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Creating and Troubleshooting a Plastic Injection Molding Process - The "Art of Injection Molding vs. Scientific Molding" - Part 1


Custom Plastic Injection Molding - The Art of Molding vs. Scientific or Decoupled Molding – In the plastic injection molding world, there are basically two schools of thought.  I’m going to sneak this information in between the process setup information I just presented in Plastic Injection Molding Basics, sections one through five, and the troubleshooting pages that will follow this.  Having been in the field for almost 25 years now, I have seen a lot of changes in the thought process relating to the way injection molding processes are developed.  In the previous pages, I have used primarily the “the scientific molding” approach, but I wanted to touch a bit more on that thought processes as well as the previously more “traditional” way of process development and control that was used for years and I’m sure can still be found in use today in some shops.


In years gone by, the process of injection molding being as young as it was, was almost a daily experiment in process redevelopment with a lot of focus on controlling the machine settings or set points.  The idea of controlling variables was not even a consideration and we spent a lot of time on the plant floor trying to get our injection molding machines to provide us with a consistent part.  The best descriptions that I can give to that method of process control were that we were a bunch of “knob turners”.  What I mean by that besides the fact that many of the machines still in use at that time, literally had knobs to turn, is that we spent our day in a constant mode of turning knobs trying to maintain our part quality.  What many of us didn’t realize at the time was that our methods were flawed and our machines not consistent enough to provide us with the type of control that we needed for our processes.  We had all been though the old “Paulson series of injection molding theory”, and had been taught in numerous seminars and other training, that this was the way to control our processes.  Most colleges hadn’t yet developed the specialized plastic processing degrees that are available today and many of us were just Mechanical Engineers or other, that happens to fall into the plastics field.  And most of us found it to be fascinating and stayed with it over the years to follow. 


Many of the molding machines produced in the 70’s were built with simple mechanical timers, limit switches, relays, and hydraulic pressure and speed control valves.  Pressure control consisted of three knobs for our injection molding process and they were a “boost or high”, hold, and back pressure control valve.  We also had an injection speed control for injection speed management.  We would set our process up, limiting our first stage or “boost” pressure to level we felt would not flash our part, and then try to control that with the injection speed control valve.  This was a balancing act to say the least and the worst thing about it, was that most of the time we never realized why this was, and just accepted it for what it was, “process variation”.  Hence the term the “art of injection molding” existed, instead of the science of injection molding.  Most of us in the field back then treated what we did as an art form instead of the scientific method it is and could be.  It was our way of saying, we don’t really understand why this but because we are good enough at the “art of

injection molding” we can conquer it and beat it into submission.  Kept us in an elite group the way.


Then came a guy called Rodney Groleau of RJG, Inc., in 1985.  He may not have been the first but he definitely became the most famous that I am aware of in the area of “scientific or decoupled molding” as he referred to it. (continued on next page)


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Written by: WM8C, August 18th, 2006.  Not for use without written permission







Up ] Plastic Injection Molding II ] Plastic Injection Molding III ] Plastic Injection Molding IV ] Plastic Injection Molding V ] [ Plastic Injection Molding VI ] Plastic Injection Molding VII ] Plastic Injection Molding VIII ] Plastic Injection Molding IX ] Plastic Injection Molding X ] Plastic Injection Molding XI ] Plastic Injection Molding XII ] Rapid Prototyping ] Electric Injection Molding Machine ] Electric Injection Molding Process ] Electric Injection Molding ]




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