Page 47 - Injection Molding Advanced Troubleshooting Guide

30 3 Decoupled or Scientific Molding
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7. The screw should normally recover to shot size approximately 2–3 seconds prior
to mold open. This may require use of a screw rotate delay to avoid rotating the
screw at extremely slow speeds.
These basic steps are the process methods that this book will reference. Note that
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the particular process method is Decoupled II molding as defined by RJG Inc.
The key is to use a scientific approach and let data help with decision making. De-
cisions should be made based on data. Use of the STOP method (see Chapter 1) will
help reinforce making the effort to gather the data. Also when using this method,
focus on recording the plastic data; for example, record fill time and fill only weight
rather than worrying about velocity set points. By using plastic data it is possible
to translate from machine to machine and process to process.
Modern scientific molding relies on establishing and documenting a process and
then always running that process. The days of process technicians having a “little
black book” of their personal processes are long gone. If someone has an idea to
improve a process it must be approached with data, proper evaluation, and valida-
tion. The process technicians should understand that even though they think the
process is better all impacts must be examined. There is a saying that the “law of
unintended consequences” can create wide-ranging impacts that were never
considered. Depending on customer and part requirements it may be necessary to
resubmit parts and extensive test data to support a process change.
From a practical standpoint everyone in the molding operation must be knowl-
edgeable in scientific molding methods. Once people understand that the process
was based on decisions that are backed with data they will tend to take more
ownership of the process variables. Quality training in the techniques is absolutely
critical to success.
Benefits of Decoupled /scientific molding include:
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1. Faster fill rates yield faster cycle times.
2. Faster fill rates yield lower and more consistent viscosity. Lower viscosity allows
easier filling of the mold and less pressure drop across the cavity.

3. Separating first-stage fill from second-stage pack/hold gives the processor the
ability to impact plastic flow rates and plastic pressurization as independent
points of control.
4. Utilizing a fill only weight of 95–98% will help prevent mold damage and flash
from rapid spikes in cavity pressure.
5. Use of gate seal studies ensures that second-stage time is not wasted packing
out runners.

6. Cavity balance studies ensure that each cavity is experiencing the same plastic
conditions throughout the process.