Thick Gauge Cooling Fixtures: Holding the Shape After Demoulding

Why thick gauge keeps moving

A 6 mm sheet formed into a large enclosure comes off the tool at 80 to 90 degrees Celsius. It is dimensionally stable enough to handle and far from stable enough to hold tolerance, because the core of the wall is still well above the glass transition temperature.

The part then cools from both faces at different rates. The face that is exposed to air cools slowly; the face that is against a fixture or a pallet cools fast. The differential produces a temperature gradient through the thickness, and a temperature gradient through the thickness produces warpage.

On top of that, the material has memory. A formed part wants to return towards flat, and the amount it moves depends on the forming temperature, the stretch ratio and the amount of internal stress locked in during forming.

So a thick gauge part is not finished when it is demoulded. It is finished when it has cooled to a stable temperature in a shape that is held. That is the job of the cooling fixture.

The fixture is a heat sink

A cooling fixture does two things at once. It holds the part in the correct geometry while it cools, and it conducts heat out of the part faster than air would. The second job is often forgotten, and it is the one that decides how long the part has to stay in.

The chart on the left is warpage after 24 hours against time in an aluminium fixture at 40 degrees Celsius, for a 6 mm HDPE enclosure 1.2 by 0.8 metres. At zero fixture time the part warps 4.8 mm. At two minutes it is 3.9, at four 3.1, at six 2.2, at eight 1.5, at ten 1.1 and by fifteen it is 1.0.

The curve flattens at about ten minutes. Beyond that the part is not cooling further in any meaningful way, and keeping it in the fixture is just tying up capital.

The right hand panel compares fixture types on time to stable dimension: 95 minutes free cooling on the floor, 60 with fan assist, 35 in a wooden fixture, 18 in an aluminium fixture and 12 in an aluminium fixture with water cooling. The material of the fixture matters more than the fact that there is one.

Fixture types and what each buys

A wooden or MDF fixture is the cheapest and is common on low volume work. It holds the shape but conducts almost no heat, so the part cools by air and the fixture time is long. It is adequate for parts with generous tolerance.

An aluminium fixture conducts heat well and holds shape accurately. It is the default for anything with a real tolerance, and it is usually made from the same tooling model as the mould so the geometry matches.

A water cooled aluminium fixture adds internal channels and a chiller connection. It roughly halves the fixture time again and it is worth it when the fixture would otherwise be the bottleneck in the cycle.

A vacuum fixture holds the part against the surface while it cools, which is useful for large panels that would otherwise sag between supports. It needs a vacuum pump and a perforated surface, and it is the most expensive option.

Table 1 — Fixture types and the time each takes.

Designing the fixture

Support the part where the mould supported it. A fixture that contacts the part in different places from the mould introduces a new set of stresses and can produce a shape that is wrong in a different way from the one you were trying to fix.

Let the part shrink. A fixture that grips the part on its outside dimensions will fight the shrinkage and produce a part that is stressed and, eventually, cracked. The fixture should locate on features that do not change, or allow sliding on the contact faces.

Provide access for air. A large flat contact face traps air between the part and the fixture, and the trapped air insulates exactly where you wanted conduction. Grooves or a perforated surface let the air out and improve contact.

And match the thermal expansion. An aluminium fixture expands and contracts with temperature, and if it is used at 40 degrees and stored at 15 the geometry changes. For a tight tolerance part, the fixture should be at a controlled temperature rather than shop temperature.

Table 2 — What separates a fixture that works from one that marks time.

Measuring whether it worked

Measure the part after 24 hours, not after ten minutes. A part that is correct when it leaves the fixture and wrong the next morning was not in the fixture long enough, or was not supported correctly.

Measure the same points every time, on a surface plate or a fixture that locates the part repeatably. A warpage measurement taken with the part resting on a bench is measuring the bench as much as the part.

Record the fixture time and the shop temperature alongside the result. A part that passes in winter and fails in summer is telling you that the fixture is marginal and the ambient temperature is doing part of the work.

And check a batch, not a part. Thick gauge warpage varies with sheet lot, with oven condition and with operator timing, and a single good part proves very little.

When a fixture is not the answer

If the part warps because it was formed too cold, a fixture will hold it straight and it will spring back later. The fix is the forming temperature, not the fixture.

If the part warps because the wall thickness is uneven, a fixture will hold a part that is internally stressed and it will crack in service. The fix is the forming and the sheet, not the fixture.

If the part warps because the design has a large flat panel with no ribs, no fixture will make it stiff. The fix is a rib, a crown or a change in the material.

A cooling fixture is a good solution to a thermal problem. It is a bad solution to a forming or a design problem, and it is often applied to those because it is the cheapest thing to try.