Where does a plastics plant use compressed air?
Walk through any moulding shop and compressed air turns up at almost every step, from the silo to the packing table. The demand is a mix of small, fast pulses at many machines and a few steady or very large users. Understanding that mix is the starting point for every sizing and treatment decision.
| User | Typical pressure | Flow pattern | Typical air quality (ISO 8573-1) |
|---|---|---|---|
| Injection-moulding machine valves, air ejection, core pulls | 5–7 bar (72.5–101.5 psig) | Short pulses every cycle, many machines | Class 1.4.2 or similar general plant air |
| Sprue pickers and take-out robots, end-of-arm grippers and vacuum generators | 5–6 bar (72.5–87 psig) | Rapid pulses synchronised with mould opening | Dry, filtered air; oil carry-over damages seals and venturis |
| Granule and regrind conveying (venturi or pressure conveying) | 3–6 bar (43.5–87 psig) | Intermittent but heavy while loaders run | Dry air; moisture spoils hygroscopic resins |
| Resin dryer purge and regeneration (compressed-air dryers) | 5–7 bar (72.5–101.5 psig) | Continuous small bleed | Low dew point, oil-free at the point of contact |
| Extrusion blow moulding (HDPE, PP containers) | 6–10 bar (87–145 psig) | Large pulse per cycle | Clean and dry; contact with inside of container |
| PET stretch blow moulding | Pre-blow 7–12 bar, final blow typically 25–40 bar (362.6–580.2 psig) | Very large, repeated volumes per cavity | Oil-free or oil-removed, dry, particle-filtered |
| Mould cleaning, blow-off guns, maintenance tools | 5–6 bar | Random, often wasteful | General plant air |
| Packing, cartoning, leak testers, label applicators | 5–6 bar | Small, steady | General plant air, local regulator |
The classes in the table are typical starting points, not universal rules. Food-contact containers, medical mouldings and pharma packaging usually need a tighter specification agreed with the customer or brand owner.
Why one header pressure does not suit every machine
The most common mistake in moulding plants is raising the whole header to satisfy the most demanding machine. A robot gripper that drops parts at 5.5 bar, or a blow mould that will not fill at the end of a long pipe, tempts the maintenance team to lift the compressor setting from 7 to 8 or 8.5 bar. Every extra bar costs roughly 7% more compressor power, and it also increases leakage across every fitting in the plant.
The better question is why that one user is starved. In our experience the answer is usually one of these:
- An undersized drop line, quick coupler or hose feeding the machine.
- A clogged filter or regulator at the machine inlet.
- Too little local storage to cover a fast pulse such as a mould-open blow or a loader cycle.
- Several machines opening valves at the same moment, causing a short, sharp pressure dip.
- A genuinely different duty, such as blow moulding, sharing a header with 6 bar users.
Only the last item justifies a different pressure, and even then the answer is a separate zone or a booster, not a higher setting for the entire factory.
Typical system architecture for a moulding plant
A well-integrated plastics plant normally ends up with three layers.
- Main plant-air station. Oil-injected or oil-free rotary screw compressors, often one variable-speed unit trimming on top of fixed-speed base machines, delivering 6.5–7 bar to a ring main. A wet receiver after the compressors, a refrigerated dryer and coalescing filtration follow.
- Machine and zone level. Drops taken from the top of the header, a filter-regulator at each machine, and local receivers near robots, loaders or blow-off stations that draw fast pulses. Zones can be isolated for mould changes without shutting the whole line.
- Special-pressure duties. PET blow moulding is supplied either by a dedicated high-pressure compressor (typically a multi-stage reciprocating or a combined screw-plus-booster package) or by a booster fed from the plant air header. High-pressure air has its own receivers, dryer and filters rated for the pressure.
Keeping these layers separate makes energy measurement easier too. If the high-pressure blow-moulding air has its own meter, the plant can see clearly what each bottle or preform costs in air, rather than hiding it inside a single electricity bill.
PET and blow moulding: the high-pressure duty
PET stretch blow moulding is the biggest air user in many packaging plants. A preform is heated, stretched by a rod and pre-blown at low pressure, then fully formed at a final blow pressure that is typically 25–40 bar (362.6–580.2 psig), depending on bottle size, wall design and machine make. After each cycle that volume of high-pressure air is exhausted, so consumption is very high.
Points we look at on high-pressure duty:
- Booster or dedicated compressor. A booster takes plant air at 6–7 bar and raises it to blow pressure. It suits plants that already have spare low-pressure capacity. A dedicated high-pressure compressor suits larger lines where the blow air dominates.
- Air recovery. Many modern blow-moulding machines can recover part of the exhaust air into the low-pressure or pre-blow network. If recovery is used, the plant-air system should be sized with that return flow in mind.
- Cooling and ambient heat. High-pressure stages run hot. In a Telangana or Andhra summer the compressor room can be well above 40 °C, which raises discharge temperatures and stresses valves and seals. Ventilation and cooler cleanliness matter as much as the compressor itself.
- Air quality. Air touches the inside of the bottle. Oil-free compression or proper oil removal, a dryer suited to the pressure, and particle filtration are normal practice; the brand owner may specify the exact class.
- Safety. Receivers, pipework, hoses and valves must be rated for the pressure, with relief valves and inspection records.
Extrusion blow moulding of HDPE or PP containers usually works at much lower pressure, typically 6–10 bar, and can often run from the main header with a local receiver close to the machine.
Granule conveying, resin drying and moisture
Many hygroscopic polymers such as PET, nylon, PC and ABS must be dried before moulding. Wet compressed air used for conveying or dryer purge can put moisture straight back into dried granules, causing splay, silver streaks, bubbles and brittle parts. The defect then looks like a material or process problem, and weeks can be lost adjusting barrel temperatures.
Practical rules:
- Use air that has passed through the main dryer for any conveying line feeding dried material. A refrigerated dryer gives a pressure dew point of about +3 °C, which suits general plant air.
- Where compressed air is used for drying or purge directly, a desiccant dryer with a dew point of typically −20 °C to −40 °C is often specified.
- Check that condensate drains on receivers and filters actually work. A failed drain in a hot, humid monsoon month sends slugs of water downstream.
- Consider whether a vacuum or blower-based conveying system would be cheaper than compressed-air venturi loaders, which are convenient but inefficient for large tonnages.
Reliability, standby and rental cover
A moulding machine stopped by low air pressure does not simply pause. Moulds cool, material degrades in the barrel, and restarting often produces scrap until the process stabilises. For a PET line, a high-pressure compressor trip stops bottle output straight away.
- Plan N+1 on the main station so that one compressor can be serviced without starving production.
- Keep critical spares for the high-pressure machine: valves, piston rings or seals, filters and gaskets, identified from the machine nameplate.
- Add the dryer to the standby plan. A dryer bypass during a breakdown sends wet air into robots and conveying.
- Arrange temporary air before a planned shutdown or expansion rather than during a breakdown. Electric or diesel rental compressors from 50 to 2,000 cfm and 2 to 30 bar can cover plant air, and a temporary dryer and filter train keeps quality acceptable.
Energy: where moulding plants lose money on air
Compressed air is often one of a moulding plant’s largest electricity costs. Typical losses we find in plastics plants:
- Header pressure set higher than any user needs.
- Leaks at quick couplers on mould-change hoses and robot fittings, running 24 hours a day.
- Open blow-off pipes used for part cooling or cleaning instead of engineered nozzles.
- Venturi vacuum generators running continuously on robots when they only need vacuum during pick-up.
- Fixed-speed compressors loading and unloading frequently on a variable demand.
- Heat from compressors vented away when it could warm water or help pre-dry air.
A measured audit with power and flow logging, pressure profiling and a leak survey shows which of these matter in a particular plant, and gives a specific power figure (kW per m³/min) to track afterwards.
The Indian context: Telangana, Andhra Pradesh and beyond
Plastics processing in Telangana is spread across industrial estates around Hyderabad and its outskirts, with injection moulding for automotive, appliance, pharma packaging and household products, and PET preform and bottle making for beverages and water. Andhra Pradesh has packaging and moulding units around its coastal and industrial corridors. Similar clusters exist across India.
Three local factors shape compressed-air design here. High summer ambient temperatures reduce cooling margin and dryer performance. Monsoon humidity increases the water load on dryers and drains. Power dips and voltage fluctuation can trip compressors and dryers, so restart behaviour and sequencing deserve attention.
Common problems and fixes
| Symptom | Likely cause | Fix |
|---|---|---|
| Robots drop parts or miss picks | Pressure dip at gripper during mould opening; clogged regulator | Local receiver, larger drop, clean filter-regulator |
| Splay or bubbles in hygroscopic material | Wet conveying or purge air | Dry air to conveying, check dryer dew point and drains |
| Blow moulds not filling consistently | Undersized high-pressure supply or long pipe run | Dedicated zone, receiver near machine, check booster capacity |
| Oil traces on parts or robot seals swelling | Oil carry-over, saturated coalescing filters | Change filters, check separator, review oil-free option |
| High-pressure compressor tripping on temperature | Hot compressor room, fouled coolers | Ventilation, cooler cleaning, check valves |
How Reddy Compressors helps
From our office at Tarnaka, Secunderabad, we work with moulding and packaging plants across Telangana and Andhra Pradesh, and supply compressors, rentals and spares across India. As an independent multi-brand specialist we sell, rent and service machines from ELGi, Atlas Copco, Ingersoll Rand, KAESER, Chicago Pneumatic and others, so our advice is not tied to one maker.
- Sales of new and used/refurbished rotary screw, reciprocating and high-pressure compressors, dryers, filters and receivers.
- Rental compressors from our own fleet for shutdowns, expansions and breakdown cover.
- Service, AMC, overhaul and breakdown repair through a team of more than 20 service engineers and technicians.
- Spare parts identified from the machine nameplate, serial number or old part, with fit confirmed before quoting.
- Air audits: demand profiling, leak detection, pressure-drop and dew-point checks.
Our track record. We have supplied compressors to leading companies in plastics moulding across Telangana, Andhra Pradesh and beyond – new and used machines, long-term rental, and breakdown cover when a plant’s own compressor fails – backed by our own service team.
What to send us
- List of moulding and blow-moulding machines, with makes and the air pressure each requires.
- Existing compressors, dryers and receivers: make, model, nameplate photos.
- Header pressure setting and the lowest pressure seen at the worst machine.
- Materials processed and any air-quality specification from your customers.
- Shift pattern, planned expansion and any recent air-related quality problems.
Frequently asked questions
What air pressure does an injection moulding machine need?
Most injection-moulding machines use compressed air at about 5–7 bar (72.5–101.5 psig) for valve gates, air ejection, core pulls and robot interfaces. The machine manual gives the exact figure. If one machine needs more, check its drop line, filter-regulator and local storage before raising the header pressure, because every extra bar costs roughly 7% more compressor power across the whole plant.
What pressure is used for PET bottle blowing?
PET stretch blow moulding usually pre-blows at around 7–12 bar and completes the bottle at a final blow pressure of typically 25–40 bar (362.6–580.2 psig). The exact value depends on bottle size, design and the blowing machine. This air is supplied by a dedicated high-pressure compressor or a booster fed from plant air, with its own receivers, dryer and filtration.
Should I use a booster or a separate high-pressure compressor for blow moulding?
A booster suits plants that already have spare low-pressure capacity and modest blow-air demand, because it raises existing plant air to blow pressure. A dedicated high-pressure compressor suits larger PET lines where blow air is the main load. The choice depends on measured demand, air recovery on the blow-moulding machine, floor space, maintenance skills and standby requirements.
Why do my moulded parts show splay or silver streaks?
Splay often comes from moisture in hygroscopic resins such as PET, nylon, PC or ABS. If the conveying or dryer purge air is wet, it can put moisture back into dried granules. Check the dew point after your air dryer, test the condensate drains, and make sure conveying lines feeding dried material take air from after the dryer.
Does a plastics plant need oil-free compressed air?
General moulding machine and robot air can usually come from a well-maintained oil-injected screw compressor with coalescing filters. Air that touches food-contact or pharma containers, especially in PET blowing, is often specified as oil-free or oil-removed to a defined ISO 8573-1 class. The customer or brand owner normally sets the requirement, and we size treatment to match.
How can a moulding plant reduce compressed air costs?
Start by measuring. Lower the header to the pressure the worst user really needs, fix leaks at quick couplers and robot fittings, replace open blow-off pipes with engineered nozzles, and control venturi vacuum generators so they run only during pick-up. Good compressor sequencing or a variable-speed trim unit then reduces unloaded running and frequent load-unload cycling.
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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.