Injection vs. Extrusion Molding: Key Differences Explained

1. Injection Molding

Injection molding is a plastic-shaping process in which molten plastic is injected under high pressure into a mold cavity, where it cools and solidifies to take the shape of the mold.

Construction Components

• Hopper: Funnel-shaped container at the top of the barrel; feeds plastic pellets.
• Barrel: Thin-walled cylinder housing the screw; features multiple heating zones.
• Screw: Comprises three zones—feed, compression, and metering; moves axially.
• Non-return valve: Fitted at the top of the screw.
• Nozzle: Connects the barrel to the mold; controls the flow.
• Cylinder for screw ram: Pushes the screw forward during the injection stroke.
• Stationary platen: Fixed mold-half section.
• Movable platen: Moves forward and backward along the rods to operate the other half of the mold.
• Mold: Two-part tool that determines the final shape.

Working Principle

• Melt preparation: The screw rotates, moving plastic pellets forward while heaters melt the material. Melt pressure pushes the screw backward as material collects. The non-return valve stays open to allow melt flow.
• Mold closing & injection: The screw stops rotating, and the non-return valve closes. Melt is injected at high pressure to fill the mold cavity completely. Packing pressure is applied to compensate for shrinkage.
• Cooling & solidification: Cold water flows through mold channels. The part cools, shrinks slightly, and takes the mold shape. The screw begins rotating again for the next shot.
• Ejection & mold opening: The movable platen moves back to open the mold. Ejector pins push the part off the core. The mold closes again for the next cycle.

Applications

• Automotive industry: Dashboard panels and interior parts.
• Electronics & electrical: Mobile and laptop casings.
• Medical industry.
• Packaging industry.

2. Extrusion Molding

Extrusion molding is a plastic-shaping process in which molten plastic is continuously forced through a die to produce a profile of constant cross-section.

Construction Components

• Hopper: Funnel-shaped container at the top of the barrel; feeds plastic granules.
• Barrel: Thick-walled steel cylinder housing the screw with a hardened inner surface; features multiple heating zones and cooling channels to prevent overheating.
• Screw: Comprises three zones—feed, compression, and metering.
• Breaker plate: Circular steel disc with multiple holes placed between the screw and the die.
• Die: Mounted at the exit of the barrel.

Working Principle

• Feeding: Plastic pellets enter the feed section from the hopper; screw rotation picks up the solid pellets and conveys them toward the die.
• Melting & compression: Heat melts the plastic; channel depth decreases to compress the material, pushing out trapped air between pellets.
• Metering: Uniform channels pump the melt steadily, ensuring thermal and chemical consistency while building pressure to push the melt through the die.
• Filtration: The melt passes through the breaker plate, which straightens the flow and removes unmelted lumps or contaminants.
• Die shaping: The melt is forced through the die opening, taking the exact shape of the die and exiting as a continuous, soft profile.

Applications

• Plumbing: Water and gas pipes.
• Packaging: Bags and wraps.
• Electrical: Wire and cable insulation.
• Agricultural: Drip tubes.
• Automotive: Seals.
• Medical.
• 3D printing.