Radiant Versus Contact Heating: Speed Against Control

Two ways to put heat into a sheet

Thermoforming needs a sheet at forming temperature all the way through its thickness. There are two ways to get there: radiate energy onto the surface and let conduction carry it inward, or press the sheet against a hot surface and let conduction do the whole job from both sides.

Almost every thermoforming machine in the world uses radiant heating, because it is flexible, it works with any sheet size, and it does not touch the surface that will become the visible face of the part. Contact heating exists in a narrower set of applications and does some things much better.

The choice is not really about speed, although speed is the visible difference. It is about what happens to the sheet while it is being heated: whether it sags, whether it is marked, and how uniform it is when it arrives at the mould.

Radiant: fast, and it only heats the surface

A radiant heater delivers energy to the surface, and the surface then conducts it inward. That means the surface is always hotter than the core during heating, and the through-thickness gradient is what limits the heating rate. Heat the surface too fast and it degrades while the core is still cold; heat it slowly and the cycle is long.

The gradient is also what causes sag. A sheet with a hot surface and a cold core is a sandwich with a low-viscosity skin on a stiff core, and under its own weight the skin stretches. Thick sheets sag more because the gradient is larger and the mass is greater, which is why heavy gauge thermoforming needs sheet supports and why sag control is a whole subject of its own.

Radiant heating has two big advantages. It does not touch the sheet, so there are no marks and no texture transfer. And it is easy to zone, so more energy can be delivered to the region that will be stretched hardest. That zoning is what makes a deep draw possible.

Contact: faster still, and it marks the sheet

Contact heating presses the sheet between heated platens. Heat enters from both faces at once, which roughly halves the distance the heat has to travel and therefore roughly quarters the conduction time, and the surfaces are held at the platen temperature so there is no gradient-driven degradation.

The measured effect is that contact heating takes about half the time of radiant at every gauge: 10 seconds against 22 at 0.5 mm, 18 against 38 at 1.0, 27 against 55 at 1.5, 36 against 72 at 2.0 and 54 against 105 at 3.0. The ratio is remarkably constant, which makes the choice easy to evaluate on cycle time alone.

The price is surface quality. A platen in contact with a sheet transfers its own texture, and any dust, any scratch and any non-uniformity in the platen appears in the part. A polished platen on a glossy sheet gives a mottled surface; a platen with a scratch gives a line. For a part where the surface matters, contact heating is usually unacceptable on the face side.

The numbers that decide it

Table 1 — Heat-up time, temperature spread and surface consequences for the four arrangements.

The spread column is the one that decides part quality, and it is the one that is most often ignored. A spread of plus or minus 14 °C on a semi-crystalline sheet is the difference between a part that forms and a part that tears at one corner, because the forming window for polypropylene is only about 15 degrees wide to begin with.

The zoning column is the other one. A radiant field with four zones can put 30 percent more power into the area that will become the walls of a deep part and 30 percent less into the base. A contact heater cannot, because the platens are a single thermal mass. That is the reason radiant dominates despite being slower.

Where contact wins

Contact heating wins in a specific set of circumstances, and when they apply it wins decisively.

• Thin gauge, high volume, simple geometry. A blister or a tray with a shallow draw does not need zoning, and halving the cycle time doubles the output of an expensive machine.
• Polypropylene and other semi-crystalline sheets, where the forming window is narrow and the sheet benefits from being held at a uniform temperature rather than heated through a gradient.
• Single-curvature parts, where the sheet only bends one way and the platen can be shaped to match.
• Parts where both faces are hidden, so surface marking does not matter.
• Lines where the sheet is thin enough that sag is not an issue, which is most thin-gauge packaging.

The thin-gauge packaging case is the big one. A form-fill-seal line running 0.4 mm polypropylene at 30 cycles a minute cannot afford the radiant heat-up time, and the part is a shallow tray where zoning buys nothing. Contact heating is the standard answer there and it has been for decades.

Table 2 — Material against heating method, and the reason behind the preference.

Where radiant wins

Radiant wins wherever the sheet is thick, the draw is deep, or the surface matters. Heavy gauge thermoforming of large enclosures is entirely radiant, because a 6 mm sheet cannot be contact heated without a platen the size of the part and a handling system that can move it.

It also wins on materials that stick. A sheet at forming temperature is tacky, and some polymers will adhere to a hot platen and tear when they are removed. Polymethyl methacrylate and polycarbonate are both prone to this, and both are usually radiant heated for that reason as much as for the surface finish.

And it wins on anything where the sheet has to be heated selectively. A part with a deep well in one corner and a flat flange everywhere else needs a heater field that is hotter over the well, and only a radiant field can do that.

Hybrid arrangements and the practical choice

There are hybrid machines that use radiant heating for the bulk of the energy and a contact platen for the final equalisation, or contact heating on one side and radiant on the other. They exist because the two methods fail in opposite directions: radiant is slow but controllable, contact is fast but blunt.

The practical decision usually comes down to four questions, and they can be answered before any machine is specified.

1. Is the draw ratio above 0.4? If yes, radiant with zoning, because the walls need more heat than the base.
2. Is the sheet above 2 mm? If yes, radiant, because a contact platen that size is impractical and sag will dominate anyway.
3. Does the visible face have to be unmarked? If yes, radiant on that side at least.
4. Is the part shallow, thin and high volume? If yes, contact, because the cycle time is the whole economics.

There is one more consideration that is rarely in the textbook. Contact heating is much more sensitive to sheet thickness variation than radiant, because a thicker region takes longer to heat and the platen cannot compensate. A sheet with a 5 percent gauge band running through a contact heater produces a part with a 5 percent temperature band, and in a narrow forming window that is enough to fail. Radiant heating with a closed-loop sensor on the sheet is more forgiving, and that forgiveness is worth something on a real production line with real sheet.