Process selection
Blow Molding vs Injection Molding
Blow molding inflates a molten tube or preform against a cooled cavity to make hollow, closed parts. Injection molding packs melt into a fully enclosed cavity under 500 to 2,000 bar. The core trade: blow molding is the only economical route to one-piece hollow parts; injection molding owns precision, tight tolerances, and solid geometry.
| Blow Molding | Injection Molding | |
|---|---|---|
| Part geometry | Hollow, closed: bottles, tanks, ducts | Solid or open: caps, housings, gears |
| Wall thickness control | ±10 to 20%, thins at corners | ±0.05 mm, designed per feature |
| Tolerance | ±0.5 mm body, ±0.2 mm neck finish | ±0.05 to 0.1 mm achievable |
| Tooling cost | $15,000 to $50,000 per blow mold | $25,000 to $150,000 per tool |
| Cycle time | 10 to 20 s per bottle, multi-cavity | 15 to 60 s, multi-cavity |
| Clamp pressure | Low, 6 to 10 bar blow air | High, 2 to 5 tons per in2 projected |
| Materials | HDPE, PET, PP, PVC mainly | Nearly all thermoplastics, filled grades |
Choose Blow Molding when
- One-piece hollow containers: bottles, jerry cans, tanks, ducts
- Lower tooling cost from low-pressure cavities, no core needed
- Handleware and complex hollow shapes impossible to core out
Choose Injection Molding when
- Tolerances under ±0.1 mm: threads, bosses, snap fits, seal faces
- Solid or open parts with ribs, inserts, and engineered walls
- Wide material choice including glass-filled and high-temp resins
The verdict
Geometry decides this one before cost does. A closed hollow body goes to blow molding; injection can only match it by molding halves and welding them, adding a tool and an operation. Everything else, especially anything with tight tolerances or engineered wall sections, goes to injection molding.
Cost comparison
A single-cavity extrusion blow mold for an HDPE bottle runs $15,000 to $40,000; making the same container by injection means two tools plus a welding operation, easily $80,000 to $150,000 in tooling. Material also favors blow molding: a 1 L HDPE bottle uses 30 to 45 g of resin, about $0.045 per part. If the part must be hollow and closed, blow molding wins at essentially any volume; injection only competes when the geometry can be opened up or split.
Common questions
Why not injection mold a bottle in two halves and weld it?
You pay for two injection tools plus a welding or spin-weld station, add a leak-prone seam, and roughly double the piece cost. A single extrusion blow mold at $15,000 to $40,000 makes the container in one 10 to 20 s cycle with no joint. Two-piece construction only makes sense when internal features demand it.
How tight can blow molding hold wall thickness?
Expect ±10 to 20% variation, with thinning at corners and pinch-off areas. Parison programming on extrusion machines and preform design on PET stretch-blow tighten it considerably, but if a wall must hold ±0.05 mm for a sealing or structural feature, that feature belongs on an injection molded component.