How does injection molding work?
Now that we’ve established what a mold is, it’s time to look at how the entire injection molding process works.
The entire process is carried out using a piece of equipment known as an injection molding machine (or injection molding press), which consists of two main sections:
An injection unit, where raw plastic pellets are melted down before being injected into the mold
A clamping unit, where the sections of the metal mold are held in place and manipulated
For clarity, we can break the injection molding process down into six main stages.
1. Mold Creation
Although this is more of a preliminary step, the first part of injection molding involves preparing the metal tooling that will be used as the mold. Molds are generally designed on a computer using Computer Aided Design (CAD) software, then fabricated from metal using CNC machines.
When designing a part for injection molding and consequently designing its metal mold, certain design principles must be followed to prevent defects.
Another preliminary step involves readying the raw injection molding material, which is usually a thermoplastic in pellet form. These pellets may need to be mixed with colored dyes before molding begins.
When the mold has been fabricated, it must be fitted securely into the clamping unit of the injection molding machine. The unit squeezes the core plate and cavity plate together, ensuring that no molten plastic can escape between the two sections.
On the injection unit of the injection molding machine, raw plastic pellets are contained in a hopper — a tapered container that can dispense pellets into the machine as required.
When the mold is ready, these pellets are dispensed from the hopper into an oblong section underneath called a barrel, where they are heated into a molten state. The barrel contains a ram or reciprocating screw, which forces the material toward the mold while simultaneously heating it up.
The molten plastic exits the heated barrel through a nozzle and into the mold cavity via a sprue — a long channel built into the mold that allows material to enter.
When the entire 'shot' of molten plastic has been injected into the mold cavity, it begins to cool and harden. This is because the metal mold is much cooler than the heated barrel where the plastic pellets are melted down. As the plastic part hardens, it also exhibits some degree of shrinkage — a factor that must be accounted for during part design.
When the plastic part has fully cooled and solidified, the mold can be opened up. During this stage, the core plate is pulled away from the cavity plate, before ejector pins are pushed into the plastic component to disengage it from the core.
When the plastic component has been removed from the mold, the two halves of the mold can be clamped together again, ready for the next shot.
6. Post-processing and assembly
When the part has been removed from the mold, it may need to be painted or subjected to surface finishing procedures. The part may also be joined to other (molded or non-molded) parts to form a complete product.