Thermoforming: how a flat sheet becomes a splitter

Put simply: we heat a plastic sheet until it softens, then press it against a mould so it takes on its shape. In our case, though, it isn’t a press that does the work – it’s vacuum. We draw the air out from under the sheet, and atmospheric pressure does the rest. It sounds simple, and the principle really is simple. The devil is in the detail: the mould, the temperature, the timing and the experience of the people who watch over the whole process.

Everything begins with an ABS sheet. ABS is a thermoplastic, which means it softens when heated, then becomes rigid again as it cools and keeps the shape it was given. That’s exactly what makes it ideal for thermoforming.

ABS didn’t end up in our workshop by accident. We started out with fibreglass laminates, but switching to ABS turned out to be a breakthrough. It’s lighter, easier to work with and allows for far more precise shapes. On top of that, it’s tough and flexible, so it copes well with everyday driving, changing weather and run-ins with high kerbs.

Before a single sheet goes into the machine, the mould has to be made. It decides how every splitter that comes out of it will look and fit. CNC milling machines cut the mould straight from the 3D design, which is based on a scan of a real car. A single mould takes around 24 hours of machine time.

But that’s not the end of it. After milling, our team refines the mould and smooths its surface by hand. Why does that matter so much? Because the mould works like a stamp. Every scratch or imperfection left on it would show up on every part made from it. An extra hour spent on the mould means hundreds of flawless splitters.

Now the actual thermoforming begins. We clamp the ABS sheet in a frame and heat it with hot air. At first nothing seems to happen, but moment by moment the plastic becomes softer and more pliable, until it starts to sag slightly under its own weight.

This is the most demanding moment of the whole process. An underheated sheet won’t press tightly against the mould and won’t pick up the fine details. An overheated one can discolour, thin out or lose its properties. Good thermoforming is all about finding that one moment when the material is ready – and here the operator’s experience matters more than the machine settings.

Once the sheet is properly heated, the most spectacular part of the show begins. The hot plastic goes onto the mould and we start drawing the air out from underneath. A vacuum forms between the sheet and the mould, and atmospheric pressure pushes the soft ABS into every recess, curve and edge. The sheet literally “wraps” itself around the mould and takes on its shape to the millimetre.

It all happens surprisingly fast, but these few moments decide whether the part will be perfect. As it does, the plastic stretches over the mould and naturally gets thinner in the deeper sections. That’s why we take into account how the material will spread right from the design stage, so the splitter is strong along its entire length – not just where it’s flat.

After forming, the sheet stays on the mould and cools down. It’s a stage you rarely see in production videos, but it’s very important. The ABS has to regain its rigidity before we take it off the mould. If we do it too soon, the plastic can warp – and then all the precision of the mould and the heating goes to waste.

Only when the material has cooled and “remembered” its shape do we remove it from the mould. What we’re holding now clearly resembles a splitter, but it isn’t one yet.

When it comes off the mould, the splitter is still part of a larger sheet, with excess plastic left around the finished shape. This is where thermoforming ends and further processing begins. The part has to be cut out of the sheet and then sanded by hand so that its outline, thickness and look are exactly right. Finally comes quality control, protection and packing.

We’ve described exactly what happens to the part in these final stages – and the whole journey of a splitter from scanning the car to the box – in our article How Maxton Design splitters are made.