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Molding Processes
Published in Richard L. Shell, Ernest L. Hall, Handbook of Industrial Automation, 2000
Gas-assisted injection molding is a relatively novel molding process invented over 20 years ago [125–127]. The process is deceptively simple to carry out but difficult to control due to the dynamical interaction between gas and polymer melt. A schematic of the process is given in Fig. 14. The process comprises three stages: cavity filling, gas injection, and packing. In the cavity filling stage, a predetermined amount of the polymer melt is injected into the cavity to partially fill it. Then, in the gas injection stage, nitrogen gas under high pressure is injected through the nozzle or mold wall into plastic. The nitrogen is typically injected at pressures ranging from 0.5 to 30 MPa. Gas can be injected sequentially or simultaneously during the cavity filling stage. The gas penetrates through the thick sections where the melt is hot and pushes the plastic melt to fill the mold. The polymer in the skin layer is stagnant due solidification upon contact with the cold mold surface. Due to the geometrical complexity of parts, multiple disconnected gas channels with a multiple gas injection system are frequently used. This allows one to transmit the pressure uniformly to various areas of the part. In the packing stage, after the cavity is filled, the gas pressure is maintained to compensate for shrinkage and just prior to mold opening, gas is vented. Advantages of the gas-assisted injection molding process in comparison with the conventional injection molding are the reduction of cycle time, part weight, shrinkage, warpage, injection, pressure, and clamping force. In addition, the process allows for the improvement of the surface finish and the elimination of sink marks.
Investigation on crystallization kinetics and solidification behavior of isotactic polypropylene (iPP) using an enthalpy transformation method
Published in Phase Transitions, 2020
Shu-Qing Wang, Bin Yang, Dan Wang, Qian-Lei Zhang, Ji-Bin Miao, Li-Feng Su, Ru Xia, Jia-Sheng Qian, Peng Chen, You Shi
Phase transition problems, also known as the ‘moving boundary’ problems, widely exist in a variety of industrial processes, were first raised and reported by J. Stefan [1]. In recent years, phase-change materials (PCMs) have also been widely used as thermal storage materials with a high storage density in many applications [2–4]. Since the ‘moving boundary’ problems are mathematically strong non-linear, very few exact closed-form solutions are currently available except in some very simplified (idealized) systems [5]. Nowadays, many numerical methods have been reported to solve this kind of problems, e.g. the variational method [6], finite-element method (FEM) [7], finite-difference method (FDM) [8], boundary element method (BEM) [9], enthalpy method [10] as well as finite volume method (FVM) [11], etc. In our recent studies [12–14], an enthalpy transformation method (ETM) was proved to reasonably describe the temperature profiles of polyolefins and their blends during real polymer processing operations (including compression molding, injection molding, gas-assisted injection molding, etc).