JP2007193443A - Method for generating surface mesh model - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a model generation method, capable of shortening the generation time of a surface mesh model. <P>SOLUTION: The method for generating a surface mesh model describing a design surface or neutral surface from a solid CAD model of an injection molded product comprises processes of setting a coordinate axis in a direction capable of overlooking the design surface of the injection molded product described by the solid CAD model; extracting one plane in which the design surface is projected to a set coordinate surface; dividing the plane without clearance to form a minute small plane group; and arranging, for each minute plane, data describing a surface of the solid CAD model corresponding to the minute plane concerned to data describing a minute surface corresponding to the minute plane concerned. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to a method for creating a surface mesh model used for CAE analysis from a solid CAD model of an injection molded product.

In order to perform CAE analysis of an injection molded product, it is necessary to create a surface mesh model in which the surface of the injection molded product described by the solid CAD model is divided into micro surface groups.
In the surface mesh model creation method of Patent Document 1, the surface of an injection-molded product is divided into groups of minute surfaces, and the shape and position of each minute surface and the plate thickness to the back surface are measured using a three-dimensional measuring means. A surface mesh model for CAE analysis is created by associating plate thickness data corresponding to data describing the surface.
JP 2005-196245 A

When creating a surface mesh model of an injection-molded product having a plurality of surfaces using general CEA analysis software, the operator sequentially designates the surfaces for creating the surface mesh model. The CEA analysis software creates a surface mesh model for each specified surface. In many cases, there is a discontinuity between the surface mesh models created individually for each face. Therefore, the operator connects these to create a surface mesh model of the entire injection molded product. This operation takes a considerable amount of time, and in particular, the longer the number of surfaces provided in the injection molded product, the more time is required.
The present invention has been devised to solve the above problems.
The present invention provides a model creation method capable of shortening the time for creating a surface mesh model.

(Invention of Claim 1)
The present invention relates to a method of creating a surface mesh model that describes a design surface from a solid CAD model that describes an injection molded product. This method includes a step of setting a coordinate axis in a direction in which the design surface of an injection molded product described by a solid CAD model can be looked over, and a step of extracting a single plane obtained by projecting the design surface on the coordinate surface of the set coordinate axis And a step of dividing the plane into minute plane groups that divide the plane without gaps, and for each minute plane, describe the minute surface corresponding to the minute plane from the data describing the surface of the solid CAD model corresponding to the minute plane. It has a process of organizing the data to be processed.
The “direction in which the design surface of the injection-molded product can be looked over” indicates a direction in which the entire design surface of the injection-molded product can be viewed at a time when it is possible. When there is no direction in which all can be seen at once, it means the direction in which most or the main part of the design surface can be seen.
A “coordinate plane” refers to a plane orthogonal to the viewing direction. For example, when the observation direction is set to the z axis, the xy plane corresponds to the coordinate plane.
An example of arranging the data describing the surface of the solid CAD model into the data describing the minute surface corresponding to the minute plane includes, for example, a minute plane whose position on the xy plane is determined. It includes a mode in which the position in the z direction is extracted from the data describing the surface of the corresponding solid CAD model, thereby arranging the data to describe the minute surface corresponding to the minute plane (giving the position in the z direction). .
According to the model creating method of the present invention, a solid CAD model is projected onto a set coordinate plane to project the entire design surface onto one plane, and a surface mesh that describes the entire design surface from the one continuous plane. You can create a model at once. As a result, if the design surface is composed of multiple surfaces, a surface mesh model is created for each surface, and the entire surface mesh model is created by continuing the discontinuous surface mesh models. Compared to the conventional method, the time required for model creation can be greatly shortened.

(Invention of Claim 2)
The present invention also provides a method for creating a surface mesh model describing a neutral plane from a solid CAD model describing an injection molded article. This method includes a step of setting a coordinate axis in a direction in which the design surface of an injection molded product described by a solid CAD model can be looked over, and a step of extracting a single plane obtained by projecting the design surface on the coordinate surface of the set coordinate axis And a step of dividing the plane into minute plane groups that divide the plane without gaps, and for each minute plane, describe the minute surface corresponding to the minute plane from the data describing the surface of the solid CAD model corresponding to the minute plane. A process of calculating the normal direction of the micro surface, a process of calculating the thickness of the injection molded product measured in the normal direction from the data of the solid CAD model for each micro surface, A step of calculating an offset minute surface offset by a distance of half the thickness in the normal direction with respect to the minute surface is provided.
“Neutral surface” refers to a surface that bisects the injection molded product described by the solid CAD model in the thickness direction.
“Thickness of injection molded product measured in the normal direction for each minute surface” is a value obtained by averaging the thickness in the normal direction for each minute surface, even if the thickness in each minute surface is not uniform. Good.
Generally, when an injection molded product is relatively large, a surface mesh model that describes a design surface for CAE analysis is used. However, when the injection-molded product is relatively small, it is known that the use of the surface mesh model describing the neutral surface has less error in the analysis result. For this reason, some software for CAE analysis has a function of creating a surface mesh model describing a neutral plane from a solid CAD model.
According to the model creating method of the present invention, a solid CAD model is projected onto a set coordinate plane to project the entire design surface onto one plane, and the surface describing the entire neutral plane from the one continuous plane. A mesh model can be created at once. A surface mesh model describing a neutral plane suitable for CAE analysis of an injection molded product having a relatively small shape can be created in a short time.

(Invention of Claim 3)
In the coordinate axis setting step, it is preferable to set the coordinate axis in the die-cutting direction of the injection molded product.
The die-cutting direction of the injection-molded product is often set in a direction in which undercut does not occur, and is often set in a direction in which the entire design surface of the injection-molded product can be seen at once. Therefore, if a coordinate axis is set in this direction, one plane on which all design surfaces are projected can be easily extracted.

(Invention of Claim 4)
In creating the surface mesh model, it is preferable to include a step of creating a solid CAD model from which ribs existing on the non-design surface are removed. First, it is preferable to calculate the thickness from the solid CAD model from which ribs have been removed.
The “rib” refers to a protrusion provided locally on the non-design surface.
If the thickness is calculated including the ribs, it is difficult to calculate the thickness, and the neutral surface tends to be unnatural. It is easier to calculate the thickness with a model with the ribs removed, and a natural neutral plane can be calculated.

(Invention of Claim 5)
It is preferable to include a step of calculating the neutral surface of the removed rib and a step of adding the neutral surface of the rib to the neutral surface calculated by removing the rib.
Since it is easy to calculate the neutral plane of the rib, the neutral plane of the rib is added to the neutral plane calculated by removing the rib, rather than calculating the neutral plane from a solid CAD model without removing the rib. It is easier to calculate by adding. A natural neutral plane can also be calculated.

(Invention of Claim 6)
After calculating an offset microfacet offset by a distance half the thickness in the normal direction, for a group of microfacets that are discontinuous in the height direction (in the direction of observation), apexes with different heights (microfacets) A step of replacing the vertex groups adjacent to each other with a connected minute surface.
According to this, it can be easily replaced with a surface mesh model describing a continuous neutral plane.

  According to the present invention, the time required to create a surface mesh model describing a design surface or a neutral surface can be shortened.

The main features of the embodiments described below are listed.
(First embodiment) A coordinate axis is set in a direction in which the entire design surface of an injection molded product can be looked over.
(Second Embodiment) The model creation method includes a step of detecting and automatically removing ribs provided on the non-design surface.
(Third embodiment) The model creation method automatically calculates the neutral surface of the rib provided on the non-design surface and automatically removes the neutral surface from the neutral surface after removing the neutral surface. Additional steps are provided.

An embodiment according to the CAE analysis method of the present invention will be described with reference to FIGS. The present embodiment relates to a numerical calculation (CAE analysis) method for performing resin flow analysis and rigidity analysis from a solid CAD model of an injection mold for injection molding a resin instrumental panel (instrument panel) of an automobile. In the present embodiment, a method for creating a surface mesh model using a CAE analysis program of a CAE analyzer (not shown) will be described.
FIG. 1 is a sectional view of an injection molded product (instrument panel) 10. FIG. 2 is an arrow view of the injection molded product 10 as viewed from the II direction shown in FIG. 3 is an arrow view of the injection molded product 10 as viewed from the III direction shown in FIG. FIG. 4 is a flowchart showing a part of the CAE analysis program. FIG. 5 is a diagram for explaining a simple model injection molded product 20. FIG. 6 is a diagram for explaining a portion where the surface obtained by offsetting the injection molded product 20 in the thickness direction is discontinuous. FIG. 7 is a diagram for explaining a method of setting the discontinuous surface in FIG. 6 as a continuous surface. FIG. 8 is a diagram illustrating a neutral surface of the injection molded product 20.

As shown in FIG. 1, the injection molded product 10 has a design surface (front surface) 11 and a non-design surface (back surface) 12. The design surface 11 is a surface that needs to be finished to the intended surface shape. The non-design surface 12 is a surface on which the surface shape is not important.
The injection molded product 10 has a substantially U-shaped cross section. The design surface 11 is provided with an upper surface (upper surface shown in FIG. 1) when attached to the vehicle as an instrument panel, and a front surface (a surface facing the driver side, the right surface shown in FIG. 1). On the surface side of the upper surface, non-slip convex portions 11a are provided in three rows in parallel. Moreover, the hole 11b for attaching members, such as a lid | cover, and instruments is provided in two places on the front surface. Regarding the method of creating a surface mesh model from a solid CAD model for the injection molded product 10 having such a shape, the processing executed by the CAE analyzer will be described based on the flowchart shown in FIG.

A program for executing the processing shown in FIG. 4 is stored in storage means (not shown) such as a ROM of the CAE analyzer. A CPU (not shown) of the CAE analysis apparatus reads a program from the storage unit and executes it as necessary.
In step S10, coordinate axes for projecting the solid CAD model of the injection molded product 10 are set. The injection molded product 10 appears as shown in FIG. 2 when viewed from the II direction shown in FIG. When viewed from this direction, the front surface of the injection molded product 10 can be seen, but the top surface cannot be seen. Further, when viewed from the III direction shown in FIG. 1, it looks as shown in FIG. When viewed from this direction, both the front surface and the upper surface of the injection molded product 10 can be seen. Both the convex portion 11a provided on the upper surface and the hole 11b provided on the front surface can be seen. The operator selects a direction in which the entire design surface 11 of the injection molded product 10 can be looked over at once in the CAE analyzer, and sets the CAE analyzer in the CAE analyzer. In the case shown in FIG. 1, the Z coordinate axis is set in the III direction. The CAE analyzer proceeds to the process of step S11.
In step S11, it is determined whether or not the non-design surface 12 has a rib. If there is a rib (YES in step S11), the process proceeds to step S12. If there is no rib (NO in step S11), the process proceeds to step S13. For example, a screen for selecting whether the display means of the CAE analyzer has a rib (display “Yes”) or not (display “No”) is displayed. When there is a rib, the operator selects the display of “Yes”. Thereby, it is set that there is a rib on the non-design surface of the injection molded product. If there is no rib, the operator selects “No” display. In this case, it is not set that there is a rib on the non-design surface of the injection molded product. In the case of an injection molded product having a rib on the non-design surface, the position of the rib is set by the operator in step S12. Then, the process proceeds to step S13. Since the injection molded product 10 is not provided with ribs on the non-design surface 12, the process proceeds to step S13.
Note that the determination of the presence or absence of the rib in step S11 and the setting of the position of the rib in step S12 can also be automatically performed by the CEA analyzer.
In step S13, a surface obtained by projecting the solid CAD model is extracted from the coordinate surface of the coordinate axis set in step S10. In the case of the injection molded product 10, a plane on which the design surface 11 is projected is extracted thereby. FIG. 3 illustrates this. At this stage, the design surface 11 that is actually three-dimensional is projected onto a plane.

Hereinafter, in order to make the explanation easy to understand, an injection molded product 20 illustrated in FIG. 5 will be described instead of the injection molded product 10.
The injection-molded product 20 has a substantially rectangular shape on the upper surface, and has a design surface 21 and a non-design surface 22. For simplicity of illustration, the design surface 21 is shown as a plane, but is actually three-dimensional and includes a three-dimensional curved surface. Further, a step 23 is provided on the non-design surface 22 side. As shown in FIG. 5, the thickness (the vertical dimension shown in FIG. 5) is different between the left part of the step 23 and the right part of the step 23. A rib 24 is provided on the non-design surface 22 side. In the case of FIG. 5, when viewed from the Z-axis direction, the entire design surface 21 that is actually three-dimensional can be seen all at once.

In order to create a surface mesh model from the solid CAD model of the injection-molded product 20, the Z axis is set in a direction in which the entire design surface 21 can be seen at once in step S10. The CAE analyzer proceeds to the process of step S11.
In step S11, it is determined whether or not the non-design surface 22 has a rib. If there is a rib (YES in step S11), the process proceeds to step S12. If there is no rib (NO in step S11), the process proceeds to step S13. In the case of the injection molded product 20, since the rib 24 is provided on the non-design surface 22, the process proceeds to step S12.
In step S12, the position of the rib 24 is set by the operator. Then, the process proceeds to step S13.
In step S13, a surface obtained by projecting the solid CAD model onto the XY plane is extracted from the coordinate axis in the Z direction set in step S10. Thereby, one plane on which the substantially rectangular design surface 21 is projected is extracted. The CAE analyzer proceeds to the process of step S14.

In step S14, the plane extracted in step S13 is divided into small plane groups each of which is a triangle as described in part on the upper surface of FIG. At this point, each minute plane has position data in the XY plane, but does not have position data in the Z direction.
Next, the CAE analyzer proceeds to the process of step S15. In step S15, the vertexes constituting the minute surface corresponding to each minute plane (also a triangle, but generally not in the XY plane but inclined with respect to the XY plane). The position data in the Z direction is calculated. At this stage, the data of the solid CAD model is used, and the Z coordinate of the position on the surface specified by the XY coordinates is calculated. As a result, the XYZ coordinates of each vertex of a minute surface (a minute surface corresponding to the minute plane) that becomes a minute plane when projected along the Z axis are obtained.
At this stage, the surface mesh model describing the design surface is completed. According to this processing method, a surface mesh model that describes the entire design surface 21 is created at one time from one continuous plane (a plane obtained by projecting the design surface 21 onto the XY plane). Conventionally, when a design surface is mathematically composed of multiple surfaces, a surface mesh model is created for each surface, and the entire surface mesh model is created by connecting the surface mesh models created for each surface. I was making it. Compared with the conventional method, the time required for creating the surface mesh model describing the design surface can be greatly shortened.

When creating a surface mesh model describing a neutral plane, the CAE analysis apparatus proceeds to the processing from step S16.
In step S16, the average of the thickness of the injection molded product measured in the normal direction is calculated for each minute surface. If the position of the rib is set in step S12, the thickness is calculated assuming that there is no thickness related to the rib (for example, the vertical dimension of the rib 24 shown in FIG. 5) (that is, the rib 24 is removed). ) Therefore, in the case of the injection molded product of FIG. 5, regarding the minute surface on the left side of the step 23, if the thickness between the design surface 21 and the non-design surface 22 is ha, the average thickness of the minute surface is ha. In addition, even in the portion where the concave portion 22d1 and the convex portion 22d2 are provided, the width (dimension in the X direction) of the concave portion 22d1 and the convex portion 22d2 and the height from the non-design surface 22 (dimension in the Z direction) are the same. The average thickness is ha. Further, regarding the minute surface on the right side of the step 23, when the thickness between the design surface 21 and the non-design surface 22 is hb, the average of the thickness of the minute surface is hb. Then, the CAE analyzer proceeds to the process of step S18.

In step S18, as shown by the alternate long and short dash line in FIG. 5, for each minute surface, an offset minute surface that is offset by a distance half the thickness in the normal direction is extracted. As calculated in step S16, the thickness in the normal direction of the minute surface is different between the left side and the right side of the step 23. Therefore, at this position, the offset minute surface group in which each minute surface is offset in the thickness direction is discontinuous. Become.
In step S20, it is determined whether or not there is a discontinuous portion in the offset minute surface group calculated in step S18. If there is no discontinuous group of minute surfaces, the process proceeds to step S24. If there is a discontinuous group of minute surfaces, the process proceeds to step S22.
In the case of the injection-molded product of FIG. 5, as shown in FIG. 6, at the stepped portion 23, the offset minute surface (for example, triangles L11, L12, L2) and the offset minute surface (triangles U11, U12, U2) are discontinuous. ing. Hereinafter, for example, the offset minute surface (L11, L12, L2) indicates a triangular surface having points L11, L12, and L2 as vertices. The offset minute surface (L12, L13, L3) and the offset minute surface (U12, U13, U3) are also discontinuous, and the offset minute surface (L13, L14, L4) and the offset minute surface (U13, U14, U4). ) Are also discontinuous, and the offset minute surfaces (L14, L15, L5) and the offset minute surfaces (U14, U15, U5) are also discontinuous. The offset surface 25a on the left side of the step 23 and the offset surface 25b on the right side are discontinuous. The white circles in the figure indicate the vertices (nodes) of the continuous minute surface, and the black circles indicate the nodes of the discontinuous minute surface. If there is a discontinuous group of minute surfaces, the process proceeds to step S22.

In step S22, the discontinuous offset minute surface is made continuous. Here, for the offset minute surface group discontinuous in the height direction, a minute surface is created by directly connecting the vertex groups adjacent to the vertices having different heights. As shown in FIG. 7, for example, for the offset minute surface (L11, L12, L2) and the offset minute surface (U11, U12, U2) that are discontinuous in the height direction, the vertex U11 and the vertex L11 (with different heights) In addition, the vertex U1 and the vertex L1 adjacent to each other are directly connected. Further, the vertex U2 and the vertex L1 are directly connected. Further, the vertex U2 and the vertex L2 are directly connected. Similarly, vertices U12 and L12 adjacent to different heights, vertices adjacent to vertex U13 and vertex L13 having different heights, vertices adjacent to vertex U14 and vertex L14 having different heights, different By directly connecting the apex U15 having a height and the apex adjacent to the apex L15, the minute surface group is replaced as shown by a one-dot chain line in FIG. As a result, the discontinuous offset minute surface group becomes the consecutive minute surface group (U1, L1, U2), (L1, U2, L2), (U2, L2, U3), (L2, U3, L3). , (U3, L3, U4), (L3, U4, L4), (U4, L4, U5), (L4, U5, L5), (U5, L5, U6), (L5, U6, L6) It is done.
The CAE analyzer returns to the process of step S20 until there is no discontinuous portion.

In step S24, the offset surface continuous in step S22 is used as a surface mesh model describing the neutral surface. As shown in FIG. 8, at this stage, a surface mesh model that describes the neutral surface 26 of the injection molded product 20 from which the ribs 24 have been removed is calculated. Then, the process proceeds to step S26.
In step S26, it is determined whether or not a rib is set in step S12. If no rib is set, the process ends. If a rib is set, the process proceeds to step S28.
In step S28, the front and back surfaces of the rib are set. In the case of the rib 24 shown in FIG. 5, the operator sets the front surface 24a and the back surface 24b of the rib. Then, the process proceeds to step S30.
In step S30, as shown in FIG. 8, the neutral surface 24c of the rib 24 is extracted. At this time, the neutral surface 26 and the neutral surface 24c of the rib 24 seen from the design surface 21 side are discontinuous. Then, the process proceeds to step S32.
In step S32, the operator adds the neutral surface 24c of the rib 24 to the neutral surface 26 using CAE analysis software. Then, the process returns to step S26.
In this way, the neutral surface of the injection molded product 20 is extracted.

For the injection molded product 10, a surface mesh model describing the neutral plane is created by the same processing as that of the injection molded product 20 described above.
In the case of the injection-molded product 10 shown in FIGS. 1 to 3, since both the front surface and the upper surface are projected into one XY plane, the plane is divided into minute plane groups without gaps, and the design is based on this. You can create a surface mesh model that describes a surface or neutral surface at once. Compared with the case where a plurality of surfaces forming the design surface are individually divided into minute surface groups and a neutral surface is created based on this, the time required for model creation can be shortened.
Further, according to the model creation method of the present embodiment, when the rib is provided on the non-design surface, the neutral surface from which the rib is removed may be extracted, and the neutral surface of the rib may be added later. . Therefore, it is possible to easily calculate the neutral surface of the solid CAD model in which ribs are provided on the non-design surface.

In this embodiment, the coordinate axis of the injection molded product 10 is set in the III direction so that the entire design surface 11 of the injection molded product 10 can be seen. The coordinate axis of the injection molded product 20 was set in the Z direction so that the entire design surface 21 of the injection molded product 20 could be seen. In the case of an injection-molded product that cannot set the coordinate axes that allow the entire view to be set, the coordinate axes are set in a direction in which most or the main part of the design surface can be seen.
In the present embodiment, the case of using a surface mesh model that describes the neutral surface of an injection molded product for use in CAE analysis has been described. However, when creating a surface mesh model that describes a design surface, Step 4 up to step S14 may be executed.
In this embodiment, when the offset microfacets that are offset by a distance of half the thickness in the normal direction are discontinuous in the height direction, as shown in FIG. 7, they are adjacent to the vertices having different heights. By substituting the vertices to be connected with the directly connected minute surfaces, the discontinuous minute surface groups in the height direction were converted into continuous minute surface groups. The method of converting the minute surface group discontinuous in the height direction into the continuous minute surface group is not limited to this example, and various methods are conceivable. For example, the midpoint of points U11 and L11, the midpoint of points U12 and L12, the midpoint of points U13 and L13, the midpoint of points U14 and L14, and the midpoint of points U15 and L15 shown in FIG. May be calculated to obtain a minute surface passing through the middle point and passing through a group of vertices adjacent to vertices having different heights.

Specific examples of the present invention have been described in detail above, but these are merely examples and do not limit the scope of the claims. The technology described in the claims includes various modifications and changes of the specific examples illustrated above.
In addition, the technical elements described in the present specification or drawings exhibit technical usefulness alone or in various combinations, and are not limited to the combinations described in the claims at the time of filing. In addition, the technology illustrated in the present specification or the drawings achieves a plurality of objects at the same time, and has technical utility by achieving one of the objects.

A cross-sectional view of an injection molded product (instrument panel) 10 is shown. It is the arrow line view which looked at the injection molded product 10 from the II direction shown in FIG. It is the arrow line view which looked at the injection molded product 10 from the III direction shown in FIG. It is a flowchart figure of the process performed when creating the neutral surface for CAE analysis with the program for CAE analysis. The solid CAD model of the injection molded product 20 in which the part from which thickness differs, and the rib 24 is provided is shown. When the injection molded product 20 is projected in the coordinate axis Z direction and offset in the thickness direction, it is a diagram for explaining a portion where the offset surface is discontinuous. It is a figure explaining the method of connecting the surface which has become discontinuous in FIG. It is a figure explaining the neutral surface of the injection molded product.

Explanation of symbols

10, 20 Injection molded product 11, 21 Design surface 11a Protruding part 11b Hole 12, 22 Non-design surface 23 Step 24 Rib 24a Surface 24b of rib 24 24 Back surface 24b of rib 24 Neutral surfaces 25a, 25b of rib 24 Offset surface 26 Rib Neutral surface of injection molded product 20 from which

Claims (6)

A method of creating a surface mesh model that describes a design surface from a solid CAD model of an injection molded product.
Setting a coordinate axis in a direction in which the design surface of the injection molded product described by the solid CAD model can be overlooked;
Extracting one plane obtained by projecting the design surface on the coordinate plane of the set coordinate axis;
Dividing the plane into small plane groups that divide the plane without gaps;
For each microplane, arranging the data describing the surface of the solid CAD model corresponding to the microplane into data describing the microsurface corresponding to the microplane;
To create a surface mesh model that describes a design surface with A method for creating a surface mesh model that describes a neutral plane from a solid CAD model of an injection molded product.
Setting a coordinate axis in a direction in which the design surface of the injection molded product described by the solid CAD model can be overlooked;
Extracting one plane obtained by projecting the design surface on the coordinate plane of the set coordinate axis;
Dividing the plane into small plane groups that divide the plane without gaps;
For each minute plane, the data describing the surface of the solid CAD model corresponding to the minute plane is organized into data describing the minute surface corresponding to the minute plane, and the normal direction of the minute surface is calculated. When,
Calculating the thickness of the injection-molded product measured in the normal direction from the data of the solid CAD model for each minute surface;
For each micro surface, calculating an offset micro surface offset by a distance half the thickness in the normal direction;
How to create a surface mesh model that describes a neutral plane with   The model creation method according to claim 1 or 2, wherein, in the coordinate axis setting step, the coordinate axis is set in a die cutting direction of the injection molded product. It has a process of removing ribs existing on the non-design surface from the solid CAD model.
4. The model creation method according to claim 2, wherein the thickness is calculated from a solid CAD model from which ribs are removed. Calculating a neutral surface of the rib removed in the removing step;
5. The model creation method according to claim 4, further comprising the step of adding a neutral surface of the rib to the neutral surface calculated by removing the rib.   After calculating the offset minute surface offset by half the thickness in the normal direction, the discontinuous minute surface group in the height direction has apexes having different heights (vertices forming the minute surface). The model creation method according to claim 2, further comprising a step of replacing adjacent vertex groups with a connected minute surface.

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