Process selection
Plastic Extrusion vs Injection Molding
Plastic extrusion pushes melt through a die continuously, producing pipe, film, and profiles with one constant cross section. Injection molding shoots melt into a closed mold and makes discrete 3D parts. The core trade is geometry versus tooling: extrusion dies cost $2,000 to $20,000, injection molds start near $15,000 and climb fast.
| Plastic Extrusion | Injection Molding | |
|---|---|---|
| Tooling cost | $2,000 to $20,000 profile die | $15,000 to $150,000 mold |
| Geometry | Constant cross section only | Full 3D, bosses, threads, snap fits |
| Output | 200 to 2,000 lb/hr continuous | 15 to 60 s cycles, discrete parts |
| Tolerance | ±0.1 to 0.5 mm on profile | ±0.05 to 0.1 mm typical |
| Materials | PVC, HDPE, PP, ABS, high MW grades | Nearly all thermoplastics, filled grades |
| Startup scrap | 50 to 300 lb per changeover | 5 to 20 shots to stabilize |
| Secondary ops | Cut to length, punch, fabricate | Degate only, near net shape |
Choose Plastic Extrusion when
- Linear products sold by the foot
- Tooling budget under $20,000
- High throughput by weight, 100,000+ lb/yr
Choose Injection Molding when
- Discrete 3D parts with end features
- Tolerances tighter than ±0.1 mm
- Volumes above 10,000 identical parts
The verdict
The geometry decides this one. If the part is a length of constant cross section, extrude it; nothing beats a $5,000 die and continuous output. If it has ends, bosses, or 3D features, injection mold it. Stop machining features into extruded stock past about 5,000 parts per year.
Cost comparison
An extrusion die for a window profile runs $5,000 to $15,000, with startup scrap of $100 to $400 per changeover. An injection mold for a similar sized part runs $30,000 to $80,000 but delivers finished parts at $0.30 to $1.50 each. If you are machining or fabricating features onto extruded lengths at $1 to $3 per part, the mold pays back between 5,000 and 20,000 parts. Pure linear product never crosses over; extrusion stays cheaper at any volume.
Common questions
Can extrusion make hollow parts?
Yes, but only hollow shapes that run the full length, like pipe, tubing, and multi chamber window profiles. The die uses a mandrel or spider to form internal voids. What extrusion cannot do is close off an end, add a boss, or change the section along the length; those features force you to injection molding or secondary fabrication.