PP Sheet: Gloss and Shrinkage Come From the Same Roll

Джон Ли
Январь 24, 2026
9:00 дп

Polypropylene sheet running over three polished chill rolls on an extrusion line, operator checking the surface — Polypropylene sheet over the chill rolls. The roll temperature sets the gloss and the forming shrinkage at the same time, in opposite directions.

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1. One roll, two properties, opposite directions
2. Why the two properties are linked
3. Choosing the setting from the part, not the sample
4. What the thermoformer has to know
5. Reading a defect back to the roll
6. What to specify

One roll, two properties, opposite directions

Polypropylene sheet is quenched against a chill roll, and the roll temperature decides how fast the surface solidifies. Fast quenching gives a smooth, glossy skin and a low level of crystallinity near the surface. Slow cooling gives a rougher, matte surface and more crystallinity through the thickness.

Gloss and forming shrinkage both follow from that one variable, and they move in opposite directions. Gloss against chill roll temperature: 88 units at 10 C, 80 at 20, 68 at 30, 52 at 40 and 38 at 50. Forming shrinkage over the same range: 1.2 percent at 10 C, 1.5 at 20, 2.0 at 30, 2.8 at 40 and 3.6 at 50.

So a customer who wants a glossy sheet and a dimensionally stable formed part is asking for two things that the chill roll cannot deliver together. That conversation has to happen before the sheet is made, not after the parts are rejected.

Why the two properties are linked

Forming shrinkage is the release of orientation and the completion of crystallisation that the extrusion process froze in. A quenched sheet has oriented molecules locked in place and low crystallinity, so when it is reheated for forming it relaxes and crystallises, and it shrinks by an amount that depends on how much was frozen in.

A slowly cooled sheet has already crystallised most of what it is going to crystallise, and its molecules are less oriented. When it is reheated it shrinks less, and it shrinks more predictably, because the structure is closer to equilibrium.

That is the whole trade. The glossy sheet is a sheet in a metastable state, and the matte sheet is closer to stable. A part made from the glossy sheet is dimensionally alive; a part from the matte sheet is dimensionally calm.

Gloss against chill roll temperature, and forming shrinkage against the same temperature — Gloss falls from 88 to 38 units as the roll warms from 10 to 50 C, while forming shrinkage rises from 1.2 to 3.6 percent.

Choosing the setting from the part, not the sample

The decision is not which sheet is better. It is which failure the application can tolerate. A display part that is judged on appearance wants gloss and can live with 3 percent shrinkage if the tool is cut to compensate. A part that has to fit a machined frame wants predictable shrinkage and can live with a matte surface.

Приложение	Chill roll	Gloss	Усадка	Почему
Display and signage	10 to 20 C	80 to 88	1.2 to 1.5 percent	Appearance is the product
Packaging, formed trays	20 to 30 C	68 to 80	1.5 to 2.0 percent	Balance of look and fit
Technical parts, tight fit	40 to 50 C	38 to 52	2.8 to 3.6 percent	Dimensional stability wins
Parts to be printed	20 to 30 C	68 to 80	1.5 to 2.0 percent	Ink adhesion prefers a slightly matte skin

Table 1 — Chill roll temperature chosen from what the part has to do

The mistake is to specify the highest gloss the line can produce and then complain about shrinkage. The two numbers are the same decision, and the sheet supplier cannot optimise both.

What the thermoformer has to know

Forming shrinkage is not a single number. It depends on the sheet's thermal history, on the forming temperature, on the draw ratio and on the tool temperature. A sheet quoted at 1.5 percent can produce parts that shrink 1.2 percent in one area and 2.0 percent in another, because the draw ratio varies across the part.

The practical approach is to form a trial tool and measure, rather than to design the production tool from a data sheet number. The trial tool does not have to be the production tool; it has to be the same sheet, the same forming temperature and a representative draw.

The second thing the thermoformer has to know is that the sheet's two faces are different. The chill roll face is glossier and more oriented than the air face, so a part formed with the roll face out looks different from the same part formed with the air face out. That is a specification item, not a preference.

Reading a defect back to the roll

Most PP sheet complaints trace to one of four causes, and three of them are on the chill roll.

Symptom	Причина	Fix
Gloss varies across the width	Uneven roll surface temperature	Balance water flow, check for channel blockage
Sheet blocks when stacked	Too glossy, no antiblock	Add antiblock, or raise the roll temperature slightly
Parts warp after forming	Orientation frozen in by over-quenching	Raise the roll temperature, accept less gloss
Shrinkage varies along the roll	Line speed or cooling drift	Stabilise the roll temperature and the line speed together

Table 2 — PP sheet defects and the setting behind each

The gloss variation across the width is the most diagnostic. A roll with uneven internal flow produces a sheet that is glossy at the edges and matte in the middle, and the difference is often 15 gloss units, which is visible to anyone. That is a roll maintenance item that gets misdiagnosed as a resin problem for months.

What to specify

A PP sheet specification has to carry both halves of the trade, because they cannot be separated.

Gloss, with a tolerance, on the face that will be visible.
Forming shrinkage, with the temperature and draw ratio it was measured at.
Which face is the chill roll face, and which face goes out on the part.
Antiblock or slip, if the sheet will be stacked.
Thickness variation across the width, which drives heating uniformity and therefore shrinkage variation.

Then agree the acceptance test before the first roll. A gloss target measured on a lab sample and a shrinkage target measured on a production part are two different specifications, and the gap between them is where the arguments live.

Myth: a glossier sheet is a better sheet. Gloss comes from quenching, quenching freezes orientation, and frozen orientation becomes shrinkage when the sheet is reheated. The glossiest sheet is the least dimensionally stable one on the line.

Why does chill roll temperature affect gloss?

It sets how fast the surface solidifies. Fast quenching gives a smooth skin and high gloss; slow cooling gives a rougher skin and a matte finish.

Why does forming shrinkage rise with roll temperature?

A slowly cooled sheet has already crystallised and relaxed more, so on reheating it has less frozen-in orientation to release and shrinks by a larger, more predictable amount.