The Injection Molding Process

Plastic is used in a huge range of products from cars and appliances to office supplies and toys. While many consumers may not realize it, the vast majority of plastic parts are manufactured through a process called injection molding. Injection molding uses a high-pressure electric or hydraulic machine that melts, injects and cools plastic to form a solid part that can then be removed from the mold. The complex manufacturing process is a highly efficient way to make a wide variety of plastic parts and components.

Injection molding is most often used to produce thermoplastic parts. Thermoplastics are polymers that turn molten at high temperatures and then solidify upon cooling. They can be reheated and remolded without degrading. Typical plastics for injection molding include polyethylene (PE), polypropylene (PP) and polystyrene (PS). Injection-molded plastic is also often reinforced with fibers or other additives to improve strength, durability or other properties.

During the injection-molding process, raw plastic pellets are fed through a hopper into a heated barrel of the injection machine. The barrel is fitted with a reciprocating screw that both turns and slides axially. The screw drives the pellets forward, heating and melting them with the shearing action and friction generated by its motion. The hotter the pellets are, the lower their viscosity and the more easily they flow.

The melted plastic flows from the barrel through a channel in the injection machine’s mold called the sprue bushing, or gate. The sprue bushing is connected to a series of channels in the front half of the injection mold called runners. Each runner is a separate passageway into the mold cavity, and each runner is narrowed by a pin or gate. The sprue gate connects to the runner and is the first place in the runner that the melted plastic enters the cavities of the mold.

Once the sprue gates have been opened, the injection pressure is replaced by holding pressure to compact and hold the molded plastic while it cools. Cooling is facilitated by a liquid or gas-based cooling system. This is a critical step in the process. Failure to optimize the holding pressure can result in voids and other defects.

During the cooling cycle, additional melt can be added to compensate for shrinkage of the molded part. The molded plastic is then ejected from the mold by the sprue lock or puller, a pin that fits into an undercut on the cavity block retainer plate. Excess sprue, runner and gate material can be recycled to form new injection-molded plastic parts. This is an advantage of the injection-molding process over other fabrication methods such as CNC machining, which generate large amounts of scrap material. Excess plastic can also be remelted and injected to create more products from the same batch of raw material, increasing the efficiency and lowering the cost of injection-molded products. This is one of the main reasons why injection molding has become a very popular and widely used manufacturing process for a variety of industrial, commercial and consumer plastic components. plastic injection

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