How does vacuum forming work, and what does the vacuum system do?
In vacuum forming, a thermoplastic sheet such as PET, PP, PS, HIPS, ABS or PVC is clamped, heated until it softens and then brought against a mould. Vacuum applied through small holes or slots in the tool removes the air trapped between sheet and mould, and atmospheric pressure pushes the sheet into every detail of the tool. The part is held while it cools, then the vacuum is vented, often with a short blow of compressed air to release the part.
The forming window is short. Once the sheet starts to cool against the tool its stretch behaviour changes, so detail, wall thickness and surface finish depend on how quickly the pressure behind the sheet falls in the first fraction of a second to a few seconds. That is why forming machines rely on a large reserve tank kept at low pressure by the pump, rather than on the pump alone.
Pressure forming, plug assist, twin-sheet and pre-stretch processes add compressed air and extra steps, but the vacuum side follows the same logic: a defined volume must be evacuated to a defined level within a defined time, cycle after cycle.
What sets the vacuum demand of a forming cycle?
Each cycle has a short, intense demand followed by a hold and a recovery period. The main factors are these:
- Volume evacuated per cycle: the space between sheet and tool, the tool’s internal passages and plenum, hoses, valves and pipe up to the reserve tank.
- Target vacuum, usually expressed as an absolute pressure in mbar or as a percentage of atmospheric. Fine detail and deep draws need lower absolute pressure.
- Time allowed to reach the target, set by the forming window of the material and part.
- Leakage at clamp frames, seals and through porous or worn tools during the hold.
- Cycle rate and whether several stations or machines draw on the same vacuum source at the same time.
Two machines with the same platen size can have very different demand because one has deep, multi-cavity tooling with long internal passages and the other a shallow single cavity.
How do you size the reserve tank and pump?
The reserve (vacuum receiver) supplies the rapid first draw-down; the pump then recovers the tank pressure before the next cycle. A simple equalisation estimate shows the principle. If a reserve tank of volume Vr at pressure pr is suddenly connected to a forming volume V at atmospheric pressure p0, and the transfer is fast, the combined pressure is roughly:
p = (pr × Vr + p0 × V) ÷ (Vr + V)
Worked example (illustrative only): a 500 litre reserve held at 50 mbar absolute is opened to a 50 litre forming volume at 1,013 mbar. The equalised pressure is (50 × 500 + 1,013 × 50) ÷ 550, about 138 mbar absolute. If the part needs 100 mbar or lower, either a larger reserve, a lower reserve pressure, a smaller dead volume or continued pumping during the hold is needed.
The pump must then bring the reserve back down in the time available. For an ideal pump of effective speed S evacuating volume V from pressure p1 to p2, the time is approximately t = (V ÷ S) × ln(p1 ÷ p2). Recovering the 550 litre system from 138 mbar to 50 mbar is ln(2.76), about 1.0, so with an effective speed of 100 m³/h (about 28 l/s) at the tank the recovery takes roughly 20 seconds, before allowing for leakage and line losses. If the cycle is shorter than that, the tank never fully recovers and each cycle starts at a worse vacuum.
These estimates are a starting point. Real speed falls as pressure falls, valves and pipes restrict flow, and leakage continues during the hold, so the final selection should be checked against the pump’s speed curve and measured cycle data.
Why do valves, pipework and tool venting matter so much?
The fastest pump and the largest tank are wasted if the path to the tool is restricted. A small vacuum valve, a long thin hose or a tool with too few or blocked vent holes will slow the first draw-down more than any pump change can fix.
- Keep the reserve tank close to the platen, with short, large-bore pipework and a fast-acting valve of adequate bore.
- Avoid sharp bends, reducers and long flexible hoses in the main path.
- Check vent holes and slots in the tool regularly; plastic residue, release agent and dust block them gradually.
- Fit an inlet filter to protect the pump from plastic fines and degraded material, and keep it clean so it does not become a restriction itself.
- Minimise dead volume that is evacuated every cycle but does not help forming.
Which vacuum pumps suit thermoforming?
| Pump type | Strengths for forming | Points to watch |
|---|---|---|
| Oil-sealed rotary vane | Good vacuum level, compact, widely used on forming machines | Oil and separator maintenance; plastic fumes in oil |
| Dry rotary vane | No oil; simple | Higher ultimate pressure; vane wear |
| Dry claw | Oil-free, robust, efficient in the forming pressure range | Higher purchase cost; filtration of fines |
| Dry or oil-flooded screw | Large capacity for central systems | Suited to bigger installations |
| Liquid ring | Tolerates vapour and fines | Seal-water use and higher energy |
For plants with several forming lines, a central vacuum system with speed-controlled pumps and a shared reserve can be more efficient than one pump per machine, provided the pipework is sized so that one machine’s draw-down does not starve another.
Why are my thermoformed parts not forming properly?
Poor detail, webbing, thin corners or inconsistent parts can come from the sheet, the heating or the tool as well as from vacuum. Separate vacuum causes using the pressure trace:
- Fit a fast-responding vacuum gauge or transducer at the tool, not only at the pump, and record the pressure during a cycle.
- Note the reserve-tank pressure at the start of each cycle. If it is drifting upwards over successive cycles, the pump is not recovering in time.
- Compare the time to reach target vacuum with the expected forming window.
- Check how well the pressure holds during cooling. A rising pressure points to leaks at the clamp frame, seals or tool.
- Inspect valves, filters and vent holes for restriction.
- Check pump condition: inlet filter, oil, vanes and the pump’s own ultimate vacuum when isolated.
| Symptom | Vacuum-related cause | Check |
|---|---|---|
| Poor definition in corners and details | Slow draw-down or insufficient final vacuum | Tool-side pressure trace; vent holes |
| Quality worsens through a shift | Reserve not recovering between cycles | Tank pressure at cycle start; pump speed |
| Parts lift or distort during cooling | Leakage during hold | Clamp seals, tool porosity |
| Pump hot and noisy | Running at high inlet pressure continuously; blocked exhaust | Duty, separator, oil |
Where does compressed air come in?
Most forming lines also use compressed air for clamp and plug cylinders, part ejection and blow-off, pressure forming, cooling and trimming. Air quality matters: oil or water from the compressed-air system can mark parts or contaminate food-contact packaging. A suitable dryer and filtration, and an oil-free compressor where the product requires it, keep the air side from undoing the work of the vacuum side. In Hyderabad’s packaging and plastics units, high summer temperatures also make dryer and compressor cooling performance a seasonal concern.
How Reddy Compressors helps
Reddy Compressors, at Tarnaka in Secunderabad, has supported industry since 2002 as an independent multi-brand compressed-air, vacuum and blower specialist. We are not an authorised dealer for any brand. We have tie-ups with all major and reputed brands. Our service teams work across Telangana and Andhra Pradesh, and we supply equipment and spares across India.
- Reviewing forming-line vacuum: pump size, reserve tank, valves, pipework and pressure traces at the tool.
- Supplying, servicing and repairing rotary-vane, claw, screw and liquid-ring vacuum pumps of many makes.
- Supplying inlet filters, separator elements, vanes, oils and other spares identified from the pump nameplate.
- Supplying, renting and servicing compressors, dryers and filtration for the compressed-air side of the line, including oil-free options.
- Advising on a central vacuum system where several forming machines are installed.
Call +91 99590 60718 or +91 92468 88812, or email reddycompressors@gmail.com. Office: Srinilayam, 12-13-200, First Floor, Street No. 1, Tarnaka, Secunderabad 500017.
What to send us
- Forming machine make, model, platen size and cycle time.
- Material, sheet thickness and the part or tool depth.
- Vacuum pump make, model and nameplate photo, and reserve-tank volume.
- Any pressure readings at the tool and tank during a cycle.
- The quality problem or capacity increase you want to solve.
Frequently asked questions
What vacuum level is needed for thermoforming?
Most vacuum forming works well when the space behind the sheet is pulled quickly to a low absolute pressure, commonly well below 200 mbar, with deeper vacuum helping fine detail. The speed of reaching that level matters as much as the final number, because the sheet cools quickly against the tool. The right target depends on material, thickness, draw depth and detail, so confirm it with trials on the actual tool.
Why do thermoforming machines use a vacuum tank?
The forming draw-down must happen in a very short time, often faster than any economical pump could achieve alone. A reserve tank kept at low pressure provides that instant flow when the valve opens, and the pump then recovers the tank between cycles. Tank volume, starting pressure and the pump’s recovery speed together decide whether every cycle starts with adequate vacuum.
How do I size a vacuum pump for a vacuum forming machine?
Start with the volume evacuated per cycle, the vacuum level needed, the time allowed and the cycle rate. Use the equalisation estimate to size the reserve tank, then check that the pump can recover the tank pressure in the time between cycles, using its real speed curve and allowing for leakage and line losses. Measured pressure traces on the existing machine are the best basis.
Why is my vacuum forming losing detail during the shift?
A gradual loss of detail over a shift often means the reserve tank is not recovering fully between cycles, so each draw-down starts at a slightly worse vacuum. Causes include a worn pump, a blocked inlet filter or exhaust element, increased leakage at the clamp frame, or a faster cycle than the system was sized for. Logging tank pressure at the start of each cycle shows this clearly.
Can one central vacuum system serve several thermoforming machines?
Yes, and it can save energy, especially with speed-controlled pumps that follow demand. The design must ensure that one machine’s draw-down does not starve another, which means short, large-bore branches, a local reserve tank or fast valve at each machine, and enough pump capacity for the realistic combination of cycles happening together. Standby capacity is also easier to arrange centrally.
Is an oil-free vacuum pump needed for food packaging thermoforming?
Not always, but it is often preferred. An oil-sealed pump with a good exhaust separator does not normally contaminate the formed part, because the vacuum draws air away from the product. The bigger risks are oil mist in the room and oil or water in the compressed air used for release and blow-off. Many food packaging plants choose dry claw or dry screw vacuum and oil-free compressed air.
Related pages
Calculators and worked examples
These tools give indicative figures for screening, comparison and for preparing an enquiry. They run in your browser and send nothing. No prices or tariffs are assumed: enter your own to see costs. Efficiencies and other figures shown are example values: replace them with your own. Results are not a performance guarantee or a design certificate; final selection is confirmed by a service engineer against the machine data sheet and the site.
Vacuum forming reserve tank and pump recovery
Check whether a reserve tank pulls the forming volume down to the vacuum the part needs, how long the pump takes to recover the tank, and where the start pressure settles if it does not recover each cycle.
The worked example below shows the method with fixed numbers. Turn on JavaScript to change the inputs.
Worked example
Inputs
- Reserve tank volume (litres): 500
- Reserve tank pressure before the draw (mbar a): 50
- Forming volume per cycle: sheet-to-tool space, passages, hoses (litres): 50
- Atmospheric pressure at site (mbar a): 1013
- Vacuum needed at the tool (mbar a): 100
- Effective pump speed at the tank (m³/h): 100
- Time available for recovery in each cycle (s): 30
Result
- Equalised pressure at the tool: 137.5 mbar a (does not meet the 100 mbar a target)
- Reserve volume needed for the target at this tank pressure: 913 litres (183% of the tank entered)
- Recovery time back to 50 mbar a: 20 s (0.334 min, 0.00557 h) for 550 litres at 100 m³/h
- Effective speed needed to recover in 30 s: 66.79 m³/h = 18.55 L/s
- Recovery within the cycle: Yes 10 s to spare
Method. Equalised pressure when the valve opens (fast, isothermal transfer): p = (p_r × V_r + p₀ × V) ÷ (V_r + V). Reserve volume needed for a target p_t: V_r = V × (p₀ − p_t) ÷ (p_t − p_r). Recovery with an ideal pump of effective speed S on the whole connected volume: t = ((V_r + V) ÷ S) × ln(p ÷ p_r). If the time available is shorter, the pressure reached is p × exp(−S t ÷ (V_r + V)); repeating that each cycle, the start pressure settles at the value where one cycle’s rise equals the pump’s recovery. Ideal pump, no leakage or line losses: check against the pump curve and measured cycle data.
What to take from it. A tank that does not recover between cycles shows as detail getting worse through the shift: log the tank pressure at the start of each cycle.
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.