Page 46 - Injection Molding Advanced Troubleshooting Guide

3 Decoupled or Scientific
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Molding

Most of the methods discussed in this chapter are those developed and taught by
Don Paulson [1], John Bozzelli [2], and Rod Groleau [3]. There is a large amount of
excellent information available on Decoupled /scientific molding, so this chapter
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will only cover the basics [4].
One of the Merriam-Webster dictionary definitions of scientific is as follows:
Conducted in the manner of science or according to results of investigation by science:
practicing or using thorough or systematic methods.
This is the foundation that Decoupled /scientific molding is built on, thorough and
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systematic, and is also a great definition for how effective troubleshooting should
be conducted.
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The basic premise of Decoupled /scientific molding follows these steps:
1. Fill the mold to 95–98% full using first-stage velocity control. Filling should be
as fast as possible while making a quality part. The molding machine must not
be pressure limited meaning that the required pressure to achieve desired fill
velocity should not reach the machine’s maximum pressure (actually anything
within 2000 psi plastic pressure should be considered pressure limited; this
allows for compensation due to viscosity variation). The 95–98% short shot is
referred to as a fill only shot.
2. Conduct a cavity balance study if running a multi cavity mold. See Chapter 12
(Cavity Balance) for more details.
3. Transfer from first-stage velocity control to second-stage pressure control by
using the transfer position setting on the machine.
4. Control pressure during second stage to compensate for plastic shrinkage
during cooling.

5. Second-stage pressure should be controlled for a set time period. This time pe-
riod should be determined by conducting a gate seal study [5]. For every mold
determine if running with gate seal provides the best quality part.
6. Cooling time will be optimized to minimize cycle time, based on part ejection
temperatures.