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Comparison of sheet-metal-forming simulation and
try-out tools in the design of a forming tool
A. ANDERSSON
Today, sheet-metal-forming simulation is a poAwerful 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, where this study has been carried out, 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 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-consuming 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. Sheet-metal-forming simulation technology is constantly developing and the results of the simulations are
more and more accurate. In the future it will also be possible to analyse more processes using sheet-metal-forming simulations. Today, the accuracy of the results in sheetmetal- forming simulation is high enough to replace the use of try-out tools to a great extent.
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 (PSM) (Rundqvist and Sta°hl 2001) together with a Process Correspondence Matrix (PCM) that has been developed especially for this study. The PSM is a matrix that categorizes 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 neutralizing or minimizing these effects can be made. The PCM has been developed through extensive interviews of senior experts in automotive component forming to analyse the correspondence between the results of sheet-metalforming simulations, the try-out tool and the quality of produced parts in actual production.
3 Process for designing a forming tool
Figure 1 shows a simplified flow of the production process of developing a forming tool at Volvo Car Corporation, Body Components (VCBC).
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.
3.1 Use of sheet-metal-forming simulation
Sheet-metal-forming simulation can be used in several stages of a tool design process:
●early in the preliminary design phase, to enable rapid verification of different proposals for the design of automotive components
●to improve an existing process.
Preliminarydesign of part
Part
layout
Hard forming tools/Process design
Try-out
tools
Sheet metal forming simulation
Figure 1. Process for designing a forming tool at VCBC.
3.1.1. Requirements for sheet-metal-forming simulation.
Sheet-metal-forming simulation requires the following:
●Simulation software.
●A computer-aided design (CAD) model of the part layout or a CAD model of the forming surfaces of the tool.
●Parameters for description of the specified sheet-metal material.
●Process parameters.
●Workstations (today the development of the personal computer (PC) is rapidly advancing so that PCs will be a strong alternative in the future).
●A competent staff that can handle the software and analyse the results of the simulation.
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 (2001), 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. The forming limit curve 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 PCs enables the clustering of several PCs, 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.
Thickness(mm)
Rp0.2yield strength(Mpa)
Rm ultimate
tensile strength
(MPa)
n value
(average)
R value
(average)
0.8
140
320
0.243
1.76
Table 1 Material data for V-1158.
3.2. Results of a sheet-metal-forming simulation
Sheet-metal-forming simulation enables the study of:
●Thickness distribution.
●Risk of fracture.
●Draw lines.
●Wrinkles.
●Drawbeads/ blankholder pressure.
●Surface defects.
●Stability of the surface.
●Springback.
●Material behaviour.
●Process surveillance.
●Draw in.
●Forming window.
●Forces (punch, blankholder).
In order to demonstrate possible results, a simulation of a Body Side Outer from a Volvo S80 has been studied. The material used for this automotive component is a mild steel with good formability (V-1158). Material data are presented in table 1.
3.2.1. Thickness distribution.
The sheet-metal-forming simulation can provide a good approximation of the thickness distribution for a part (see 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.
Figure 2. Thickness distribution. The scale shows blue for 20% thinning and red for 10% thickening.
3.2.2. Risk for fracture. Risk for fracture during the forming process could be evaluated by means of a forming limit curve, which was described earlier in this section.
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)
3.2.3. Draw lines. Draw lines occur when a visible section of an exterior part has been gliding over a radius during forming. A plot of how a point on the part surface moves during the simulation (see figure 4) illustrates these lines. Draw lines are not acceptable on a visible surface on an exterior part.
In figure 5, which describes 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. This is a simplified analysis. A more detailed analysis would include the interaction between stresses and strains for the complete part.
Figure 4. The blue dark 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.
Figure 5. The images show an example of surveillance of the process.
It is easy to follow how the wrinkles develop during the forming process.
3.2.4 Wrinkles. Visible wrinkles are not allowed on a part. These can be detected with sheet-metal-forming simulation (see figure 6).
3.2.5 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.
3.2.6 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 stress strain distribution.。
3.2.7 Stability of the surface. Stable surfaces are required in order to increase the stiffness of the part to prevent the part from becoming unstable and vibrating. Sheet-metal-forming simulation can be used for detecting risk areas through analysis of the strain distribution. Figure 6 describes a simplified analysis. A more detailed analysis would include the interaction between stresses and strains for the complete part.。
Figure 6. The upper image shows the formability.
The grey areas in the upper image indicate unstable surfaces and the pink area indicates wrinkles. In the lower image the surfaces with small strains are marked blue, which indicates compression. If these areas are located on a visible surface of an exterior part, there is a risk for unstable areas.
3.2.8 Springback. Springback is a phenomenon that could be described as a change in geometry that occurs after the parts have been removed from the forming tool. This g eometry change causes mismatch for the part when it is assembled with other parts.
3.2.9 Process surveillance. In sheet-metal-forming simulation, the process can be followed in detail by means of animations. Figure 5 illustrates this.
3.2.10 Draw in. To minimize material consumption, it is important to optimize the shape of the blank. Sheet-metal-forming simulation can facilitate optimization of the blank by analysing the draw in (see figure 7).
3.2.11 Forming window. A forming window could be described as the allowable variation of the process parameters in order to keep the quality of the produced parts.
3.3. Use of try-out tools
Try-out tools are used when the design of the process is to be verified (see figure 1).Based on this design the try-out tools are then cast in kirksite, for example. Prototype parts are then produced from this try-out tool. There are several differences between a try-out tool and a production tool. One is that the try-out tool wears out much faster than a production tool. Therefore, it is not possible to produce so many parts in a try-out tool. Another difference is that a try-out tool is much cheaper than a production tool. However, since there are differences between the two types of tools, there is no guarantee that the parts produced in the two types of tools will have the same quality..
The PSM can be used to determine which parameters have significant effects on the stability of the process. It is also possible to determine the extent of an effect. This provides valuable help in the identification of the most severe problems. These severe problems are especially interesting since they are the most cost-effective when solved. A more detailed description of the PSM is presented by Rundqvist and Sta°hl (2001). An example where the PSM is applied is presented in Pettersson (1991), where the PSM is used to analyse different processes at VCBC.
Figure 7. The cyan line shows the sheet position after blankholder closing.
The draw in can then easily be measured by a comparison with the line in the bottom position.
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.
5.1 Study of agreement of predicted process with production process
The PCM allows a clear comparison between try-out tools and simulation regarding correspondence with the production process. Table 2 presents the different fields of applications for the different techniques together with the ability to predict behaviour in the production process. The values in table 2 have been determined through extensive interviews with senior forming experts.
In table 2 the following scale is used:
5 The results show perfect agreement with the production process.
4 The results show good agreement with the production process. Special cases can deviate.
3 The results show good agreement in most cases with the production process.
2 The results show good agreement in certain cases with the production process. Indirect interpretation of the results is needed.
1 The results show no agreement with the production process. It cannot be used for process prediction or verification.
Comments on table 2 include the following:
● The difference between risk for fracture and actual fracture is that risk for fracture shows areas that 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.
process
Thickness distribution
Risk for fracture
Fracture
Draw lines
Wrinkles
Surface defects
Stability of the surface
springback
Material properties
Process surveillance
Draw in
Force-punch
Draw beads
Blankholder force
Forming window
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
Table 2. Process Correspondence Matrix (PCM): correspondence with the production process.
5.2 Study of which factors in the production process are possible to analyse
The concept of grouping different factors that are typical for the production process into different factor groups has been used in this study according to the PSM model. In a previous study (Andersson et al. 1999), different factors concerning the forming of aluminium were studied. This work has been modified in order to facilitate a comparison between the two techniques for prediction and verification considered in this study; namely, sheet-metal-forming simulation and try-out tools. See table 3 for the results.
In table 3 the following scale is used:
3 The results show perfect prediction of production process.
2 The results show direct prediction of production process.
1 The results show indirect prediction of production process.
0 The results cannot predict production process at all.
5.3. Restriction /expansion of test possibilities
An analysis of tables 2 and 3 shows several advantages of using sheet-metal-simulation in the tool design process. However, one of the biggest advantages of sheetmetal- forming simulation is that it enables the testing of many different designs of the part, tool or process, which generates substantial savings in costs and time. In this respect, try-out tools are more limited and expensive, which means that only a minimum number of try-out tools are produced. The use of try-out tools contributes to a restriction of test possibilities while the use of sheet-metal-forming simulation contributes to an expansion in test possibilitie
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.
The investment requirements are relatively small when starting to implement sheet-metal-forming simulation. It is necessary to invest in a workstation and software, which cost about SEK 500,000. In addition, it is necessary to have competent personal for handling the sheet-metal-forming simulation. Compared with the investment for one try-out tool (.SEK 500,000 per tool), it is clear that there is a lot to gain in reducing cost and time if sheet-metal-forming simulation is used when it is suitable.
Factor gruups
Sheet-metal-forming
simulation
Thy-out tools
A
Tooling
A1
Tool geometry
2
2
A2
Microgeomertril/Surface
0
1
A3
Drawbeads
1
2
B
Material
B1
Thickness distribution
2
2
B2
Risk for fraction
2
2
B3
Draw lines
2
2
B4
Wrinkles
2
2
B5
Surface defects
1
2
B7
Surface stability
1
2
B8
Springback
1
2
B9
Material properities
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
Lubrication
1
2
C4
Forces-punch
2
2
C5
Forces-blankholder
2
2
C8
Forming-window
2
2
D
Human factor
D1
Control
1
2
D2
Change frequency
1
2
E
Maintenace
E2
Press maintenace
1
1
F
Special factors
F1
Tool cleaning
0
2
G
Misc equipment
G1
Handling equipment
1
3
Table 3. The possibilities to predict different factors (parameters) in the production process
compared in a Production Reliability Matrix (PSM)
Assuming the possibility of measuring forces in the try-out tool.
As stated earlier, 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. The use of try-out tools in the tool design process may be necessary for some time to come to verify some process parameters, but the following advantages are closely associated with sheet-metal-forming simulation:
● Deeper insight into the process at significantly earlier stages.
● Greater flexibility in testing designs for the part, the tool and