Design & Manufacturing

Korean Society of Die & Mold Engineering (한국금형공학회)
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Study on sink-mark reduction and gas venting of injection molded parts using compressed air

압축공기를 사용한 사출성형품의 싱크마크 저감 및 가스 벤팅에 관한 연구

  • Se-Ho Lee (Dep. of Aeronautical & Mechanical Design Eng., Korea Nat'l Univ, of Transportation) ;
  • Ho-Sang Lee (Dep. of Aeronautical & Mechanical Design Eng., Korea Nat'l Univ, of Transportation)
  • 이세호 (한국교통대학교 항공기계설계학과) ;
  • 이호상 (한국교통대학교 항공기계설계학과)
  • Received : 2024.09.04
  • Accepted : 2024.09.30
  • Published : 2024.09.30
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Abstract

Sink marks are a common defect that occurs due to differences in shrinkage in areas with significant thickness variations in injection-molded parts. In this paper, we investigated the reduction of sink marks and the improvement of gas venting in injection molding processes using External Gas Injection (EGI). A mold was designed with considerations for EGI core pins, O-ring grooves to prevent gas leakage, and ejector-pin sealing. The sink marks were then examined through a series of experiments. When the delay time for injecting compressed air was set to 2.2 seconds, the depth of the sink marks was minimized. However, when the delay time was either too short or too long, the depth of the sink marks increased. There was almost no difference in the depth of the sink marks at discharge pressures of 30 and 50 bar of compressed air, but the sink marks were significantly reduced at a discharge pressure of 70 bar. Under the conditions of a 2.2-second delay time and a supply pressure of 70 bar, the smallest depth, 0.594 ㎛, was observed when the supply time was between 6 and 7 seconds. This represents a reduction of approximately 94% compared to the sink mark depth of 10.078 ㎛ observed with conventional injection molding. To verify the gas venting effect of compressed air injection, an experiment was conducted using non-dried PC. The silver streaks that appeared on the exterior of the molded part were completely eliminated when the air supply pressure was set to 20 bar. This indicates that by injecting compressed air into the mold cavity before injecting the resin, the appearance quality of the injection-molded part can be improved without the need to dry the resin in advance.

Keywords

Acknowledgement

본 연구는 2024년 한국교통대학교 산학협력단 지원을 받아 수행하였음

References

  1. Chang, R. Y. and Tsaur, B. D., "Experimental and theoretical studies of shrinkage, warpage, and sink mark of crystalline polymer injection molded parts," Polym. Eng. Sci., Vol. 35, No. 15, pp. 1222-1230, 1995.
  2. Kim, D.-E. and Lee, C.-K., "A study on the reduction method of sink marks for plastic products with T-shape," J. Korea Society of Die & Mold Engineering, Vol. 12, No. 2, pp. 22-26, 2018.
  3. Shen, C. Y., Wang, L. X., Cao, W. and Qian, L., "Investigation of the effect of molding variables on sink marks of plastic injection molded parts using Taguchi DOE technique," Polym-Plast Technol Eng, Vol. 46, pp. 219-225, 2007.
  4. Mathivanan, D. and Parthasarathy, N. S., "Prediction of sink depths using nonlinear modeling of injection molding variables," Int J Adv Manuf Technol, Vol. 43, pp. 654-663, 2009.
  5. Gruber, D. P., Berger, G., Pacher, G. and Friesenbichler, W., "Novel approach to the measurement of the visual perceptibility of sink marks on injection molding parts," Polym Testing, Vol. 30, pp. 651-656, 2011.
  6. Tursi, D. and Bistany, S. P., "Process and tooling factors affecting sink marks for amorphous and crystalline resins," J. Inject. Molding Technol., Vol. 4, pp. 114-119, 2000.
  7. Iyer, N. and Ramani, K., "A study of localized shrinkage in injection molding with high thermal conductivity molds," J. Inject. Molding Technol., Vol. 6, pp. 73-90, 2002.
  8. Nakayama, K., Komoda, M. and Motoichi, Y., "Optimization of rib design and possibilities of weight reduction for ABS molding," Annual Tech. Conf., Society of Plastic Engineers 41st Published by SPE, pp. 690-693, 1983.
  9. An, K. H., "Analysis of the void growth mechanism in partial frame process," The Korean Society of Rheology, Vol. 9, No. 2, pp. 60-65, 1997.
  10. Lee, H. S., 2001, "A Study on the unified molding for a box shaped thick part using gas-assisted injection molding," Transactions of Materials Processing, Vol. 10, No. 5, pp. 402-410, 2001.
  11. Kim, H. S. and Lee, D. K., "A case study on development of automotive interior parts using gas assisted injection molding process," Transactions of Materials Processing, Vol. 14, No. 5, pp. 452-459, 2005.
  12. Kang, S. H., Woo, C. K. and Kim, O. R., "Research on gas injection mold using CAE analysis of steering wheel parts," Journal of the Korea Academia-Industrial Cooperation Society, Vol. 16, No. 11, pp. 7729-7735, 2015.
  13. Mapleston, P., "External gas new variant of low-pressure injection", Mod. Plast., Vol. 71, pp. 23-25, 1994.
  14. Osswald, T. A., Turng, L. and Gramann, P. J., Injection Molding Handbook, Carl Hanser Publishers, Munich, 2001.
  15. Su, H.-Y., Nian, S.-C. and Huang, M.-S., "Reducing ghost marks in injection-molded plastic parts by using external gas-assisted holding pressure," Inter. Comm. in Heat and Mass Transfer, Vol. 66, pp. 1-10, 2015.
  16. Chen, S. C., Lin, Y. C. and Huang, S. W., "Study on the packing effects of external gas-assisted injection molding on part shrinkage in comparison with conventional injection molding," Polym. Eng. Sci., Vol. 50, N0. 11, pp. 2085-2092, 2010.
  17. Jiang, S., Zheng, W., Zhang J. and Li, J., "Experimental study on the influence of parameters on the plastic parts quality of external gas-assisted injection molding", Appl. Math. Inf. Sci., Vol. 6, pp. 665-671, 2012.