What changes when the part gets big
Small parts are governed by the forming step: the vacuum, the plug, the cooling. Large parts are governed by the heating step, because the time to heat a thick sheet is long, the sheet sags while it is hot, and the handling between the heater and the tool becomes a process step in its own right.
The scale also changes the material behaviour. A 10 mm sheet at forming temperature is a heavy, soft slab that carries its own weight and sags under it. A 1 mm sheet is a film that follows whatever the vacuum does. The same machine and the same tool produce completely different results because the sheet is a different structure.
And it changes the economics. A large tool costs more, a large machine costs more, and a rejected part wastes several kilograms of material rather than a few grams. The tolerance for getting the process right the first time is much tighter.
Heating time against thickness
The chart on the left plots heating time against sheet thickness at three heater powers. At 20 kW per square metre a 3 mm sheet takes 45 seconds to reach forming temperature and a 12 mm sheet takes 198. At 40 kW per square metre the same sheets take 25 and 109 seconds.
The relationship is close to linear with thickness, not quadratic, because the limiting factor is the surface heat flux rather than conduction through the sheet. That is good news for thick sheets: doubling the thickness doubles the time, not quadruples it, provided the heater power is adequate.
The catch is that the heater has to be able to deliver the flux. A heater that is undersized will not reach forming temperature at all on a thick sheet, whatever the time allowed, because the losses from the surface balance the input. That is why large machines have high power densities and short distances between the heaters and the sheet.
Heating also has to be even. A large sheet is close to the heaters at the centre and further at the edges on some machine designs, and the temperature difference across the sheet shows up as a thickness difference in the part. Zoned heaters, with separate control for the centre and the edge, are the standard solution.
Sag is the constraint nobody plans for
A sheet heated to forming temperature has almost no stiffness. On a large span it sags under its own weight, and the sag is greatest at the centre of the sheet where the span is longest and the temperature highest. A sag of 50 mm is normal on a 1,500 mm span, and it changes the distance to the heaters and the shape of the sheet as it enters the tool.
Sag is controlled in three ways. The first is support: a sag frame, a support table or a series of rods under the sheet. The second is speed: heat fast and form before the sag develops. The third is temperature: heat the sheet from both sides so that the core is hot and the surface is not overheated, which reduces the differential that drives sag.
The interaction with the tool matters. A sagging sheet enters the cavity with a shape that is already curved, and the vacuum has to pull that curve flat before it pulls it into the detail. On a deep part that is an advantage; on a shallow one with fine detail it is a problem, because the detail fills last and fills worst.
The practical measurement is a sag check: clamp a sheet, heat it for the production time, and measure the deflection at the centre before forming. A repeatable number, recorded with the temperature, is the only way to know whether the process is stable from shift to shift.
Cooling and demoulding
A large part holds a lot of heat, and it has to be cool enough to keep its shape when it is lifted. The cooling time scales with the square of the thickness, so a 10 mm sheet takes four times as long as a 5 mm one, and on a large part that is minutes rather than seconds.
Cooling is usually done with air, sometimes assisted by water mist or by a cooled tool. Air is slow and even; water is fast and can mark the surface. On a part with a cosmetic surface, air cooling with fans on both sides is the usual compromise, and the cycle time is what it is.
Demoulding a large part needs either a lifting frame, a tilting table or two operators. A part that is lifted by one corner while still warm will distort, and the distortion will be blamed on the tool. The handling has to be designed as part of the process, with support points that match the part's stiffness.
| Thickness | Heating time at 30 kW/m2 | Cooling time | Handling |
|---|---|---|---|
| 3 mm | 32 s | 20 to 30 s | One operator |
| 5 mm | 55 s | 40 to 60 s | One operator |
| 8 mm | 92 s | 2 to 3 min | Lifting frame |
| 10 mm | 116 s | 3 to 4 min | Lifting frame |
| 12 mm | 142 s | 5 min | Frame and two operators |
Table 1 — Time budget for a large part, single sided heating
Sizing the machine
The machine has to be big enough for the sheet, not for the part. The clamp frame, the heater bank and the platen all have to accommodate the sheet with its margin, and a machine that is just big enough for the part will be too small for the sheet that makes it.
The heater power is the second number. A machine with 20 kW per square metre will form a 6 mm sheet in a reasonable time and will struggle with 12 mm. The power density, not the total power, is what matters, and it is a function of the heater to sheet distance as well as of the elements.
The third number is the handling. A large sheet weighs 10 to 30 kg, and it has to be loaded, heated, formed and unloaded without being dropped or distorted. Vacuum lifters, roller tables and tilting frames are not accessories, they are part of the process.
| Requirement | Rule of thumb | Why |
|---|---|---|
| Frame size | Sheet size plus margin plus 50 mm | The margin is not part of the part |
| Heater power | 30 kW/m2 minimum for 8 mm | Below this the sheet will not reach temperature |
| Heater zones | At least 3 across the sheet | Compensates edge losses |
| Cooling | Fans on both faces | Air is even, water marks |
| Handling | Lifting frame above 8 mm | A warm part distorts if lifted by a corner |
Table 2 — Sizing a machine for a large part
Tooling for a large part
A large tool is usually aluminium, cast or machined, with a water circuit for temperature control. The tool has to be stiff enough not to deflect under the vacuum, and it has to be heated to 60 to 80 C so that the part does not chill on contact and mark.
Vent holes have to be placed where the vacuum reaches last, which on a large part is the deepest point and the far corners. A tool with vents only in the centre will form the centre first and trap air at the edges, and the edges will be short or blistered.
The tool also has to allow for shrinkage, which on a large part is a large absolute number. A 1,200 mm part in PP shrinks about 18 mm, and the tool has to be made larger by that amount. Getting it wrong on a large tool is expensive, because the correction is a weld and a re-cut.
How long does it take to heat a 10 mm sheet?
About two minutes at 30 kW per square metre, and three minutes at 20. The time is roughly linear with thickness, provided the heater power is adequate; an undersized heater will never reach temperature however long it is left.
How do I stop a large sheet sagging?
Support it from below with a sag frame or rods, heat it faster so there is less time to sag, and heat from both sides so the core is hot without overheating the surface. Measure the sag as part of the process record.
Why does my large part have thin corners?
Usually uneven heating: the corners are further from the heaters or lose more heat to the frame, so they are cooler and stretch less. Zoned heaters and an insulated frame improve it, as does a plug.
What machine size do I need for a 1,200 mm part?
A machine that takes the sheet, which is the part plus the clamp margin plus trim allowance, so around 1,350 mm. Sizing the machine to the part is the most common and most expensive mistake in large part thermoforming.
How much does a large PP part shrink?
About 1.5 percent, which on a 1,200 mm part is 18 mm. The tool has to be made larger by that amount, and the shrinkage varies with the draw ratio and the cooling, so the first article is where the number is confirmed.
Can large parts be trimmed on the machine?
Some machines have in-line trim, but most large parts are trimmed off-line on a CNC or a five axis router, because the trim path is complex and the part is too big to handle in the machine. That adds a handling step and a fixture.
Large formed part in development?
Send the part drawing, the sheet size and the material. We will tell you the heating time, the machine size you need and where the sag will cause trouble.
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