Blow Molding

Blow molding forms hollow plastic parts by inflating a heated parison inside a mold, enabling lightweight containers and ducts at high production rates.

Overview

Blow molding makes hollow thermoplastic parts by extruding or injection-forming a preform (parison), then inflating it against a cooled mold to set the shape. It excels at bottles, tanks, and ducts where you need an enclosed volume, low mass, and fast cycle times.

Choose blow molding for medium to very high volumes, consistent wall sections, and geometries that can be split by a mold parting line without complex internal features. Typical strengths are low piece price, good surface finish, and the ability to integrate handles, neck finishes, and pinch-off seams.

Tradeoffs: tolerances are looser than injection molding, wall thickness varies (especially at corners and along the parison stretch), and design freedom is limited for sharp details, threads (outside of neck finishes), and tight flatness requirements. Tooling is still a significant up-front cost, and secondary trimming/deflashing is common.

Common Materials

  • HDPE
  • LDPE
  • PP
  • PET
  • PVC
  • ABS

Tolerances

±0.010" to ±0.030"

Applications

  • HDPE bottles and jugs
  • Automotive washer fluid reservoirs
  • Industrial chemical containers
  • Medical and consumer squeeze bottles
  • HVAC/automotive air ducts
  • Fuel tanks (multi-layer)

When to Choose Blow Molding

Blow molding fits parts that must be hollow, lightweight, and produced in repeatable high volumes, especially containers with defined neck finishes. It’s a strong choice when moderate dimensional variation is acceptable and the design can tolerate a parting line and pinch-off seam. Expect the best economics when the same tool runs for long production campaigns.

vs Standard Injection Molding

Choose blow molding when the part must be hollow (closed volume) and weight/cost matter more than tight tolerances and crisp detail. Injection molding is better for solid parts with ribs, bosses, threads, and tight interfaces; blow molding wins on container-style geometries and faster, cheaper parts at scale.

vs Overmolding

Choose blow molding when you need a single-material hollow body and the main requirement is a sealed volume (bottles, tanks, ducts). Overmolding is the right fit when the product needs bonded soft-touch grips, seals, or multi-material functionality on a primarily solid molded substrate.

vs Insert Molding

Choose blow molding when the geometry is primarily a hollow shell and you don’t need embedded metal/plastic inserts carrying loads or providing threads. Insert molding makes sense when fasteners, electrical contacts, or reinforcement must be captured accurately with controlled positional tolerances.

vs Rotational Molding

Choose blow molding for higher volumes, better surface finish, and faster cycle times on small-to-mid size hollow parts. Rotational molding is more flexible for very large parts and thicker walls, but cycle times are long and fine detail/tolerances are typically worse.

vs Compression Molding

Choose blow molding for hollow thermoplastic containers and ducts where the tool defines the exterior and cycle time drives cost. Compression molding is typically used for solid parts (often thermosets or rubber) where fiber loading, heat resistance, or low-stress forming is the priority.

Design Considerations

  • Keep wall thickness as uniform as possible and avoid sharp section transitions that amplify thinning during inflation
  • Design the parting line and pinch-off areas early; add trim features and allow access for deflashing tools
  • Use generous radii and smooth external contours to reduce localized thinning and improve parison distribution
  • Define critical dimensions on features controlled by the mold (e.g., neck finish) and relax tolerances on free-blown surfaces
  • Add ribs or stiffening features where possible without creating deep draws that trap material or cause thin spots
  • Call out material, color, target weight, and any leak/pressure requirements to help suppliers set process windows and quote accurately