Gas-assisted Injection Molding Filling / Filling Process Flow Numerical Simulation

An innovative molding technique, gas assisted injection molding (GAIM) has received extensive attention in recent years. In this process, molten plastics are injected into a mold followed by an injection of pressurized nitrogen gas into the core of the melt to produce hollow parts. The pressurized gas follows the path of least resistance and penetrates toward the melt front through the thickest sections of the part. The melt is pushed to the extremities of the cavity by the advancing gas. The required injection pressures and the tonnages employed in GAIM are much lower than in conventional injection molding. In addition, with properly designed gas channels, the pressure distribution in GAIM is more uniform than in conventional injection molding so that warpage is reduced. The GAIM process is capable of producing parts having both thick and thin sections, with structured rigidity, and not sacrificing surface quality . It may also reduce equipment cost, material usage and cycle time .GAIM is, however, inherently more complex than conventional injection molding. On the one hand, it involves many additional process parameters associated with gas injection, such as gas delay time, gas pressure and gas injection time . On the other hand, to achieve a stable gas penetration, usually additional ribs are incorporated into part designs as gas channels to guide the gas flow. The addition of gas channels on a part will significantly alter the melt flow behavior and subsequently the gas penetration dynamics. While it is closely linked with the quality of the final molded parts, gas penetration is also well known for its sensitivity to various surrounding conditions, such as slight variations in process conditions or melt-property disturbances.Computer simulation is expected to become an important and required tool to help with part design and process evaluation in the coming age. The study employes a unified theoretical model to simulate the filling/posting stages of the GAIM. Implementation of such a model is based on a hybrid FEM/FDM numerical solution of the generalized Hele-Shaw flow of a compressible viscous fluid under nonisothermal conditions. The shear viscosity of the polymeric material is represented bya cross model for the shear-rate dependence and a WLF-type functional form for the temperature and pressure dependence, whereas the specific volume is modeled in terms of a double-domain Tait equation. The analysis also handles variable specific heat and thermal conductivity of the polymer as a function of temperature. Complex thin parts of variable thickness can be moldeled and discrietized by flat, triangular finite elements which may have arbitary orientation in 3D space, whereas runners are reprented as ID circular-tube elements, and the gas channel of a semicicrcular cross section is approximated by a model which uses a circular pipe of an equivalent hydraulic diameter superimposed on the thin part. A scheme based on the control-volume/finite-element method combined with a dual-filling parameters technique suitable for the tracing of two-component flow-front advancements is utilized and numerically implemented to predicted both melt and gas-front advacements during the melt-filling and the gas-assisted filling/post-filling processes.The capabilities of the numerical model are demonstrated through the analysis of several illustration cases. The influence of different molding parameters such as the dimension of gas channel and the percentage of polymer fill is investigated for these cases.