Comparison of Sheet-Metal-Forming Simulation and Try-Out Tools in The Design of A Forming Tool
Comparison of Sheet-Metal-Forming Simulation and Try-Out Tools in The Design of A Forming Tool
Comparison of Sheet-Metal-Forming Simulation and Try-Out Tools in The Design of A Forming Tool
forming tool
Andersson, Alf
Published in:
Journal of Engineering Design
DOI:
10.1080/09544820410001697598
2004
Link to publication
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Accepted for publication in Journal of Engineering Design, 2004, Vol. 15, No. 3.
Comparison of sheet-metal-forming
simulation and try-out tools in the design of a
forming tool
Andersson A*,**
*
Volvo Car Corporation, Body Components
**
Division of Production and Materials Engineering, Lund University
Abstract
Today, sheet-metal-forming simulation is a powerful technique for predicting
the formability of automotive parts. Compared with traditional methods such
as the use of try-out tools, sheet-metal-forming simulation enables a significant
increase in the number of tool designs that can be tested before hard tools are
manufactured. Another advantage of sheet-metal-forming simulation is the
possibility to use it at an early stage of the design process, for example in the
preliminary design phase. Today the accuracy of the results in sheet-metal-
forming simulation is high enough to replace the use of try-out tools to a great
extent. At Volvo Car Corporation, Body Components (VCBC), where this
study has been done, sheet-metal-forming simulation is used as an integrated
part in the process of tool design and tool production.
1 Introduction
Traditionally, try-out tools are used in order to verify that a certain tool design
will produce parts of the required quality. The try-out tools are often made of a
cheaper material, e.g. kirksite, than production tools in order to reduce the try-
out costs. This is a very time- and cost consuming method. However, today
another more efficient technique is available – sheet-metal-forming simulation.
This new technique is based on the simulation of the forming process, and
could result in a cost reduction of factor 10 and a time reduction of factor 15
for each hard tool.
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A. Andersson
2 Method
The purpose of this study is to analyse and compare the benefits and
drawbacks of the use of sheet-metal-forming simulation and try-out tools in
the design of forming tools. The method employed in this study is based on the
Production Reliability Matrix (Rundqvist and Ståhl, 2001) together with a
Process Correspondence Matrix which has been developed especially for this
study. The Production Reliability Matrix (PSM) is a matrix that categorises the
effects of different factors (parameters) in the process into different factor
groups. The effect of each factor (parameter) is then assessed according to a
scale of 0-3. Based on the results of the matrix, the parameters that have the
most considerable effects on the production process can be extracted and a
priority list for neutralising or minimising these effects can be made. The
Process Correspondence Matrix (PCM) has been developed through extensive
interviews of senior experts in automotive component forming to analyse the
correspondence between the results of sheet-metal-forming simulations, the
try-out tool and the quality of produced parts in actual production.
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Comparison of sheet-metal-forming simulation and try-out tools
Try-out
tool
Sheet-
metal-
forming
simulation
The process of the design of a forming tool includes a try-out phase where
different designs of the tool are tested. This is a very important stage in the
tool design process, in order to verify that the part will fulfil the required
quality. It is very difficult to predict the result of a forming operation, but by
using sheet-metal-forming simulation there is a possibility to gain valuable
insight into the outcome of the forming operation.
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A. Andersson
Simulation software
Today there is a variety of commercial software available on the market. In
order to find suitable software the area of use must be analysed. The software
package is different with regard to user-friendliness and flexibility.
At VCBC, where this study was performed, two different software packages
are used. One is Autoform (2001) which is user-friendly and provides fast
results. This software is used for the iterative process of finding the proper tool
geometry. The other software is LS-DYNA, which is used at VCBC to verify
the results of Autoform.
CAD-model
In order to analyse a part or a tool design using sheet-metal-forming
simulation, a CAD-model of the part or tool is needed. This model can be
created in most CAD-programs, for instance CATIA, which is used at VCBC.
Different simulation software demand different qualities of the CAD-models.
Material parameters
Uniaxial tensile tests are used to describe the material parameters. There is
also a need for describing the risk of fracture in the material. Data regarding
risk for fracture are obtained by creating a forming limit curve (FLC). The
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Comparison of sheet-metal-forming simulation and try-out tools
FLC is a curve in the plane of principal strains that indicates the maximum
allowed strain values before fracture occurs. A more thorough description is
presented in Pearce (1991).
Process parameters
Sheet-metal-forming simulation requires proper process parameters e.g.
drawbeads.
Workstations
The simulation models that are used in sheet-metal-forming simulation are
generally so large that they require a workstation in order to achieve
reasonable calculation times. However the development of PC’s enables the
clustering of several PC’s which can be an alternative to workstations.
Competent personnel
In order to interpret the results of a sheet-metal-forming simulation it is
necessary to enter the correct input data, and possess the ability to understand
the results. This requires competent personnel. The competence should consist
of both forming knowledge and simulation knowledge since that gives a
natural connection between the production process and the interpretation of the
results.
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A. Andersson
Thickness distribution
The sheet-metal-forming simulation can provide a good approximation of the
thickness distribution for a part, see appendix 1 figure 2. In the automotive
industry there are requirements concerning the maximum allowable reduction
in thickness, in order to ensure safety margins in the event of a crash.
Draw lines
Draw lines occur when a section of an part has been gliding over a radius
during forming. A plot of how a point on the part surface moves during the
simulation (see appendix 1, figure 4) illustrates these lines. Draw lines are not
acceptable on a visible surface on an exterior part
Wrinkles
Visible wrinkles are not allowed on a part. These can be detected with sheet-
metal-forming simulation, see appendix 1 figure 6.
Forces
In order to dimension the process in an accurate way it is necessary to know
which forces are necessary to form the part. The data for these forces can be
obtained from the results of a sheet-metal-forming simulation.
Surface defects
Exterior automotive parts are sensitive to deflections of the surface that can
occur during forming. These deflections can be very small but can still be
visible after the part is painted, which means that the part must be scrapped.
The defects can be detected by the human hand as it moves gently across the
surface. Sheet-metal-forming simulation can be used for detecting risk areas
through analysis of the strain distribution.
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Comparison of sheet-metal-forming simulation and try-out tools
simulation can be used for detecting risk areas through analysis of the strain
distribution, see appendix 1 figure 6.
Process surveillance
In sheet-metal-forming simulation the process can be followed in detail by
means of animations. Figure 6 in appendix 1 illustrates this.
Draw in
To minimise material consumption, it is important to optimise the shape of the
blank. Sheet-metal-forming simulation can facilitate optimisation of the blank
by analysing the draw in.
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A. Andersson
5 Result
The technique of using try-out tools has been compared with the technique of
using sheet-metal-forming simulation from two aspects. The first aspect is a
comparison of the ability to predict the different parameters of the production
process, mentioned in section 3. The second aspect is the ability to verify
which process parameters should be studied.
Process surveillance
Material properties
Blankholder force
Forming window
Risk for fracture
Stiffness in part
Surface defects
Forces-punch
Springback
Drawbeads
Draw lines
Wrinkels
Fracture
Draw in
Process
Simulation 4 4 4 4 4 2 2 2 4 4 4 3 2 2 4
Try-out tool 3 3 4 3 4 4 4 3 2 3 4 3 4 3 3
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Comparison of sheet-metal-forming simulation and try-out tools
Comments to table 2.
• The difference between risk for fracture and actual fracture is that risk for
fracture shows areas which have not cracked but where necking has
appeared.
• The parameter “Material characteristics” refers to the ability to predict the
quality of the part depending on variation in the material quality.
• Process surveillance enables the monitoring of how different parameters
change during the process.
• The forming window is an aid for detecting how sensitive the process is to
disturbances.
• The values for the tool-forces are based on the assumption that it is
possible to measure the forces in the try-out press.
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A. Andersson
Sheet-metal-forming
Try-out tool
simulation
Factor groups
A Tooling
A1 Tool geometry 2 2
A2 Microgeometri/Surface 0 1
A3 Drawbeads 1 2
B Material
B1 Thickness distribution 2 2
B2 Risc for fracture 2 2
B3 Draw lines 2 2
B4 Wrinkels 2 2
B5 Surface defects 1 2
B7 Surface stability 1 2
B8 Springback 1 2
B9 Material properties 2 2
B10 Draw in 2 2
B11 Surface roughness/galling 0 2
C Process
C1 Press velocity 1 2
C2 Temperature 0 1
C3 Lubricant 1 2
C4 Forces - punch 2 2*
C5 Forces - blankholder 2 2*
C6 Forming window 2 2
D Human factor
D1 Control 1 2
D2 Change frequence 1 2
E Maintenance
E2 Press maintenance 1 1
F Special factors
F1 Tool cleaning 0 2
G Misc. equipment
G1 Handling equipment 1 3
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Comparison of sheet-metal-forming simulation and try-out tools
6 Conclusions
The use of sheet-metal-forming simulation leads to a significant reduction in
both cost and time compared with the use of try-out tools. The requirement is
that the respective parameter for study (see section 3.1.2) demonstrates good
correspondence between simulation and actual production processes. Sheet-
metal-forming simulation is also superior to try-out tools with regard to
predicting and verifying the forming process.
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A. Andersson
7 Comments
At VCBC, where this study was done, sheet-metal-forming simulation is today
a natural part of the tool design process. Sheet-metal-forming simulation has
been used in manufacturing since 1995 and the experiences have been very
good. Today all processes that are so complex that it is difficult to choose
process conditions based on experience are simulated. During the development
of the Volvo S80, which was the first car project to use simulation technology
in full scale, it was established that there was a significant decrease in
problems in the process when it was introduced in actual production for the
first time.
Acknowledgement
I would like to express my gratitude to my colleagues at VCBC, who have
contributed much valuable information and interesting discussions during this
work. I would also like to thank Professor Jan-Eric Ståhl (Division of
Production and Materials Engineering, Lund University) and Professor Kjell
Mattiasson (Chalmers University of Technology) for their support and
constructive criticism of the article.
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Comparison of sheet-metal-forming simulation and try-out tools
References
Andersson A., Assarsson J. and Ingemansson A., 1999, Front Fender –
Aluminium versus steel, a study in PROPER’s research areas at VOLVO,
Working paper: LUTMDN/(TMMV-7023)/1-18/2001, University of Lund.
Lund, Sweden.
Pearce R., 1991, Sheet Metal Forming, Adam Hilger, IOP Publishing Ltd,.
Great Britain.
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A. Andersson
Figure 2: Thickness distribution. The scale shows blue for 20% thinning and red
for 10% thickening.
Figure 3: Risk for fracture. In this image, cracks are shown in red. To the right is
the forming limit curve represented by the black line. Shown also are the results
of the simulations (blue points).
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Comparison of sheet-metal-forming simulation and try-out tools
Figure 4: The blue line in the image shows how the material has flowed during
the forming operation. If the material has flowed over a radius, a draw line will
appear on the part. If the draw line appears on a visible surface of an exterior
part, the part will be rejected for quality reasons.
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A. Andersson
Formability
Figure 5: In the images above, which describe formability, surfaces with enough
strains to be stable can be seen. By studying these images together it is possible to
estimate the stability of the surfaces. The upper image shows the formability. The
grey areas in the lower image indicate unstable surfaces and the pink area
indicates wrinkles. In the lower image the surfaces with small strains are marked
with blue, which indicates compression. If these areas are located on a visible
surface of an exterior part there is a risk for unstable areas.
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Comparison of sheet-metal-forming simulation and try-out tools
Process after
blankholder closing
Process 170mm
from bottom
Wrinkles
Process 93mm
from bottom
Process at bottom
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A. Andersson
Figure 7: The line shows the sheet position after blankholder closing. The draw
in can then easily be measured by a comparison with the line in bottom position.
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