Pellets or granules are fed from a hopper into a heated cylinder.

Melt is forced into the mold by a hydraulic plunger or rotating screw system.

Injection moulding process steps: Material preparation $\rightarrow$ Feeding into hopper $\rightarrow$ Injection $\rightarrow$ Mould holding and cooling $\rightarrow$ Ejection.

Working Process

Material granules melt in the barrel (surrounded by heaters).
Rotating and reciprocating screw builds up polymer in front of the sprue bushing, pushing the screw backward. Rotation stops when enough polymer has built up.
Melted material is injected into the mold cavity, and the mold is held under pressure until the material cools and hardens.
Once hardened, the mold opens, and the component is ejected by a runner plate.
Thermoplastics: Molds kept cool ($\approx 90^\circ C$).
Thermoset parts: Molded in heated molds ($\approx 200^\circ C$) for polymerization and cross-linking.

Sprue Bushing

Connects the machine's nozzle to the mold, allowing molten plastic to enter the mold cavity.

Cycle Time

Consists of Injection Time (Packing Time, Discharge Time, Sealed Cooling) and Cooling Time (Mold reset time).

Mould

Tool used to produce components.
Constructed from hardened steel, pre-hardened steel, aluminium, and beryllium copper alloy.
Steel molds are costly but offer longer lifespan.
Manufactured by CNC machining or electrical discharge machining (EDM).

Mould Design Considerations

Shrinkage Allowance: Depends on material properties, core, and cavity size.
Cooling Circuit: Water circulates through drilled holes in core and cavity plates to reduce cycle time.
Ejection Gap: Gap between ejector plate face and core black plate face must hold dimension within the core and allow full removal of the component.
Mould Polishing: Core, cavity, runner, and sprue should have a good surface finish, polished along material flow direction.
Mould Filling: Gate placed to fill component from thicker to thinner section.
Draft: Required in core and cavity for easy ejection.

Mould Storage

Custom molds are protected due to high costs.
Maintained at perfect temperature and humidity to ensure longest lifespan.
Rubber injection molds stored in controlled environments to prevent warping.

Mould Cooling

Constant mold temperature for uniform quality.
Reduced cycle time, improved surface finish, longer mould life.
Avoids warpage (distortion caused by non-uniform cooling).

Injection Moulding Defects

Blister: Thin film of plastic bubbles up from the surface.
Flash: Excess plastic forms on the surface of the part.
Burn marks: Discoloration due to overheating.
Warping: Molded plastic part becomes distorted or deformed.
Weld lines: Visible lines where plastic melt fronts meet.
- Cause: Melt enters cavity at high speed, touches surface, cures, then pushed by subsequent melt, leaving snaking marks.
- Solution: Reduce number of gates, adjust gate position, reduce mold release agents.
Color streaks: Uneven color distribution.
Voids: Empty spaces within the molded part.

Advantages of Injection Moulding

Fast production (5-60 seconds cycle time).
Low labor costs, design flexibility,