Recently I was asked to provide some information regarding the use of lightweight design concepts for automotive vehicle design and more specifically, the use of fixings and engineered plastics for these purposes.
Automotive plastics have come a long way recently. Previously confined to cheap flimsy interior parts many modern-day vehicles are now using plastics for exterior body panels, such as the BMW i8, granted the BMW also uses a large amount of CFRP.
As with any parts for a vehicle. they have to be fixed into place some how. With Metals parts many of the components are bolted together or welded together. With plastic parts this can sometimes cause problems.
Plastics, of course, cannot be welded. So what is the alternative? Automotive adhesive solutions for a start. Modern adhesive technology allows for the bonding of parts from the interior to main chassis components as you can see from the above video of the BMW i8 Manufacturing process.
McLaren also use bonding solutions for various parts of the construction of their vehicles. Not only for cosmetic parts but also structural parts.
As you can see though there is still a lot of fastening required for the subframes front and back using high strength metal fasteners. When fastening composites such as carbon fibre using metal fasteners there are a few things that engineers must consider.
- Differential expansion of the fastener in the composite.
- The effect of drilling on the structural integrity of the material, as well as de-lamination caused by fasteners under load.
- Water intrusion between the fastener and composite.
- Electrical continuity of the composite and arcing between fasteners.
- Possible galvanic corrosion at the composite joint.
- Weight of the fastening system.
- Fuel tightness of the fastening system, where applicable.
Whilst there is not the element of corrosion caused by glass fibre composites as opposed to carbon fibre composites the other structural issues occur such as expansion differentials and the de-lamination of the fibre structures.
It is possible to work around these issues with metal fasteners using either a sleeve material which is bonded to the structure or to bond the fastener directly to the surface of the part without the need for drilling. When considering the use of a sleeve material the problems found with the interaction of the materials must still be addressed, such as structural integrity and the expansion differential. It does however, allow the engineer to use a more desired fastener with the sleeve only acting as a barrier and not a structural component. When bonding the fastener to the surface of the material the adhesive acts as a barrier therefore the corrosive effects are not as apparent. The connection between the two components is now only as strong as the bonding material so consideration must be made for the purpose of the fixing.
The problem with using composites is of course, cost. For large-scale production, composites are just not a cost-effective solution and even though companies such as BMW are using them for mass production, the cost of an i8 or i3 is much more than the cost of a similar class vehicle. For this reason automotive plastics are used where weight saving is required but at low-cost.
Automotive plastics offer the ability to design parts without the worry of corrosion and also without the worry of thermal expansion differentials compared with composites or other plastics. Secondly we can use plastic fasteners and fixings to negate effects of corrosion commonly found with metal fasteners, using plastic fasteners where applicable also aids in weight saving. whilst the saving on a single fastener is small, when considering the fasteners used throughout a vehicle and replacing these with plastic alternatives, that weight saving quickly adds up. Finally using automotive plastics allows the design of parts which do not require separate fixing elements, This is because of the extra manufacturing flexibility offer by injection moulding techniques. These components are called Adjacents.
Adjacents are predominantly injection moulded parts which will snap into place. Ordinarily they are used as interior parts however they are not just restricted to this. Thermoplastics offer functionality for parts to be used in the engine bay for areas such as the engines induction system and engine covers.
Plastics are also used for exterior panels, The SMART car being one of the pioneers in this sector with BMW using this same philosophy on their ‘i’ range.
Going forward where can we continue to use thermoplastics and injection moulded parts for the purposes of saving weight?
The answer to this question is pretty much anywhere you can think. Provided the material selection is fit for purpose, engineers can use plastic components throughout the vehicle. From the engine bay to structural parts or interior and exterior cosmetic parts. It is simply a matter of the design process:
- Material Selection
- Design and Implementation
As someone with an engine design focus I would lean toward developing components for induction systems, cylinder head parts and covers as well as oil lines and fixings. However with the rising popularity of Electric Vehicles, ICE components are going to phase out. Therefore other areas of consideration can be components such as motor covers, suspension caps and cooling elements for battery packs or the electric motor. All of these could be designed or fixed using engineered plastics. It’s just a matter of design and desire.