Plastics : Comparison of Injection Molding and Extrusion

Below is a comprehensive comparison of Injection Molding and Extrusion:

INJECTION MOLDING

Definition:

A manufacturing process where molten plastic is injected into a mold cavity under high pressure and allowed to cool and solidify

Advantages:

• Used for high precision and complex geometries 
• It has an excellent surface finish
• Used for high production rates for large volumes
• It has a consistent part quality
• Multiple materials can be used
• Can produce complex parts with features like threads or snap-fits

A snap-fit is a self-locking joint or fastening mechanism molded directly into plastic parts that allows them to be quickly assembled by pushing the components together until they "snap" into place.

Disadvantages:

• High initial tooling costs
• Long lead times for mold creation
• Not economical for small production runs
• Material waste from runners and sprues
• Size limitations based on machine capacity
• Higher maintenance costs

Runners and sprues are channels in injection molding. 

Sprues are the main channels that connect the injection nozzle to the runners that are the smaller channels that distribute molten plastic from the sprue to the actual part cavities in the mold.
Together, they form the pathway for molten plastic to flow into the mold, but become waste material once the part is complete.

Common uses:

• Electronic housings
• Automotive parts
• Medical devices
• Consumer products
• Containers and packaging
• Toys
• Household items

EXTRUSION

Definition:

A continuous manufacturing process where molten plastic is forced through a die to create a constant cross-sectional profile

which is the shape and dimensions you see when you cut through an object perpendicularly. It's essentially the 2D outline you get when making a straight cut through a 3D object.

Advantages:

• Continuous production process
• Lower tooling costs compared to injection molding
• Good for long, continuous profiles
• Excellent for high-volume production
• More material efficient
• Can produce very long parts
• Lower startup costs

Disadvantages:

• Limited to constant cross-section designs
• It has less precise tolerances than injection molding
• Cannot create complex 3D shapes
• The surface finish may be less refined
• Limited material options compared to injection
• Post-processing often required

Cross-section designs refer to the consistent 2D shape or profile that runs continuously along the length of a product or part. It is the shape you'd see if you cut straight through something.

Common uses:

• Pipes and tubing
• Window profiles
• Wire and cable insulation
• Film and sheet
• Weather stripping
• Plastic lumber
• Trim and molding

Weather stripping is a strip of material (often rubber, plastic, or foam) installed around doors and windows to seal gaps and prevent air, water, and dust from entering, helping with insulation and energy efficiency.

Key differences:

Process type:

• Injection molding: Batch process
• Extrusion: Continuous process

Part complexity:

• Injection molding: Complex 3D shapes possible
• Extrusion: Limited to consistent cross-sections

Production volume:

• Injection molding: Better for medium to high volumes of discrete parts
• Extrusion: Ideal for continuous products in very high volumes

Initial investment:

• Injection molding: Higher initial costs
• Extrusion: Lower initial investment

Design freedom:

• Injection molding: Greater design flexibility
• Extrusion: Limited to profiles that can be extruded

Material usage:

• Injection molding: More waste from runners/sprues
• Extrusion: More efficient material usage