【正文】 ns. For example, the casing for disposable cameras is made via injection molding. The geometry for those pieces include several plicated curves, ribs, holes, protruding cylinders, and other elements. Even so, new applications and modifications of injection molding are being developed to increase the usefulness, applicability, and profitability of injection molding. One such emerging technology is that of coinjection molding.There are two main types of coinjection molding. The first is referred to as two color molding. In twocolor molding a part is molded using one resin, and then a second resin is molded into the first once it is sufficiently cooled after some part of the mold retracts or the part rotates to a second larger cavity. This technology is used in products such as puter keys and multicolored automotive taillights. The second, and more mon, type of coinjection molding involves the injection of a skin material and a core material. This is also known as “sandwich molding” because the core material is sandwiched in between the skin material. Sandwich molding is a variation of injection molding in which two different plastics are either simultaneously or in rapid sequence injected through the same gate. One material makes up the core of the product and the other makes up the skin. Through the bination of two different materials several benefits can be extracted that traditional injection molding can not supply by itself. Figure 1: Sandwich molding machine with a mon nozzle and a switchieadThe sandwich injection molding process was patented in 1969. It was first invented to be an alternative to the structural foam process. Traditionally structural foam needed postprocessing operations such as polishing and painting. The sandwich molding process could use a cellular core with a solid skin to obtain a good surface finish. The process has been mercially used since 1975. The sandwich molding process is achieved by a machine that has two separate and individually controlled injection units. Since the two materials are injected through the same gate, they must have a mon injection nozzle and a switching head. Figure 1 illustrates a sandwich molding machine. The sandwich molding process is more mon than the twocolor process, so the rest of this paper will focus on sandwich molding (referred to just as coinjection molding throughout the rest of the report).The Coinjection Molding Process:The first possibility for injecting the two materials is to do so sequentially. This is achieved using one channel. Figure 2 illustrates the steps in the sequential coinjection process. Initially, a shot of the skin material is injected into the mold but does not pletely fill it. This is known as a “short shot.” In the next step the core material is injected. A frozen layer begins to form at the cold mold walls. In the next stage, a shot of the core material is injected to almost pletely fill the part. The phenomena known as the fountain flow effect of polymer inside the mold will cause the skin material that is still in the melt phase near the center of the mold thickness to be pushed out against the mold walls creating the 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 divided into four regions: 1) the skin material is injected, 2) the skin injection stops and the core material is injected, 3) the core flow front reaches the skin flow front but does not break through (the two material advance together), 4) the core flow front breaks through the skin flow front (the core material will appear at th