What Is Blow Molding?
Blow molding makes hollow plastic parts. A tube of molten plastic (called a parison) is inflated with air pressure inside a mold cavity, forcing the material against the mold walls. When it cools, you have a hollow shape with uniform wall thickness.
Every plastic bottle you’ve ever used was blow molded. So are fuel tanks, drums, ducting, kayaks, and automotive fluid reservoirs.
Three Types of Blow Molding
1. Extrusion Blow Molding (EBM)
The simplest and most common variant. A continuous tube of molten plastic is extruded downward, a mold closes around it, and air is blown in to inflate the shape.
- Best for: bottles, containers, tanks, irregular shapes
- Materials: HDPE, PP, PVC, PETG
- Can produce very large parts (100+ gallon tanks)
- Tooling: $5,000 – $50,000 (much cheaper than injection molds)
2. Injection Blow Molding (IBM)
A preform is injection molded onto a core pin, then transferred to a blow station and inflated. Produces parts with precise neck finishes and no flash.
- Best for: small bottles, pharmaceutical containers, precision cosmetic packaging
- Materials: PET, PP, HDPE
- Higher tooling cost than EBM but tighter tolerances
3. Injection Stretch Blow Molding (ISBM)
The process behind every PET water bottle. A preform is heated, stretched axially with a rod, then inflated. The biaxial stretching orients the molecular chains, dramatically improving strength and clarity.
- Best for: carbonated beverage bottles, water bottles, high-clarity containers
- Materials: PET (primarily)
- Highest tooling and equipment cost but lowest per-unit cost at very high volumes
How It Compares to Injection Molding
| Factor | Injection Molding | Blow Molding |
|---|---|---|
| Part geometry | Solid or partially hollow | Hollow (required) |
| Tooling cost | $15K – $500K+ | $5K – $75K |
| Wall thickness control | Excellent | Good (varies with shape) |
| Surface finish | Excellent both sides | Good outside, rough inside |
| Part size range | Small to medium | Small to very large |
Design Considerations
- Wall thickness. Unlike injection molding, you can’t precisely control internal wall thickness. Corners and deep draws stretch thinner. Design with generous radii and avoid extreme aspect ratios.
- Pinch-off (flash). EBM produces a pinch line where the mold halves close on the parison. This flash must be trimmed — factor it into part design and secondary operations.
- Blow ratio. The ratio of the largest cross-section to the parison diameter. Keep it under 3:1 for uniform walls. Higher ratios cause thin spots.
- Parting line. The seam where mold halves meet is visible. Place it strategically for cosmetics.
- Internal features. Limited. You can’t put ribs, bosses, or complex geometry on the inside. If you need internal structure, consider a two-piece injection molded assembly instead.
When to Use Blow Molding
- Hollow parts (containers, tanks, ducts, enclosures)
- Large parts where injection mold tooling would be prohibitive
- Moderate to high volumes (1,000+ for EBM, 100,000+ for ISBM)
- Applications where internal surface finish doesn’t matter
When to Consider Alternatives
- Solid parts: Injection molding
- Very large hollow parts with complex geometry: Rotational molding
- Low volumes (under 500): Urethane casting or fabrication
- Uniform wall thickness critical: Rotational molding (coats evenly)
Bottom Line
If you need a hollow plastic part, blow molding is almost certainly how it gets made. Tooling is cheaper than injection molding, part sizes can be much larger, and the process is fast at volume. The trade-off is less control over wall thickness and internal geometry — so design to the process’s strengths rather than fighting its limitations.