Plastic injection molding is one of the classic industrial users of compressed air. Machines run 24/7 in production, with multiple molding cells, robot loaders, part ejectors, mold cooling assists, and packaging downstream — all driven by air. Demand is high and constant, so the compressor side is usually a proper screw setup, often multi-compressor as the plant grows.

What buyers most often get wrong: they size for today's demand and forget that injection molding plants almost always add lines. And when they do add capacity, they make the piping mistake — which is the most expensive lesson in industrial compressed air. More on that below.

What injection molding needs from the air

Pressure: Standard 7-8 bar (100-115 psi). Some machines or specialty operations (high-pressure mold purging, certain ejector systems) want more, but for the bulk of plant air, standard is fine.

CFM: Big variation depending on plant size:

Plant scale Typical CFM
Single small molding cell 5-15 CFM
Single industrial machine + auxiliaries 20-50 CFM
Small plant (3-5 machines + robots + conveyors) 80-200 CFM
Medium plant (10+ machines, multi-line) 300-800+ CFM
Large plant 1,000+ CFM, multi-compressor

The numbers add up faster than people expect because each molding cell has a base demand (machine controls, robot grippers, part ejection), plus periodic bursts (mold cooling assist, blowing parts clear) that overlap across machines during continuous production.

Air quality: Standard plant air is fine for the machine controls and robots. It gets interesting if you're molding food-grade packaging — if compressed air contacts the inside of a bottle or container that will later hold food/beverage, the air quality framework shifts. See the food/pharma section below.

How much compressor (and how many)?

Real customer setup I've seen many times: medium injection molding plant making consumer or food packaging — 3 to 4 oil-injected screws in the 75-90 kW range (GA75 / GA90 territory), separate dryers, proper compressor room, running 24/7 except maybe Sunday. That's the standard mid-size mold shop configuration.

Going larger: 5+ screws in the 90-150 kW range, with one VSD for trim duty and the rest fixed-speed for base load. At this scale, multi-compressor sequencing controls are mandatory.

Type: it's screws, basically always

Injection molding plants run continuously. That's the textbook screw application:

  • Continuous duty
  • Predictable demand (varies with machine cycles but stays in a band)
  • Multiple compressors with rotation
  • Long maintenance intervals planned around production schedules

Pistons don't fit at this scale. Even a 50-machine plant with a 7.5 kW piston for occasional auxiliary air is rare — by the time you have multiple molding cells, you're firmly in screw territory.

Oil-free? Usually not. Standard oil-injected screw + proper filtration is fine for machine controls and robotic handling. The exception is when air directly contacts product that will hold food/beverage — and even then, most food packaging plants run oil-injected with food-grade filtration rather than oil-free compressors.

VSD: probably one, in a multi-compressor setup

If you're running 3+ compressors, one VSD as trim makes sense. It modulates to fill demand variation while the rest run flat-out at their efficient operating point. Don't make every compressor a VSD — that's expensive and unnecessary, and a roomful of VSDs all modulating to a target pressure tends to hunt and waste energy.

For a single-compressor plant, fixed-speed is usually fine. Injection molding demand is fairly stable when production is running. See VSD vs fixed speed for the framework.

The piping mistake — don't make this one

This is the most common expensive mistake I've seen in injection molding plants, and it's worth a story.

A plastic injection molding plant making food packaging had three GA75 oil-injected screws in their original compressor room with separate dryers — a proper setup running 24/7. They expanded production with a new line on the other side of the factory. Demand went up. They bought a GA90 VSD with integrated dryer (Workplace style, dryer built in) and dropped it into a converted storage room near the new line.

Result: constant pressure fluctuations across the whole plant. The interconnect piping between the two compressor rooms wasn't sized for the new flow. Pressure in the two halves of the factory wouldn't equalize. The four compressors fought each other for control. Production suffered.

The fix would have been a bigger header pipe between the two rooms, not another compressor. They didn't realize until after the new compressor was installed and running.

Before you buy another compressor when production grows: measure your pressure at the discharge AND at the equipment. If discharge is fine but the equipment side is low, it's piping. Adding more compressor won't fix piping — see adding capacity to existing system for the diagnostic and fix.

Application-specific gotchas

Plan for expansion from day one. Injection molding plants always add machines. Size your compressor room, your electrical supply, and especially your piping for where you'll be in 5-10 years, not for today's machine count.

Weekend operation patterns. Most plants run reduced shifts on Sunday (or full off). Plan for one compressor handling the weekend skeleton crew demand — instrument air, building services, occasional packaging — while the rest are off. A proper sequencing controller handles this automatically.

Receiver sizing matters. Each molding cell creates demand spikes (mold cooling blow-off, part ejection blast). A larger central receiver (1000-3000L for a multi-machine plant) absorbs the spikes and lets the compressors run smoother.

Backup matters for production-critical applications. If your plant stops when the compressor stops, plan for N+1 redundancy. A spare compressor that can be brought online for a planned maintenance window or a sudden failure is much cheaper than an unplanned production shutdown.

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