Taking its fabrication roots from the processes associated with metal die-casting, injection molding begins by heating plastic to a fluid state. The liquid materials are then force injected into a cavity that bears the negative shape of a given product design. After cooling, the solidified material is removed from the mold. It seems a simple process, and is used to produce many types of everyday goods such as children’s toys, audio CDs, and plastic drink bottles. But, is injection molding as simple as it appears?
In the metal die-casting process, the materials are poured in the mold. However, a simple pouring process cannot work with polymers. The elevated viscosity of plastic polymers causes the materials to contract as they solidify. This means that additional melted materials must be forced into the mold cavity. Therefore the plastic molding process must include a method of continuous forced material input.
Touching on the Technical Definition of the Injection Molding Process
Though wording and terms may vary from vendor to vendor, in general, the injection moulding process starts with a hopper that leads into the injection molding press. The material, typically in pellet form, is placed into the hopper, and then heated to a fluid state by friction and band heaters. In its liquid state, the material is forced through the nozzle, enters the bushing, passes through the runner systems and the gate, and finally enters the cavity of the molding press. Pressure is maintained until the materials solidify. At this point, the molded part, the gate, and the runner are ejected from the molding press unit.
The primary components of an injection-mould machine include the following:
- A plasticating unit that addresses the shearing, melting, and injection process.
A hydraulic clamping unit that binds and seals the mold to withstand the injection pressure.
The mold is the shape of the cavity that produces the desired geometric unit.
A control system to manage the plasticating temperature, the material shearing process, including velocity and positioning, and the mold temperature.
A tempering system to manage the cooling process.