How Does the Plastic Injection Molding Process Work?

Injection molding is a production method based on heating and melting thermoplastic pellets, injecting the melt into a mold cavity under high pressure, and ejecting the part once it has cooled and solidified.

The process consists of feeding (pellets filling the hopper), plasticizing (melting in the screw-barrel assembly), injection (pushing the melt into the mold), packing/holding (maintaining pressure briefly to compensate for shrinkage), cooling, and finally ejection from the mold; this entire loop is called a 'cycle.'

The duration of an injection cycle depends largely on the cooling stage — the thicker the part's wall section, the longer the cooling time required; for this reason, part design aims for the thinnest and most uniform wall thickness possible.

The material's MFI also directly affects cycle time: a higher-MFI (more fluid) PP or PE fills the mold faster and at lower pressure — this is the main reason higher-MFI grades are preferred, especially for thin-walled parts with complex geometry (see the 'What Is MFI' article).

On the PP side, the homopolymer/copolymer distinction comes into play here: homopolymer is preferred when stiffness and dimensional stability are the priority, while block copolymer PP is preferred when impact strength and cold-environment performance matter (see the related article).

On the PE side, YYPE is preferred for rigid molded parts such as buckets, crates and container caps, while AYPE is preferred for more flexible closures or soft parts. The right grade choice directly affects both the part's final performance and production efficiency.

Sink marks are depressions that form on the surface where thicker sections shrink as the material cools. Warpage is the part deviating from the mold geometry due to uneven cooling rates across different regions.

Short shot is the material solidifying before the mold is completely filled, generally caused by low pressure, low temperature, or selecting a material with too low an MFI. Flash is melt escaping through the gap between mold surfaces, generally associated with excessive injection pressure or insufficient clamping force. Most of these issues can be prevented through correct material selection and optimized process parameters.