Vacuum forming pulls a sheet of heated plastic down onto a mould by evacuating the air between sheet and tool. The pump for this job is chosen on draw-down speed above all — the sheet is soft for only a few seconds, so the vacuum has to arrive fast, at the moment of forming, while the plastic is still formable. That makes a vacuum reservoir the key design element and the pump the device that keeps it charged. For forming that also needs blow air, the combined YBVP vacuum-and-pressure pumps do both jobs from one unit.
The forming cycle and why speed rules
A vacuum forming cycle runs: heat the sheet → seal it over the mould → evacuate fast → the sheet draws down and takes the mould's shape → cool → release. The forming window is short because the sheet only stays formable while hot. If the vacuum arrives slowly, the sheet cools and stiffens before it fully forms, and you get poor detail, webbing, and incomplete draw into corners.
So the governing requirement is volumetric draw-down rate at the forming instant — pull the air out of the sheet-to-mould space quickly enough to form the part while it's still soft.
Why a reservoir does the heavy lifting
A pump alone rarely evacuates fast enough at the exact forming instant — its instantaneous flow is limited. The standard answer is a vacuum reservoir (surge tank):
- The pump pre-evacuates the reservoir to a deep vacuum during the heating part of the cycle, when there's time.
- At the forming instant, a valve opens and the reservoir dumps its stored vacuum into the mould near-instantly, drawing the sheet down fast.
- The pump then recharges the reservoir before the next cycle.
This decouples the fast draw-down from the pump's steady flow. The pump is sized to recharge the reservoir within the cycle time, not to supply the whole draw-down instantaneously — a much smaller, cheaper pump than trying to form directly off the pump would need.
| Design element | Sized by |
|---|---|
| Reservoir volume | Mould/part volume × required draw-down speed |
| Reservoir vacuum level | Detail and depth of draw needed |
| Pump displacement | Recharge the reservoir within cycle time |
| Pump vacuum | Must reach the reservoir's target level with margin |
Sizing the pump
- Set the reservoir vacuum level. Deeper vacuum forms finer detail and deeper draws. A moderate-to-deep vacuum suits most forming.
- Size the reservoir for the part volume and the draw-down speed the plastic demands — larger parts and faster forming need a bigger tank.
- Size the pump to recharge that reservoir from its post-form pressure back to target within the available cycle time, with margin. Remember pumping speed derates as vacuum deepens, so the recharge slows as the tank approaches target — allow for it. The method is in choosing a pump by CFM and vacuum level.
- Confirm the pump's ultimate is comfortably deeper than the reservoir target so it isn't recharging on the dead end of its speed curve.
For straight vacuum forming where oil is acceptable, the ACME high-vacuum pumps — 750 mmHg, 10 to 302 m³/hr — pull deep and recharge a reservoir well:
| Model | Power | Displacement | Vacuum |
|---|---|---|---|
| ACME-040 | 2.0 HP | 40 m³/hr | 750 mmHg |
| ACME-063 | 3.0 HP | 63 m³/hr | 750 mmHg |
| ACME-100 | 4.0 HP | 100 m³/hr | 750 mmHg |
| ACME-140 | 5.0 HP | 140 m³/hr | 750 mmHg |
| ACME-202 | 6.0 HP | 202 m³/hr | 750 mmHg |
When you need pressure too — plug-assist and blow
Many thermoformers use pressure air as well as vacuum: to plug-assist the sheet into deep moulds, to pressure-form against the tool for sharper detail, or to blow-release and eject the part. For those machines the combined YBVP vacuum-and-pressure pumps — −600 mbar vacuum and +600 mbar pressure from one unit — cover both sides without a second machine:
| Model | Power | Displacement | Vacuum | Pressure |
|---|---|---|---|---|
| YBVP 40 | 1.85 kW | 40 m³/hr | −600 mbar | 600 mbar |
| YBVP 60 | 3 kW | 60 m³/hr | −600 mbar | 600 mbar |
| YBVP 100 | 5.5 kW | 100 m³/hr | −600 mbar | 600 mbar |
Being oil-free, the YBVP also suits forming food and pharma packaging where the airstream must stay clean — the same logic as the YBDVP dry pumps used on hygienic packaging lines.
Matching pump to forming style
Different forming methods lean on the vacuum system differently, and the right pump follows the method.
- Simple sheet forming (trays, blister backs, shallow parts) draws the hot sheet down with vacuum alone. A reservoir charged by an ACME high-vacuum pump is the classic setup — deep, fast draw-down, no pressure side needed.
- Deep-draw forming pulls the sheet into tall or steep moulds where plain vacuum leaves thin, webbed corners. A plug-assist mechanically pushes the sheet in first, and pressure air can help form it — the combined YBVP pump supplies both from one unit.
- Pressure forming uses positive pressure above the sheet as well as vacuum below for the sharpest detail and texture reproduction. Here the pressure side is not optional, and a vacuum-and-pressure pump is mandatory.
- Continuous / roll-fed thermoforming indexes film through a heated station at speed; the reservoir recharge time becomes the pacing constraint, so size the pump generously so it never becomes the bottleneck.
Read the mould depth and the required detail first: shallow and simple → vacuum-only ACME; deep or high-detail → vacuum-and-pressure YBVP.
Multiple cavities and shared reservoirs
A multi-cavity tool forms several parts per cycle, multiplying the volume the reservoir must dump and the pump must recharge. Size the reservoir for the total cavity volume and the pump to recharge it within the cycle, not for a single cavity. Where several forming stations share one vacuum system, size for the worst case of stations forming simultaneously — the same coupled-demand logic that governs any shared vacuum system. Get the reservoir right and even a modest pump forms crisp parts; get it wrong and no amount of pump fixes a slow, cold draw-down.
Practical notes
- Keep the reservoir close to the mould and use generous, short piping — a restrictive line throttles the fast dump and slows draw-down no matter how good the pump.
- Filter the pump inlet. Forming pulls trim dust and plastic fines; keep them out of the vanes.
- Watch recharge time as a health check. If the reservoir takes longer to reach target than it used to, suspect worn vanes or (on oil pumps) tired oil — see vane inspection.
The plastics context — forming, PET, extrusion — is on the plastics and PET applications page, and packaging-specific forming is in vacuum pumps for packaging machines.
Summary
Vacuum forming is won on draw-down speed at the forming instant, so the design centres on a vacuum reservoir the pump keeps charged — you size the pump to recharge the tank within the cycle, not to form directly, allowing for the speed that derates as the tank fills. Use ACME high-vacuum pumps for straight vacuum forming, and the oil-free combined YBVP pumps when the machine also needs plug-assist or blow air — and when the formed pack is food or pharma.
Send us the part size, cycle time and whether you need blow air and we'll size the pump and reservoir. WhatsApp +91 9311693322 or email sales@yashblowers.org.