Page 22 - Plastics News July 2021

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The method dubbed GIMIM entails the addition of gas traps at each stage of molding, various plasticization
stages

ngineering thermoplastics supplier Polyplastics Polyplastics lays out the negative cycle offgassing can
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formation during injection molding and ultimately can and short shots leads to the installation of larger vents on
help reduce mold deposits. WKH WRRO 7KHVH ODUJHU YHQWV FDQ OHDG WR ÁDVKLQJ IRUFLQJ
the vents to be reduced in size, which once again creates
Dubbed Gas Investigation Method in Injection Molding
(GIMIM), the concept entails the addition of gas traps gas issues within the tool.
at each stage of molding: plasticization, metering, and GIMIM makes it possible to measure high-molecular-weight
injection. Once captured, gas chromatography mass substances that can cause mold deposits. Since this
spectrometers (GC/MS) are used to qualitatively and method directly analyzes gases that are formed inside the
quantitatively analyze the gases produced in the various PROG GXULQJ ÀOOLQJ WKH FRPSDQ\ EHOLHYHV LW \LHOGV UHVXOWV
plasticization stages. that are closest to real-world conditions.
Prior to GIMIM, ascertaining the source and output of
pyrolysis gas largely fell to two methods, neither of which
captured the actual conditions in the tool. One method
involved heating pellets and analyzing the resulting
volume and composition of decomposition gases. The
other technique involved running a tool and material and
then analyzing the resulting mold deposits that adhered
to the mold.
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and chemical adsorption where in the former mold
deposits are adsorbed to positively charged metal surfaces
and in the latter electrons are shared between surface
The high speeds, temperatures and pressures of injection layer atoms in covalent bonds and hydrogen bonds. In
molding can lead to the deterioration of plastic additives further research, Polyplastics plans to examine other
and the subsequent release of decomposition gases. Mold problems caused by pyrolysis gas such as short shots and
deposits, gas burns, and short shots caused by pyrolysis gas burn defects, as well as mold deposits.
gas create quality issues and higher defect rates.

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converging fronts, jetting, or mold/machine problems
such as non-vented core pins, poor venting (try vacuum
venting), too much decompression, or resin degradation
due to overheating or long residence times.

The gas may appear from water vapor, volatiles from
the resin, or decomposition byproducts. Air trapped in
ribs or non-vented projections off the nominal wall will
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of bubbles. Determining where the gas is coming from is
more important than knowing the composition of the gas,
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