Pressure Box Forming: Positive Pressure on a Simple Tool

What a pressure box actually is

A pressure box is a chamber that clamps over the back of the sheet and fills with compressed air. The sheet is heated as usual, the box closes, and air pressure pushes the sheet onto the mould from behind. Vacuum is still applied on the mould side, so the total driving pressure is the box pressure plus the atmospheric pressure that vacuum provides.

The attraction is that the box is a relatively simple add-on. It does not need a pressure vessel around the whole machine, it does not need a hydraulic clamp rated for the full force, and it can be fitted to a machine that was designed for vacuum forming. That is why it exists as a category between vacuum forming and full pressure forming.

The limitation is the same thing: the box has to be clamped against the sheet, and the clamp has to carry the full pressure over the full area. That force does not go away because the box is simple.

What pressure buys

The achievable draw ratio rises with pressure but with diminishing returns. At 100 kPa, which is essentially vacuum alone, the practical draw ratio is about 0.35. At 200 kPa it is 0.50, at 300 it is 0.62, at 500 it is 0.75 and at 700 it is 0.85.

The shape of that curve matters for specification. The step from 100 to 300 kPa buys 0.27 of draw ratio; the step from 300 to 700 buys 0.23 for four times the pressure. So a box that delivers 300 kPa captures most of the benefit, and a box that delivers 700 kPa is buying the last 0.1 of draw ratio at a significant cost in clamp force.

That is why many pressure boxes are rated at 200 to 400 kPa rather than at the highest pressure the sheet will take. They are solving the detail problem, not trying to reach a draw ratio that really belongs to a different process.

Table 1 — Box pressure against draw ratio and the clamp force it requires on a half square metre.

Force is pressure times area

The clamp force is the box pressure multiplied by the area it acts on, plus a margin for the seal. A 0.5 square metre part at 600 kPa needs 300 kilonewtons, which is thirty tonnes. That is not a light frame; it is a press.

The force scales with area, which is the trap. A box that works well on a 0.2 square metre part at 400 kPa needs 80 kilonewtons. The same box pressure on a 1.2 square metre part needs 480 kilonewtons, which is a different machine entirely. Shops that scale a pressure box job up in size discover this the hard way.

The other consequence is that the tool and the platen have to carry the load without bowing. A tool that deflects under 300 kilonewtons will produce a part that is thick in the middle and thin at the edges, because the gap between the sheet and the mould is larger where the tool has bowed away. Tool stiffness matters as much as machine tonnage.

Sealing and venting

A pressure box only works if it is sealed. The seal is usually a silicone cord in a groove around the box, compressed against the sheet or against the clamp frame. Any leak is a loss of pressure and a jet of air across the sheet, which shows up as a local cooling mark or a distorted area.

The seal has to cope with the sheet moving as it forms. As the sheet draws into the cavity, its edge pulls inward, and a seal that was gripping the edge can lose contact. That is why the seal is normally against the clamp frame rather than against the sheet, and why the clamp frame is wider on a pressure box machine than on a vacuum machine.

Venting is the other half. The air that was in the cavity has to leave as the sheet is pushed into it, and it has to leave fast enough that it does not get trapped and compressed into a blister. Vents at the low points, and vacuum pulled at the same time as the pressure is applied, are both standard.

What changes in the tool

Table 2 — Tool changes between a vacuum tool and a pressure box tool.

The surface finish point catches people out in a good way and a bad way. A pressure box reproduces the tool surface more faithfully, so a good polish looks better and a scratch looks worse. A tool that was acceptable under vacuum may need re-polishing before it runs under pressure.

Where it sits between vacuum and pressure forming

There are three points on this spectrum and they are worth keeping distinct. Vacuum forming gives up to about 100 kPa and is the cheapest. A pressure box gives 200 to 400 kPa on a simple add-on and captures most of the detail benefit. Full pressure forming gives 300 to 700 kPa with a purpose-built press and a sealed tool.

• Choose vacuum when the part is large, simple and tolerant of a 6 mm radius.
• Choose a pressure box when the part needs 2 to 3 mm radii and a better wall, and the machine can carry the clamp force.
• Choose full pressure forming when the part needs 1 mm detail, a crisp texture, or a draw ratio above 0.75.
• Choose none of them if the part needs a sharp edge and a moulded-in insert, because that is injection moulding.

The pressure box is the option that gets skipped, because it is neither the cheapest nor the most capable. In practice it is often the right answer: a shop with a vacuum machine and a job that needs better detail can add a box for a fraction of the cost of a pressure former, and get 80 percent of the result.

The safety case

A pressure box stores energy. A 0.5 square metre box at 600 kPa holds about 300 kilonewtons of force, and if the clamp fails or the seal lets go the box can move. That is why pressure box machines have interlocks, why the box is not opened until the pressure is vented, and why the seal is inspected regularly.

The other hazard is the sheet itself. A sheet that bursts under pressure sends fragments, and a sheet that is too hot or has a flaw can fail at a much lower pressure than expected. Guards and a pressure relief valve are not optional, and neither is a procedure that vents before opening.