Free Forming: Shaping Without a Mould Surface

No mould, no cavity, no contact

Free forming clamps a heated sheet over a circular opening and applies air pressure from one side. The sheet bulges into a dome, and the dome is the part. There is no mould surface, so nothing touches the material except the clamp ring, and the shape is decided entirely by the pressure and the sheet's own resistance.

The geometry that forms is close to a spherical cap. Its height depends on the pressure, the sheet thickness, the clamp diameter and the material's stiffness at the forming temperature. Because there is no tool to stop it, the process is entirely a balance between the pressure pushing and the material resisting.

This is the oldest form of thermoforming in the laboratory and one of the newest in production. It is used as a standard test of formability, and it is also used to make parts where a perfectly smooth, tool-mark-free dome is the product: light diffusers, radomes, speaker grilles, inspection windows.

Pressure sets the height

On a 1.5 mm polycarbonate sheet over a 200 mm clamp ring at 165 C, the bubble height goes from 30 mm at 20 kPa to 55 mm at 40, 80 mm at 60, 110 mm at 80 and 145 mm at 100 kPa. The curve steepens rather than flattening, because as the dome grows the radius of curvature increases and the same pressure produces a larger increment of height.

That is the opposite of what most people expect from a pressure-driven process, and it is the reason free forming is hard to control open loop. A small pressure error early in the blow produces a small height error; the same error late in the blow produces a large one. The practical consequence is that the pressure has to be ramped, and the last 20 percent of the height needs a much finer control than the first 20 percent.

The clamp diameter matters as much as the pressure. Doubling the ring diameter at the same pressure roughly doubles the height, because the sheet has more unsupported area to bulge. That is why a formability test has to fix the clamp diameter as carefully as it fixes the temperature.

Table 1 — Bubble height against pressure on a 1.5 mm polycarbonate sheet, 200 mm clamp ring.

The pole thins as it rises

A bubble grows by stretching the sheet, and the stretch is not uniform. The pole, the point furthest from the clamp, stretches the most because it is where the material has travelled furthest and where the wall is least supported. Its thickness falls from 85 percent of the original at a height-to-diameter ratio of 0.2 to 72 percent at 0.4, 58 percent at 0.6, 46 percent at 0.8 and 36 percent at 1.0.

That is a much gentler thinning than a deep vacuum draw, and it is one of the arguments for free forming in optical parts. A hemisphere formed by vacuum into a cavity typically loses 60 to 70 percent of its thickness at the pole; a free-formed bubble of the same proportions loses about 50 percent, because the material is stretched over a smooth surface rather than dragged into corners.

The distribution is still the design constraint. If the finished part has a minimum wall requirement, the sheet has to be thick enough that 36 percent of it still meets the number. A part that needs 0.8 mm at the pole at a 1.0 ratio needs a 2.2 mm sheet, and that is what sets the material cost.

Table 2 — Pole thickness against the shape of the bubble.

Free forming as a material test

The most valuable use of free forming is not making parts. It is measuring whether a sheet can be formed at all. Blow a bubble at a fixed temperature and record the height at which it bursts, and you have a direct measure of the material's biaxial extensibility at that temperature.

Run the same test across a temperature range and you get a formability window: the band of temperatures where the sheet stretches far enough to form the part without bursting, and does not sag so much that it touches the heater. That window is what a thermoformer needs to know before quoting a new material, and it takes an afternoon rather than a tool.

The test also catches moisture. A polycarbonate or a PET sheet that has absorbed water will burst early and show a hazy, bubbly pole, because the water flashes to steam at the forming temperature. A drying problem that would have shown up as a batch of rejects in production shows up as a short bubble in the test.

Where it is used in production

Free forming makes parts where the shape is the function and there is no need for a moulded feature. A light diffuser for a ceiling fitting is a shallow dome with a smooth outside, and a tool would only add cost and tool marks. A speaker grille cover, a machine inspection window and a radome are the same argument.

It is also used to pre-stretch. A bubble blown before the vacuum is applied gives a more even wall in a subsequent vacuum form, which is the principle behind bubble-assisted forming. The bubble does the thinning in a controlled way, over the whole sheet, instead of letting the cavity corners do it locally.

The third production use is prototyping a shape before a tool exists. A free-formed dome tells a designer what a 1.2 mm sheet will actually do at a given draw, and whether the wall at the pole will meet the requirement. That answers a question that would otherwise need a tool to settle.

What it cannot give you

Free forming cannot produce a flat face, a sharp edge, a rib, a boss or a mounting flange. Everything it makes is a smooth, doubly curved surface, and any feature has to be added afterwards by trimming, bonding or a secondary operation.

It cannot hold a dimension. The height of a bubble depends on pressure, temperature, sheet thickness and the clamp, and the tolerance on all four adds up to a height tolerance of a millimetre or two. A part that has to fit a mating component needs a mould, or it needs to be trimmed to a datum.

It also cannot form a deep shape without a pressure that approaches the burst strength of the sheet. The practical limit is a height-to-diameter ratio of about 1.0, and beyond that the pole becomes so thin that it fails in service even if it survives the blow.