Counterpressure for microcellular injection molding
10.1002/spepro.000055
Counterpressure for microcellular injection molding Shia-Chung Chen
Applying different gas pressures at the melt stage improves parts production by controlling foaming and viscosity. Microcellular injection molding (MuCell) is an innovative manufacturing technology in which supercritical (i.e., subject to extreme pressure) fluid (SCF) is dissolved into a polymer melt within a specially designed screw.1 During injection, the SCF vaporizes and becomes gas bubbles (foam) within molded parts. Because the bubbles can in principle reach micron sizes, the process is known as microcellular foaming. MuCell offers many advantages over conventional injection technologies, such as less shrinkage, light weight, low materials cost, and fewer sink marks. In addition, MuCell-produced parts show less warping and closer tolerances, and in many cases also cool more quickly. Despite these benefits, widespread application of MuCell is hampered by inadequate control over the microcellular foaming process. When foaming occurs along the melt front, advancement frequently introduces streaklike flow marks on the molded surface. These imperfections can be remedied using co-injection or in-mold decoration technology,2 but at a high cost. The SCF foaming also causes changes in melt viscosity and other physical properties. This leads to nonuniform bubble sizes, which limits the use of MuCell for optical parts such as light guide plates that rely on diffusion. Here, we describe a gas-counterpressure (GCP) method that controls foaming by applying different gas pressures at the melt-injection stage (see Figure 1).3 The injection speed is determined by the difference between the screw pressure (Pscrew ) and that of the gas (Pgas ). When Pscrew is slightly higher than Pgas , and both parameters are high enough, the SCF-dissolved melt flows into the mold cavity without foaming. Setting Pscrew higher than Pgas , and Pgas lower than the critical pressure, results in partial foaming. Finally, the appropriate choice of Pscrew , Pgas , and pressure difference combined with dynamic mold temperature enables more precise control of bubble size. A preliminary test shows that it is quite possible to eliminate flowinduced streaks using the GCP technique. We also measured how foaming affects the viscosity of the melt with dissolved SCF.4, 5
Figure 1. Schematic drawing of the microcellular injection molding (MuCell) process using gas counterpressure (GCP). P: Pressure. Pg: Gas pressure.
We built a slit rheometer (see Figure 2) to investigate the rheological (flow) behavior of a polystyrene melt dissolved with 0.4wt% SCF of N2 under different mold temperatures (185, 195, and 205◦ C) (see Figure 3), injection speeds (5, 10, and 15mm/s screw speed), and GCPs (50, 100, 200, and 300 bar). The measured shear rate is within the 3000–11000s−1 range. We found that the glass-transition temperature, Tg , can be reduced from 96 to 50◦ C when the GCP is 300 bar. In addition, compared with conventional injection molding, melt viscosity goes down by about 30% when the GCP is increased from 50 to 200 bar. When the GCP is 300 bar, the viscosity of the single-phase injection melt without any foaming can be reduced by as much as 50%, depending on the injection conditions. This suggests the potential for energy savings owing to lower pressure and temperature requirements. As a next step, we plan to use GCP to optimize the temperature and pressure of the mold cavities during the MuCell process. We will also
Continued on next page
10.1002/spepro.000055 Page 2/2
Author Information Shia-Chung Chen Center for Mold and Molding Technology and Department of Mechanical Engineering Chung Yuan Christian University Chung-Li, Taiwan
Figure 2. Schematic drawing of MuCell injection molding using a GCP system. N2 : Nitrogen gas. CO2 : Carbon dioxide. SCF: Supercritical fluid.
Shia-Chung Chen is professor of mechanical engineering and dean of the Engineering College at Chung Yuan Christian University, where he also directs the R&D Center for Mold and Molding Technology. In addition, since 2005 he has served as chair of the board of directors of the Society of Advanced Molding Technology. References 1. D. F. Baldwin, D. E. Tate, C. B. Park, S. W. Cha, and N. P. Suh, Microcellular plastics processing technology, J. Japan Soc. Polym. Process. 6, pp. 187–256, 1994. 2. L. S. Turng and H. Kharbas, A novel microcellular co-injection molding process, Proc. ANTEC, pp. 535–539, 2004. 3. S. C. Chen, R. P. Yang, Y. W. Lin, P. S. Hsu, and S. S. Hwang, Study on pressure control device to improve foaming uniformity for the injection molding microcellular foaming process, Proc. ANTEC, pp. 699–703, 2007. 4. J. R. Royer, J. M. DeSimone, and S. A. Khan, High pressure rheology and viscoelastic scaling predictions of polymer melts containing liquid and supercritical carbon dioxide, J. Polym. Sci. B Polym. Phys. 39, pp. 3055–3066, 2001. 5. C. Kwag, C. W. Manke, and E. Gulari, Rheology of molten polystyrene with dissolved supercritical and near-critical gases, J. Polym. Sci. B Polym. Phys. 37 (19), pp. 2771–2781, 1999.
Figure 3. Measured viscosities of a polystyrene melt under different foaming conditions. Pa-s: Pascal seconds.
investigate ways of making bubble size more uniform and try to improve the surface quality of components by controlling the drift of fluid along the melt front.
c 2009 Society of Plastics Engineers (SPE)
Counterpressure for microcellular injection molding Shia-Chung Chen
Applying different gas pressures at the melt stage improves parts production by controlling foaming and viscosity. Microcellular injection molding (MuCell) is an innovative manufacturing technology in which supercritical (i.e., subject to extreme pressure) fluid (SCF) is dissolved into a polymer melt within a specially designed screw.1 During injection, the SCF vaporizes and becomes gas bubbles (foam) within molded parts. Because the bubbles can in principle reach micron sizes, the process is known as microcellular foaming. MuCell offers many advantages over conventional injection technologies, such as less shrinkage, light weight, low materials cost, and fewer sink marks. In addition, MuCell-produced parts show less warping and closer tolerances, and in many cases also cool more quickly. Despite these benefits, widespread application of MuCell is hampered by inadequate control over the microcellular foaming process. When foaming occurs along the melt front, advancement frequently introduces streaklike flow marks on the molded surface. These imperfections can be remedied using co-injection or in-mold decoration technology,2 but at a high cost. The SCF foaming also causes changes in melt viscosity and other physical properties. This leads to nonuniform bubble sizes, which limits the use of MuCell for optical parts such as light guide plates that rely on diffusion. Here, we describe a gas-counterpressure (GCP) method that controls foaming by applying different gas pressures at the melt-injection stage (see Figure 1).3 The injection speed is determined by the difference between the screw pressure (Pscrew ) and that of the gas (Pgas ). When Pscrew is slightly higher than Pgas , and both parameters are high enough, the SCF-dissolved melt flows into the mold cavity without foaming. Setting Pscrew higher than Pgas , and Pgas lower than the critical pressure, results in partial foaming. Finally, the appropriate choice of Pscrew , Pgas , and pressure difference combined with dynamic mold temperature enables more precise control of bubble size. A preliminary test shows that it is quite possible to eliminate flowinduced streaks using the GCP technique. We also measured how foaming affects the viscosity of the melt with dissolved SCF.4, 5
Figure 1. Schematic drawing of the microcellular injection molding (MuCell) process using gas counterpressure (GCP). P: Pressure. Pg: Gas pressure.
We built a slit rheometer (see Figure 2) to investigate the rheological (flow) behavior of a polystyrene melt dissolved with 0.4wt% SCF of N2 under different mold temperatures (185, 195, and 205◦ C) (see Figure 3), injection speeds (5, 10, and 15mm/s screw speed), and GCPs (50, 100, 200, and 300 bar). The measured shear rate is within the 3000–11000s−1 range. We found that the glass-transition temperature, Tg , can be reduced from 96 to 50◦ C when the GCP is 300 bar. In addition, compared with conventional injection molding, melt viscosity goes down by about 30% when the GCP is increased from 50 to 200 bar. When the GCP is 300 bar, the viscosity of the single-phase injection melt without any foaming can be reduced by as much as 50%, depending on the injection conditions. This suggests the potential for energy savings owing to lower pressure and temperature requirements. As a next step, we plan to use GCP to optimize the temperature and pressure of the mold cavities during the MuCell process. We will also
Continued on next page
10.1002/spepro.000055 Page 2/2
Author Information Shia-Chung Chen Center for Mold and Molding Technology and Department of Mechanical Engineering Chung Yuan Christian University Chung-Li, Taiwan
Figure 2. Schematic drawing of MuCell injection molding using a GCP system. N2 : Nitrogen gas. CO2 : Carbon dioxide. SCF: Supercritical fluid.
Shia-Chung Chen is professor of mechanical engineering and dean of the Engineering College at Chung Yuan Christian University, where he also directs the R&D Center for Mold and Molding Technology. In addition, since 2005 he has served as chair of the board of directors of the Society of Advanced Molding Technology. References 1. D. F. Baldwin, D. E. Tate, C. B. Park, S. W. Cha, and N. P. Suh, Microcellular plastics processing technology, J. Japan Soc. Polym. Process. 6, pp. 187–256, 1994. 2. L. S. Turng and H. Kharbas, A novel microcellular co-injection molding process, Proc. ANTEC, pp. 535–539, 2004. 3. S. C. Chen, R. P. Yang, Y. W. Lin, P. S. Hsu, and S. S. Hwang, Study on pressure control device to improve foaming uniformity for the injection molding microcellular foaming process, Proc. ANTEC, pp. 699–703, 2007. 4. J. R. Royer, J. M. DeSimone, and S. A. Khan, High pressure rheology and viscoelastic scaling predictions of polymer melts containing liquid and supercritical carbon dioxide, J. Polym. Sci. B Polym. Phys. 39, pp. 3055–3066, 2001. 5. C. Kwag, C. W. Manke, and E. Gulari, Rheology of molten polystyrene with dissolved supercritical and near-critical gases, J. Polym. Sci. B Polym. Phys. 37 (19), pp. 2771–2781, 1999.
Figure 3. Measured viscosities of a polystyrene melt under different foaming conditions. Pa-s: Pascal seconds.
investigate ways of making bubble size more uniform and try to improve the surface quality of components by controlling the drift of fluid along the melt front.
c 2009 Society of Plastics Engineers (SPE)
