Plastic gets a pasting from new seam-free car models

Stylists and designers who model new cars for a living are being offered another way to show off their traditional craft skills. Instead of using Ureal, a plastics material from Ciba-Geigy, or some similar brand to create their models, they will be able to choose another process. Models made this way will be of a […]

Stylists and designers who model new cars for a living are being offered another way to show off their traditional craft skills.

Instead of using Ureal, a plastics material from Ciba-Geigy, or some similar brand to create their models, they will be able to choose another process.

Models made this way will be of a higher quality and surface finish than those from the plastics material, it is claimed. For example, there are no visible joint lines, which occur in larger models with several blocks of material joined together.

Models are also lighter and retain their structural stability and dimensional accuracy over a long period.

That is a key requirement since models are used as a reference against which actual finished parts are measured.

Swiss company Sika, based in Stuttgart, has developed the method, now being used by one car maker at its German and Spanish plants and most recently in the UK where it has just installed the first plant.

At the heart of the system is a paste similar to household filler, according to Shaun Hope, a director with Sika’s UK agent Mason Chemical in Croydon.

This is dispensed by machine on to a core which has been machined on a five-axis milling machine using CAD/CAM data. This could be a full-scale model for a car side panel.

With this process, the core could be made from cheap polyurethane foam. For extra strength, honeycomb aluminium could be used. Both are lighter than solid plastics.

The core is machined about 20mm undersize and that is a key part because the paste coating will be used to create a slightly oversize model.

It is the coating, described as a two-component polyurethane thixotropic system, that gives the core its stability and excellent finish. Sika considered working with an epoxy instead, but that heats up during curing and causes problems.

When the paste has hardened, within 24 hours, it can be machined to the exact size required of the model using surface milling techniques to produce a polish-like finish. The material’s machinability is said to be excellent.

Sika had to overcome the problem of getting the thixotropy staying power under stress of the paste just right. It had to stay put on a vertical wall in thicknesses up to 40cm.

Mason Chemical expects to sell 100 machines world-wide with around 10 going to the UK market. So far four have been sold.

With a price tag of £40,000, there will be no rush to buy. But Hope says customers can expect a cost saving on raw materials.

Mason will be looking to profit long term from continuing paste sales.