How PET bottle blowing uses compressed air
In stretch blow moulding, an injection-moulded preform is reheated in an infrared oven until the PET is soft enough to stretch. The preform is transferred into a split mould, a stretch rod pushes it lengthways, and compressed air expands it sideways against the cold mould wall. The bottle cools against the mould, the air is exhausted and the mould opens.
The air does three different jobs in that cycle, and each has its own pressure and quality need:
- Pre-blow – a short, lower-pressure pulse that starts the expansion while the stretch rod is moving, so the material distributes evenly.
- Final blow (high-pressure blow) – the main charge that forces the PET hard against the mould so the bottle picks up the full shape, base detail and thread finish.
- Control and actuation air – valves, cylinders, mould locking, transfer grippers and other machine functions, usually on the plant’s normal low-pressure network.
Every bottle blown means a volume of high-pressure air is vented after the cycle. That is why PET blowing is one of the most air-intensive processes in a packaging plant and why compressor sizing, energy and reliability get so much attention.
What pressure and flow does a PET blowing machine need?
The figure that counts is the one on the blowing machine’s datasheet. Pressure and flow depend on bottle size, wall thickness, neck finish, base design, number of cavities, cycle speed and the machine maker’s valve block design. Small water bottles on a modern machine may blow well at the lower end of the high-pressure band; carbonated soft drink bottles with petaloid bases and hot-fill containers often need more pressure and tighter control.
As typical industry practice, plants see these bands:
| Air service | Typical pressure | Flow pattern | Usual source |
|---|---|---|---|
| Machine control and actuation | 6–8 bar (87–116 psig) | Steady, modest | Plant low-pressure screw compressor |
| Pre-blow | Up to about 10–12 bar (145–174 psig) depending on machine | Pulsed, every cycle | Low-pressure network or regulated from high-pressure line |
| Final blow, small and medium bottles | Typically 25–35 bar (362.6–507.6 psig) | Large pulsed demand | High-pressure piston compressor or booster |
| Final blow, carbonated and larger containers | Typically 35–40 bar (507.6–580.2 psig) | Large pulsed demand | High-pressure multi-stage piston compressor or booster |
Flow is normally quoted by the machine maker as free air delivery (FAD) in m³/min or cfm at the required pressure for a stated output. Ask for it in writing, with the bottle size and output rate it applies to. If the plant will run several bottle formats, take the worst case, not the most common one.
Compressor architectures for PET blowing
Three system layouts are common. Each can work well; the right one depends on how many machines you run, how much low-pressure air you already have and how much standby you need.
| Architecture | How it works | Where it suits | Points to watch |
|---|---|---|---|
| Dedicated high-pressure piston compressor | A multi-stage reciprocating compressor takes atmospheric air and compresses it in three or four stages to final-blow pressure | Single lines, plants without spare low-pressure capacity | Intercooler and valve condition, cooling, vibration and foundation |
| Low-pressure screw plus booster | A screw compressor delivers air at about 7–10 bar; a reciprocating booster raises part of it to final-blow pressure | Plants already running screw compressors; multiple machines | Booster inlet pressure must stay stable; total energy of both stages |
| Integrated combined package | Screw low-pressure stage and piston high-pressure stage built into one package | New lines where footprint and single-supplier service matter | One machine carries both services, so standby planning matters |
Piston technology dominates the high-pressure stage because positive displacement in several cooled stages handles 25–40 bar efficiently and robustly. Each stage keeps its pressure ratio moderate, and intercooling between stages keeps discharge temperatures within limits and saves power. See our pages on multi-stage reciprocating compressors and booster compressors for the mechanics.
Air quality: oil-free or oil-lubricated?
PET bottles carry water, soft drinks, juice, edible oil and pharmaceuticals. Blow air is in direct contact with the inside of the container, so oil aerosol, oil vapour, particles and water are all unwelcome. Many brand owners and machine makers specify oil-free compression for the high-pressure stage, often with reference to ISO 8573-1 classes for particles, water and oil.
Two approaches exist:
- Oil-free high-pressure compressors – the compression chambers run without oil, so there is no lubricant in the air path to remove. This is the lower-risk route for food and beverage packaging.
- Oil-lubricated compressors with filtration – coalescing and activated-carbon filtration remove most oil, but the plant then depends on filter condition every hour of every shift. Filter failure means contaminated bottles.
Whichever is chosen, the air still needs drying. Water in blow air can affect valve life and, in some products, container quality. A refrigerated dryer giving a pressure dew point of about +3 °C is common on the low-pressure side, while the high-pressure stage often uses a dryer and filters rated for that pressure. All treatment equipment on the high-pressure side must carry a pressure rating above the system maximum. Our pages on food-grade compressed air and oil-free piston compressors go deeper.
How to size a PET blowing compressor system
- Collect the machine data. High-pressure and low-pressure FAD, required pressure at the machine inlet, permitted pressure variation and any air-quality requirement, for each bottle format.
- Set the production plan. Number of machines, shifts per day, days per week and planned format changes. Continuous three-shift running changes the choice of machine and the need for standby.
- Allow for pressure losses. Dryers, filters, pipework, valves and the machine’s own inlet assembly all take pressure. The compressor discharge setting must cover the machine requirement plus those losses.
- Size the storage. High-pressure receivers next to the machine smooth the pulsed final-blow demand and protect pressure stability. They must be designed and certified for the working pressure.
- Decide on standby. If one compressor stops, does the line stop? For most bottlers the answer is yes, so N+1 capacity or a booster plus a spare is usually justified.
- Check the low-pressure side. If a booster is used, the screw compressor feeding it needs enough spare capacity for the booster inlet plus all the plant’s other users.
- Review the room. Cooling air or cooling water, ventilation, foundation, noise and access for valve and piston-ring service.
Do not size from motor kW alone or from a neighbour’s installation. Two machines with the same motor can deliver quite different FAD at 40 bar.
Energy: where PET blowing air is wasted
High-pressure air is expensive to make. Compressing to 40 bar takes far more energy per cubic metre than compressing to 7 bar, so every litre that is vented, leaked or over-pressurised costs real kWh. Typical energy levers include:
- Air recovery – many modern blowing machines can recover part of the final-blow exhaust into the low-pressure network. Where the machine offers this, the low-pressure compressor load reduces.
- Correct set pressure – running the high-pressure system higher than the bottle needs wastes energy on every cycle. Trial the lowest stable pressure that gives good bottles, with the machine maker’s guidance.
- Leaks – high-pressure fittings, valve blocks and hoses leak at a far higher rate per hole than low-pressure joints. Regular leak checks pay back quickly.
- Dead volume – reducing unnecessary volume between the blow valve and the preform cuts air use per bottle; this is a machine-design matter to raise with the machine supplier.
- Heat recovery – intercooler and aftercooler heat can sometimes be used for washing or process water, depending on the plant.
As a rule of thumb across compressed air, about 7% more power is needed for each extra bar on a low-pressure system. On the high-pressure side the effect is less linear, but the principle is the same: generate only the pressure the process needs.
Common problems on PET blowing air systems
| Symptom | Likely causes | What we check |
|---|---|---|
| Bottles not fully formed, weak base detail | Low final-blow pressure at the machine, undersized receiver, pressure drop in treatment | Pressure at the machine during the cycle, filter differential, receiver volume |
| High discharge temperature on one stage | Leaking suction or delivery valves, fouled intercooler, poor cooling water or room ventilation | Interstage pressures and temperatures, valve condition, cooler condition |
| Interstage safety valve lifting | Valve failure in the next stage, blocked intercooler, incorrect setting | Interstage pressures against normal, valve and cooler inspection |
| Oil or water traces in bottles | Filter element overdue, drain failure, oil carry-over from a worn lubricated machine | Drains, filter condition, dryer dew point, compressor oil consumption |
| Excess vibration or knocking | Worn bearings or crosshead, loose foundation bolts, valve damage | Running clearances, foundation, vibration readings, valve inspection |
| Rising energy per bottle | Wear lowering FAD, leaks, higher set pressure than needed | FAD check, leak survey, pressure setting review |
Valve and piston-ring condition decide most of a high-pressure piston compressor’s performance. A disciplined preventive maintenance plan, with interstage pressures and temperatures logged every shift, catches problems before bottles are rejected.
How Reddy Compressors helps
We are an independent multi-brand compressed-air specialist based at Tarnaka, Secunderabad, serving industry since 2002. For PET bottling plants and packaging units we:
- Review the blowing machine’s air specification with you and propose a low-pressure, high-pressure or booster architecture to suit it.
- Supply new and used/refurbished compressors, boosters, dryers, filters and pressure-rated receivers from brands including ELGi, Atlas Copco, Ingersoll Rand, KAESER, Chicago Pneumatic and others. We are not an authorised dealer of any brand. We have tie-ups with all major and reputed brands.
- Service, overhaul and repair high-pressure piston compressors and the screw compressors that feed them, with valve, ring, bearing and cooler work carried out by our team of more than 20 service engineers and technicians.
- Offer AMC and preventive maintenance plans built around three-shift production.
- Identify spare parts from the machine’s make, model, serial number and nameplate, and confirm fit before quoting.
- Provide rental compressors for low-pressure duty and pressures up to 30 bar (435.1 psig) where that suits a breakdown or a trial; final-blow duty above that needs a dedicated machine.
Our service teams cover the whole of Telangana and Andhra Pradesh, and we supply compressors and spare parts across India. Call +91 99590 60718 or +91 92468 88812.
What to send us
- Blowing machine make, model and number of cavities
- Bottle formats, sizes and target output per hour
- Required high-pressure and low-pressure FAD and pressure from the machine datasheet
- Existing compressors, dryers and receivers, with nameplate photos
- Shift pattern and whether standby is needed
- Air-quality requirement from the brand owner or machine maker
- Any current problem: pressure dips, high temperatures, rejects or breakdowns
Frequently asked questions
What pressure is needed for PET bottle blowing?
Typical final-blow pressure is 25–40 bar (362.6–580.2 psig), with machine control air at about 6–8 bar (87–116 psig) and pre-blow up to about 10–12 bar (145–174 psig). Small water bottles often blow at the lower end of the high-pressure band, while carbonated drink bottles and larger containers usually need more. The exact figure must come from the blowing machine maker for each bottle format.
Which compressor is best for a PET blowing machine?
There is no single best machine. Most plants use a multi-stage high-pressure piston compressor, a screw compressor feeding a piston booster, or a combined package. The choice depends on the number of blowing machines, available low-pressure capacity, shift pattern and need for standby. We compare these options against your machine’s air data before recommending a supply.
Does PET blowing need oil-free compressed air?
Blow air touches the inside of the bottle, so many brand owners and machine makers specify oil-free compression, often with ISO 8573-1 quality classes. Oil-lubricated machines with coalescing and carbon filters are also used, but they depend on filter maintenance. For food, beverage and pharmaceutical containers, oil-free high-pressure compression is the lower-risk choice.
How do I calculate compressor size for a PET blowing machine?
Start with the high-pressure and low-pressure FAD figures from the blowing machine datasheet for your worst-case bottle and output. Add pressure losses in dryers, filters and pipework, allow for receiver storage close to the machine, and add standby capacity if a stoppage would halt production. Motor kW alone is not a reliable sizing basis.
What is a booster compressor in PET blowing?
A booster takes air that is already compressed, usually at about 7–10 bar from a screw compressor, and raises it to final-blow pressure, typically 25–40 bar. Because the inlet air is already dense, a booster can be compact. Its performance depends on a stable inlet pressure, so the low-pressure compressor must have enough spare capacity.
Why does my PET compressor run at high temperature?
High stage temperatures usually point to leaking suction or delivery valves, a fouled intercooler or aftercooler, poor cooling water flow or poor room ventilation. Summer ambient heat in Telangana and Andhra Pradesh makes marginal cooling worse. Logging interstage pressures and temperatures helps locate the faulty stage before damage spreads to rings and bearings.
Can I rent a compressor for a PET plant breakdown?
We rent compressors from 50 to 2,000 cfm at 2–30 bar (29–435.1 psig), diesel and electric, which can cover low-pressure plant air and some boosted duties. Final-blow pressures above 30 bar need a dedicated high-pressure machine, so tell us the blowing machine’s requirement and we will advise what can realistically be covered.
Related pages
Technical content reviewed by D. Srinivas Reddy, Managing Partner and service engineer, Reddy Compressors. Last updated 7 November 2026. How our engineering content is reviewed.