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Why Plastic Injection Molding is like An Ecosystem

May 3, 2019

As any high school biology can tell you, an ecosystem is  “a complex network or interconnected system,” according to one definition.  The inescapable conclusion from this is that no one creature or natural process is more important than any other.

In effect, plastic injection molding is its own ecosystem because, like in a natural ecosystem, there is a series of interconnected process parameters which are mutually dependent — if one parameter is altered, the entire process is either altered too, or simply breaks down altogether.

This simple but profound truth forms the rationale for Scientific Molding, which puts a priority on process outputs rather than the set-points.

Plastic Injection Molding Machine
Just one example will illustrate this point.  Let’s say a screw is retrieving the full shot a few seconds before the cooling time is completed, before the mold even starts opening.  A Competent Molder might try to optimize the process to decrease the residence time of the material by lowering the screw RPM.   This should increase the recovery to what looks to be a more optimal time— right before the mold opens.  With this one simple change,  the Competent Molder has also:

  • lowered the material melt temperature
  • lowered the screw heat
  • increased injection pressure
  • increased viscosity
  • contributed to difficulty with velocity control during fill

Do you get the point? One minor alteration  — lowering the screw RPM — led to five very significant changes to the entire molding process.

But the changes don’t stop there:  lowered melt temperature means molded products are going to cool faster— after all, less heat in, less time needed to retrieve product.  One problem with faster cooling: the plastic part shrinks to the core.  This potentially could result in larger part dimensions.

Another side effect of the lower screw RPMs:  less material mixing, which means a nonuniform melt.  This means a very nonuniform color dispersion and other issues.

Yep, one minor change can lead to major headaches!



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