PET Blow Moulds and Extrusion Blow Moulds Explained

Open PET blow mould halves shown with clear PET bottles of different sizes

Bottles, jars, cans and drums are hollow, and they can't be made in an ordinary two-half injection mould with a solid core. They are blow moulded instead: a hot plastic shape is inflated with air against the walls of a cooled mould. Two blow moulding families cover most packaging in India. PET stretch blow moulding makes clear bottles and jars, and extrusion blow moulding makes HDPE and PP containers such as jerry cans. The moulds for each work differently, and knowing the difference helps you brief a toolmaker properly.

PET blow moulds: from preform to bottle

PET bottles start as a preform, a small test-tube-like part whose threaded neck is already finished. Preforms are made by injection moulding, often in multi-cavity PET preform hot runner moulds. In the blow moulding machine, each preform is heated, a stretch rod pushes it lengthwise, and air pressure expands it outward against the mould. Stretching in two directions, called biaxial orientation, gives PET its clarity, stiffness and gas barrier.

Two process routes are common:

  • Two-stage (reheat) process: preforms are made on one machine, stored, and then reheated and blown on another. This is common for high-volume water and beverage bottles.
  • Single-stage process: preforms are injected and blown on one machine while they still hold their heat. This is often used for jars, pharma bottles and shorter runs.

What matters in a PET blow mould

  • Neck and preform match: the mould must hold the preform's finished neck accurately so the closure seals properly.
  • Base design: petaloid bases for carbonated drinks and champagne-style or flat bases for still products. The base insert often has its own cooling.
  • Cooling: even cooling around the cavity keeps the bottle's shape and helps the cycle time.
  • Venting: fine vents let trapped air escape so embossed details and label panels form cleanly.
  • Cavity-to-cavity consistency: in multi-cavity moulds, each bottle must match the others in volume and weight.

Our PET blow moulds are built for 100 ml to 2 L bottles for water, beverages, pharma and edible oil. The range also includes PET stretch blow moulds, jar, wide-mouth and multi-cavity designs.

Extrusion blow moulds: from parison to container

Extrusion blow moulding starts differently. An extruder pushes out a hot, hollow tube of plastic called a parison. The two mould halves close around it, pinching and sealing one end, and a blow pin inflates the parison to fill the cavity. After cooling, the mould opens and the excess plastic, called flash, is trimmed off.

This process suits HDPE, PP and similar materials. It can also form features that stretch blow moulding can't, such as integrated hollow handles on jerry cans and cans with offset necks.

Open extrusion blow mould for a jerry can, with a finished blue plastic jerry can beside it

What matters in an extrusion blow mould

  • Pinch-off design: the pinch-off edges cut and weld the parison with every cycle. Their geometry decides the strength of the bottom seam, and they take heavy wear, so they are often made as hardened inserts.
  • Flash pockets: these relieve areas for excess material and help keep trimming simple.
  • Wall distribution: corners and handles stretch more and thin out. Mould geometry and parison wall programming on the machine work together to control this.
  • Cooling: thick sections such as the neck and handle usually set the cycle, so cooling needs to reach them well.
  • Machine layout: a mould built for a single-station machine won't necessarily suit a double-station or accumulator-head machine, so the machine type shapes the design.

At Madhav Plasto Mould, our extrusion blow moulds have hardened pinch-offs. They include jerry can blow moulds covering HDPE and PP containers from 5 L up to 25 L for fuel, water and chemicals.

PET or extrusion blow: a quick guide

  • Clear, glossy bottles for water, drinks or edible oil: PET stretch blow
  • Opaque, tough containers with handles: extrusion blow in HDPE or PP
  • Large volumes such as cans and drums: extrusion blow, often with an accumulator head
  • Very precise necks on small bottles: injection blow moulding, a third route worth considering

What to send when you ask for a blow mould quote

  • A bottle drawing, 3D model or physical sample
  • Volume, target weight and material
  • Neck finish or closure specification
  • Machine make and model, with the number of cavities it can hold
  • Label panels, embossing or special base requirements

Planning a new bottle or container?

Madhav Plasto Mould designs and machines blow moulds in Ahmedabad for packaging manufacturers across India. Share your drawing or sample through our contact page, or call or WhatsApp +91-7383314026 to start with a DFM review.

 

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