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.

Continued on next page

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.

References

1. M. J. Gordon Jr., Total Quality Process Control for Injection Molding, Wiley, NewYork, 2010.

2. M. F. Lacrampe and J. Pabiot, Defects in surface appearance of injection mouldedthermoplastic parts: a review of some problems in surface gloss distribution, J. Inj.Molding Technol. 4, pp. 167–176, 2000.

3. A. M. Grillet, A. C. B. Bogaerds, G. W. M. Peters, and F. P. T. Baaijens, Numericalanalysis of flow mark surface defects in injection molding flow, J. Rheol. 46, pp. 651–669, 2002. doi:10.1122/1.1459419

4. J. Bozzelli, Injection molding: you must dry hygroscopic resins, Plast. Technol. 57,p. 27, 2011.

5. M. J. Moran and H. N Shapiro, Fundamentals of Engineering Thermodynamics,6th ed., Wiley, New York, 2008.

6. S. B. Driscoll and W. F. Gacek, Gas counter pressure structural foam molding versusconventional low-pressure SF molding - a comparison of mechanical properties, Plast.Eng. 40, p. 34, 1984.

7. J. S. Wu and M. J. Lee, Studies on gas counter pressure and low pressure structuralfoam molding, III. Effect of processing conditions on mechanical properties of moldedparts, Plast. Rubber Compos. 21, pp. 163–171, 1994.

8. S. Djoumaliisky, D. Christova, N. Touleshkov, and E. Nedkov, Morphol-ogy and orientation of PP structural foam moldings, J. Macromol. Sci.Part A – Pure Appl. Chem. 35 (7), pp. 1147–1158, 1998.doi:10.1080/10601329808002108

9. S. Djoumaliisky, M. L. Cerrada, T. Dobreva, and P. Zipper, Development ofˇ and ˛ isotactic polypropylene polymorphs in injection molded structuralfoams, Chem. Pap. 64 (2), pp. 246–254, 2010. doi:10.2478/s11696-009-0107-6

Continued on next page

10.1002/spepro.004323 Page 3/3

10. A. K. Bledzki, H. Kirschling, G. Steinbichler, and P. Egger, Polycarbonate microfoamswith a smooth surface and higher notched impact strength, J. Cell. Plast. 40, pp. 489–496, 2004. doi:10.1177/0021955X04048423

11. G. Menges, W. Michaeli, and P. Mohren, How to Make Injection Molds, 3rd ed.,Hanser, New York, 2001.

12. R. B. Johnson and M. Caropreso, Designing for counterpressurefoam molding, Polym.-Plast. Technol. Eng. 25, pp. 187–207, 1986.doi:10.1080/03602558608070083

c� 2012 Society of Plastics Engineers (SPE)