How Does the Plastic Injection Molding Process Work?
Understanding the injection molding cycle helps you optimize part design and production efficiency.
The plastic injection molding process follows a precise, repeatable cycle consisting of four main stages:
1. Clamping
The two halves of the injection mold (cavity and core) are securely clamped together by the molding machine with significant force (measured in tons) to withstand the injection pressure. Proper clamping prevents flash and ensures dimensional accuracy.
2. Injection
Plastic pellets are fed from a hopper into a heated barrel where they are melted by heater bands and mechanical shear from a rotating screw. The molten plastic (melt) is then injected at high speed and pressure through a nozzle into the mold cavity via the runner system and gates.
3. Cooling
Once the cavity is filled, the plastic begins cooling immediately upon contact with the mold surfaces. Cooling channels within the mold circulate water or oil to extract heat efficiently. This is typically the longest phase of the cycle. Pack and hold pressure is maintained to compensate for shrinkage.
4. Ejection
After sufficient cooling, the mold opens and ejector pins push the solidified part out of the mold cavity. The part falls or is removed by robot, and the cycle begins again.
Cycle times range from under 10 seconds for small, thin-walled parts to several minutes for large, thick components.
Need to Know
Scientific molding principles optimize each phase of this process to ensure maximum quality and consistency.