Injection Molding of Polymers and Polymer Composites: Process Modeling and Simulation
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Moreover, as for many other manufacturing processes, the lack of fundamental scientific understanding has made the operation (from mold design to process control) unpredictable. The complex properties of polymeric materials make the situation even more difficult than for most other materials. As a result, injection molding has long been essentially an art rather than a science. Mold design has depended heavily on the designer's prior experience with particular types of products or materials; setting up processing parameters has required extensive trial-and-error experiments to achieve an acceptable product. Furthermore, the industry has been dominated by small custom molders who operate over 50% of the injection-molding machines.2 No significant research was conducted on the subject until the early 1970s.3"14 Since then, considerable effort has gone into understanding the physics of the process so that the flow and solidification of the polymer melt in the mold throughout the entire molding operation can be mathematically represented and analyzed.
Simulation of the Mold-Filling Process Substantial progress has been made over the past 17 years in modeling and simulating the injection-molding process.15 This is evidenced by the material contained in recent books16,17 concerning the application of CAE (computer-aided engineering) to injection molding as well as the fundamental treatment of the process. A growing number of commercial CAE software
packages have also become available in recent years. Even though these computer programs provide engineers with a useful tool, success based on the accuracy of predictions varies widely from case to case. The underlying reasons are many, from lack of basic understanding of the materials' behavior during the process to the inability to represent the dynamic process mathematically. During injection molding, the viscoelastic polymer melt undergoes a thermomechanical process in a very short period of time, a few seconds in most cases. In terms of pressure history at any location in the mold cavity, Figure 1 shows qualitatively that the process consists of three distinctive stages. Stage I from point A to F represents the complete filling of the cavity. A short packing Stage II from F to B follows to bring the cavity pressure abruptly to a much higher level in order to bring out any fine details (e.g., sharp corners or impressions) and compensate for any shrinkage due to cooling. The partially solidified material is subject to continuous cooling during the holding phase, Stage III from B to D, and at some point C the gate freezes and no more new material can be further packed into the cavity. Such a molding cycle can be correspondingly represented in a PVT (pressure-volume-temperature) diagram shown in Figure 2, which characterizes the thermophysical properties of the material during the molding process. Theoretically speaking, if an equation of state of a given material and desirable PVT path for that material can be established, one should be able to predict (and thereby contro
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