Plastics Technology

SEP 2018

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A balanced fill ensures that each cavity fills and packs at the same point in time; it ensures that each cavity is experiencing similar process conditions. It is a fundamental on which a process is built, so it's importance is self-evident, but calling it the "most important" parameter is still difficult. • Fill Time: Maybe the most important process parameter is fill time. Let me be clear again—this is an output not a setpoint. The word parameter is defined as "a quantity whose value is selected for the particular circumstances." We do select a fill time, but we do so by adjusting machine parameters. The fundamentals of molding that many of us have adopted are focused on outputs of the process. The entire premise of "what is the most important param- eter" is questionable, considering that many parameters contribute to the process outputs. There are many molders that would say fill time is the foundation of your process and there are many experiments used to establish it. I typically run a study on rheology, viscosity linearity, fill balance, and machine variation before identifying a fill time for a process. Did I say fill balance? I did. Fill balance is greatly affected by the fill rate and will change based on how fast or slow we inject. An injection molding process is a connected process, meaning that multiple variables and parameters will have a direct or indirect impact on one another. Very little about an injection molding process is isolated and won't have an impact on something else. With fill time, some processers may complete fewer experiments and some might even add to the list. Regardless of how fill times are identified, it is critical to ensure that we maintain that same fill time throughout the life of that mold or part. If we maintain a fill time, we ensure molecular-orien- tation equivalency, which results in filling consistently run to run. If we fill consistently run to run, this will contribute to a consistent shrinkage, shear rate, and fill balance. Again, there are several variables that affect how a part shrinks, and I am only pointing to one. There are many experts in our industry that would say the fill time is the foundation of a robust process, but is it really the most important? Now if we ask, "What do I need to ensure a repeatable fill time?" the answer is the right injection pressure. So. does that mean injection pressure is the most important parameter? • Injection Pressure : Injection pressure is critical to a repeatable process, and although you do expect some variation in this output, if you have insufficient pres- sure, it can be catastrophic to part quality. Pressure-limited processes lack the ability to adjust themselves based on variation in the material viscosity. If material viscosity increases beyond the available machine pressure, our pressure curve will abruptly flatten out and our fill time will increase. It will take longer for the machine to hit the V-to-P transfer point, if at all. There are several defects this can cause, most notably short shots or underfills. When we refer to Scientific Molding or Decoupled Molding, we are really refer- ring to a velocity-controlled first stage, but we are completely unable to control that velocity if we have a limit on the amount of pressure we have to achieve it. I know of several processors that kind of ignore injection pressure when it comes to process monitoring, reasoning that "as long as it is not limited, it is not a critical process output," and that we expect to see variation. I agree that we expect to see variation, but how much is too much and why is that important? I would say that too much varia- tion is anything outside established "normal" variation during validation runs. Depending on the product and customer specifi- Looking for the "most important" parameter may be futile. But one thing is certain: Your people are your most important resource— more important than the equipment. (Photo: Prism Plastics) The entire premise of "what is the most important param- eter" is question- able, considering that many param- eters contribute to the process outputs. PTonline.com 36 SEPTEMBER 2018 Plastics Technology I N J E C T I O N M O L D I N G K now How

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