Blow Molding

Blow molding forms hollow, thin-wall plastic parts from extruded or injected parisons, ideal for bottles, tanks, and ducts at medium to very high volumes.

Overview

Blow molding produces hollow plastic parts by inflating a hot tube of plastic (parison or preform) inside a cooled mold. The process excels at making thin-wall, closed containers and complex ducts with continuous internal volumes, such as bottles, reservoirs, and small tanks.

Use blow molding when you need lightweight, hollow parts with relatively uniform walls and moderate dimensional precision at scale. It shines at high production volumes, where cycle times are short and per-part cost is low, but tooling is a significant upfront investment. Expect good repeatability on necks and critical interfaces, with looser control on general wall and free-blown areas. Very sharp details, thick structural sections, and tight flatness are harder or expensive to achieve. If your design can tolerate modest tolerances and is mainly a hollow shell, blow molding is often one of the most economical processes available.

Common Materials

  • HDPE
  • LDPE
  • PET
  • PP
  • PVC

Tolerances

±0.005" to ±0.015"

Applications

  • Plastic beverage bottles
  • Detergent and household chemical bottles
  • Automotive fuel tanks and fluid reservoirs
  • HVAC and automotive air ducts
  • Cosmetic and pharmaceutical containers
  • Small plastic drums and jerry cans

When to Choose Blow Molding

Choose blow molding for hollow, mostly uniform-thickness plastic parts such as bottles, containers, ducts, and small tanks at medium to very high volumes. It fits parts where weight, cost, and throughput matter more than extremely tight tolerances or crisp cosmetic details on every surface.

vs Standard Injection Molding

Choose blow molding instead of standard injection molding when the part is primarily a hollow shell and you want to avoid complex cores or multi-part tools. Blow molding is usually more economical for bottles, tanks, and ducts, especially as volumes climb and wall thickness can be relatively thin and uniform.

vs Overmolding

Choose blow molding over overmolding when you just need a single-material hollow container or duct without soft-touch or multi-material regions. Blow molding avoids the extra tooling, added cycle time, and bonding complexity of overmolding while delivering low-cost, high-volume hollow parts.

vs Insert Molding

Use blow molding instead of insert molding when metal or plastic inserts are not critical to the part’s function and the design is essentially a standalone hollow body. Blow molding keeps tooling simpler and cycles faster, and you can add secondary operations like drilling, spin-welding, or threaded fittings only where needed.

vs Rotational Molding

Pick blow molding over rotational molding when you need higher volumes, thinner walls, and better cycle times on relatively smaller hollow parts. Blow molding delivers smoother surfaces, more consistent wall thickness, and lower per-part cost once tooling is in place, but is less suited to very large, very thick-walled tanks.

Design Considerations

  • Maintain as-uniform-as-possible wall thickness to reduce sink, warpage, and uneven material distribution during blowing
  • Avoid sharp corners and transitions; use generous radii so the parison can stretch and contact the mold consistently
  • Define neck finishes, threads, and sealing surfaces clearly with tighter tolerances than the general body, and call these out on drawings
  • Place logos, ribs, and text away from high-stretch regions to prevent thinning and readability issues
  • Keep critical features away from the pinch-off area, and specify acceptable flash locations and trimming requirements
  • Provide target part weight, resin grade, annual volume, and any drop or pressure requirements to help shops size tooling and process windows accurately