How Plastic Injection Molding Works
Plastic injection molding is an efficient, economical and versatile way to produce many different types of finished products. This process has the ability to produce complex shapes that would be difficult or impossible to make by other means, and it can do so quickly and consistently. In addition to speeding up production time, it also produces a more precise product and reduces the amount of waste material.
The plastic injection molding process is a common method of producing items such as wire spools, cable wrappers, bottle tops, toys, combs, chairs and small furniture pieces, and mechanical parts and components. It is also increasingly being used to manufacture a wide range of medical devices and pharmaceutical containers. Injection moulding is most often carried out using thermoplastic polymers, but thermosetting polymers can also be injected.
The process involves placing a pelletised raw material into a heated barrel of a plastic injection machine where it is sheared and mixed by a reciprocating screw. This increases the temperature of the polymer to a point where the Van der Waals forces that resist relative flow of individual chain molecules are weakened, causing it to flow. It is then pushed forwards into the injection mold with the force of the screw and the pressure of the injection unit. This compresses the cavity sides of the mould and allows molten plastic to be injected into it.
When the melted plastic reaches the cavity, it will fill the entire area and stay in place under clamping pressure as the two halves of the mould close together. A sprue bushing seals tightly against the nozzle of the injection machine and directs the molten plastic to a set of channels (also called runners) that have been machined into the faces of the A and B plates. The runner will lead the melted plastic to the cavity images and create a seal that is known as the gate.
It is important that the melted plastic maintains consistent temperatures within the mold cavity as this is what determines the final shape of the product. This is especially true with hygroscopic materials such as PE, PVC and ABS. This is because internal moisture will deteriorate their performance and appearance properties.
A steady stream of coolant flows through passages in the injection mold to ensure it maintains a constant temperature, allowing for accurate injection. The injection machine will then deliver a shot of molten plastic through the sprue and into the cavities to form the finished product.
To avoid jetting, which is a snake-like stream of polymer melt sucked into the cavity through restrictive areas such as the sprue, the injection machine should be set to inject at a low rate and with a good gate design. Frequently jetting can be the result of poor mould design, lack of venting, masterbatch being added too quickly or the injection speed being too high. It can also occur due to a lack of clamping force, a problem with the machine or the tool or contamination from other coloured material introduced by a bad recycling or regrind policy.
