Plastics : Difference between Injection Molding and Extrusion

Injection Molding: manufacturing process where melted plastic is injected into a mold cavity, allowed to cool and solidify, then ejected as a finished part with complex geometry.

Extrusion: manufacturing process where melted material is continuously forced through a die opening to create products with a consistent cross-sectional profile, like pipes, sheets, or profiles.

Injection Molding:

Process Mechanics:

• Material (usually plastic pellets) is fed into a heated barrel. A reciprocating screw or ram melts and homogenizes the material
• The molten material is injected under high pressure (500-2000 bar) into a closed mold
• The material cools and solidifies inside the mold
• The mold opens and the part is ejected
• The cycle repeats

Key Components:

• Injection unit (barrel, screw, nozzle)
• Clamping unit (holds mold closed against injection pressure)
• Mold (typically two or more parts that form a cavity)
• Control system

Characteristics:

• Cycle time: typically 15-120 seconds per part
• Part complexity: can produce highly complex geometries with intricate features
• Dimensional accuracy: excellent (±0.05mm possible)
• Surface finish: very good, with texture options
• Part size: limited by machine size, typically small to medium
• Material waste: minimal (runners and sprues can be recycled)
• Production volume: medium to high volumes economical
• Tooling cost: high (complex molds are expensive)

Best Applications:

• Complex parts with varying wall thicknesses
• Parts requiring high precision
• Products needing surface texture or fine detail
• Components requiring assembly features (snap-fits, threads)
• Products with inserts or multiple materials

Extrusion:

Process mechanics:

• Material is fed into a heated barrel
• A rotating screw continuously melts, mixes, and pressurizes the material
• Molten material is forced through a die opening
• The extruded profile is cooled (typically by water, air or rollers)
• The continuous product is cut to length or wound onto spools

Key components:

• Feed hopper
• Barrel and screw system
• Die (determines the cross-sectional shape)
• Cooling system
• Haul-off system (pulls material at consistent speed)
• Cutting or winding system

Characteristics:

• Production rate: continuous process, measured in kg/hour or m/min
• Part complexity: limited to constant cross-sections (though post-forming is possible)
• Dimensional accuracy: good (±0.1mm typical)
• Surface finish: good, but limited texturing
• Length: theoretically unlimited
• Material waste: very low in continuous operation
• Production volume: high volumes economical
• Tooling cost: lower than injection molding

Best applications:

• Products with constant cross-sections
• Continuous lengths (pipes, tubing, weather stripping)
• Sheets and films
• Profiles (window frames, trim)
• Fibers and filaments

Key differences:

Process type:

• Injection molding: Cyclical/batch process
• Extrusion: Continuous process

Part geometry:

• Injection molding: complex 3D shapes with varying wall thickness
• Extrusion: consistent cross-sectional profiles

Production volume economics:

• Injection molding: higher startup costs but efficient for medium to high volumes
• Extrusion: lower tooling costs and excellent for high-volume continuous products

Material considerations:

• Both can process similar thermoplastics
• Injection molding: Better for materials with narrow processing windows
• Extrusion: Better for heat-sensitive materials due to shorter heat exposure

Post-processing requirements:

• Injection molding: often minimal (trim gates, remove flash)
• Extrusion: may require cutting, forming, or additional operations

Design flexibility:

• Injection molding: allows for complex features like undercuts, threads, living hinges
• Extrusion: limited to shapes that can be pulled from the die in one direction