Injection Blow Molding Process, Step by Step

What is injection blow molding? Injection blow molding (IBM) is a process that makes small, accurate plastic bottles in one machine. Plastic is injected onto a core rod to form a preform with the neck already finished, the preform is blown into the bottle shape, and the bottle is ejected. It runs PP, PE and PS, for bottles from 2 ml up to 1 litre.

Watch: the process running

What you will see: the turret of a 3-station injection blow molding machine turning, with core rods carrying white, slim bottles out of the mould.

This page explains how the injection blow molding (IBM) process works. If you make bigger containers, jerry cans or bottles with a handle, read the extrusion blow molding process instead. AMAN supplies one-step injection blow molding machines, built by its sister factory, and cuts the matching injection blow moulds in its own workshop. We have done this since 2007.

The five steps of the injection blow molding cycle

Figure 1. The three stations. Each core rod carries the preform from injection to blowing to ejection.

1. Inject the preform onto a core rod. The neck and the thread are formed here, in steel. Plastic granules (PP, PE or PS) are melted and injected into a closed mould that has a steel core rod standing in the middle. The melt fills the space around the rod and forms a small test-tube-shaped piece called a preform. Because steel forms the neck, an injection-blown bottle has an accurate neck.

Figure 2. Step 1: the preform forms around the core rod.

2. Open and turn to the blow station. Nothing has to be reheated. The injection mould opens and the core rod, still carrying the warm preform, turns to the next station. The preform never leaves the rod and has not had time to cool down. On a 3-station machine the three stations are injection, blow and eject.

3. Close the blow mould and blow. Air through the core rod inflates the preform to the shape of the bottle. A bottle-shaped blow mould closes around the preform. Compressed air is blown in through the core rod and inflates the preform outward until it presses against the mould wall. The preform is not stretched lengthwise, so this is not the "stretch blow" used for clear PET bottles.

4. Cool. The blow station is normally the slowest step. The bottle is blown and then held against the cooled mould wall until it is firm enough to keep its shape, and the other stations wait for the blow station. The time needed depends mainly on the wall thickness.

5. Eject. On a 3-station machine, one rod is injected, one blown and one ejected in the same cycle. The core rod turns to the eject station, where the finished bottle is stripped off, and then turns back to the first station. Each station works on a different core rod at the same time.

Nothing to trim. The preform is injected, not pinched, so there is no weld seam on the bottom and no flash to cut off, and very little plastic is left over to regrind.

Figure 3. The five steps of the cycle.

Why the neck is so accurate

In injection blow molding the neck is formed by injection into steel, not by blowing air, so the thread and the sealing surface come out repeatable. On a bottle this small that matters: the thread and the sealing surface have to be right every time, or the cap leaks or will not sit properly. That is why IBM is used for eye-drop and dropper bottles, which have to seal and take a dropper insert or a child-resistant cap.

What injection blow molding is good for — and what it is not

IBM runs PP, PE and PS, for bottles from 2 ml up to 1 litre. It suits small cosmetic, pharmaceutical and food bottles: eye-drop and dropper bottles, medicine bottles, roll-on and deodorant bottles, shampoo and lotion bottles at 100 to 400 ml, eyeliner and glue bottles.

Bottles with a handle are not made this way; for those, choose extrusion blow molding. Clear PET bottles use a different route, injection stretch blow molding — see PET injection stretch blow molding machines.

Injection blow, extrusion blow and stretch blow side by side

Figure 4. How the hollow shape starts in each method.

Which machine for your bottle

On our injection blow molding page each machine lists a maximum shot — the most plastic (in HDPE) the machine can inject in one cycle. The rule is simple: weight of one bottle × number of cavities must not be more than the maximum shot.

Our Bangladesh project below ran 10–30 ml HDPE and PP eye-drop, dropper and medicine bottles on an AM 35 injection blow molding machine.

The mould is a matched set

An injection blow mould is not one tool but a set: the injection cavity, the core rods, the neck rings and the blow cavity are cut together, so they fit each other. AMAN cuts the whole set in its own workshop, and every set gets a trial shot before packing; the trial bottles are shipped with the mould. Blow and injection blow moulds are delivered in 25–35 days from approved drawings.

A real example

A pharmaceutical-packaging manufacturer in Bangladesh needed neat, consistent 10–30 ml HDPE and PP eye-drop, dropper and medicine bottles. AMAN supplied a 3-station injection blow molding machine, set up the cycle for that size range and tested neck dimensions and bottle consistency before shipment. Read the case: Injection blow molding for pharma eye-drop and dropper bottles, Bangladesh.

FAQ

Related: cosmetic bottles · pharmaceutical bottles · chemical bottles · food and beverage bottles · injection blow moulds · extrusion blow molding vs injection blow molding