For a shop of injection moulding machines, you size the compressor by adding up the air used around each machine: take-out robot, mould blow-off, vacuum cups and cylinders. As a rough guide, one machine with its robot uses around 100 to 200 L/min. Eight machines on three shifts therefore need about 1,400 L/min including a margin, which is an 11 kW screw compressor; from ten machines upwards, you move to 22 kW.
This article walks through the calculation. For the other air needs of a plastics plant, see our page on compressors for plastics processing.
What uses air around a moulding machine
The machine itself is usually hydraulic or electric. Its compressed air mainly feeds the equipment around it.
- The take-out robot or sprue picker. Its cylinders move on every cycle. Their consumption is calculated like any cylinder, from bore, stroke and cycle rate: our article on pneumatic cylinder air consumption gives the formula.
- Venturi vacuum cups. They create vacuum by passing compressed air through a nozzle. They blow for as long as the part is held, which often makes them the largest consumer.
- Mould blow-off. A nozzle that ejects the part or cleans the cavity uses about as much as a blow gun, around 150 L/min while open.
- Pneumatic valves, cores and shut-offs, depending on the mould.
The exact figures are on the data sheets of the robot, the vacuum cups and the mould. The ones below are typical orders of magnitude, to be replaced with yours.
The calculation for one machine, then for the shop
Step 1, one equipped machine (indicative values):
| Item | Consumption during the cycle | Share of time | Average |
|---|---|---|---|
| Take-out robot, cylinders | 60 L/min | 100 % | 60 L/min |
| Two venturi vacuum cups | 50 L/min | 60 % | 30 L/min |
| Mould blow-off nozzle | 150 L/min | 25 % | 38 L/min |
| Valves and cores | 10 L/min | 100 % | 10 L/min |
| Total | 138 L/min |
Step 2, the shop. In injection moulding, the machines run at the same time, so you add them up with no diversity factor. Eight machines: 8 × 138 = 1,104 L/min.
Step 3, the margin. Add about 25 % for leaks, wear and one more machine: 1,104 × 1.25 = 1,380 L/min at working pressure.
Step 4, read the data sheet. The S 110 delivers 1.7 m³/min at 8 bar, or 1,700 L/min: it covers the demand with headroom.
Which compressor for which size of shop?
| Shop | Demand incl. margin | Model |
|---|---|---|
| 3 to 5 machines, one or two shifts | up to about 1,000 L/min | K 110, 4-in-1 with 340 L receiver, dryer and filtration |
| 6 to 9 machines on shifts | up to 1,700 L/min | S 110, with receiver and dryer sized to the flow |
| 10 to 20 machines on three shifts | up to 3,600 L/min | S 220 PM, 3.6 m³/min at 8 bar |
| Beyond that | industrial project on quotation |
The S 220 PM runs at variable speed. Between cycles, mould stops and job changes, demand moves all the time: it slows down instead of idling. On three shifts, that is where the electricity bill is decided, as our guide to compressor energy costs explains.
Pitfalls specific to injection moulding
Vacuum cups left open. A venturi that blows through the whole cycle when it only holds the part for two seconds doubles its consumption. Check the timing on the robot before buying a bigger compressor.
Pressure. Robots and vacuum cups usually work at 6 bar. An 8 bar version is enough and gives the best flow. Only go to 10 bar if a machine requires it on its data sheet.
Water. Water in the network makes robot cylinders stick and floods the venturis. A refrigerated dryer after the compressor and drains at low points are the baseline.
Heat. Moulding machines heat the shop. Install the compressor in a ventilated room outside the production hall: our article on compressors in hot workshops sets out the rules.
Power supply. These compressors run on 380 V three-phase, like the moulding machines.
For your own shop, list the machines and their equipment, then run the numbers in our configurator or send us the list with a quote request: we will check the flow with you.
For your trade: Compressor for plastics processing.