Development of DURACON® POM metallic finish automobile part
― Joint effort for a knob and handle at Shanghai and Fuji facilities ―
Around the end of 2010, Polyplastics proposed metallic finish parts molded from DURACON® POM to the Pan Asia Technical Automotive Center (PATAC), which undertakes automotive design in Shanghai. While PC/ABS is superior from a simple material cost perspective, the cost associated with coating is non-existent for this POM grade because the electrodeposition coating process can be done away with. A further advantage is that yields can be significantly increased. A typical calculation based on an approximately 20% reject rate for PC/ABS due to coating nonuniformity and contaminant adhesion, 25% higher material and associated molding and coating costs means that even if the unit material cost for POM is doubled, it is possible to reduce overall costs by more than 30%.
２.The challenge of tests on an actual machine
Recognizing the potential for substantial cost savings, the opportunity arose for evaluation using an actual machine at PATAC, and preparation of evaluation samples for strength and other tests was started. While preparation of samples through borrowing a tool from a Tier I molder was contemplated, the opportunity for molding prototype parts using a tool for PC/ABS that was being used for mass production did not present itself and in the end, Polyplastics Shanghai Ltd. made the decision to fabricate a tool that could be used for evaluation from scratch. Fabrication of a prototype tool based on the shape of a door handle already present in the market was commenced but this first iteration of the tool resulted in conspicuous flow marks and sink marks in the POM prototype part because the gate position and shape was the same as that used for PC/ABS. The outcome was that this was not an appropriate situation for evaluation by the customer and further work was thus undertaken to change the gate shape, remove wall thickness and change R (corner radii) and thereby obtain acceptable metallic finish POM molded parts. During this process, staff at Polyplastics’ China Technical Solution Center (C-TSC) utilized Moldflow to verify the filling pattern and optimize the processing parameters, and thereby acquire numerous countermeasures to prevent defects in external appearance.
Evaluation samples were structurally analyzed at Polyplastics’ Fuji Technical Solution Center (Fuji TSC) and together with these results, the samples were tested using apparatus also used for testing of products already in the market. The POM molded sample parts were designed in a shape that matched the mechanical property standards of PC/ABS, and both structural analysis and tests using actual test equipment confirmed that the POM parts exhibited good strength and durability. The findings obtained from tests on actual test equipment also suggested that product design based on the premise of POM being the molding material could lead to further weight reductions.
Figure 1. Gate design for test samples
*Mold filling proceeds in the direction of blue to red. Gate positioning is such that weld lines (convergences of resin flow fronts) do not form on the surface due to holes that reduce wall thickness.
３.Consideration of a new volume production shape
The development objective of a DURACON metallic finish colored part was achieved so efforts were stepped up for actual adoption in vehicles starting from the end of 2012. Based on the surface shape proposed by PATAC, we worked on the reverse surface and intermediate rib structure and furthermore looked at the gate position. The main point was to avoid any problems with external surface appearance cropping up through preventing the formation of weld lines and sink marks on the surface while ensuring the required strength was retained. Work was shared between the C-TSC and the Fuji TSC and the task of designing the ribs to ensure strength was completed relatively easily. However, it wasn’t easy to make delicate modifications to the curved surface of the handle that prioritized design aspects and an enormous amount of time was spent on flow analysis to verify the filling pattern. Besides modifications on gate position, a proposal was made to form weld lines on the reverse surface by making the front slightly thicker and with this, PATAC once again carried out final evaluation of the part shape.
Figure 2. Structural analysis of test samples
*Yellow sections are where force is applied due to opening and closing motions (enduring over 45,000 repetitions was verified)
４.Fabrication of samples in volume production form
The knob and handle parts that were to be mass produced were actually molded from PC/ABS and POM using an injection molding machine in order to carry out the final assessment. We needed to demonstrate that the part could directly enter volume production with no issues surrounding, needless to say, mechanical strength, but also external appearance.
Two-cavity tools for left and right knobs and handles were fabricated in China. A toolmaker with extensive experience in door handles cooperated with us to mold prototypes but while the weld lines formed on the reverse surface as anticipated, the parts were not 100% acceptable, with some black markings appearing caused by flow marks. Thereupon, we carried out molding trials repeatedly and by localized refinement of R shapes and surface roughness, we are able to mold samples that could be presented to the client.
５.Volume production support at Tier I molder
PATAC approved the external appearance of our POM metallic finish colored part samples and POM came to be adopted in some variations of the planned vehicle model. Thereupon, we were then asked to fabricate mass production tools at a the Tier I molder and support optimization of processing parameters. What was particularly difficult was elimination of the black marks resulting from flow marks. While the high-performance all-electric injection molding machine possessed by the C-TSC boasted good repeatability and could make fine injection speed adjustments, the conventional hydraulic machines that were to be used for mass production from time to time resulted in external appearance defects. Thereupon, we once again carried out detailed adjustments to the surface roughness of the cavities and also engaged in identifying the processing parameters that could be used to obtain on-spec products with good repeatability. It was around early summer in 2013 that, after many iterations of tooling modification and prototyping, that we finally reached the objective of mass production.
6.POM metallic finish colored part merits and points to remember
POM metallic finish colored parts boast much better short-term and long-term mechanical properties compared to PC/ABS. Conventional injection molding machines can be used for molding while no specialized equipment is required in terms of pellet driers, mold temperature regulators, and other peripherals. However, the drawback is that flow marks and weld lines tend to be conspicuous on account of the aluminum pigments contained in the compound. Flow marks can potentially be improved by using a mold temperature of more than 100°C and using a slow injection speed. However, the issue of weld lines cannot be improved by adjusting molding parameters. We needed to involve the customer in discussions from the early stages of development and thoroughly study gate positioning and product shape.
In addition, the surface gloss of the molded product varies greatly in accordance with the surface roughness of the molding tool and we understand that the impression of a metallic finish can vary greatly so detailed consideration of the tool specifications including surface finish is required. In other words, these products require extensive technical support, from product design through tool design, to optimization of processing parameters. However, through the efforts of the C-TSC and the Fuji TSC directed at two product shapes, technical levels have been substantially raised and it looks like moving forward, we can anticipate further technical development of products with stringent external appearance requirements.
■DURACON® is a registered trademark of Polyplastics in Japan and other countries