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In injection molding, the polymer melt at high temperature is injected into the mold under high pressure [1]. Thus, the mold material needs to have thermal and mechanical properties capa¬ble of withstanding the temperatures and pressures of the mold¬ing cycle. The focus of many studies has been to create the injection mold directly by a rapid prototyping (RP) process. By eliminating multiple steps, this method of tooling holds the best promise of reducing the time and cost needed to create low-volume quantities of parts in a production material. The potential of integrating injection molding with RP technologies has been demonstrated many times. The properties of RP molds are very different from those of traditional metal molds. The key differ¬ences are the properties of thermal conductivity and elastic mod¬ulus (rigidity). For example, the polymers used in RP-fabricated stereolithography (SL) molds have a thermal conductivity that is less than one thousandth that of an aluminum tool. In using RP technologies to create molds, the entire mold design and injection-molding process parameters need to be modified and optimized from traditional methodologies due to the completely different tool material. However, there is still not a fundamen¬tal understanding of how the modifications to the mold tooling method and material impact both the mold design and the injec¬tion molding process parameters. One cannot obtain reasonable results by simply changing a few material properties in current models. Also, using traditional approaches when making actual parts may be generating sub-optimal results. So there is a dire need to study the interaction between the rapid tooling (RT) pro¬cess and material and injection molding, so as to establish the mold design criteria and techniques for an RT-oriented injection molding process.19748
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2 Integrated simulation of the molding process
2.1 Methodology
In order to simulate the use of an SL mold in the injection molding process, an iterative method is proposed. Different soft¬ware modules have been developed and used to accomplish this task. The main assumption is that temperature and load bound¬ary conditions cause significant distortions in the SL mold. The simulation steps are as follows:
1 The part geometry is modeled as a solid model, which is translated to a file readable by the flow analysis package.
2 Simulate the mold-filling process of the melt into a pho¬topolymer mold, which will output the resulting temperature and pressure profiles.
3 Structural analysis is then performed on the photopolymer mold model using the thermal and load boundary conditions obtained from the previous step, which calculates the distor¬tion that the mold undergo during the injection process.
4 If the distortion of the mold converges, move to the next step. Otherwise, the distorted mold cavity is then modeled (changes in the dimensions of the cavity after distortion), and returns to the second step to simulate the melt injection into the distorted mold.
5 The shrinkage and warpage simulation of the injection molded part is then applied, which calculates the final distor¬tions of the molded part.
In above simulation flow, there are three basic simulation mod¬ules.
2.2 Filling simulation of the melt
2.2.1 Mathematical modeling
Computer simulation techniques have had success in predicting filling behavior in extremely complicated geometries. However, most of the current numerical implementation is based on a hybrid finite-element/finite-difference solution with the middleplane model. The application process of simulation packages based on this model is illustrated in Fig. 2-1. However, unlike the surface/solid model in mold-design CAD systems, the so-called middle-plane (as shown in Fig. 2-1b) is an imaginary arbitrary planar geometry at the middle of the cavity in the gap-wise direction, which should bring about great inconvenience in applications. For example, surface models are commonly used in current RP systems (generally STL file format), so secondary modeling is unavoidable when using simulation packages because the models in the RP and simulation systems are different. Considering these defects, the surface model of the cavity is introduced as datum planes in the simulation, instead of the middle-plane.