A PET bottle blow moulder needs air at up to 40 bar for final blowing. Our compressors do not go that high: 13 bar at most for the S and K series, 16 bar for the L series. They cannot supply the blow moulder's high-pressure circuit and are not a substitute for it. Their place is alongside: an 8 bar compressor supplies the service air for the line, and this article shows how to calculate that flow.
For all the air needs of a plastics plant, see our page on compressors for plastics processing.
Why PET blow moulding needs 40 bar
A PET preform is heated, stretched by a rod, then blown into a mould. Blowing happens in two stages:
- pre-blow, at a lower pressure, which starts shaping the bottle;
- final blow at high pressure, up to 40 bar, which presses the PET against the cold walls of the mould.
It is this high pressure that gives the bottle its exact shape: the base, the relief, an even wall thickness. At 8 or 13 bar, the PET does not take the shape of the mould. The exact pressure of each stage is in the blow moulder manufacturer's documentation.
What we do not supply
Let us be precise, because the question comes up often.
- Our compressors do not produce air at 40 bar.
- We do not offer a high-pressure compressor or any equipment to reach that pressure.
- The blow moulder's high-pressure circuit is chosen with its manufacturer and a supplier specialised in that field.
Our article on 10, 13 and 16 bar compressors shows where our range stops.
What the 8 bar compressor does alongside
Around the blow moulder, the line uses air at 6 or 7 bar, like any plant:
- the blow moulder's cylinders and pneumatic controls, where its data sheet provides a low-pressure connection;
- the labeller, shrink wrapper, case packer and palletiser;
- the valves on the filler and capper;
- blow guns for cleaning and maintenance.
Some blow moulders recover part of the blowing air into the low-pressure network. Ask the manufacturer: it reduces the flow the 8 bar compressor has to deliver.
Calculating service air, step by step
Example of a water bottling line, with indicative values to be replaced with those on your machines' data sheets:
| Station | Consumption |
|---|---|
| Blow moulder pneumatic controls | 100 L/min |
| Filler and capper | 120 L/min |
| Labeller | 80 L/min |
| Shrink wrapper | 200 L/min |
| Palletiser | 100 L/min |
| Two blow guns, half the time | 150 L/min |
| Total | 750 L/min |
With a 25 % margin: 750 × 1.25 = about 940 L/min, at 8 bar.
Which compressor for this service air?
- The line in the example: the K 75. 1.1 m³/min at 8 bar, with a 420 L receiver, refrigerated dryer and filtration on the same frame.
- A faster line or a second line: the K 110. 1.7 m³/min at 8 bar.
- A site that already has its dryer and network: the S 110, a standalone screw compressor, 1.7 m³/min at 8 bar.
Points to watch
Two separate networks. The 8 bar network and the high-pressure circuit must never be connected. Each circuit has its own pipework, fittings and relief valves, rated for its pressure.
Air that touches the packaging. If 8 bar air is used to blow or rinse empty bottles, it touches the inside of the packaging. Our compressors are lubricated: the ISO 8573-1 air class for that station is defined with our technicians, and the treatment follows. Our article on compressed air in breweries shows the station-by-station approach.
Continuity. A bottling line without service air stops just as surely as one with a failed blow moulder. Keep service kits in stock: AIRSTAHL parts remain available for 15 years.
Send us the list of machines on your line, excluding the high-pressure blow moulder, with a quote request: we will size the service air with you.
For your trade: Compressor for plastics processing.