Injection molding is a widely used process to produce parts with variable dimensions. An injection molding machine consists of the following four components:
The purpose of the injection unit is to heat and melt the polymer, inject the melt into the cavity, and apply pressure during the cooling phase. The most common type of injection molding machine is the reciprocating screw. In this type of machine, the screw rotates to plasticize the polymer, moving backward to deposit a volume of polymer melt ahead of the screw (shot). Once the correct shot size has been built, the screw then moves forward to inject the melt into the mold. Injection molding is a discontinuous process, and the clamping unit allows for the mold to open and close for part removal and to provide pressure as the cavity is filled. This is depicted in Fig. 1.19.
The purpose of an injection mold is to give the shape of the part (cavity), distribute the polymer melt to the cavities through a runner system, cool the part, and eject the part. During the injection molding cycle, the polymer flows from the nozzle on the injection unit through the sprue, then to the runners, which distribute the melt to each of the cavities. The entrance to the cavity is called the gate and is usually small so that the runner system can be easily removed from the part. A typical feed system for injection molding is shown in Fig. 1.20.
Figure 1.21
depicts a number of gate configurations. Molding conditions for a wide variety of thermoplastics are given in Table 1.7.
The molding process itself can have a large influence on the final properties of the part. The polymer chains undergo orientation in the flow direction during the mold filling phase of the injection cycle as shown in Fig. 1.22.
The amount of orientation in the final part depends on how much orientation was induced during filling minus the amount that was been removed through molecular relaxation. This is particularly true of the surface of the part, where hot material reaches the cool walls of the mold with rapid solidification coupled with the highest shear and induced orientation at the mold surface. Orientation can result in parts with anisotropic properties and should be accounted for during part design. Mechanical properties are thus typically higher in the direction of orientation. For semicrystalline polymers, the injection molding process parameters can have a large impact on the degree of crystallinity. As the cooling rate increases, the degree of crystallinity will decrease. Cooling rate effects can cause a gradient of crystallinity across the thickness of a part where interior portions of the part may have higher crystallinity due to their slower cooling rates compared to the surface. The crystalline morphology will also be influenced by the cooling behavior. Slower cooling rates result in larger spherulites, while more rapid cooling rates result in a larger number of smaller spherulites. A number of specialized injection molding processes also exist and are outlined below.
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2 comments to "Injection Molding"
August 20, 2013 at 5:35 AM
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March 17, 2019 at 11:31 PM
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