Korea-Australia Rheology Journal

The Korean Society of Rheology (한국유변학회)
권호 표지

A study of birefringence, residual stress and final shrinkage for precision injection molded parts

  • Yang, Sang-Sik (Department of Mechanical Engineering Pohang University of Science and Technology) ;
  • Kwon, Tai-Hun (Department of Mechanical Engineering Pohang University of Science and Technology)
  • Published : 2007.12.31
Download PDF
⟨ Previous Next ⟩

Abstract

Precision injection molding process is of great importance since precision optical products such as CD, DVD and various lens are manufactured by those process. In such products, birefringence affects the optical performance while residual stress that determines the geometric precision level. Therefore, it is needed to study residual stress and birefringence that affect deformation and optical quality, respectively in precision optical product. In the present study, we tried to predict residual stress, final shrinkage and birefringence in injection molded parts in a systematic way, and compared numerical results with the corresponding experimental data. Residual stress and birefringence can be divided into two parts, namely flow induced and thermally induced portions. Flow induced birefringence is dominant during the flow, whereas thermally induced stress is much higher than flow induced one when amorphous polymer undergoes rapid cooling across the glass transition region. A numerical system that is able to predict birefringence, residual stress and final shrinkage in injection molding process has been developed using hybrid finite element-difference method for a general three dimensional thin part geometry. The present modeling attempts to integrate the analysis of the entire process consistently by assuming polymeric materials as nonlinear viscoelastic fluids above a no-flow temperature and as linear viscoelastic solids below the no-flow temperature, while calculating residual stress, shrinkage and birefringence accordingly. Thus, for flow induced ones, the Leonov model and stress-optical law are adopted, while the linear viscoelastic model, photoviscoelastic model and free volume theory taking into account the density relaxation phenomena are employed to predict thermally induced ones. Special cares are taken of the modeling of the lateral boundary condition which can consider product geometry, histories of pressure and residual stress. Deformations at and after ejection have been considered using thin shell viscoelastic finite element method. There were good correspondences between numerical results and experimental data if final shrinkage, residual stress and birefringence were compared.

Keywords

References

  1. Baaijens, F. P. T. 1991, Calculation of residual stresses in injection molded products, Rheol. Acta 30, 284-299 https://doi.org/10.1007/BF00366642
  2. Ballman, R. L. and H. L. Torr, 1960, Modern Plastics 38, 113
  3. Batoz, J. L., K. J. Bathe and L. W. Ho, 1980, Study of three-node triangular plate bending elements, International Journal for Numerical Methods in Engineering 15, 1771-1812 https://doi.org/10.1002/nme.1620151205
  4. Bergan, P. G. and C. A. Felippa, 1985, Triangular membrane element with rotational degrees of freedom, Computer Methods in Applied Mechanics and Engineering 50, 25-69 https://doi.org/10.1016/0045-7825(85)90113-6
  5. Bushko, W. C. and V. K. Stokes, 1995a, Solidification of thermoviscoelastic melts. part I: formulation of model problem, Polym. Eng. Sci. 35, 351-364 https://doi.org/10.1002/pen.760350409
  6. Bushko, W. C. and V. K. Stokes, 1995b, Solidification of thermoviscoelastic melts. part II: effects of processing conditions on shrinkage and residual stresses. Polym. Eng. Sci. 35, 365-384 https://doi.org/10.1002/pen.760350410
  7. Chiang, H. H., C. A. Hieber and K.K. Wang, 1991a, A nuified simulation of the filling and postfilling stages in injection molding. part I: formulation, Polym. Eng. Sci. 31, 116-124 https://doi.org/10.1002/pen.760310210
  8. Chiang, H. H., C. A. Hieber and K.K. Wang, 1991b, A nuified simulation of the filling and postfilling stages in injection molding. part II: experimental verification, Polym. Eng. Sci. 31, 125-139 https://doi.org/10.1002/pen.760310211
  9. Chiang, H. H., K. Himasekhar, N. Santhanam and K. K. Wang, 1993, Integrated simulation of fluid flow and heat transfer in injection molding for the prediction of shrinkage and warpage, J. Eng. Mat. And Tech. 115, 37-47 https://doi.org/10.1115/1.2902155
  10. Coxon, L. D. and J. R. White, 1979, Measurement of internal stresses in chemically cross-linked high-density polyethylene, J. Mater. Sci. 14, 1114-1120 https://doi.org/10.1007/BF00561295
  11. Coxon, L. D. and J. R. White, 1980, Residual stresses and aging in injection molded polypropylene, Polym. Eng. Sci. 20, 230-236 https://doi.org/10.1002/pen.760200311
  12. Dietz, W. and J. L. White, 1978, Ein einfaches Modell zur Berechnung des Druckverlustes wahrend des Werkzeugfullvorganges und der eingefrorenen Orientierung beim Spritzgiessen amorpher Kunststoffe, Rheol. Acta 17, 676-692 https://doi.org/10.1007/BF01522040
  13. Flaman, A. A. M., 1993a, Buildup and relaxation of molecular orientation in injection molding. part I: formulation, Polym. Eng. Sci. 33, 193-201 https://doi.org/10.1002/pen.760330402
  14. Flaman, A. A. M., 1993b, Buildup and relaxation of molecular orientation in injection molding. part II: experimental verification, Polym. Eng. Sci. 33, 202-210 https://doi.org/10.1002/pen.760330403
  15. Friedrichs, B., M. Horie and Y. Yamaguchi, 1996, Simulation and analysis of birefringence in magneto-optical discs. part A: formulation, J. Materials Processing & Manuf. Sci. 5, 95-113
  16. Ghoneim H. and C. A. Hieber, 1997, Incorporation of density relaxation in the analysis of residual stresses in molded parts, Polym. Eng. Sci. 37, 219-227 https://doi.org/10.1002/pen.11664
  17. Greener, J. and G. H. Pearson, 1983, Orientation residual stresses and birefringence in injection molding, J. Rheol. 27, 116-134
  18. Hastenberg, C. H. V., P. C. Wildervanck, A. J. H. Leenen and G.G. J. Schennink, 1992, The measurement of thermal stress distributions along the flow path in injection-molded flat plates, Polym. Eng. Sci. 32, 506-515 https://doi.org/10.1002/pen.760320708
  19. Isayev, A. I. and C. A. Hieber, 1980, Toward a viscoelastic modelling of the injection molding of polymers, Rheol. Acta 19, 168-182 https://doi.org/10.1007/BF01521928
  20. Isayev, A. I., 1983, Orientation development in the injection molding of amorphous polymers, Polym. Eng. Sci. 23, 271-284 https://doi.org/10.1002/pen.760230507
  21. Isayev, A. I. and D. L. Crouthamel, 1984, Residual stress development in the injection molding of polymers, Polymer-plastics technology and engineering 22, 177-232 https://doi.org/10.1080/03602558408070038
  22. Janeschitz-Kriegl, H., 1983, Polymer melt rheology and flow birefringence, Springer-Verlag, Berlin
  23. Kaliske, M. and R. Rothert, 1997, Formulation and implementation of three-dimensional viscoelasticity at small and finite strains, Computational Mechanics 19, 228-239 https://doi.org/10.1007/s004660050171
  24. Kamal, M. R. and V. Tan, 1979, Orientation in injection molded polystyrene, Polym. Eng. Sci. 19, 558-563 https://doi.org/10.1002/pen.760190806
  25. Famili, N. and A.I. Isayev, 1991, Modeling of Polymer Processing, Hanser Publisher, New York
  26. Kim, I. H., S. J. Park, S. T. Chung and T. H. Kwon, 1999a, Numerical modeling of injection/compression molding for center-gated disk: part I: injection molding with viscoelastic compressible fluid model, Polym. Eng. Sci. 39, 1930-1942 https://doi.org/10.1002/pen.11586
  27. Kim, I. H., S. J. Park, S. T. Chung and T. H. Kwon, 1999b, Numerical modeling of injection/compression molding for center-gated disk: part II: effect of compression stage, Polym. Eng. Sci. 39, 1943-1951 https://doi.org/10.1002/pen.11587
  28. Lee, Y. B. and T. H. Kwon, 2002a, Numerical prediction of residual stresses and birefringence in injection/compression molded center-gated disk. part I: basic modeling and results for injection molding, Polym. Eng. Sci. 42, 2246-2272 https://doi.org/10.1002/pen.11114
  29. Lee, Y. B. and T. H. Kwon, 2002b, Numerical prediction of residual stresses and birefringence in injection/compression molded center-gated disk. part II: Effects of processing conditions, Polym. Eng. Sci. 42, 2273-2292 https://doi.org/10.1002/pen.11115
  30. Lee, Y. B. and T. H. Kwon, 2001, PPS 17th Annual meeting, PPS-17
  31. Leonov, A. I., 1976, Nonequilibrium thermodynamics and rheology of viscoelastic polymer media, Rheol. Acta 15, 85-98 https://doi.org/10.1007/BF01517499
  32. Leonov, A. I., E. H. Lipkina, E. D. Paskhin and A. N. Prokunin, 1976, Theoretical and experimental investigation of shearing in elastic polymer, Rheol. Acta 15, 411-426 https://doi.org/10.1007/BF01574496
  33. Mandell, J. F., K. L. Smith and D. D. Huang, 1981, Effects of residual stress and orientation on the fatigue of injection molded polysulfone, Polym. Eng. Sci. 21, 1173-1180 https://doi.org/10.1002/pen.760211711
  34. Pham, H. T., C. P. Bosnyak and K. Sehanobish, 1993, Residual stresses in injection molded polycarbonate rectangular bars, Polym. Eng. Sci. 33, 1634-1643 https://doi.org/10.1002/pen.760332408
  35. Russell, D. P. and P. W. R. Beaumont, Structure and properties of injection-moulded nylon-6 - part 2 residual stresses in injection-moulded nylon-6, J. Mater. Sci., 15, 208-215 https://doi.org/10.1007/BF00552446
  36. Santhanam, N., 1992, PhD thesis, Cornell university, Ithaca, N.Y
  37. Sandilands, G. J. and J. R. White, 1980, Effect of injection pressure and crazing on internal stresses in injection-moulded polystyrene, Polymer 21, 338-343 https://doi.org/10.1016/0032-3861(80)90278-5
  38. Shyu, G. D. and A. I. Isayev, 1995, Residual stresses and birefringence in injection molded disks, SPE ANTEC Tech. Papers 41, 2911-2917
  39. Shyu, G. D., 1993, PhD thesis, The university of Akron
  40. Siegmann, A., A. Buchman and S. Kenig, 1982, Residual stresses in polymers - 3. the influence of injection-molding process conditions, Polym. Eng. Sci. 22, 560-568 https://doi.org/10.1002/pen.760220908
  41. Treuting, R. G. and W. T. Read Jr., 1951, A mechanical determination of biaxial residual stress in sheet materials, J. Appl. Phys. 22, 130-134 https://doi.org/10.1063/1.1699913
  42. Wales, J. L. S., Ir J. Van Leeuwen and R. Van Der Vijgh, 1972, Some aspects of orientation in injection molded objects, Polym. Eng. Sci. 12, 358-363 https://doi.org/10.1002/pen.760120508
  43. White, J. L., 1991, Principles of polymer engineering rheology, John Wiley & Sons, New York