Novel fixing technique has potential beyond the automotive sector

Written by: Tim Fryer | Published:

There are various ways of making cars perform better. You could gain speed by utilising a jet engine, but that may be counterbalanced by the cost of installing and running it.

Aerodynamics becomes more significant as speed increases, but even then automotive designers have put enough lumps of plasticine in enough wind tunnels to have a pretty good idea of what is a good shape for a car aerodynamically speaking.

Another obvious contributing factor is weight, but while it is well recognised that a lighter car will be able to use less fuel to go faster, this is an area that still has much untapped potential. One company, heavily involved in providing fastening solutions to the automotive industry, is Bollhoff and it has been focussing on 'lightweighting' activities for half a dozen years.

Andy Witts is managing director of Bollhoff Fastenings UK and he outlined the background: "We recognised that there was going to be a shift to more lightweight materials - we were very involved with the McLaren SLR project which was the first ever real total carbon fibre car."

The company did have fasteners in that car but they were existing products designed for use in a carbon steel bodied car rather than one constructed of carbon fibre reinforced plastic (CFRP). "It was workable but it wasn't optimised for a CFRP vehicle," stated Witts. "So we decided that we would try and create something that would allow us to work more easily with those materials and to provide some additional benefits for the customer."

Inside a traditional steel car body as it passes down the assembly line there will be an array of fixings which will be used for non-structural purposes like anchoring pipes and hoses, engine dressings and so on. These will be applied to the panels by stud (resistance) welding – a common, cheap process that allows studs to be placed anywhere by appropriate programming of a robot. The process – pick, place and weld – is ideal for mass production.

Carbon fibre cars are only just making the transition from the technology demonstrator environment of the supercar to the practicalities of a production vehicle. Witts acknowledged: "Carbon vehicles have been the domain of Top Gear – the supercars that are ultra lightweight and ultra expensive. They are test beds basically for the volume manufacturers." But that transition to the production environment is now underway.

Unlike the stud welding process, attaching fastenings to CFRP has not always been of satisfactory quality, nor is it as easily automated and therefore as cheap. The standard method involves drilling a hole in the substrate and inserting a mechanical fastening, which typically needs an action on both sides of the panel and the stud itself is relatively expensive. Another issue is the difficulty in creating a clean hole when drilling, with delamination of the CFRP substrate always a danger.

There was therefore a void between existing practices used on metallic cars and the current 'best option' for use in CFRP vehicles. Bollhoff set up a development programme to address the issue, and it did so with another German manufacturer, Delo, that specialises in advanced adhesives.

"They had a breakthrough with a light curing product which we found very interesting," said Witts. "They were really looking for something to do with it and we saw the potential of using our fastening systems with their adhesive system together to make a two component fixing. And that is where ONSERT came from."

ONSERT is both a product and a process. It consists of a mechanical fixing, which at the moment is typically a metallic threaded or unthreaded stud. This stud has a 'foot' which is over-moulded around it and then adhesive is applied to the flat base of the foot. The part can then be offered to the panel and is flashed with blue light for four seconds to cure the adhesive. The light source, at 400nm, is at the same point of the spectrum as regular daylight so if the product is not fully cured then it will continue to cure in daylight.

Witts claimed: "Basically what we have done is effectively duplicated the stud welding process. We have equipment that feeds the stud with the over-moulded foot, dispenses the adhesive, puts it in the magazine and then the robot picks it up, places it and cures it. So the process effectively a replica of stud welding except using adhesive and CFRP rather than steel and welding."

It is a system that has already been used to good effect in a production vehicle environment. BMW launched its range of all-electric cars with the i3 city car and will shortly add the i8 sports car. In terms of being available from the show room, these could lay claim to be the first carbon fibre production cars. The company has taken 18 ONSERT systems for use in the manufacture of these electric cars and equally have been valuable partners in the development of the system over the past year.

Witts commented: "We focussed our development with the ONSERT product on the i3 and i8 programme because there is a definite volume need for that. What it has done for BMW is it has streamlined their process. It does away with quality risk and enables them to use familiar technologies on the same production line."

It is a system that is not restricted to applications with CFRP. The first application in the UK is with a well known luxury 4x4 manufacturer. Its new model, due to be released in January 2015, created a roof channel so that they could keep the join between aluminium roof and side panels out of sight. This channel needed a finishing strip to cover it and the company developed a solution that involved bonding 24 ONSERT studs into the roof channel and then attaching the finishing strip to them. "We can bond to most substrates," added Witts. "We are having a few problems with organic materials like wood, but most plastic, metallic and ceramic materials are fine. Delo will alter the acrylic adhesive material to suit requirements."

Bollhoff has a long history in the automotive sector and so it was an obvious starting point for implementing this technology, but Witts sees endless opportunities for designers.

"It can be in any configuration – applications are as broad as your imagination," he said. "Obviously from our point of view, we like to produce as many of the same parts as we can so that the customer can get the economies of scale in manufacturing, but from the customers point of view designers can design whatever suits them. We create the metal part, put it into the mould machine and mould the attachment foot. That has to be clear to allow the blue light to shine through it. It is interesting that the handful of applications we have got are mostly based on a 5mm stud with a thread or a Christmas tree type fixing. But we can do pretty much what the designer wants."

Beyond some of the higher volume potential uses in aerospace and white goods, for example, the development of a hand tool to do the curing, essentially an LED torch, opens the door for both prototyping and low volume applications.

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