Plastics Technology

SEP 2018

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I was recently asked a question that provoked a serious emo- tional reaction from me due to its utter disregard for the scien- tific principles that are the foundation of a molding process. For me, the ques- tion—"What is the most important process parameter of a molding pro- cess?"—is akin to asking which one of my children I loved more. My first reaction, which I was (fortunately) able to keep from verbalizing: "Are you kidding me?" A little background: I sat down with a group from a major medical manufacturer to discuss injec- tion molding medical process validations and the principles of Scientific Injection Molding. So, to be asked which param- eter is the most important, especially by someone from the medical side of our industry, was surprising, to say the least. Considering that in the medical market, the molding process itself is essentially a customer specification, as critical to shipping product as the part dimensions, I was not expecting this type of question. Whether it is medical or pharma, the customer typically requires that you show evidence that the product you ran was molded within the process-control limits established during the validation. I would argue that the OQ or Operation Qualification is the stage of the IQ/OQ/PQ protocol where the molder's process is developed, while the PQ (Performance Qualification) sets your upper and lower control limits, but that is a different article. This conversation was the motivation for this column, where I figured I would argue the points for several process parameters Is There a 'Most-Important' Process Parameter? as "most important," and with that maybe help identify whether there is in fact a "most important" process parameter. What better place to start than the answer I reluctantly gave at the time, after making it very clear that all outputs of an injection molding process are important. • Fill Balance: I made it abundantly clear that fill balance on its own is not technically a parameter but actually an output of many parameters. That said, if I had to pick something, fill bal- ance would be one of the starting points of process develop- ment. I almost lost my composure when the follow-up question was, "Okay, so what's the most important parameter in control- ling fill balance?" My response: "Really?" Literally hundreds of variables can affect fill balance. Whether it is thermodynamics, percentage of regrind, mold dimensions or shear, it is impossible to pick one of these as being more important than the others. That said, a debate can be had about the importance of having a balanced fill and, more importantly, the effects unbalanced filling will have on part quality and dimen- sional repeatability, specifically in regard to the medical requirements to hit Cpk and Ppk numbers of 1.33 and 1.67, respectively. The Cpk of a dimen- sional specification in the medical molding industry is simply the comparison of one cavity to another. So it helps deter- mine the variation from Cavity One to, let's say, Cavity 32 on a 32-cavity mold and so on. Fill balance is absolutely critical from this perspective. If you can't fill the parts the same, you can't pack the parts the same, and thus Cpk values can become extremely low. Cavities that fill on first stage will be packed out less because the viscosity in these cavities will become extremely high. Don't forget the basic rule of plastic flow: Plastic flows in the direction of least resistance. If we have a two-cavity mold and one cavity is full on first stage and the other is 80% full, the path of least resistance becomes the cavity that isn't completely full. If you can't fill the parts the same, you can't pack the parts the same, and thus Cpk values can become extremely low. A case can be made for multiple variables—fill balance, fill time, injection pressure, cavity pressure—as most important. But there is something else altogether that is essential to successful injection molding. Get more insights on Injection Molding from our expert authors: Learn more at KNOW HOW INJECTION MOLDING 34 SEPTEMBER 2018 Plastics Technology K now How By Robert Gattshall INJECTION MOLDING

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