molding process is believed to have been first made com-
mercially available by the invention of Friederich, which is
disclosed in US Patent No. 4,101,167 issued July 18, 1978.
The Friendrich patent solved the problem of molding hollow
shape bodies in a single injection molding operation [5].
Specifically, during those early years, the industry paid
most of its attention on the use of structural foam as a spe-
cial process used for molding relatively thick-sectioned ar-
ticles. These parts are light in weight and have acceptable
surface finish, i.e., without sink marks that are associated
with conventional plastic injection molding. In recent years,
attention has been concentrated on the use of gas assistance
with conventional plastic injection molding to achieve high
product quality and productivity. Good surface quality, short
cycle times, lower clamp tonnage, material saving, weight
reduction and minimization of part distortion or warpage
can all be achieved with proper utilization of gas assistance
into a conventional plastic injection molding process.
There are two methods in conventional GAIM. The one
is “short shot”. The short shot is sequentially done by fol-
lowing a simple three-step process. In the short shot pro-
cessing, a molten polymer is initially filled in cavity about
75–98% by ram speed control of the injection molding ma-
chine. After a short delay period, compressed nitrogen gas
cores out the molten polymer, filling the remainder of the
mold. The next step is the gas packing stage that compen-
sates for the volumetric shrinkage of the polymer melt. As
the plastic solidifies, the gas expands into volume created by
shrinkage, locally packing out the part [6]. The short shot
method is used for thick section moldings, typically handles
and tubular components. The advantage of the short shot is
reduction in molded plastic weights. However, surface de-
fects such as hesitation mark [7] may be visible when the
gas is injected too late or the initial gas pressure is too low.
The other is “full shot”. The full shot is injected to fill or
nearly fill the mold cavity, but the plastic is not packed by
an injection molding machine. After a selected time delay,
first phase gas is injected. Second phase gas penetration oc-
curs to compensate for volumetric shrinkage of the plastic
as it cools. A uniform gas pressure is applied throughout the
plastic. Gas is exhausted to atmosphere or for recovery be-
fore mold opens. Plastic refill commences after the nozzle
valve is closed or after the plastic feed gate has solidified.
The ‘full shot’ method is normally applicable for compo-
nents in which there are thick and thin sections. The gas
flows into the path of least resistance in the thicker sections
where the plastic interior is still in a molten state [5]. The
pushed melt needs to expel from the cavity to another place.
The place is called overflow and wholly wastes material.
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