Watching paint dry

Sensor designed to measure thickness of paint while still wet and rubber uniformity on tyres could help car manufacturers reduce waste in the production of crucial components.

A sensor that can measure the thickness of paint before it has dried could help car manufacturers reduce waste when producing components crucial to car safety, according to its manufacturer, German instrumentation specialist Micro-Epsilon.

Using a combination of electrical phenomena and laser measurement, the system can also measure the thickness of the rubber layer on the composite materials used to make tyres.

The sensor was originally developed for a Belgian company, Recticel, which makes plastic components, such as dashboards and seating, for cars. The manufacture of many of these components includes a paint-spraying process, where the outer skin is made by spraying polyurethane paint from a spray nozzle mounted on a robot arm, into a heated nickel-coated mould.

The thickness of the paint skin is obviously crucial to the quality of the part — tight control of materials is central to all manufacturing processes. But for the section of the dashboard that covers the airbag, known as the cladding, the thickness is even more important. Too thick, and the airbag will be slowed down when it is deployed during a crash, which could be catastrophic for the car’s driver and passengers.

All component makers, therefore, need some sort of system to measure the thickness of the paint coating. Generally, they use eddy current sensors, where the sensor generates a magnetic field which induces electrical current in the mould whose strength is proportional to the distance from the sensor to the mould. But for these to work, they have to actually touch the surface of the paint, which means that the paint must be dry before the sensor can be used; even then, the sensor can damage the surface when it makes contact.

The manufacturers would prefer to be able to measure the thickness during the process itself, as this would save time and reduce the need to keep the painted skins until they had set, when some would have to be scrapped anyway.

It was the problem of high levels of scrapping that prompted Recticel to approach Micro-Epsilon. The company responded by combining an eddy current sensor with a laser measurement device. The sensor sits above the painted surface, with the laser component measuring the distance from sensor to the top of the paint surface, and the eddy current sensor measuring the distance to the surface of the mould. Subtracting one from the other gives the thickness of the paint. The system works up to 15mm from the surface, has a resolution of 1µm and an accuracy of 5µm.

‘Recticel had been using a laser sensor that just wasn’t performing correctly,’ said Micro-Epsilon product manager, Erich Winkler.

‘After installing our system, the company’s reject and scrap levels have been significantly reduced and their production process is faster and more efficient. The robot arm sprays a couple of layers then inspects the coating while it’s still wet.’

The system is also being used in tyre production. Tyres are made from layers of a fabric woven from strands of metal, which is then rolled between two drums covered with molten rubber. The rubber is transferred from the rollers to the fabric in an even coating, the thickness and uniformity of that determines the strength and mouldability of the final material.

Because of the way the drums are positioned, it has previously not been possible to measure the rubber thickness during the process; it has to be checked post-production which, once again, can lead to high levels of scrapping. However, Micro-Epsilon says, their sensors can give an accurate measurement of the rubber thickness as soon as the coated material emerges from the drums; what is more, the high resolution capability of the laser sensor means that the surface structure of the rubber coating can also be inspected.