In recent years, there has been increasing activity among automotive manufacturers to develop cars specifically for Asian markets. The Shanghai GM Group's Pan Asia Technical Automotive Center (PATAC) is one such company, proactively designing and developing vehicles for the Chinese market. We spoke to Mr. Pinyi Tang (Tang), Material Engineer in PATAC's Body & Exterior department, about the development of inner door handles and knobs using Polyplastics' metallic-finish DURACON® POM. Zhengan Wang (Wang), Sales Manager at Polyplastics (Shanghai) Ltd., also gave his perspective on the development.
Body & Exterior Dept.
Mr. Pinyi Tang
In Search of New Technologies and Plastics Materials
Tang Competition in automobile development in Asia is becoming increasingly fierce. Here at the Shanghai GM Group as well, we are engaged in a constant pursuit of cost reduction that will achieve better fuel efficiency, improved comfort, and affordable prices for consumers. At the end of 2010, Polyplastics came to us with a proposal to develop inner door handles and knobs using its metallic-finish POM. Their proposal was to replace the material that was in use at the time, a coated PC/ABS alloy (a multicomponent polymer of polycarbonate and acrylonitrile butadiene styrene) with its metallic-finish POM, which could only be produced with injection molding. Because the material would not need to be coated, it would eliminate the costs associated with coating and greatly reduce the rejection rate due to coating nonuniformity and contaminant adhesion. Therefore, although the POM material itself was more expensive than the PC/ABS in a simple comparison of material costs, an effective cost reduction of around 30% would be possible, which made Polyplastics' proposal very attractive.
Testing on Actual Machinery, Collaboration with China TSC and TSC (Japan)
Wang PATAC kindly gave us the opportunity to evaluate our product on actual machinery, so we set about preparing samples for evaluation. The molds used for the PC/ABS were unavailable because they were in use for mass production, so we decided to fabricate our own prototype mold. We started this work in Shanghai, basing the mold on the shape of a door handle already available in the Chinese market. Because the position and shape of the gate (where the material enters the mold cavity) on the initial prototype were the same as for the PC/ABS, the conspicuous flow marks and sink marks resulted in the POM. We modified the shape of the gate and made a number of other changes, such as reducing wall thickness and changing the corner radii (R), and we were eventually able to obtain metallic-finish POM molded parts for evaluation. During this process, China Technical Solution Center (China TSC) also performed resin flow analysis using CAE* to verify the filling pattern and optimize the processing parameters. This work gave us a great deal of insight into how to counter those external appearance defects.
We immediately delivered the metallic-finish POM parts to PATAC for evaluation on equipment used for testing of products already in the market. We also furnished them with the results of CAE structural analysis that TSC in Japan had undertaken while we were developing the prototype.
* Computer Aided Engineering
Full-Scale Product Development of the Premise of Adoption of POM
Tang The design of the shape had been based on the mechanical properties of the PC/ABS material, and Polyplastics' structural analysis and our own testing using actual equipment confirmed that the POM parts offered good strength and durability. Our tests showed that we could achieve further weight reduction if we designed the product using POM. Consequently, from the end of 2012, we moved onto full-scale product development of parts for adoption in the vehicle models in which we planned to install the product.
Thorough Flow Analysis in Pursuit of Better Appearance
Wang Based on the surface shape proposed by PATAC, we started looking at the shape of the reverse surface and intermediate rib structure and at the position of the gate. The main objective was to prevent problems with the external surface appearance, including avoiding weld lines and sink marks, while also ensuring the necessary strength. Sharing the CAE work between TSCs in China and Japan made it a relatively easy task to design the rib structure to secure the required strength. However, making fine modifications to the surface curves for aesthetic design reasons proved more difficult, and we spent a tremendous amount of time on flow analysis to verify the filling patterns. In addition to modifications to the gate position, we proposed making the front slightly thicker so that the weld line would form on the reverse surface, and PATAC once again carried out a final evaluation of the shape of the part.
Metallic-finish POM handle
Decision Made to Adopt Metallic-Finish POM
Tang To help us make a final decision on whether to stay with the PC/ABS material or switch to POM, we decided to conduct a final evaluation using a prototype of the shape planned for mass production. We needed to verify whether it was ready to enter immediate mass production without any issues related not only to its mechanical properties, but also its external appearance. Polyplastics fabricated a prototype two-cavity mold for left and right handles. The evaluation samples in metallic-finish POM that were provided were all acceptable in terms of both mechanical properties and external appearance, and it was proved that mass production with the two-cavity mold was possible, so the samples were deemed acceptable.
Assistance with Mass Production at a Tier I Molder
Wang Once PATAC had accepted our material, an official decision was made to adopt POM parts for some versions of the planned vehicle model. Next, we were asked to fabricate a mass production mold at a Tier I molder and to assist with the optimization of processing parameters. The most difficult issue was eliminating the gray lines caused by flow marks. We were able to fine-tune the processing parameters using China TSC's high-performance all-electric injection molding machine, but those kinds of slight adjustments were not possible with the conventional hydraulic injection molding machines that would be used for mass production, so appearance defects occurred frequently. To remedy this, we again made repeated slight adjustments to the surface roughness of the mold cavity in pursuit of processing parameters that would result in conforming products. After repeated mold modifications and prototyping, we were finally ready for mass production around early summer in 2013. Through the teamwork of China TSC and TSC in Japan, we were able to provide our customer with satisfactory technical support. We are deeply grateful for the cooperation of our team members.
Impressive Willingness to Assist with Development
Tang Adoption of metallic-finish POM was a new challenge for us, but Polyplastics not only developed the new material, they kept the development of the product moving forward by collaborating with their TSCs in China and Japan to optimize the shape using CAE, and conducting repeated trials using their own prototype molds. These efforts led to our decision to adopt the material. With the mass production molds as well, their support helped us to successfully develop a metallic-finish POM part that was very strong and durable and had a good external appearance free of flow marks or sink marks. I was extremely impressed by how very proactive and enthusiastic all of the salespeople and technical people in China and Japan were throughout the entire duration of the project.
We are always in search of new products and technologies. I sincerely hope that Polyplastics will continue to support us as an excellent partner in development.
PATAC's Mr. Pinyi Tang (right) and Polyplastics' Zhengang Wang (left) outside PATAC.