A thermoformer's compressor is sized first on the forming air: the volume of the pressure box, multiplied by the pressure and by the cycle rate. A pressure-forming machine working at 4 bar with a 10 litre box, at 15 cycles per minute, needs about 750 L/min on its own. With part release and cylinders added, an 11 kW 4-in-1 screw compressor covers the demand; a vacuum-only machine manages with 7.5 kW.
For the other uses of air in a plastics plant, see our page on compressors for plastics processing.
Vacuum or pressure, two very different needs
The vacuum that pulls the hot sheet onto the mould comes from a vacuum pump, not from the compressor. A vacuum-only thermoformer therefore only uses compressed air for its cylinders, part release and some blow-off.
A pressure thermoformer adds compressed air above the sheet to reproduce the mould detail better. On every cycle, it fills a pressure box and then empties it. This is what sets the size of the compressor.
Calculating forming air, step by step
Step 1, the box volume. This is the volume of air above the sheet at the moment of forming. It is on the machine's data sheet or can be obtained from the manufacturer. Example: 10 litres.
Step 2, the forming pressure. Also on the data sheet, often a few bar. Example: 4 bar gauge. To be on the safe side, count in absolute pressure, as for a cylinder: 4 + 1 = 5 bar.
Step 3, air per cycle. 10 litres × 5 = 50 litres of free air per cycle.
Step 4, the cycle rate. At 15 cycles per minute: 50 × 15 = 750 L/min.
Step 5, the rest of the machine (indicative values, to be replaced with those on your data sheets):
| Item | Consumption |
|---|---|
| Forming air | 750 L/min |
| Part release by blow-off | 60 L/min |
| Cylinders on the trim press and stacker | 120 L/min |
| One cleaning blow gun, intermittent | 75 L/min |
| Total | 1,005 L/min |
Step 6, the margin. 1,005 × 1.25 = about 1,260 L/min, at 8 bar.
Which compressor for which machine?
- A pressure thermoformer like the one in the example: the K 110. 1.7 m³/min at 8 bar, with a 340 L receiver, refrigerated dryer and filtration built in.
- A vacuum-only machine, or a small sheet-fed machine: the K 75. 1.1 m³/min at 8 bar.
- Two pressure-forming lines on shifts: the S 220 PM. 3.6 m³/min at 8 bar, variable speed, with receiver, dryer and filters sized to the flow.
If the data sheet calls for a forming pressure close to 7 bar, check that there is enough margin to the compressor's setting. The S and K series are available at 10 bar, with slightly lower flow: the K 110 then delivers 1.4 m³/min.
The buffer receiver next to the machine
Forming air is drawn in bursts: the box fills in a fraction of a second. Without a reserve nearby, pressure drops at the moment of forming and parts come out badly formed. A receiver placed next to the thermoformer, in addition to the compressor's own, supplies these peaks. Our air receiver size tool helps you choose it.
Thermoforming pitfalls
Water on the hot sheet. A drop of water or oil carried by the forming air marks the part. A refrigerated dryer, drains and a fine filter close to the machine prevent these defects. Our article on fine filters explains the order of installation.
Food trays. When the formed part is food packaging, the forming air touches its inner face. Our compressors are lubricated: the ISO 8573-1 class for that station is defined with our technicians, and the filtration follows.
Box leaks. A worn box seal lets air escape on every cycle. Consumption rises and forming deteriorates: replace it before blaming the compressor.
Send us your thermoformer's data sheet with a quote request: we will check the flow and the receiver with you.
For your trade: Compressor for plastics processing.