Virtual processing – application of rapid prototyping for visualization of metal forming processes

Virtual processing ± application of rapid prototyping forvisualization of metal forming processes

Suwat Jirathearanat*, Victor Vazquez, Ciro A. RodrõÂguez, Taylan Altan

ERC for Net Shape Manufacturing, The Ohio State University, Columbus, OH 43210, USA

Abstract

The objective of the study was to build physical models (prototypes) of a product based on ®nite element (FE) process simulations to aid

product engineering in the evaluation of the manufacturing ability of the product. A methodology and two computer programs were

developed to translate the FE geometrical information into a geometric representation acceptable by most rapid prototyping (RP) systems.

In the case of the ERC/NSM, the prototypes are built with a fused deposition modeling (FDM) machine. This report has been focused in the

production of physical models based on the FE simulation results for sheet and billet forming processes. However, the programs can be used

with other process simulations as well. # 2000 Published by Elsevier Science S.A. All rights reserved.

Keywords: Virtual processing; Prototyping; Sheet metal forming processes; Forging process

1. Introduction

During the last decade, process modeling has been

accepted as a powerful tool in the metal forming processes.

FE simulations are used to verify parameters of the forming

process, such as stress and strain distributions in a formed

part. This could be done for various steps of the deformation

sequence. This method may help the engineers to ensure a

successful forming process before expensive tooling is

manufactured. Different steps of the forming sequence

can be observed as a result of FE process simulation.

However, visualization of the intermediate forming steps

is dif®cult due to the complexity of most industrial products.

As an alternative, physical models of formed metal parts

from FE process simulations can assist in visualizing the

formability of the parts. Being able to physically handle the

formed parts is more desirable and eases the understanding

of the simulation results [1].

Recently, the methods to generate physical models by RP

technology have been drastically improved. Models can be

prototyped accurately and directly from 3D solid models in a

highly automated fashion. Integrating RP technology and

the process simulations performed in the die design process

could reduce lead time in die design and manufacturing

process. To achieve this objective it is necessary to manu-

facture solid models of the ®nite element meshes that are the

results of process simulations.

1.1. Previous work on virtual processing

Virtual processing is a process of generating physical

models of intermediate steps of metal forming process FE

simulations. Literature review on the virtual processing

showed only one study that applied RP technology in virtual

processing of metal forming process FE simulation [1].

First, the authors obtained the geometric information of

sheet metal FE mesh from FE simulation results. Second,

they converted it into a geometric representation acceptable

by RP systems. Finally, the data was fed into a laser

stereolithography system for prototyping the formed parts.

1.2. Objective

The objective of the present work was to apply RP

technology to reduce lead time in the development of

manufacturing processes, focusing mainly on forging and

sheet metal forming. Two conversion programs were

designed and developed at the ERC/NSM the Ohio State

University: FEMSTL [2] and PAMSTL [3]. FEMSTL works

with geometric data created either in I-DEAS or DEFORM.

I-DEAS is a commercial CAD program that has capabilities

to simulate structural process and DEFORM is a commercial

program used mainly for the simulation of bulk forming* Corresponding author.

Journal of Materials Processing Technology 98 (2000) 116±124

0924-0136/00/$ ± see front matter # 2000 Published by Elsevier Science S.A. All rights reserved.

PII: S 0 9 2 4 - 0 1 3 6 ( 9 9 ) 0 0 3 1 2 - X

processes. PAMSTL works with geometric data of a shell FE

mesh from PAMSTAMP. This is commercial FE simulation

software for the simulations of the sheet metal forming

processes. Both FEMSTL and PAMSTL convert the FE

mesh data into STL representation, which is the standard

input format for RP systems. Fused deposition modeling

(FDM) system (see Fig. 1) is used to fabricate the virtually

processed parts.

1.3. Fused deposition modeling system

The FDM system builds parts from plastics such as ABS,

nylon and investment casting wax. The material ®lament is

fed into the head (extruder) where the material is lique®ed

with a temperature just above its melting point. The semi-

liquid plastic is then deposited onto the base to make the ®rst

layer of the part. Then, the platform moves down a distance

of one layer thickness for the construction of the next layer.

The temperature of the previous layer has to be maintained

just below the melting point of the material for good

adhesion with the next layer. The successive layers are built

in the same way until the entire part is completed.

2. Process simulations for sheet metal forming andforging operations

At the ERC/NSM, 3D sheet metal forming simulations are

performed with PAMSTAMP. PAMSTAMP is dynamic

explicit ®nite element analysis software for sheet metal

forming process simulations. Elastic±plastic shell elements

are used for the sheet metal mesh, while rigid shell elements

are used for the tooling mesh. Fig. 2 shows the FE meshes of

tooling and sheet for simulations of a sheet hydroforming

process. The tooling and sheet metal meshes are assembled.

The material properties are assigned in the preprocessor

(GENERIS). Boundary conditions such as blank holder

force, punch speed, and friction are also de®ned.

The results are displayed in the postprocessor (PAM-

VIEW). Undesirable part defects such as splitting and

wrinkling can be displayed. The splitting can be detected

from the distributions of strains in the part. If necessary, the

forming process can be re®ned by changing the process

parameters (i.e., boundary conditions) or the tooling geo-

metry.

Bulk forming simulations are performed mainly with

DEFORM 2D and DEFORM 3D. DEFORM 2D is used

in the simulation of two-dimensional cases, that can be

simpli®ed with axisymmetric and plane strain models. As

shown in Fig. 3, the forward extrusion can be simulated with

an axisymmetric model. Fig. 4 shows the forming of con-

necting rod I-beam section, which can be simpli®ed with a

plane strain model. The FE mesh of these models consists of

quadrilateral elements, which have four nodes each.

Fig. 1. Fused deposition modeling system [4].Fig. 2. FE Mesh of a hydroforming simulation [3].

Fig. 3. Axisymmetric extrusion of cylindrical shaft [2].

S. Jirathearanat et al. / Journal of Materials Processing Technology 98 (2000) 116±124 117

Some problems that must be simulated with DEFORM 3D

are shown in Fig. 5. It shows the basic geometry and

simulation model of a cross groove inner race for a constant

velocity joint (CVJ). These FE simulations were performed

using 3D solid elements. These elements can have a brick or

a tetrahedral shape.

3. Methodology to transform FE model into STL model

The process of transforming the results of FE models from

sheet metal forming and forging simulations into STL

models is described in Fig. 6. In general, both types of

FE meshes (i.e., 3D shell FE mesh from sheet metal forming

simulation and 3D solid FE mesh from forging simulation)

undergo similar procedures to become STL models. Due to

the fact that 3D shell FE mesh could be an open surface,

PAMSTL was developed to generate a closed surface FE

mesh and transform the FE mesh into an STL model. Since

FE meshes from forging simulations are already solid

models, the closed surface FE meshes can be readily

extracted from the original solid models in I-DEAS or

DEFORM. Therefore, FEMSTL was only designed to trans-

form shell meshes into STL models (see Fig. 6).

After the STL models have been generated, QuickSlice

(RP tool path generator of FDM machine) is used to prepare

the models for prototyping. At this stage, in QuickSlice,

minor defects such as missing triangular facets on the STL

model can be repaired. The FDM machine follows the tool

paths generated by QuickSlice to build the virtually pro-

cessed parts.

3.1. Methodology to transform shell FE mesh into STL

model (sheet forming simulation, PAMSTL)

The STL ®le format allows the representation of an open

surface geometry. However, the software packages for RP

tool path generation require closed surface geometry. This is

due to the nature of the building process of RP systems,

where a slice of the part geometry is taken and then ®lled

with a material.

In PAMSTAMP, the mesh does not represent the actual

thickness of the part. Therefore, this mesh is an open surface.

Fig. 7 shows the methodology to generate a closed surface

mesh from a resulting mesh of a deep drawing operation. A

closed surface mesh of the part, which would represent the

part thickness, is created by ®rst offsetting the existing

surface mesh along the local normal directions. Then, the

offset and original surfaces are `̀ stitched'' together by

surfaces along the boundaries. Finally, the closed surface

is transformed into STL ®le format.

Step 1: Offsetting of surface mesh. GENERIS has the

capability to offset the surface mesh. In PAMSTAMP, a shell

FE mesh is de®ned as the middle surface of the real sheet

thickness. Ideally, to create a closed surface mesh of a real

sheet metal part from the simulation results, one has to

create offset surfaces on both sides of the original surface.

Sheet metal parts are generally thin (in the neighborhood of

1±2 mm). Therefore, an offset surface on only one side of

the original surface is a reasonable approximation for

visualization purposes (see Fig. 7).

Fig. 4. Plane strain 2D simulation of the I-beam section of a connecting

rod and the FEM model [5].

Fig. 5. 3D simulation model of a cross groove inner race for a constant

velocity joint inner race [6].

Fig. 6. Virtual processing flow chart for sheet metal and bulk forming

processes.

118 S. Jirathearanat et al. / Journal of Materials Processing Technology 98 (2000) 116±124

There are some limitations to offsetting of surface mesh.

Normally, curved surfaces such as rounded and ®lleted

corners cannot be offset by distances larger than their

minimum radii. Thus, deformed sheet FE meshes with

intricate geometry (such as those formed with severe wrink-

ling) are dif®cult to offset.

By offsetting the existing surfaces to create the sheet

thickness, a prototyped part will have the same thickness

over the entire part. These prototyped parts fail to represent

the thickness of the real deformed parts, which varies accord-

ingly to strain distributions over the surfaces. However, these

prototyped parts are still valuable for visualization purposes.

After surface mesh offsetting, GENERIS generates a PS

®le that contains the geometry of both the original mesh and

offset mesh. The boundary closing and STL ®le generation is

performed by PAMSTL. The main functions of PAMSTL

are illustrated in Figs. 8±10 (a circular plate FE mesh with

quadrilateral elements is used as an example to describe the

functions of PAMSTL).

Step 2: Determination of the boundary edges of the mesh.

In this step, PAMSTL searches for all nodes located on a

boundary of an original FE mesh. These boundary nodes will

later be used for closing the gap between the original FE

mesh and its offset mesh. Fig. 8 shows how PAMSTL

searches for the boundary nodes on the original FE mesh.

The concept is to use unit vectors calculated from the lines

connecting the boundary node candidates to the adjacent

nodes for determining the boundary nodes.

Step 3: Closing the gap between original mesh and offset

mesh. In this step, PAMSTL `̀ stitches'' the original FE mesh

and its offset mesh together by creating surfaces closing the

gap between them (see Fig. 9). Straight lines are generated

Fig. 7. Methodology to generate a closed shell FE mesh and transform it

into STL model [3]. Fig. 8. PAMSTL ± boundary node searching operation [3].

Fig. 9. PAMSTL ± gap closing operation [3].


to connect corresponding boundary nodes between the

original mesh and the offset mesh. The nodal numbers of

the offset mesh are always offset by a certain number, such

as the total number of the nodes on the original mesh.

Step 4: Converting FE mesh into STL representation. In

this step, PAMSTL converts the original mesh, the offset

mesh and the closing surfaces into STL format. In the

meshes, every element (quadrilateral shell element) is bro-

ken into two triangular facets and written into the equivalent

STL facets. Fig. 10 shows the conversion of FE mesh into

STL representation.

Accuracy issues. STL ®les converted from FE mesh have

some accuracy related drawbacks in de®ning their solid

models. The ®rst reduction of accuracy is caused by the

nature of meshing process (discretization) itself. Discretiza-

tion process is a process of breaking a solid model into a

number of very small elements. Therefore, it is always an

approximation, where the accuracy of the model depends on

the sizes of the elements used.

Transformation of the deformed FE mesh into STL format

using PAMSTL further reduces the accuracy. This is because

a quadrilateral shell element would not remain planar once

deformed. Consequently, splitting the quadrilateral element

might generate two triangular facets on different planes.

This results in faceted appearance. One way to improve the

accuracy and the faceted surface effect is using very small

elements in the discretization process of the FE simulation.

This could also be accomplished by remeshing the part at the

®nal forming step, thus avoiding the penalty in calculation

time caused by using a large number of elements.

3.2. Methodology to transform solid FE model into STL

model (forging simulation, FEMSTL)

In general, the outer surface of a 3D FE mesh could have

either quadrilateral or triangular facets. These facets can be

considered as the units that form the surfaces of a solid

model in the STL format. The surface boundary is extracted

from a model consisting of either tetrahedral or brick

elements, the result is a closed shell FE mesh. This is formed

by triangular or quadrilateral facets, respectively. If the

triangular facets are obtained, the normals can be calculated

for each facet and the STL ®le can be written using this

information. If the quadrilateral facets were extracted from

the solid, each of the rectangular facets have to be divided

into two triangular facets and also the normal of each facet

should be calculated to write the STL ®le. FEMSTL was

developed to transform the FE meshes with both triangular

elements and quadrilateral elements into STL models.

4. Applications of virtual processing

PAMSTL was designed to transform a shell FE mesh into

four different shapes in STL representation mainly based on

the quality of the sheet FE mesh, see Fig. 11. The sheet

surface option is suitable for FE meshes with relatively

simple geometry so that successful offset for the meshes

can be generated. The die surface option is suitable when the

successful offset mesh cannot be created, especially in the

case of intricate FE meshes. The open surface option should

be used to transform troublesome sheet FE meshes (i.e.,

sheet FE meshes with missing elements) into STL format.

Then, it can be repaired and processed for prototyping with

QuickSlice, FDM front-end software. The following exam-

ples are the applications of some of the different outputs

from PAMSTL in sheet metal forming. Some application

examples of FEMSTL in forging operations are presented as

well.

Fig. 10. PAMSTL ± operation to convert FE mesh to STL representation

[3].

Fig. 11. PAMSTL ± inputs and outputs [3].


4.1. Virtually processed sheet metal parts ± sheet shape

To test the program PAMSTL, simulation results of

viscous pressure forming (VPF) process were selected.

The idea was to use PAMSTL, as part of virtual processing,

to generate prototyped parts of the successful and unsuc-

cessful VPF process simulations.

The prototyped parts from both simulations physically

show the effects of initial process parameters on the ®nal

products (see Fig. 12). In this case, the geometry and

position of the blank relative to the die cavity at the begin-

ning of the process determined the success or failure of the

forming process. Not having enough material to be formed

into the cavity resulted in wrinkles on the defective parts. By

properly modifying the blank geometry and repositioning

the blank at the beginning of the process, a good part could

be obtained.

Another example from a stamping process simulation is a

decklid outer panel, which was simulated for predictions of

lose material, see Fig. 13. Due to the small wrinkles of the

part, the offset surface could not be generated successfully.

However, to create the thickness of the part, a duplicate of

the original surface was generated and translated from the

original in Z direction. This physical model reveals the

wrinkles, which were not easy to recognize on the ¯at

screens. The physical inspection on this model eases the

process engineers understanding of the process defects,

hence, improvements can be achieved by modifying the

process.

4.2. Virtually processed sheet metal parts±die shape

As mentioned earlier, a successful offset of surface mesh

is not always easy to generate. In cases where the FE mesh

Fig. 12. Virtually processed deep drawn part using viscous pressure (VPF) forming technique (sheet-shaped prototype) [3].

Fig. 13. Virtually processed deck lid (sheet-shaped prototype) [3].


has an intricate geometry (local radii smaller than the offset

distances), the offset surface mesh tends to have defects.

In this example, a sheet FE mesh of a fender was

transformed into an STL ®le, and the prototyped part was

made, Fig. 14. This FE mesh had intricate geometry and

large size, too large to be built in the FDM machine.

Therefore, the part was scaled down for prototyping. In this

case, the offset distance required had to be relatively large to

compensate for the scaled-down version of the part. Unfor-

tunately, the successful offset could not be generated with

the required offset distance.

An alternative to create a closed surface from such a

troublesome FE mesh is provided in PAMSTL. It can

generate a die-shaped surface, in STL format, from an open

FE mesh. With this technique, the troublesome FE mesh can

still be useful for visualization purposes. The die-shaped part

was built with the FDM machine, Fig. 14.

4.3. Virtual processing to visualize defects in axisymmetric

forged components

In the case of axisymmetric simulations, the models are

simpli®ed with a 2D mesh of axisymmetric solid elements.

These elements lie on a plane and are geometrically refer-

enced by an axis of revolution. However, sometimes the

user may want to make a physical model from these

simulations when a speci®c defect appears. Fig. 15 shows

the sequence of deformation for semisolid forging of an

aluminum alloy. Notice that at the end of deformation

the material folds onto itself, thus producing a defect similar

to a crack. The severity of this defect would depend on the

application for which the forged part has been designed.

Fig. 16 shows how the 2D mesh has been revolved around

the axis of symmetry to create a solid mesh, which is later

transformed into a STL solid. Fig. 17 shows the fold defect

as displayed by the FDM part.

4.4. Virtual processing of complex multiaction forged

component

3D FE simulation of forging processes requires the sim-

pli®cation of a complex part to reduce computation time.

Depending on the symmetry planes, a part can be divided

into two or more parts. Thus, it would only be enough to

simulate a half or less of the model in order to get reliable

results. In these cases it is not possible to obtain the surface

for the whole object from the simulation. Hence, it is

necessary to ®rst generate the whole part using the symmetry

planes to re¯ect the simpli®ed geometry. Fig. 18 shows the

sequence of deformation of the FE meshes of a tripod (part

commonly used as inner race in universal joints or differ-

entials). Fig. 19 shows the FDM parts of a simulated tripod

formed in different steps.

Fig. 14. Virtually processed Fender (die-shaped prototype) [3].

Fig. 15. Sequence of deformation in the axisymmetric simulation of

semisolid forging of an aluminum alloy [7].

Fig. 16. Axisymmetric mesh and solid brick mesh created based on the

axisymmetric mesh [2].


5. Discussion

In present work, the objective was to implement virtual

processing with the application of RP physical models for

the geometric visualization of formed parts, mainly from

sheet metal forming and forging processes. Virtual proces-

sing of formed parts bene®ts the process of die design and

manufacturing as follows:

� In cases where die geometry is complex, the FE simula-

tion results could be more visible to manufacturing

engineers through physical models.

� Die try-out costs could be reduced due to the capability to

generate sound die design in the early stages.

� Die production lead time could be reduced due to

enhanced communication among the people involved

in die design and manufacturing.

The work completed in this study includes:

� A methodology to generate closed surface geometry from

open surface geometry (sheet metal FE mesh) was devel-

oped.

� Software (PAMSTL) was designed and implemented to

convert sheet metal FE mesh (from PAMSTAMP) into

STL representation.

� Software (FEMSTL) was designed and implemented to

convert bulk FE mesh (from DEFORM) into STL repre-

sentation.

References

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Fig. 17. Virtually processed part of the axisymmetric mesh with a fold defect.

Fig. 18. Deformation sequence for cold multiple action forging of a tripod

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objects (bottom row) [8].

Fig. 19. Virtually processed parts at different deformation steps of the cold

multiple action forging of a tripod for a CVJ.


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Virtual processing – application of rapid prototyping for visualization of metal forming processes

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