The Extrusion Blow Molding Process, Step by Step

This page explains how the process works — and, further down, which type of blow molding machine fits your bottle size. If you have already settled on the process and now need an extrusion blow moulding specialist for the machine, AMAN has supplied extrusion blow molding machines, built by its sister factory, and made the moulds that go in them since 2007.

The six steps of the extrusion blow molding cycle

1. Extrude the parison. Plastic granules (usually HDPE or PP) are melted in a heated extruder and pushed through a die head, which forms the melt into a hollow tube called a parison that hangs down between the open mould halves. The die gap sets the wall thickness.

2. Close the mould. The two water-cooled mould halves close around the parison and pinch it shut at the base, sealing one end so it can hold air. This pinch line becomes the weld seam on the bottom of the container. Where that seam sits, and how well it seals, are both designed into the blow mould itself.

3. Blow to shape. Compressed air is injected through a blow pin, inflating the soft parison outward until it presses against the inside of the mould and takes the exact shape of the container — bottle, jerry can, drum or tank, handles and all. The same six steps run on a jerry can blow molding machine and on a 50–100 L drum blow molding machine; only the die head, mould and clamping size change.

4. Cool. The part is held against the cooled mould walls until it solidifies. Cooling is normally the longest part of the cycle and depends mainly on the wall thickness; larger and thicker containers take longer.

5. Open and eject. The mould opens and the finished container is taken out, either dropped onto a conveyor or removed by a take-out arm.

6. Trim the flash. The excess plastic left at the pinch-off and neck (the flash) is trimmed away — in-line automatic deflashing on a full line, or at a separate station — and the trimmings are reground and reused. The cycle then repeats automatically, around the clock.

Single station, double station and multi-cavity

Output is raised in three ways: run two stations that alternate (one cools while the other blows), cut several containers per cycle with a multi-cavity mould and multi-die-head, and match the screw and die to the material and container. AMAN cuts the matching blow mould in-house, its sister factory builds the machine, and AMAN sizes both to your container, wall weight and target output (measured in bottles per hour).

The blow molding process: one principle, three methods

Every plastic blow molding process works the same way at heart: soften the plastic, form it into a hollow tube or preform, close a mould around it and blow compressed air inside until the plastic takes the shape of the mould. After cooling, the mould opens and a finished hollow container comes out. What changes from one method to the next is how that hollow starting shape is made — and that decides which bottles each method is good at.

Extrusion blow molding (EBM)

The process explained on this page. An extruder pushes out a hanging tube of molten plastic, the parison; the mould pinches it shut and air blows it to shape. Because the parison is shaped and pinched inside the mould, extrusion blow molding can form hollow handles, offset necks and very large parts — from 100 ml bottles to 1000 L IBC tanks. For drums and IBC tanks, an accumulator head stores the melt and pushes out a heavy parison in one stroke. Mainly HDPE and PP.

Injection blow molding (IBM)

The injection blow moulding process starts by injection-moulding a preform onto a core rod. The core rod carries the preform straight into a blow mould, where air inflates it, and the bottle is then ejected — inject, blow and eject in one machine. The neck is injection-moulded, so it comes out accurate and finished, with no flash to trim and very little scrap. Best for small, precise bottles and vials from 2 ml up to 1 litre, in cosmetic, pharma and food packaging. Bottles with a handle are not made this way; for those, choose extrusion blow molding.

Stretch blow molding (PET)

The method for clear PET bottles. A heated PET preform is pushed lengthwise by a stretch rod while air blows it outward, so the plastic is stretched in two directions — that is what gives PET bottles their clarity and strength. There are two routes:

• Two-step: preforms are made on a PET preform injection molding machine, then reheated and blown on a separate stretch blow machine. This is the usual route for 0.5–2 L water and beverage bottles.
• One-step (injection stretch blow): the preform is injected, stretched and blown in one machine, so there are no preforms to buy, store or reheat. Bottles from 100 ml up to 5-gallon (19 L) water bottles in PC, depending on the model.

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