Surface Marking: Chill Marks, Drag Lines and the Sheet That Was Too Cold

Three marks that look alike and are not

A rejected part usually arrives with a description like scratched or marked, and that description hides three different faults with three different causes. Chill marks are a thermal signature. Drag lines are a friction signature. Vacuum hole dimples are a tooling signature. They look similar in a photograph and completely different under a raking light.

Chill marks are dull, slightly matte patches with a soft edge, usually where the sheet first contacted the mould and often on the steepest part of the draw. Drag lines are thin, sharp scratches that run in the direction of the draw and stop abruptly. Dimples are circular depressions at a fixed pitch, matching the hole pattern in the tool.

The reason the distinction matters is that the three faults respond to opposite adjustments. Raising mould temperature removes chill marks and makes drag lines worse, because a warmer mould is a stickier mould. Polishing the mould removes drag lines and does nothing for chill marks. Only tooling changes remove dimples.

Under a raking light the difference becomes obvious in seconds, and that is the cheapest inspection tool in a forming shop. A torch held at ten degrees to the surface will show a chill mark as a broad dull bloom and a drag line as a bright thin line. A straight-on view shows neither.

Chill marks: the mould is the cold object

A chill mark forms because the sheet cools faster than it can be drawn. The polymer skin in contact with the mould drops below its glass transition or its crystallisation point within milliseconds, and from that moment it stops stretching. The rest of the wall continues to draw, and the frozen skin is left as a matte patch with a different orientation and a slightly different thickness.

The depth of the mark is set by the temperature difference between the sheet and the mould, and by how long the sheet sits in contact before the vacuum pulls it into shape. Both are controllable, and the chart on the left shows how much each one is worth.

At a mould temperature of 20 C, a sheet at 150 C shows marks about 41 micrometres deep. Raising the mould to 80 C brings that to 8 micrometres, which is below the threshold where a mark is visible under paint. Raising the sheet from 150 to 180 C at the same 20 C mould only halves the mark, from 41 to 28, because the sheet is still hitting a cold surface.

That comparison is the whole argument for mould temperature control. Sheet temperature has to stay inside a narrow forming window, so there is little room to buy mark quality there. Mould temperature has a wide range and a large effect, and on most machines it is the only lever that is actually free.

Drag lines: friction, not temperature alone

A drag line is a scratch, and a scratch needs relative motion under load. When the sheet is drawn over a mould feature, the contact pressure is high, the sheet is still hot enough to be soft, and the surface either yields or abrades. The result is a line that follows the draw direction and ends where the contact ends.

Three things make it worse. A rough mould surface, a high draw ratio and a fast forming speed all increase the chance. The mechanism is that the sheet grips the mould locally, stretches around the grip point, and then releases with a snap that leaves a line.

The usual fix is to reduce friction rather than to change temperature. Polishing the mould to a finer finish helps. A release agent helps for a few cycles and then builds up. The most reliable answer on a difficult draw is a combination of a warmer mould, a slower forming speed and a small amount of air cushioning before full vacuum.

The diagnosis is straightforward. If the lines are all parallel to the draw and stop at a feature edge, it is drag. If they wander and appear in patches, it is more likely a surface defect in the sheet itself, and the sheet supplier should be involved before the tool is touched.

Vacuum hole dimples and where they come from

Every vacuum hole leaves a small depression, because the sheet is pulled into the hole by the pressure difference. The depth depends on the hole diameter, the sheet thickness and how long the vacuum is held after the sheet has stopped moving. A 1 mm hole in a 0.8 mm sheet gives a dimple around 36 micrometres deep, which is visible on a glossy surface.

The problem is not the dimple itself but where it lands. On a hidden face it does not matter. On a visible face it is a defect, and the usual response is to move the hole, which is a tooling change and often not possible after the tool is cut.

There are alternatives, and they are covered in more detail in the article on mould marks. The short version is that hole size, hole position and the surface finish of the tool all interact, and a sintered aluminium tool with distributed porosity removes the mark almost entirely at a significant cost premium.

What matters at the design stage is to decide which face is cosmetic before the tool is cut. A tool designed with all the vacuum on the non-cosmetic side is cheaper than a tool designed without that thought and then modified.

Orange peel and stress whitening

Orange peel is a fine, uniform dimpling that covers a whole area rather than a point. It comes from the sheet surface itself, either from the extrusion process or from moisture. Moisture is the more common cause in a forming shop: PET and PA absorb water, and the water flashes to steam at forming temperature, leaving a micro-blistered surface that reads as orange peel.

Drying the sheet before forming is the fix, and it has to be done to the resin supplier's schedule. A PET sheet at 0.6 mm typically needs four hours at 120 C in a desiccant dryer. Skipping the dry because the sheet looked fine is the single most common cause of a hazy, pebbled part.

Stress whitening is different again: it is a milky patch where the polymer has yielded. It appears at tight radii, at the base of a deep draw, and around the rim where the sheet was clamped. It is a sign that the draw was too aggressive for the temperature, and the fix is more heat or a slower draw, not a surface treatment.

Table 1 — Reading the mark before touching a setting

What mould temperature actually buys

Mould temperature is the most misunderstood setting on a thermoformer, because it is usually uncontrolled. Many machines have no mould heating at all, and the mould sits at whatever the shop air and the last cycle left it at. On a two minute cycle with a 1.5 mm sheet, an aluminium mould can swing 15 C between the first part of the shift and the middle of it.

That swing is enough to move the mark depth by a third, which is why the same tool produces acceptable parts in the morning and rejects after lunch. Cartridge heaters in the mould, or a water circuit, hold the temperature within a few degrees and remove the drift entirely.

The table below is the practical version of the chart. It is worth noting that the gain from 20 to 40 C is larger than the gain from 60 to 80 C, because the first part of the range is where the sheet is dropping below its transition temperature fastest.

Table 2 — Mould temperature against mark depth and cycle cost

What actually fixes it on a running line

When a line starts producing marks, the operator usually has three knobs: heater time, cycle time and vacuum. None of them addresses a chill mark, and turning up the heater to compensate is how a mark problem becomes a blister problem.

The order that works is: identify the mark under a raking light, then check the mould temperature at the affected face, then check the sheet temperature, then look at the tool. That sequence takes five minutes and avoids the loop of adjusting the heater until something else fails.

For a running line with no mould temperature control, the practical fix is often to slow the cycle slightly and let the mould stabilise, or to add a simple air blast to even out the mould temperature across the face. Neither is elegant, and both are better than rejecting parts for a mark that a EUR 200 heater circuit would remove.

The parts that pass a visual check should still be checked under a raking light at the start of every run. A mark that is 10 micrometres deep passes a glance and fails a customer's paint adhesion test three weeks later, and by then the tool and the process have both moved on.