Plastic production : difference between injection molding and extrusion

Simple Definitions:

- Injection Molding = Making ice cubes (separate pieces in a mold)
-Extrusion = Squeezing toothpaste (continuous shape through an opening)

Basic Process: 

Injection Molding:

• Plastic melts in barrel
• Screw inside barrel moves back & forth
• Molten plastic shoots into closed mold
• Plastic cools in mold shape
• Mold opens, part falls out
• Repeats cycle

Extrusion:

• Plastic melts in barrel
• Screw turns one direction only inside the barrel
• Molten plastic pushes through shaped opening called die
• Continuous product comes out
• Product cools on conveyor/cooling line
• Process runs continuously

Equipment Differences:

 

Injection Molding:

• Has clamping system
• Uses two-part molds
• Reciprocating screw
• Higher pressure system
• More complex controls

A reciprocating screw in injection molding machines that both rotates and moves back and forth (reciprocates) to melt, mix, and inject plastic.

Extrusion:

• No clamping needed
• Uses die instead of mold (shapes plastic as it flows through)
• Constant rotating screw
• Lower pressure system
• Simpler controls

A die is precision mold that gives plastic its final shape as it flows through, similar to how a pastry nozzle shapes dough, but much more complex and precise.

Products Made:

Injection Molding:

• Discrete parts
• Complex shapes possible
• Both thick and thin walls
• Detailed features
• Examples: Plastic containers, Car parts, Toys, Housing components

Extrusion:

• Continuous shapes
• Constant cross-section
• Uniform thickness
• Examples: Pipes, Window profiles, Film/sheet, Wire coating

Process Characteristics:

Injection Molding:

• Cycle time: 15-60 seconds typical
• Higher tooling costs
• Better for high volume
• More precise dimensions
• More complex setup

Extrusion:

• Continuous runtime
• Lower tooling costs
• Better for long runs
• Simpler dimensions
• Easier setup

Material Considerations:

Injection Molding:

• Wider range of materials
• Higher melt temperatures
• Material must flow well
• Can use more additives

Extrusion:

• Limited material range
• Lower melt temperatures
• Material must hold shape
• Fewer additives used

Quality Control:

Injection Molding:

• Part-to-part checking
• More quality points
• Complex measurements
• Each part can be inspected

Extrusion:

• Continuous monitoring
• Fewer quality points
• Simple measurements
• Sample testing only

Cost Factors:

Injection Molding:

• Higher mold costs
• More machine complexity
• Higher operating costs
• Better for large volumes

Extrusion:

• Lower die costs
• Simpler machines
• Lower operating costs
• Better for continuous runs

Remember:

- Injection molding = making separate parts
- Extrusion = making continuous shapes

Choose of method is based on:

- Product shape needed
- Production volume
- Cost requirements
- Quality needs