Thermoforming-Grade Sheet: What the Former Needs From the Extruder

The same datasheet, two different sheets

Two sheet suppliers can quote the same polymer, the same thickness and the same width, and deliver sheet that behaves completely differently in a former. One sags evenly and forms at a wide range of oven settings; the other sags in the middle, forms only in a narrow window, and produces parts with a consistent shrink that varies from roll to roll.

The datasheet describes the polymer, not the sheet. What differs is how the sheet was made: the quench rate, the draw-down ratio, the roll temperatures and the winding tension. Those set the residual stress in the sheet, and residual stress is what the former is actually fighting.

A thermoforming-grade sheet is therefore a sheet made with the forming step in mind. It is not a different polymer and it is often not a different formulation; it is the same material run so that it arrives at the oven flat, stress-relieved and predictable.

Residual stress is the thing that moves

A sheet that is quenched fast on a chill roll freezes a skin while the core is still hot. As the core cools it contracts, and because the skin is already solid, the contraction is resisted and locked in as stress. The sheet leaves the line flat enough to wind, but it is not in equilibrium.

When the former heats the sheet, the stress is released. The sheet moves, and it moves differently in different places. That is why a stressed sheet sags unevenly, why it draws to one side, and why parts from the same roll can differ.

The consequence that matters commercially is shrink after forming. A sheet with a high residual stress index shrinks 2.7 percent after forming; a well-relieved sheet shrinks 0.3. The difference is not cosmetic: it changes the trim line, the fit in an assembly and the repeatability of a stacked part.

The forming window is what you are buying

The forming window is the range of sheet temperature over which a part forms acceptably. A general-purpose sheet might have a 12 C window; a thermoforming-grade sheet 26 C; a high melt strength sheet 34 C. The width of that window is the difference between a process that runs itself and a process that needs an operator watching every cycle.

Window width comes from two things: how sharply the sheet's stiffness falls with temperature, and how uniform the sheet is. A sheet with uniform stress and uniform thickness has a wide window because the whole sheet reaches the same state at the same time. A sheet with a stress gradient reaches different states in different places, and the window collapses to whatever overlap there is.

This is why the window is a better purchasing specification than any single point measurement. It describes the sheet's behaviour across the range the former will actually use, and it cannot be faked by a supplier who only controls one setting.

Table 1 — What each grade actually delivers to the former

What the extruder has to do

Three things. First, cool the sheet slowly enough that the core has time to contract before the skin locks it, which usually means warmer rolls and a longer cooling section rather than a colder roll. Second, keep the draw-down ratio low, because stretching the sheet in the machine direction aligns the molecules and stores that alignment as stress. Third, wind at low tension, because a tightly wound roll creeps and takes a set.

All three cost output. Warmer rolls mean a longer line; a lower draw ratio means a wider die for the same thickness; low winding tension means bigger rolls and more handling. That is the honest reason thermoforming-grade sheet costs more than general-purpose sheet of the same polymer, and it is not a marketing premium.

The fourth thing is consistency. A sheet that is well relieved on Monday and stressed on Wednesday is worse than a sheet that is consistently mediocre, because the former cannot compensate for a variable it cannot see.

Table 2 — The settings that differ, and what they cost

How to specify it

Specify shrink after forming, not residual stress, because shrink is what the former can measure. A sample formed at a defined temperature and draw, measured before and after, gives a number that both sides can agree on.

Specify the forming window as a range, and define how it is measured. A window quoted without a method is a number that will be argued about at the first failure.

Specify thickness and width tolerance together with the stress requirement, because a supplier can meet the stress target by making the sheet thick and slow, and that is not what was wanted.

When the sheet is blamed and is not at fault

The most common misdiagnosis is blaming the sheet for an oven problem. An oven with uneven zones heats one part of the sheet faster, and the resulting uneven sag looks exactly like a stress problem. The test is to rotate the sheet 90 degrees in the oven: if the sag pattern rotates with the sheet, it is the sheet; if it stays with the oven, it is the oven.

The second is blaming the sheet for a tool problem. A tool with uneven venting or a cold spot draws the sheet towards it, and the resulting thin patch looks like a material variation. Sectioning a part and measuring wall thickness distribution separates the two quickly.

The third is a genuine sheet problem that only appears at the end of the roll. Winding tension increases towards the core, so the last few metres are more stressed than the first. A former who samples from the outside of the roll and forms from the inside will see a difference that is real and is caused by winding, not by the extrusion.