外文翻譯---共射注塑成型技術(文件)
【正文】 e skin layer. Once the mold is almost filled, a small amount of skin material is injected to purge the core material from the spruce to eliminate any possibility for it to show up at the surface of the next shot. The problem with the onechannel method is that stagnation occurs at the switchover which results in a “hesitation line” in the shape of a dull ring. Also, it is difficult to achieve a uniform distribution of the core material. For this reason, the onechannel method is usually reserved for thickwalled parts with a foam core.Figure 1: Illustrations of the stages in sequential coinjection molding through a single nozzleThe other possible coinjection method is simultaneous injection of the two materials. This method utilizes nozzles with concentric flow channels such as those shown in figure 3. Simultaneous injection has the advantage of maintaining a constant flowfront velocity which helps to avoid flow marks and differences in gloss.Figure 2: Concentric injection channels used for simultaneous coinjection moldingThe ratio and distribution of the skin and core material depends on the materials used. The primary factors are the viscosity ratio of the two materials and the volume ratio, but it is also affected by mold geometry and the processing conditions (. injection speed). Figure 4 illustrates the effect that viscosity ratio can have on the skin/core distribution. It plays an important effect in determining the thickness uniformity and length of core penetration. The most uniform thickness distribution can be achieved by injecting a core that has a slightly higher viscosity than the skin. The viscosity ratio also had a significant effect on the mechanical properties of the final product. Using an improper volume ratio can give rise to the core material breaking through the skin material to the surface of the product.Figure 4: Illustrating the effects of the viscosity ratio on the thickness distribution of the core materialThe breakthrough phenomenon is a mon defect in coinjection molding. Besides the volume ratio it is dependent on the processing conditions such as injection speed, injection time of the core and skin materials, melt temperature and mold temperature. Figure 5 illustrates the relationship between flow length of both the skin and core materials for a sequential coinjection molding process for a case where the breakthrough phenomenon occurs. The graph is divide
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