify the points of the piece where there are changes in thickness and the points where there are no changes in thickness.

According to a preferred embodiment of the invention, it is determined that one point of the piece has a change in thickness if at least one of the thickness increases between said point and each of the adjacent points is different from the others.

5) Determine as candidate points to expulsion points those points without change of thickness whose first surrounding surface does not contain any point with change of thickness.

The term "first surrounding surface" refers to a certain surface that surrounds each point without changing thickness. In principle, this surface can be defined through a certain geometric shape, such as a circle centered at the point in question.

In this operation, it is verified that there is no point with change of thickness very close to the candidate points to expulsion points, with the purpose of excluding those points located very close to nerves, edges, edges, and in general other structures of the piece that are not suitable for the location of ejection rods.

6) Calculate the number of points with changes in thickness contained in a second surface surrounding each candidate point at the point of expulsion, where the second surrounding surface is larger than the first surrounding surface.

The term "second surrounding surface" refers to a surface normally of the same geometric and concentric shape in relation to the first surface, although larger in size.

According to a preferred embodiment of the invention, the first surrounding surface and the second surrounding surface of a candidate point to expulsion point are formed as circumferences centered at said point. By

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For example, the diameter of the first surrounding surface can be essentially equal to the distance between the nodes of the grid. In another preferred embodiment of the invention, the diameter of the second surrounding surface is essentially twice the diameter of the first surrounding surface.

In this operation, it is determined which of the previous candidate points are located close enough to structures of the piece such as nerves, edges or edges, since a certain proximity to these structures provides rigidity to this area of the piece and therefore causes These points are especially suitable for the location of the ejection rods.

7) Select as expulsion points those candidate points whose number of points with change in thickness contained in the second surrounding surface constitutes a maximum local.

That is, those candidate points whose second surrounding surface contains the maximum number of points with change in thickness are selected. This implies that, from among points that are located sufficiently close to structures of the piece such as edges, edges or nerves, those with a greater number of such structures are chosen sufficiently, since they will be the most rigid among the “eligible” points ”For the location of the ejection rods.

8) Optionally, the process of the invention preferably comprises a last step consisting in the manufacture of the injection mold provided with ejection rods located at the ejection points determined in the previous step.

The process of the invention is specially designed to be carried out with the help of a processing means such as a computer. Therefore, the present invention extends to computer programs adapted to carry out the method of the invention. The program may have the form of source code, object code, an intermediate source of code and object code, for example, as in partially compiled form, or in any other form suitable for use in the implementation of the processes according to the invention . The carrier can be any entity or device capable of supporting the program.

Computer programs may be arranged on or within a carrier. By

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For example, the carrier could include a storage medium such as a ROM, a CD ROM or a semiconductor ROM, or a magnetic recording medium, for example, a floppy disk or a hard disk. In addition, the carrier can be a transmissible signal, for example, an electrical or optical signal that could be transported through electrical or optical cable, by radio or by any other means.

When the program is incorporated into a signal that can be transported directly by a cable or other device or medium, the carrier may be constituted by said cable or other device or means.

As a variant, the carrier could be an integrated circuit in which the program is included, the integrated circuit being adapted to execute, or to be used in the execution of, the corresponding processes.

BRIEF DESCRIPTION OF THE FIGURES

Figs. 1a and 1b respectively show an example of the real M piece and the M ’piece in discrete format after scanning the real piece (M) with a measuring device.

Fig. 2 shows the surfaces I ’of the piece M’.

Fig. 3 shows an Nk nodal mesh for the M ’piece.

Fig. 4 shows a representation of the lines Rt and Rt ++ 1

Fig. 5 shows a set of intersection points Pij represented on the piece M ’itself.

Fig. 6 shows a representation of a line Rq.

Fig. 7 shows an example of obtaining the maximum thickness Eij of the piece M ’at a generic Pij point.

Fig. 8 graphically shows the calculation of the variation in thickness between nodes adjacent to the Pij node.

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Fig. 9 shows a representation of the nodes Pijv and Pijr on! ’.

Fig. 10 shows the location of the Pijexp points.

Fig. 11 shows the representation of the Pijexp points.

Fig. 12 shows the procedure for locating ejectors on! ’.

Fig. 13 shows an example of the estimation of the number of Pijr nodes per Pijexp node.

Fig. 14 shows the concentration of Pijr nodes per Pijexp node indicated in grayscale in an analysis performed with A / 2 precision and where the ejection points are represented by white spheres on the M ’piece.

PREFERRED EMBODIMENT OF THE INVENTION

In the following, the present invention is described in more detail with reference to the attached figures.

Initial data

The procedure described in this document takes three data as input:

1) A discrete geometry of a piece M ’that is formed by a set of triangular flat elements and nodes, as shown in Fig. 1b. In this specific example, the discrete geometry of part M ’has been obtained by a digital scanning process of a real part M represented in Fig. 1a. In any case, as discussed earlier in this document, it would also be possible to obtain the discrete geometry of the M ’piece from a M part geometry generated by CAD in B-Rep continuous format.

2) A set of surfaces I ’in discrete format in a preferred embodiment belonging to the part M’ that will be moldable by the lower mold cavity, figure 2.

3) A value A that determines the precision of the two-dimensional nodal mesh (distance

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between two contiguous nodes, on the X and Y axes) coinciding with the accuracy of the analysis. It will always be convenient to adapt the value of A to half the dimension of the smallest geometric detail of the piece.

Generation of a grid on the surfaces of the plastic part removable by the lower mold cavity

During this phase a grid of flat Nk nodes (2D nodal mesh) is generated on a plane parallel to the XY plane with a value of Z = cte. The input data for the creation of the nodal mesh are the measurements of the prism circumscribed to the piece M 'and the precision of the analysis A. In figure 3 the nodal mesh Nk for the piece M' has been represented, for a precision A .

Next, a straight line Rt is drawn parallel to the Z axis, starting at each node Nk of the 2D nodal mesh. In figure 4 two lines Rt and Rt + 1 belonging to the set of lines that cross the piece M 'are shown as an example.

Then the intersection points Pij between each straight line Rt and the surfaces I ’of the moldable part are obtained by the lower cavity of the mold. These points Pij are stored in a matrix A. In figure 5 the set of intersection points Pij on the surface I ’has been represented.

Calculation of the differences in the thickness variations of the plastic part M ’for each Pij node belonging to the mesh A

In this phase of the procedure, the maximum thickness of the piece M ’is obtained for each Pij point. To do this, starting at each point Pij belonging to A, a straight line Rq is drawn parallel to the Z axis that crosses the mesh M ’calculating the intersection points (Pij, ..., Pij n) between each line Rq and M’. The value of the maximum thickness Eij, of the plastic piece at each point Pij is calculated as the difference in height in the direction of the Z axis, between the intersection point Pij n of greater dimension and the point Pij located on I ’.

Pij = [Pij x, Pij y, Pij z]

Pijn = [Pij xn, Pij yn, Pij zn] Eij = (Pij n z - Pij z)

To identify in an automated way geometric areas of the piece that incorporate

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Nerves, protrusions, and corners of the procedure, analyze the variation in the difference in thickness between each pair of contiguous nodes of the mesh Pij and Pi ± 1 j ± 1 separated by a distance A equal to the precision of the analysis Figure 8.

This analysis is carried out bidirectionally following the direction of the coordinate axes X and Y. For each node Pij of the mesh A the value AE is evaluated with all the nodes adjacent to it (Pi-1j, Pi + 1j, Pij-1, Pij +1) separated from each other by a distance A. Each node can have up to 4 adjacent nodes with which the values of thickness variations AEA, AEB, AEC, AED are obtained. Thus, if the variation in the node-node thickness difference remains constant in one of the directions of the coordinate axes X or Y, that is, if the condition AEA = AEB or AEC = AED is met, these nodes are grouped in a new matrix nv rename as Pijv.

On the contrary if there is any inequality, between the values AEA, AEB, AEC, or AED then the Pij node presents a variation of thickness in a nearby environment. In this case the analyzed node Pij belongs to an area of the geometry of the piece characterized by incorporating, ribs, projecting corners etc. The node with the greatest magnitude of thickness is stored in a new matrix nr becoming Pijr. Figure 9 shows the result of this stage of the procedure for a precision A representing the Pijv nodes with a circle and the Pijr nodes with a square.

Localization of optimal regions for the expulsion and determination of the ejectors on the removable geometry through the lower mold cavity

For each Pijv node belonging to nv a circular area of radius R = A is generated, where A is the value of the precision of the mesh and it is checked, if any Pijv node belonging to nv is included in the circular area of center Pijv and radius R = A

If this criterion is met, the Pijv node is a possible point of expulsion, since it is located in a region without changes in thickness being stored in a new nexp matrix, and naming Pijexp. This procedure is intended to detect those nodes separated from edges, edges, and areas that are not optimal for the situation of an ejector. Figure 10 shows an example of the location of the Pijexp points for the M ’piece and in figure 11 the set of Pijexp points marked with a cross.

Next, the position of the ejectors on the demouldable geometry is determined

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by the lower cavity of the mold. For each Pijexp node belonging to nexp a circular area of radius R = 2 * A is generated, A being the value of the mesh precision. Next, it is checked if any Pijr node belonging to nr is included in the circular area of center Pijexp and radius R = 2 * A. If this criterion is met, the Pijexp node will be a possible point of expulsion since it meets the requirement of being near walls, nerves, corners, finally areas with thickness changes.

A value of Cn associated to each Pijexp node is defined to indicate the number of Pijr nodes that are in the circular area of Pijexp center and radius R = 2 * A. An example of location of the ejectors on ! ', Figure 13 shows two examples of estimating the number of Pijr nodes per Pijexp node. Next, the maximum local values of Cn are obtained for all Pijexp points. The local maximum will be the optimal points of expulsion for the plastic piece since they are close to very rigid areas of the piece in order to avoid deformations.

Figure 14 shows the result of the procedure for the example piece M ’with an accuracy of analysis A / 2; an expulsion map is indicated with the concentration of Pijr nodes per Pijexp node, in grayscale as well as the final location of the ejection points using white circles on the M ’piece.

Finally, the final step of manufacturing the injection mold can be carried out with the ejection rods located in the positions calculated by the above procedure. Specifically, this injection mold would specifically include five ejector rods located respectively at the points marked by the white circles of Fig. 14: four on the periphery and one in the central area.

Claims (13)

5 10 fifteen twenty 25 30 35 1. Procedure for the design of the ejection rod system for a mold from a three-dimensional model (M ') of the part and the surface (!') Moldable by a mold cavity, which allows to select points (Pijexpo ) of optimal expulsion for the location of some ejector rods of the piece, characterized in that it comprises the following steps: create a grid of flat nodes (Nk) located on a plane perpendicular to the direction of movement of said mold cavity; determine the points (Pij) of the piece (M ’) corresponding to the intersection with a line (Rt) perpendicular to said plane that passes through each node (Nk) of the grid; calculate the thickness (Eij) of the piece corresponding to each point (Pij) of intersection; identify the points (Pijr) of the piece where there are changes in thickness and the points (Pijv) where there are no changes in thickness; determine as points (Pijexp) candidates for expulsion points those points (Pijv) without change of thickness whose first surrounding surface does not contain any point (Pijr) with change of thickness; calculate the number of points (Pijr) with thickness changes contained in a second surface surrounding each point (Pijexp) candidate for expulsion point, where the second surrounding surface is larger than the first surrounding surface; Y select as points (Pijexpo) of expulsion those points (Pijexp) candidates whose number of points (Pijr) with change of thickness contained in the second surrounding surface constitutes a maximum local. 2. Procedure according to revindication 1, which also includes the previous step of obtaining the three-dimensional model of the part (M ’) through a digital scanning process of a real part (M). 3. Method according to any of the preceding claims, which further comprises the step of manufacturing the injection mold provided with ejection rods located at the ejection points (Pijexpo) determined in the previous step. 4. Method according to any of the preceding claims, wherein the distance (A) between the nodes (Nk) of the grid is chosen to be approximately the 13 5 10 fifteen twenty 25 30 35 half of the dimension of the smallest geometric detail of the piece (M ’). 5. Method according to any of the preceding claims, wherein the thickness (Eij) of the piece is calculated as the distance in the direction of the line (Rt) between the point of intersection (Pij n) furthest from the plane and the intersection point (Pij) located on the surface (! ') moldable by a mold cavity. 6. Method according to any of the preceding claims, wherein it is determined that a point (Pij) of the piece exhibits a change in thickness if at least one of the thickness increases between said point (Pij) and each of the points Adjacent (Pi + 1 j, Pi-1 j, Pi j + 1, Pi j-1) is different from the others. 7. Method according to any of the preceding claims, wherein the first surrounding surface and the second surrounding surface of a point (Pijexp) candidate for expulsion point are formed as circumferences with center at the point (Pijexp). 8. Method according to claim 7, wherein the diameter of the first surrounding surface is essentially equal to the distance (A) between the nodes (Nk) of the grid. 9. Method according to any of claims 7-8, wherein the diameter of the second surrounding surface is essentially twice the diameter of the first surrounding surface. 10. Computer program comprising program instructions for making a computer practice the method according to any of the preceding claims. 11. Computer program according to claim 10, incorporated into a carrier. 12. Computer program according to claim 11, wherein the carrier is a storage medium. 13. Computer program according to claim 11, wherein the carrier is a transmissible signal.