PLASTICS PERFORM IN LOW VOLUME VEHICLES

John Watkins, the group technical manager of Thompson Plastics, explains why thermoformed plastic components have established their place in off-road, farm and specialist vehicles. JCB has already used them to advantage

Plastics were first incorporated into tractors in the early 1970s when they began to replace metal and wood interior facias and trims. The movement from metals to plastics developed from the inside out, so that, today, interiors are largely plastic and so are the majority of exterior panels. Manufacturers can obtain literally dozens of components from thermoforming specialists, including interior trim, instrument consoles, dashboards, exterior body panels, roof assemblies and radiator fan cowls.

Externally, the strength of plastic now compares extremely well with metal and fibre glass. Impact strength is such that thermoformed roof panels pass the most stringent roll over and falling object or ‘bullet and bomb’ tests. And unlike metal which dents easily, plastics will spring back into shape following impact.

As plastic is self-coloured, components can also be accurately colour matched. General colour fastness and resistance to UV light mean that plastics retain their condition. It is even possible, by using co-extruded plastic sheets, to have different colours on either side of the component, addressing the issue of using recycled materials in non visible areas.

Engineering thermo plastics offer heat resistance up to 120iC. Bonded in heat and sound insulation panels mean that temperatures reached in and around engine areas can be withstood comfortably.

Plastic panels can also be bonded onto the steel framework of a vehicle with the same Sikaflex adhesive used to secure glass windscreens. This provides a constant seal between the panel and the steel frame, which simultaneously accommodates the thermal expansion of the panel. Traditional mechanical fixing processes, on the other hand, require very accurate alignment of fixings and can induce stress in panels.

The use of thermoformed plastics in specialist vehicle interiors is now driven by the demand for commercial, working vehicles to offer more of the comfort and appearance of a motor car. Unfortunately, the low production volumes involved in most models of off-road, farm and customised vehicles mean that, because of high tooling costs, injection moulding plastic parts is simply not cost effective.

Several techniques can be used to manufacture plastic parts for vehicles. These include injection moulding, vacuum forming and pressure forming. With the injection moulding process, molten polymer is forced, under high pressure, into closed steel moulds. Whilst a minimal amount of scrap plastic is generated, the high cost of tooling, makes the process uneconomic unless the part is to be produced in high volumes (10,000/year and above) and has a long life cycle.

In the vacuum forming process, a heated sheet of plastic is pulled, using vacuum, over a forming mould, usually cast in aluminium. Whilst the item cost is more expensive, this is offset by the low tooling investment cost, meaning that an engineered plastic part is available for volumes lower than 10,000/year or which will be subject to frequent design or styling changes. Using five axis CNC routing machines, a number of trim variants can be realised from one basic moulding.

Vacuum formed plastics have established their place in vehicle interiors due to their performance characteristics (in particular regarding impact and temperature resistance), aesthetic qualities (bringing high gloss surfaces without painting) and comparatively low production costs. Lower start up costs achievable through the low cost tooling make vacuum forming a more appropriate method for producing plastic interior and exterior components for low volume specialist vehicles whilst advances in thermoforming technology, and in plastic materials, mean that vacuum formed components can replace almost all non-structural parts.

By vacuum forming, rather than injection moulding the plastic components, it is possible to produce a whole range of different finishes. As only one surface is in contact with the mould during thermoforming, much more can be achieved with the surface. The gloss level of the finish can be tailored to meet individual manufacturers’ preferences or a texture added. A ‘soft touch’ finish is possible through coextrusion and different plastics can be combined, using co-extruded sheets, to give different finishes to each side without changing tooling. Leathergrain, suede, haircell, or pebbled surface finishes can be applied to the sheet when it is extruded. Manufacturers can also design a surface finish unique to their product.

In addition to the savings in tooling costs achieved by choosing vacuum forming rather than injection moulding production technology, it is possible to design tooling to make economical use of plastic sheet, producing both interior and exterior components with little material waste, for instance, incorporating smaller mouldings in aperture areas. In other applications, exterior edge trim can be recycled to produce interior mouldings.

Whilst very accurate detail may still in some cases require the injection moulding rather than vacuum forming process, the gap between the abilities of the two methods to achieve precision has narrowed considerably as vacuum forming technology and finishing techniques have advanced. Vacuum forming now uses 3D CADCAM systems and five axis trimming machines, so that the repeatable accuracy of finished products is more precisely controlled. Machine capability enables complex, large area components up to 8m2 in size to be formed. Dimensions can be controlled up to tolerances of 60.5mm.

Thompson Plastics worked closely with JCB in the design and development of the Fastrac tractor to develop and produce a number of exterior components. Traditionally, this type of vehicle would have comprised metal panels on a steel frame. But it would have been difficult to achieve the level of styling desired by JCB using metal, and it would certainly have involved compound shapes, whilst the cost of press tools would have been very high compared with the tooling requirements of plastic vacuum forming. The cost differential between tooling costs for metal pressings versus vacuum forming could be as high as 20 fold.

Though glass fibre, plastic and resin injection moulding were considered, relatively low production volumes meant that vacuum forming was the appropriate technology for the Fastrac panels and cab roof. The requirement for a high gloss level finish for the roof, body and bonnet panels ruled out rotational moulding, while vacuum forming also had an edge over competitive processes in terms of component weight temperature, performance and recyclability.

A TENTH OF THE COST

Using stylists drawings of the Fastrac cab frame allowed full size panels in structural foam to be modelled. These were applied to the prototype frame. From there, the styling details and the practicalities of fitting the panels were addressed.

Prototypes were developed from wooden moulds and, once accepted, aluminium moulds were created. (Injection moulding would have required costly steel moulds from the start, whereas moulds for thermoforming prototypes can be made from wood and these can, in many cases, become the casting patterns for the final aluminium production moulds.) The total roof tooling investment was less than £30,000 whilst the similar package for the injection moulding process may have exceeded £300,000.

Acrylic-capped ABS was specified primarily for its impact resistance, weatherablity and comparatively low cost. Thompson Plastics carried out falling object tests to prove the strength of the driver’s cell.

Clearly, acrylic-capped ABS also provided the corrosion resistance that was looked for and, of course, it could be obtained self coloured in the distinctive corporate yellow.

Some projects, especially those involving undercut designs, do present challenges for vacuum forming when male moulds are used. This is caused when the form calls for the material to change direction back onto itself which effectively prevents that part from being demoulded. Mechanical moulding solutions can often overcome these difficulties through hinging and allowing the tooling to rock or by incorporating movable pieces in the tool.

However pressure forming, with its female form, is emerging as an alternative which combines the low costs of vacuum forming with the precision of injection moulding. Although a well established technology, the use of pressure forming for producing vehicle components heralds a new developmental stage in plastics applications in vehicles.

By using pressure in addition to a vacuum, it is possible to obtain a more defined shape with a multi textured surface incorporating manufacturers logos or user instructions. Pressure formed sharpness of detail is closer to that achieved by injection moulding. It gives an injection moulding look-a-like, but tooling costs can be a tenth that of injection moulding. Along with consumer electronics and the medical industry, the low volume automotive market is a key potential area for this technology.

Suitable for interior components, such as dashboards, and for exterior parts, as with vacuum forming, there is little restriction on material choice. Again, as with vacuum forming, pressure forming is a one sided process, but the texture is created by the mould, like injection moulding. Although the process tends to be used for smaller more detailed components it is possible to produce large area components. For someone looking for injection moulding definition, but who is unable to justify the tooling cost, pressure forming is a very viable option.

Now established as an essential material, there is still scope for expanding the use of plastic in vehicle design. Clearly, there is also potential for applying plastics to other types of specialist, low volume vehicles.

Figure 1: A comparison between different types of plastics processing

Figure 2: Five axis CNC trimming equipment ensures repeatable accuracy

Figure 3: Assembly of vehicle roof

Figure 4: Engine side panels in polycarbonate