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Looking Down The Barrel: Things That Can Go Wrong

January 2, 2020

Like true romance, there’s a whole host of variables that have to be just-so in the plastic injection molding process if the marriage of resin to mold is going to birth a new plastic part. It’s oh-so-easy for something – anything – to go awry and ruin the part.

But again, just like in romance, it all seems so easy at first: using the capacity of the mold as your guide, you calculate the shot size that can fill and “pack the part,” right?  And work up a schedule from there. The first few times everything seems normal, the variables are stable, the cycle is repeatable, and everything look rosy. The parts even look nifty. Unfortunately, when you send the parts to quality control, a certain percentage of them meet specs and others don’t, while a certain number flunk performance tests, owing to size, appearance or even warp problems. One possible culprit for challenges like these is either either too small or too large a shot size.

But then there’s the quality of the melted plastic itself. Is it uniform in viscosity, composition and temperature? If there is “un-melt” or partially melted granules within the manufactured part, you’ve got a big problem. Ditto for swirls, color streaks or shade differences. These issues may be difficult to measure, but they will play havoc with performance and production. Simply put, if the plastic granules are not melted properly, you are practically begging for problems such as the above. The most important thing you can do is pay strict attention to certain key variables that influence the melting process and melt quality. The three most important are: 1) percentage of barrel capacity and 2) resin type (amorphous or semi-crystalline 3) Filler type, if any.

More could be written, and will, in a future blog post!


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