PET Sheet Crystallinity: Why APET Sags and CPET Does Not

Two sheets, one polymer

APET is amorphous polyethylene terephthalate. It is extruded and quenched fast enough that the polymer chains freeze in a disordered state, which makes it clear, tough and easy to form at moderate temperature.

CPET is the same polymer with enough crystallinity to hold its shape at oven temperature. The chains are partly ordered, the sheet is opaque and white, and it survives 200 degrees Celsius in a domestic oven without collapsing.

Between them sits a range of partly crystalline sheet, and that range is where most forming problems live. A sheet that is 8 percent crystalline behaves almost like APET when it is heated but hazes when it is stretched, and the haze is blamed on the former.

So the first question about any PET forming job is not the temperature. It is what the sheet actually is.

Crystallinity sets the sag point

Amorphous PET softens at around 70 degrees Celsius. In a radiant heater that means the sheet starts to sag before the surface has reached forming temperature, and the sag has to be managed with clamp frame timing, sag bands or a plug.

Crystalline PET holds its shape until much higher temperatures. The chart on the left shows the sag onset going from 72 degrees at zero crystallinity to 182 degrees at 40 percent, which is why CPET can be heated to 160 degrees and still be flat when it arrives at the mould.

The curve is not linear. The first few percent of crystallinity barely move the sag point, and then it rises steeply between 10 and 30 percent. That steep section is why crystallinity control on the extrusion line has to be tighter than the tolerance suggests.

The right hand panel shows the practical consequence: the usable forming window. Amorphous sheet gives about 40 degrees of window, PETG about 55, CPET only 25 and thin PP about 60. A narrow window is not a defect, it is a process that has to be controlled.

Drying, IV and the hidden variable

PET hydrolyses. Every hour in a hot, damp environment breaks chain length, and chain length is what makes the sheet tough. Sheet dried at 160 degrees for four hours is fine; sheet dried at 160 degrees for twelve hours in a dryer that is not actually at 160 is not.

The measurable consequence is intrinsic viscosity. Sheet at 0.78 IV forms and survives a drop test; the same sheet at 0.68 IV forms and cracks at the corner. The forming line cannot tell the difference, which is why IV is a sheet specification and not a process check.

Drying also interacts with crystallinity. Amorphous regrind dries differently from crystalline virgin material because the diffusion path is different, and a blend of the two can be dry on average and wet in the amorphous fraction.

The practical rule is to dry at 150 to 170 degrees for four to six hours, measure dew point at the hopper rather than at the dryer outlet, and never run a hopper that has been sitting full over a weekend without re-drying.

Table 1 — Sheet types and where each one can be used.

Defects that come from the sheet

Hazing after forming is the most common. A clear APET part turns milky where it was stretched, which means the strain induced crystallisation went too far. The cause is usually too cold a sheet at too high a stretch ratio, not the heater.

Corner cracking is the second. The corner is where the material is thinnest and the strain is highest, and a crack there is a sign of low IV or of forming below the glass transition plus margin.

Whitening at the rim is the third. The rim is clamped and does not stretch, so it cools and crystallises in place. A rim that is white on an APET part means the clamp frame is too cold or the cycle is too slow.

Sag that cannot be controlled is the fourth. Amorphous sheet at 0.8 mm in a large frame will sag no matter what the heater does, and the answer is a support, a plug or a different sheet.

Table 2 — Sheet related defects and what to change.

Heating a sheet that is changing state

Amorphous sheet heats predictably until it reaches the glass transition, then it softens quickly and the surface can overheat while the core is still cold. That is why APET likes short wavelength heaters close to the sheet and short heating times.

Crystalline sheet absorbs differently and heats more evenly through the thickness, which is why CPET tolerates longer heating cycles and higher surface temperatures without surface damage.

A practical check is to measure the sheet surface and the sheet back with an infrared thermometer at the end of the heating cycle. A difference of more than 15 degrees means the heater is too aggressive for the sheet, whatever the sheet is.

For a mixed schedule running APET and CPET on the same machine, the heating recipe has to change completely between them. Running one recipe on both is the single most common reason a line that forms APET well produces hazy CPET.

Specifying the sheet

Specify crystallinity or at least the sheet type by name, and specify IV with a tolerance. Those two numbers explain most of the difference between a sheet that forms reliably and one that does not.

Specify thickness tolerance tightly. A 0.6 mm sheet at plus or minus 10 percent has a 0.54 to 0.66 range, and the thin end will form a part with a thin corner no matter how good the tool is.

Specify the surface. A sheet with one polished side and one matte side forms and stacks differently from a sheet that is matte on both, and the difference shows up in the packaging line rather than in the former.

And agree the acceptance test. A drop test on a formed part, a haze reading and an IV check on incoming sheet cover almost everything that will actually go wrong.