Savvy Seal Software

Specialized analysis tool aids initial design of TPE glass run channel

Targeted software played a key role in the design of the first unsupported, all-TPE glass run channel that fits against the top of the moveable windows to provide a tight seal against air, noise or water. The desire to switch from existing rubber and metal designs arises from the fact that TPE is lighter, has better long term durability and weathering characteristics, lowers overall systems costs, and lasts longer than EPDM. TPE sealing components do not require metal carriers and mechanical attachments so they can be produced as a single molding without additional assembly or painting, thereby lowering manufacturing costs. And another key advantage of using this material over EPDM is that it maintains original physical properties after accelerated aging at elevated temperatures.

The continuing challenge for suppliers is the long lead-time required to design a leak-proof seal using conventional trial-and-error methods. When Nicholas Plastics, Inc., Allendale, MI, set out to develop and produce this run channel, the company selected Thermovin 170-H8NC material from Vi-Chem Corporation, Grand Rapids, MI. This material provides a smooth surface without the pits or blemishes that are normally associated with EPDM. It also offers long-term gloss control and retention while avoiding chalking, blooming and cracking. They can reduce wind noise by as much as 6.4 dB and are completely recyclable.

Designing a seal with an entirely new material is not a simple process. The forces that act through the complicated geometry of the seal are too involved to be determined intuitively or through manual calculations. Conventional prototype testing provides little feedback as to why the design passed or failed. Ron Woltjer, design group leader for Nicholas Plastics, estimated that if this traditional approach had been used, it would have taken about one year to create a TPE glass run channel that met their customer’s requirements.

“We needed to look for a faster and more effective way to design the glass run channel,” Woltjer said. “Of course, there are many CAE software packages available. The problem is that the high deformations involved in seal operation mean that the most expensive and difficult-to-use nonlinear FEA software is required to analyze them. The cost of purchasing such a program and hiring an analyst to run it is a major commitment. Then we heard about a new software package specifically for seal design that provides all the analytical tools required with a much simpler user interface, costing far less than general purpose programs with the same capabilities. A new design can be created and evaluated in less than half an hour by a design engineer, making it possible to evaluate more design iterations than was ever possible in the past in a fraction of the time. This allows a variety of ‘what if’ studies and provides a better understanding of the seal performance.”

The software they used, EASi-SEAL from EASi Engineering Inc., Madison Heights, MI, captures the complete workflow of the seal design process, including geometry definition, simulation and results viewing. Analysis takes minutes on a personal computer, or a user can run several jobs using the software queuing system, which solves multiple iterations overnight. This software is written in JAVA, so that it can run on most hardware platforms, including PCs and UNIX workstations. It enables the designer to achieve an optimal balance between sealing pressure and closing force (e.g., the requirement to prevent air and water leakage directly conflicts with minimizing the closing effort), with less complication than traditional analysis.

Woltjer began by importing the initial seal geometry, based on the existing EPDM design, and entering the properties for Thermovin 170-H8NC. The program performed a nonlinear FEA and provided CLD curves, seal deformation, stress/strain contours and contact pressure distributions. A stepwise animation of the seal cross-section provided an intuitive understanding of the seal performance, which helped improve the design through subsequent iterations.

Since the initial design failed to provide the desired results, Woltjer modified and analyzed each iteration. From the results, he identified optimal positions to locate hinge points, thinner wall sections that encourage the seal to bend in a desired direction. Physical prototyping verified the software simulation predictions. Nicholas Plastics then installed the prototype in a production vehicle to demonstrate function and aesthetics to their customer.

“The automobile manufacturer was very pleased with the fact that we were able to produce a TPE glass channel that worked exactly the same as existing steel and rubber parts in much less time than they expected,” comments Woltjer. “Because of its lower weight, lower assembly cost and longer life, TPE is expected to replace EPDM in this application over the next several years. The fact that our company was the first in North America to build a functional TPE part will be a big advantage in helping us win a major share of this new market.”

—SG