The process of injection moulding may seem simple enough: liquid plastic is injected into a mould, cooled and ejected. In reality, it is a process of great integration of multiple parts playing a role.
An injection moulding machine has three main parts: the injection unit, the mould, and the clamp. The integrated process works as follows:
The injection unit: The process of injection moulding starts with plastic pellets that are fed into the barrel of the injection unit. What is interesting is that inside the injection unit is a barrel that has a reciprocating screw that transports the pellets forward to the mould. As the plastic is being moved forward, there are heated bands or coils in the injection unit that warms up the plastic.
The friction of the reciprocating screw creates the majority of the heat. The heat is so significant that, by the time the plastic reaches the front of the barrel, it is completely molten. Once there is enough molten plastic in the front of the screw it plunges forward injecting into the mould.
The cavity image: Inside the mould, there is a cavity image. Initially, the cavity image is filled with air but as the molten plastic is injected the air escapes through vents that are about 4-5 microns deep to allow the plastic to flow through. Once the cavity image is filled; the plastic solidifies again with the help of coolant, typically water, which is just on the surface of the mould.
The ejector pins: When the mould is opened, the plastic in the cavity shrinks as it cools, which can cause it to get stuck to the core half of the mould. Ejector pins are usually located behind the mould cavity on the clamp side, which helps to push the plastic part off of the mould.
The result of this action is that it creates ejector pin witness marks on moulded products that can be seen as circular blemishes on the plastic product. Whenever you look at a plastic cap, be sure to look for the circular blemishes made by the injection mould.
The whole process can take a minute, depending on the product moulded, and is repeated. Injected moulding products are everywhere, and it is likely that the device you are using to read this post has injected moulding parts with ejector pin witness marks or parting lines.
Plastic & Chemical Trading together with international partner KraussMaffei Berstorff deliver world-class plastics, injection moulding, and extrusion solutions.