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A Plastic Injection Molding Process Problem – Troubleshooting Methods
Custom Plastic Injection Molding - Troubleshooting Techniques - For this part of this series, we will start out with a very basic injection molding problem and some possible ways of solving it. This approach will assume you have developed a basic understanding of the injection molding process and that you also have a basic grasp on the terminology used. For use in this exercise, I will be using actual problems that have been encountered and solved by myself or people I have worked with. You must also keep in mind that not all situations are identical and that in many cases, the same end resulting defect can be the result of numerous combinations of process parameters for you specific situation. We will try to explore the most common and likely to occur.
Defect scenario #1 Part Defect: Splay – very tiny streaks, usually less than 5mm in length Time of year: Late summer Part description: Single cavity trim strip for a grille (28” across, ˝” width, 3mm nominal wall stock, smile configuration) Material type: PC/ABS - platable grade Machine size: 700 ton Machine Shot size: 70 oz. Part weight: 120 grams Runner Weight: 30 grams Feed System: Central loading with 25 lb. capacity hoppers on the machines. Mold: Hot Runner system with 5 valve gates. 1 gate in center, 2 each side of center, balanced configuration. Fill: Sequenced from center
We had been running this part for about 6 months and through defect analysis we had eliminated the majority of the common defects and taken the scrap from about 13% at launch to less than 8% after 6 months. This was still quite high from our goal of 3% total system scrap. Data taken at that time showed that of the remaining scrap issues, “splay” was our number one reason for defects after the injection molding operation. After speaking with our Technicians, it was confirmed that this was a common daily defect on this part. We used an is/is not matrix to define this data as could you, if you so desired to in your own operations.
Further investigation revealed the following facts. Out of 8% total scrap, approximately 5% was due to “splay”, and the number two item was “contamination” or “black specks”. If we could drastically reduce the #1 and #2 reasons, we would eliminate the majority of our defects ion this part. Looking at the potential causes for these injection molded defects, we narrowed it down to 5 likely causes and they were in order of likely hood – wet material, burning material due to the material residence time in barrel, hot runner over heating or other malfunctions, excessive or incorrect fill speed, and valve gate timing. Once again, speaking with the technicians we learned a couple of things about the process they were using. The most important piece of information came from a statement that when ever the splay or burning problem became an issue, they purged out the barrel and the problem seemed to go away for up to two hours or so. (Continued on next page)
Written by: WM8C, August 28th, 2006. Not for use without written permission
More to come...stay tuned
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