How PET forming uses compressed air
A stretch blow moulder heats a preform, stretches it with a rod and inflates it in two stages. Pre-blow at relatively low pressure starts the expansion; final or high-pressure blow then forces the plastic hard against the cold mould to set the shape, threads and base. The air is then exhausted, and in most modern machines part of that high-pressure exhaust is recovered and reused for pre-blow or for the low-pressure network.
Each blow lasts a fraction of a second, but many cavities blow in sequence. The result is a pulsating demand with high peaks, which the compressor, booster, receivers and pipework must serve without letting pressure at the forming valve sag below what the machine maker specifies. Pressure sag shows directly in the product: poorly formed bases, thin shoulders, weak threads and rejects.
The air system from intake to forming valve
| Element | Role | Key engineering points |
|---|---|---|
| Low-pressure compressor | Supplies plant air and feeds the booster | Typically oil-free or well filtered; sized for booster intake plus machine utilities |
| Treatment | Dries and filters air before boosting | Dryer and filters to the bottle maker’s air-quality requirement, often oil-free air for food and beverage packaging |
| Booster | Raises pressure to forming level | Usually reciprocating; capacity quoted at a given inlet pressure, so a low feed pressure cuts output |
| Aftercooler and separator | Removes heat and condensate after boosting | Hot, wet high-pressure air harms valves and moulds |
| High-pressure receiver | Buffers blow peaks | Volume, pressure rating and inspection regime set by pressure-vessel rules |
| Distribution and recovery | Delivers air to the machine and returns recovered air | Short, correctly sized lines; check valves; recovery set-points coordinated with the machine |
Some plants use a single high-pressure compressor instead of a low-pressure compressor plus booster. Either architecture works; the choice depends on line count, available plant air, maintenance capability and energy. A booster fed from an existing low-pressure system is common when expanding lines.
Why average flow is not enough
Machine data often quotes a high-pressure air consumption in normal m³/h at a stated bottle rate. That is an average. The forming valve actually sees the pressure in the local reservoir at the instant of each blow. What matters is:
- The peak instantaneous draw when several cavities blow close together.
- Usable storage: only the volume between receiver pressure and the minimum forming pressure is available to the machine.
- Recovery time: how quickly the booster and storage restore pressure before the next peak.
- Pipework loss between receiver and machine, including filters and check valves.
- Recovered air: how much of the exhaust is returned and at what pressure, which changes the booster’s load.
Air per bottle also changes with bottle size, wall design, preform and blow profile. A change from a 500 ml to a 2 litre format, or a new lightweight design, changes demand even when bottles per hour stay the same.
Sizing method
- Collect machine data. Machine model, cavities, formats, rated bottles per hour, high-pressure and low-pressure consumption per format, required inlet pressures and allowed pressure variation.
- Define operating states. Production, start-up, format change, idle and purge. Note which consumes most high-pressure air.
- Size the booster. Required high-pressure flow at the worst format, net of air recovery as the machine maker states it, with a margin for wear. Confirm the booster’s capacity at the actual inlet pressure it will see.
- Size the low-pressure supply. Booster intake plus machine low-pressure utilities, plus other plant users, at the right pressure.
- Size storage. Use the usable pressure band and peak draw to set receiver volume near the machine.
- Check energy. Every extra bar costs about 7% more power at the compressor; at forming pressures, running higher than the process needs is a significant kWh cost.
Air recovery and the low-pressure network
Recovery systems capture part of the high-pressure exhaust and return it for pre-blow or to the low-pressure header. This can reduce the load on the low-pressure compressor considerably. The interface must be designed carefully:
- Recovered air enters the low-pressure network at a controlled pressure through a pressure-reducing and check arrangement.
- If the low-pressure system pressure is set above the recovery pressure, recovery will not flow; set-points must be coordinated.
- Recovered air can carry moisture or lubricant traces from the machine; filtration before it enters other users may be needed.
- The compressor controls must respond to recovered air without unloading and loading excessively.
Common problems and their causes
| Problem | Typical cause | What we check |
|---|---|---|
| Poor base or shoulder definition | High-pressure sag at the blow valve | Pressure trend at the machine inlet, receiver volume, booster output |
| Booster cannot keep up after line speed increase | Demand exceeds booster capacity at its actual inlet pressure | Booster inlet pressure, valves, rings, cooler condition |
| Water in high-pressure lines | Aftercooler or drain failure, dryer overload | Drains, separator, dryer dew point, cooler fouling |
| Low-pressure compressor overloaded | Recovery not returning or booster intake larger than expected | Recovery set-points, check valves, flow balance |
| High booster temperatures | Hot room, fouled coolers, worn valves | Ventilation, cooling water or air flow, valve condition |
In Telangana and Andhra Pradesh, summer ambient temperatures of 40–45 °C make cooling and ventilation a recurring cause of booster trouble. Coolers that cope in winter may struggle in May.
Maintenance priorities for PET air systems
High-pressure equipment on a PET line works hard and continuously, often through three shifts. A structured routine keeps forming pressure stable and avoids unplanned stops during peak season.
- Booster valves and piston rings: inspect and replace at the maker’s intervals; worn valves reduce output and raise discharge temperature long before a failure.
- Intercoolers and aftercoolers: clean air-side fins or check cooling-water flow and quality; fouled coolers are a common summer problem.
- Condensate drains: test daily; a stuck drain floods high-pressure filters and sends water to the moulds.
- Filters and dryers: change elements on time and check dew point, since wet air shortens valve life on the blow moulder.
- Receivers and relief valves: follow statutory inspection and keep relief valves tested and correctly set.
- Pressure logging: record pressure at the machine inlet and booster discharge regularly to see wear trends before rejects rise.
Plan major booster overhauls during format changes or planned shutdowns, with a rental or standby unit arranged in advance where the line cannot stop. Keeping a record of booster discharge temperature, run hours and pressure recovery time gives early warning that an overhaul is due and helps justify the timing to production.
Where several blow moulders share one high-pressure system, review the combined peak when a new machine or faster format is added. A booster that coped with two machines may not hold pressure with three running in step.
Safety and pressure equipment
Forming pressures carry a lot of stored energy. Receivers, piping, hoses and fittings must be rated for the pressure with appropriate margins, fitted with correctly set relief valves and inspected according to pressure-vessel rules. Isolation and depressurisation procedures are essential before maintenance, and high-pressure hose runs should be restrained. These requirements sit with the machine maker, the plant’s engineering team and the inspection authority; we follow them in the work we do.
How Reddy Compressors helps
We supply, rent and service compressed-air equipment for PET lines as an independent multi-brand specialist, working from our office at Tarnaka, Secunderabad. Our team of more than 20 service engineers and technicians covers all of Telangana and Andhra Pradesh, and we supply equipment and spares across India.
- New and used/refurbished low-pressure, high-pressure and booster compressors from ELGi, Atlas Copco, Ingersoll Rand, KAESER, Chicago Pneumatic and other makes.
- Selection of booster, receiver and treatment from the blow moulder’s data for each format.
- Pressure logging at the machine inlet to diagnose sag, recovery and cycling problems.
- Service, overhaul and AMC for reciprocating boosters and screw compressors, including valves, rings, coolers and drains.
- Rental compressors from our own fleet to cover breakdowns or overhauls, within our 2–30 bar range.
- Spare parts identified from nameplate and old part, with fit confirmed before quoting.
What to send us
- Blow moulder make, model, cavities and bottle formats with rated speeds.
- Machine maker’s high- and low-pressure air requirements and pressures.
- Nameplate photos of existing compressors, booster, dryer and receivers.
- Pipework sketch from compressor room to machine.
- Any pressure readings at the machine and when rejects occur.
- Planned line additions or format changes.
Frequently asked questions
What air pressure is needed for PET bottle blowing?
Final blow pressure for PET bottles is typically in the 25–40 bar range (about 360–580 psig), depending on bottle size, design and machine. Pre-blow and machine utilities usually use low-pressure air around 7 bar (101.5 psig). The blow moulder maker’s specification for each format is the reference, and the pressure must hold at the forming valve during peak blow events.
Booster or high-pressure compressor for PET blowing?
Both are used. A booster takes air from an existing low-pressure supply and raises it to forming pressure, which suits plants with good plant air or that are adding lines. A dedicated high-pressure compressor suits standalone installations. Compare total power, maintenance, redundancy and how air recovery integrates before choosing; neither is universally better.
Why are my PET bottles not forming properly?
If the base, shoulders or threads are poorly defined, check pressure at the forming valve during production. Common air-side causes are booster wear, a low booster inlet pressure, too little high-pressure storage near the machine, restrictive filters or valves and water in the lines. Process causes such as preform heating also matter, so log air pressure alongside reject data.
How does air recovery work on a blow moulder?
Part of the high-pressure exhaust after each blow is captured and reused, usually for pre-blow or fed into the low-pressure network through a regulating and check arrangement. This reduces the load on the low-pressure compressor. Set-points must be coordinated so that recovered air can actually flow, and the compressor controls must accept the varying recovered volume smoothly.
Does PET blowing need oil-free compressed air?
Air for food and beverage packaging is expected to be clean, dry and free of oil that could contact the bottle. Many plants use oil-free compressors and boosters, while others use oil-injected units with high-efficiency filtration. Follow the bottle customer’s and machine maker’s quality requirements and verify air quality to ISO 8573-1 classes by measurement.
Can I rent a compressor if my PET booster fails?
Yes, we can supply rental compressors from our own fleet, within our 2–30 bar range, to cover breakdowns or planned overhauls, subject to matching the machine’s pressure and flow. Send the machine and booster details so we can confirm whether a direct high-pressure unit or a low-pressure feed with your existing booster suits best.
Related pages
Calculators and worked examples
These tools give indicative figures for screening, comparison and for preparing an enquiry. They run in your browser and send nothing. No prices or tariffs are assumed: enter your own to see costs. Efficiencies and other figures shown are example values: replace them with your own. Results are not a performance guarantee or a design certificate; final selection is confirmed by a service engineer against the machine data sheet and the site.
Receiver storage sizing
Estimate the receiver volume needed to ride through a short burst of demand without the pressure falling below the limit.
The worked example below shows the method with fixed numbers. Turn on JavaScript to change the inputs.
Worked example
Inputs
- Burst demand above compressor output (m³/min free air): 5
- Burst duration (s): 20
- Allowed pressure fall (bar): 0.7
- Receiver volume you already have (litres; 0 to skip the hold-up check): 1520
Result
- Receiver volume needed: 2,412 litres
- 2.41 m³
- Practical size (with 25% margin): 3,016 litres
- Hold-up time of the receiver you have: 12.6 s before the pressure falls by 0.7 bar
Method. V = Q × t × P_atm / ΔP, where Q is the shortfall in free air (m³/min), t the time (min), ΔP the allowed pressure fall (bar) and P_atm = 1.013 bar. The same relation gives the hold-up time of an existing receiver: t = V × ΔP / (Q × P_atm). Isothermal; for a real system add a margin.
What to take from it. A receiver buys time. It does not create capacity: once it is spent the pressure keeps falling until the compressors catch up.
Technical content reviewed by D. Srinivas Reddy, Managing Partner and service engineer, Reddy Compressors. Last updated 17 October 2026. How our engineering content is reviewed.