How does injection molding work?

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How does injection molding work?
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Overview of Injection Molding

Injection moulding is a method of manufacturing parts from plastic. Plastic pellets are placed in a large hopper, which feeds the pellets into a heated chamber. The chamber melts the pellets and injects the liquefied plastic into a variety of moulds, where the plastic cools and hardens into the appropriate shape. Once the plastic has cooled, the piece is removed from the mould. Excess plastic is scraped away and recycled, and the finished piece is polished and cleaned. Injection moulding takes place in large machines, and the necessary moulds are purchased from speciality toolmakers, who build the moulds from steel or aluminium.

Injection Molding Process

The first step in the injection moulding process is to feed plastic pellets into the hopper, which then feeds the pellets into the barrel. The barrel is heated and contains a reciprocating screw or a ram injector. A reciprocating screw is typically found in machines that produce smaller parts. The reciprocating screw crushes the pellets, making it easier for the plastic to be liquefied. Towards the front of the barrel, the reciprocating screw propels the liquefied plastic forward, thereby injecting the plastic through a nozzle and into the empty mould. Unlike the barrel, the mould is kept cool in order to harden the plastic into the correct shape. The mould plates are held closed by a large plate (referred to as a movable platen). The movable platen is attached to a hydraulic piston, which puts pressure on the mould. Clamping the mould shut prevents plastic from leaking out, which would create deformities in the finished pieces.

How does injection molding work?
Diagram of Injection Molding Process

Issues that Arise in the Injection Molding Process

A number of complications can arise during the injection moulding process, including burnt parts, deformities, surface imperfections and brittle parts. Parts become burnt when the moulds in the injection moulding machine are not kept cool or if the melting temperature in the barrel is too high. Additionally, if the reciprocating screw becomes jammed or is not rotating fast enough, liquefied resin will remain in the barrel too long and become scorched. Surface imperfections and deformities occur when the surface temperature of the mould is uneven, if the moulds are not clamped tightly enough or if the melting temperature is too high. Brittle pieces are formed when not enough liquefied resin is injected into the mould or if the plastic hardens before the mould can be filled. Regular testing and calibration of injection moulding machines is critical to ensure that the process runs smoothly.

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