Design Tool for Achieving Temperature Uniformity on a Forming Die

Introduction:

In the manufacture of molded body panels for automotive applications, continually changing body styles and the desire to improve Just In Time (JIT) production flow results in the need to accommodate new designs quickly while also assuring the highest yield and throughput possible. This desire for speed and quality requires the molding process to facilitate rapid design and set-up of new dies; as well as, maintain very tight temperature uniformity across the face of the die during the forming operation.

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Customer Challenge:

To reduce the complexity and cost of new dies, the customer historically used cartridge heaters evenly spaced along a horizontal cross section of the 45" x 65" forming die in an effort to distribute heat evenly along the upper work surface. This resulted in a temperature variation on the work surface of approximately +/-50 deg. F as shown in Figure 1. While this approach resulted in a die configuration that was relatively simple to manufacture (low design & equipment costs) the large temperature variation did not produce consistent quality parts (poor yield) and required lengthy set-up times (high maintenance & use cost). While the customer recognized they needed to improve control of the manufacturing process, they lacked an effective means for doing so other than trial and error adjustment of equipment settings. To meet the temperature uniformity specification requires a complete understanding of the system including mechanical design, environmental conditions and measurement/validation techniques employed.

Through thermal modeling and analysis, Single Iteration was able to determine the exact type and placement of heaters and sensors to exceed the customer's requested +/- 10 deg. F uniformity requirement (+/- 8 deg. F as shown in Figure 2), a substantial improvement over the original temperature uniformity of +/- 50 deg. F without requiring a chang to the control system hardware configuration. This approach required the customer to test the results on one die in order to verify that the tighter temperature uniformity effectively increased the yield of good parts.

Once the benefit of improved temperature uniformity was verified, we looked at how we could help improve the die making process. Concept development initially resulted in a split die set design whereby a "universal" die base plate could be used with multiple forming dies. Based on a survey of historical and projected die dimensions and surface contours, it was determined a fairly complex multi-zone control with corresponding greater up-front equipment development and hardware cost would be needed. Cost/benefit analysis of the projected new equipment development cost verses the projected cost savings per each new die design and die change determined the return on investment for this approach did not satisfy minimum requirements.

As a lower cost alternative, a parametric simulation tool was developed for the customer to use when creating a new die design. This tool was based on standard numerical analysis software and utilized a custom user interface to enable the customer's tooling designer to quickly perform the complex thermal analysis needed to optimize the power rating and position of standard cartridge heaters for a new die design. In addition, the tool facilitated selection of appropriate heaters from an approved supplier stock list.

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Single Iteration Solution:

Through this development, Single Iteration enabled the customer to shift from the timely and expensive method of trial and error required to achieve a barely acceptable 11% variation in temperature uniformity during set-up of a new die to being able to use an automated tool that guaranteed less than 2% variation on the first run. While this solution did add complexity and cost to design and manufacture of the die itself, the improved yield and dramatically reduced set-up time recovered the investment cost on the very next die change. Once the benefit of improved temperature uniformity was verified, we looked at how we could help improve the die making process. Concept development initially resulted in a split die set design whereby a "universal" die base plate could be used with multiple forming dies. Based on a survey of historical and projected die dimensions and surface contours, it was determined a fairly complex multi-zone control with corresponding greater up-front equipment development and hardware cost would be needed. Cost/benefit analysis of the projected new equipment development cost verses the projected cost savings per each new die design and die change determined the return on investment for this approach did not satisfy minimum requirements.

As a lower cost alternative, a parametric simulation tool was developed for the customer to use when creating a new die design. This tool was based on standard numerical analysis software and utilized a custom user interface to enable the customer's tooling designer to quickly perform the complex thermal analysis needed to optimize the power rating and position of standard cartridge heaters for a new die design. In addition, the tool facilitated selection of appropriate heaters from an approved supplier stock list.

Through this development, Single Iteration enabled the customer to shift from the timely and expensive method of trial and error required to achieve a barely acceptable 11% variation in temperature uniformity during set-up of a new die to being able to use an automated tool that guaranteed less than 2% variation on the first run. While this solution did add complexity and cost to design and manufacture of the die itself, the improved yield and dramatically reduced set-up time recovered the investment cost on the very next die change.