The final quality of sheet metal formed parts like the front fender of the all-new Skoda Fabia is strongly determined by the type of sheet material, tooling material, lubrication type and lubrication amount used in actual production. For Skoda Auto, optimizing the process and lubrication conditions in try-out and mass production is one of the ways to make high-quality parts at an increasingly competitive way. The TriboForm software shifts this working procedure from the production environment to the simulation environment, saving on process development time and costs.
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The TriboForm software is applied to determine the optimal lubrication conditions in the production of the Skoda Fabia front fender. The commonly used constant coefficient of friction is replaced in sheet metal forming simulations with extensive and highly accurate friction data simulated by the TriboForm software. This enables Skoda Auto to perform sheet metal forming simulations including the actual lubricant properties and conditions as used in production at Skoda Auto. Moreover, by simulating the stamping process for varying lubrication amounts, the optimal conditions can be determined numerically to achieve high quality parts in mass production. A selection of technical case results and business case results considering friction and lubrication modeling in stamping simulations of the Skoda Fabia front fender are described below.
Application of the TriboForm software to the front fender of the all-new Skoda Fabia
The quality of the front fender is strongly dependent on the friction and lubrication conditions that are acting in the actual production process. This is well known in try-out and mass-production and often used as a working procedure to optimize the quality of the part by changing the lubrication conditions. For varying lubrication amounts, the front-fender part can either show wrinkling, sink marks or fractures. Moreover, in the course a production run, these quality issues can either appear or disappear based on the changing production and lubrication conditions like tooling temperature or pooling of lubricant during the production run.
The effect of lubrication amount in sheet metal forming simulations using the TriboForm software is shown in the following video’s:
Front fender simulation using a low lubrication amount: fractured parts in production (rejected)
Front fender simulation using a high lubrication amount: wrinkled parts in production (rejected)
Front fender simulation using an optimal lubrication amount: high quality parts in production (accepted)
TriboForm software approach
The TriboForm software is applied to simulate the friction conditions for farying lubrication amounts, i.e. 0.6 g/m2 and 2.0 g/m2. See the figure below. For a lubrication amount of 0.6 g/m2, the friction behavior decreases for increasing contact pressures. The effect of the relative sliding velocity on the frictional behavior is minimal. By increasing the lubrication amount to 2.0 g/m2, a clear decrease of the frictional bahavior for increasing contact pressures and velocities is visible.
The TriboForm software enables the simulation of friction dependent on:
- local contact pressure;
- relative sliding velocity;
- straining in the sheet material;
- and temperature.
The two friction files generated by the TriboForm Analyzer are easily implemented in the sheet metal forming simulation software ESI Pam-Stamp using the TriboForm FEM Plug-In. Although friction is of key importance in the production process, it is currently not considered in detail in industrial sheet metal forming simulations. The current industrial standard is to use a constant (Coulomb) coefficient of friction. Using TriboForm, this constant coefficient of friction is now replaced by the two generated friction files.
Next, two Pam-Stamp simulations of the forming of the Skoda Fabia front fender are performed. The friction conditions in the Pam-Stamp simulation is now dependent on the actual lubrication conditions in production. See tje figure below. For a lubrication amount of 0.6 g/m2, a higher time- and location dependent friction behavior is observed compared to the lubrication amount of 2.0 g/m2. This will strongly affect the simulated part quality as demonstrated below.
Time- and location dependent friction behavior for two lubrication conditions using the TriboForm FEM Plug-In in Pam-Stamp.
Do you want to know more about applying the TriboForm software? Check out the following video:
Application of the TriboForm software
Validation of the simulation results is performed based on front-fender parts taken from the press line at Skoda Auto. The results are presented below for 3 situations:
- Experimentally measured results
- Simulation results using a constant (Coulomb) coefficient of friction in Pam-Stamp
- Simulation results using a TriboForm FEM Plug-In in Pam-Stamp
Validation results: Strain
The front fender part shows fractures in case it is produced with a lubrication amount of 0.6 g/m2 pre-applied on the coil. Clearly these parts are rejected. The lubrication amount has to be increased to 2.0 g/m2 in production to result in high quality front fender parts. The measured strains of an accepted part is show at the top left corner in the figure below.
The Pam-Stamp simulation using a constant coefficient of friction of µ = 0.11 is shown at the bottom figure. It shows an acceptable strain distribution, based on which the part will be approved for production. However, parts from production show fractures when produced using pre-applied lubrication amount of 0.6 g/m2. This demonstrates the discrepancy between the simulation restults and press-shop conditions when using a constant coefficient of friction in stamping simulations.
The Pam-Stamp simulation results using the TriboForm FEM Plug-In show a good correspondence with the front fender part taken from the press-line at Skoda Auto. A critical strain distribution is predicted when using a lubrication amount 0.6 g/m2, whereas the simulation shows an acceptable strain distribution when using a lubrication amount 2.0 g/m2 . The predicted strain distribution also corresponds well with the experimentally measured strains, demonstrating the improved prediction accuracy of stamping simulations using the TriboForm software.
Validation: Experimentally measured and simulated strain results for the door-inner part
Validation results: Draw-in
A comparison between the experimentally measured draw-in of the real part and the simulated draw-in is shown in the figure below. In general, the Pam-Stamp simulation results using the TriboForm FEM Plug-In show a good correspondence with the experimental draw-in results. The simulation results using constant coefficient of friction show an underprediction of the draw-in. It should be noted here that there is still a discrepancy between experimental and simulation results. This is likely caused by the use of artificial line-beads in the forming simulations which limits the simulation accuracy (see the Volvo XC90 reference).
Validation: Experimentally measured and simulated draw-in for the front-fender part
The project results demonstrate the improved prediction accuracy of stamping simulations using the TriboForm software with respect to:
- Critical quality criteria: splits, sink marks and wrinkles
It also demonstrates the enriched simulation functionalities provided by the TriboForm software:
- Simulate the effect of lubrication type and amount on product quality, and determine the optimal combination for stamping production
- Simulate spot lubrication by applying a lubricant distribution in your sheet metal forming simulation
- Numerically optimize lubrication conditions as a guide line for try-out and mass-production
In general, this reference case demonstrate the strong influence of tribology, friction and lubrication conditions on both the part quality and the overall production stability. The TriboForm software enables to accurately simulate and integrate these effect in metal forming simulations. This results in faster, more accurate and complete metal forming simulations, which improves operational efficiency, results in cost and time savings and ensures high product quality.
The technical case results demonstrate that TriboForm enables more reliable and realistic stamping simulations with respect to critical quality factors like formability (splits, wrinkles), draw-in and springback. This enables Skoda Auto to simulate the actual try-out and mass production conditions in the development process of new automotive parts more accurately which, enabling time and cost savings.
Looking at the technical case results for the front fender stamping process, it is demonstrated that optimizing the lubrication conditions in the forming process using the TriboForm software results in an improved part quality. Looking at the forming process of a front fender part, this leads to the following business case:
- Reduced scrap rate in mass production from 1,1% to 0,26%
- Reduced unplanned production downtime (e.g. in case of splits) from 3,1 hours/day to 1,4 hours/day
As a result, the following savings are achieved:
- Total Cost of Ownership front-fender part at the standard lubrication conditions: € 10,83*
- Total Cost of Ownership front-fender part at the optimized lubrication conditions: € 10,39*
- At an production volume of 180.000 front fender parts/year
- The total savings are: € 79.200,- (for a single part, for a single car model)
With multiple critical parts per car model, and an increasing number of car models being developed, the TriboForm software delivers even higher savings. The technical case demonstrates that the TriboForm software empowers its users to quickly understand, simulate and solve tribology-related problems in the metal forming industry. The business case demonstrates that TriboForm enables to shrink time-lines and reduce development costs of new vehicles and results in a high Return On Investment (ROI).
[* ULSAB Cost model stamped parts, Steel association, 2013]
For more information, please contact Dr. Ir. J. Hol, CTO.
Courtesy of Skoda Auto.