injection molding without the drying
TRANSCRIPT
10.1002/spepro.004323
Injection molding without the
drying
Sang-Won Woo, Yeong-Eun Yoo, and Sun Kyoung Kim
Using a gas counter to raise the pressure during injection molding of
plastics prevents blisters forming by suppressing evaporation of water
and cuts costly drying steps from the manufacturing process.
The injection molding industry is struggling to improve quality at a
time when costs need to be reduced.1 While efforts to reduce costs
focus largely on reducing energy consumption, most quality issues are
raised about the appearance of molded parts.2, 3 Although other matters
such as dimensional integrity and mechanical performance are impor-
tant, the appearance of a part after ejection from a mold is always the
first thing to be checked.
The most serious appearance defects are caused when water
evaporates during injection molding, which creates bubbles. As most
thermoplastic resins take up water, bubble formation can be prevented
by drying the polymer pellets prior to molding.4 Such drying not only
wastes tremendous amounts of energy but the pellets can become con-
taminated during handling and moving. Moreover, the drying process
requires time and space as well as an initial investment in equipment.
Thus, being able to perform injection molding without prior drying
would have several commercial benefits.
We recently developed a processing technology that enables high-
quality molding without prior drying of polymer pellets. The idea is
not to let the water evaporate to form bubbles inside the mold by keep-
ing the polymer hydrated throughout the molding process. This can be
done by keeping the cavity pressure above the saturation pressure of
water for a particular melt temperature until the polymer solidifies.5
Increasing the pressure pushes up the saturation temperature to sup-
press evaporation. We can achieve this using gas counter pres-
sure (GCP) technology, which has long been used to improve the
mechanical properties of foam-molded structures.6–10
Unlike conventional injection molding processes, GCP requires a
good venting scheme. Venting has tended to be done through gaps be-
tween the pin and mold or through the parting line.11 However, for a
mold cavity to be pressurized, it needs to be sealed from the ambient
air then connected to a pressurized reservoir. We designed and built
such a mold and GCP system to conduct the injection molding without
prior drying.12 We developed a control system to maintain the cavity
Figure 1. Poly(methyl methacrylate) parts molded without drying (a)
without gas counter pressure (GCP) and (b) with GCP.
pressure, which needs to be adjusted to a preset value to cope with a
change in the unfilled cavity volume and melt pressure. Furthermore, a
reservoir with sufficient volume is needed.
We evaluated the performance of our GCP molding method by
checking the visual quality and the mechanical properties of parts
molded with poly(methyl methacrylate) (PMMA) and polycarbonate
(PC). We used an electric injection molding machine (Sumitomo SE-
50D) to test the proposed system. For all cases, we chose a mold
temperature of 60ıC and an injection speed of 50mm per second. Ten-
sile test parts made with undried PMMA and no GCP contained a large
number of bubbles whereas those made with GCP were free from bub-
bles (see Figure 1). The GCP has a similar role in suppressing the
formation of voids as in previous foam-molding studies.7
We used our GCP system to manufacture a commercial product part
made entirely from PC recycled from scraps, which are not dried prior
to molding (see Figure 2). The part molded without GCP has a large
number of visible splashes on the surface whereas the part molded
under the GCP has an unblemished, glossy surface. The part quality
would be acceptable for commercial applications.
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10.1002/spepro.004323 Page 2/3
In summary, we developed an injection molding technology that
facilitates manufacture of perfect parts without the need for an energy
intensive drying process by keeping the pressure above that of satura-
tion to raise the saturation temperature. The GCP system prevents ab-
sorbed water from evaporating out of the polymer during molding and
can be transferred to commercial processes with ease. We are currently
investigating the vaporization mechanism during injection molding. An
easier mold modification technique for the GCP is also under develop-
ment.
The Small and Medium Business Administration, Korea, funded the
system development. This study has also been conducted through the
Development Project of Large Surface Micro-Machining System Tech-
nology funded by the Ministry of Knowledge Economy, Korea. The
Industry Academic Cooperation Foundation at Seoul National Uni-
versity of Science and Technology has also supported this study. We
thank Dr Kun Sup Hyun and Dr Myung Ho Kim for discussions at the
SPE Korea meeting. Uni-Solution Plus, Korea, has commercialized our
whole system, which is called UNIMAS.
Figure 2. A commercial part made from 100% recycled polycarbonate
(a) without GCP and (b) with GCP.
Author Information
Sang-Won Woo
NID Fusion Graduate School
Seoul National University of Science and Technology
Seoul, South Korea
Sang-Won Woo is a graduate student working on injection molding.
Yeong-Eun Yoo
Nano-Mechanical Systems Research Division
Korea Institute of Machinery and Materials
Daejeon, South Korea
Yeong-Eun Yoo was a senior scientist at LG Chemistry and is currently
a principal researcher. His work focuses on roll-to-roll forming and in-
jection molding of nano- and microstructures.
Sun Kyoung Kim
Seoul National University of Science and Technology
Seoul, South Korea
Sun Kyoung Kim’s research focuses on polymer characterization and
processing. He is the director of industry-academia collaborations for
SPE’s Korea Section.
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