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May 1, 2002

5 Min Read
Analysis allows for perfect parts from day one

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Figure 1. An ABS wheel cap for the GMC Tracker in its short-shot (left) and chrome-plated (right) stages. Filling analysis helped molder Hunjan Molded Products achieve zero scrap rates from the start of production.

Still not sold on the fact that moldfilling analysis works? Check out the following field report before you choose your final answer. Tom Peeler, Design Center manager at PolyOne Distribution, shares one of his recent projects with IMM to illustrate how tricky design problems can be solved via the virtual prototyping of moldfilling analysis.

Case Specifics
The subject of the field report was filling analysis of a wheel center cap for the GMC Tracker (Figure 1). The part is molded by Hunjan Molded Products (Waterloo) Ltd. (Waterloo, ON) for CAMI Automotive Inc. (Ingersoll, ON). The objective of the analysis was to determine if filling problems would occur in the cap using the side gate location. Designers were concerned about trapped air or a severe weldline opposite the gate. Additional objectives of the analysis were to minimize part stress to help achieve acceptable plating, to check gate orifice diameter and gate tab thickness, eliminate downtime, and to produce a part with no runout (see sidebar, below). The material to be used was Dow Magnum 3490 ABS. Molding conditions for part production include a melt temperature of 465F, a mold temperature of 150F, and a first-stage injection time of 1.5 seconds.

Runout defined

When a part is said to have no runout, what exactly does this mean? Peeler's answer: When looking at a part print at a certain view—let's say a side view of the wheel cap—there could be a datum line running along the straight (flat) edge of the cap from which all the dimensions in this view are referenced. The total deviation from this datum line in the molded part is the runout. In other words, the part has to be flat (no warpage) when laid on a flat surface; otherwise, it would cause the assembly to rattle and possibly come apart at the wheel's higher rotation levels. No runout (no warpage) is sometimes difficult when gating on the side of a part that ideally should be gated in the center.

Analysis Results
The analysis, performed with Moldflow Plastics Insight (MPI), showed that the material flows over the top of the cap sufficiently so that any trapped air is pushed to the vents at the parting line. This matched well with the actual short shots (Figure 2).

Designers selected various fill times and melt temperatures until the lowest stresses were achieved both near the gate and at the end of fill. Due to the low shear stress in actual molding, the parts plated well with no visual defects.

PolyOne also performed an iteration with a tab thickness of .080 inch, up from .060 inch, to see if any reduction in stress would result in the part. The software indicated no reduction in stress in the part itself, only in the tab. Therefore, the .060-inch tab thickness was deemed satisfactory and was used for moldbuilding. Also, with stress restricted to the tab and minimized in the part, optimum chrome plating could be ensured.

The only places that were slightly above the recommended shear stress were areas where the tab meets the part and at the end of fill. These were the lowest values that could be achieved in the analysis with this geometry, and they proved to be low enough to provide good plating (Figure 3).

The shear rate of 13,000 sec-1 in the .090-inch-diameter subgate was less than the recommended maximum value for this material; consequently, no excessive shearing occurred (see Figure 4).

Conclusion
Due to the OEM constraints on the start of production, Hunjan did not have time to make a prototype, nor did it have room for error. The company pays for every part it sends to the chrome plater, even if the part is subsequently scrapped.

Fortunately, as a result of the analysis, virtually no scrap was made. The chrome plater needed parts with extremely low stress, and proof of that came from virtually flawless plated parts. The plater also needed to rack (grab) the parts for plating, something that the tab provided. Confirmation from the filling analysis that gating into the tab would work gave the plater time to develop the rack during the toolbuilding stage.

The parts are high tolerance and almost no runout is allowed. This was a concern using the proposed side gate location, as the part could distort. Hunjan and CAMI needed a high degree of confidence that the parts would be flat and that no retooling would be required. In fact, the parts turned out to be extremely flat.


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Figure 2 (upper left). At a simulated 95 percent fill, the Moldflow MPI results mirrored actual short shots.
Figure 3 (lower right). A shear stress simulation shows that stress is relatively constant and low across the entire part, an important parameter for successful chrome plating.



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Figure 4. Shear rate simulation shows values less than the maximum for this material, predicting that there will be no excessive shearing in the part.



Contact information
PolyOne Distribution,
  Design Center
Suwanee, GA
Tom Peeler; (678) 546-6867
www.polyone.com
[email protected]

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