01 April 2013
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Fully bonded caravan is lighter and drier

Elddis has raised the bar in caravan construction by switching fabrication to adhesive bonding. FAST describes how the first fully bonded caravan was manufactured.

Last year, the Consett based company Elddis radically changed its methods of manufacture, favouring a fully bonded structure over one whose elements were mechanically fixed. Out went approximately 800 screws to build 'the box' along with butyl sealants to prevent water ingress and in came three different adhesives.

The outcome has been revolutionary, creating a shell that is stronger, lighter and drier than ever before. And it's all thanks to a highly successful partnership between the Elddis business development team and the engineering adhesives specialists at Henkel.

The way a caravan is manufactured has changed little in 60 years but customer expectation continues to drive innovation. To date the differences between brands have been largely cosmetic and functional but Elddis wanted to do much more. It created a dedicated research and development team whose sole purpose was to push boundaries.

In recent years, caravan builders had focussed on creating lighter weight structures and Elddis had also been exploring the use of composites to cut weight. By making its products lighter, customers could consider caravanning without the need to use a heavy tow car. Interior design flexibility would improve, as would the ability to add accessories without exceeding weight limits.

Leak paths

Another key aim was to eliminate any leak paths that were always the main bugbear of a screw-fixed construction, whilst remaining cost competitive. But Elddis wanted more than lightweight and watertight construction. Its goals also included supreme design flexibility and good aesthetics so future products could be any size, shape or colour, unhindered by the construction technique.

"The commercial drivers were to create a stronger, lighter, drier product that could be continuously developed and we were willing to explore any method to achieve that," explained business development manager, Alan Beel. "Structural bonding had clear potential but we knew this would be a massive hurdle for us as adhesives are not traditionally accepted in our industry."

Elddis had explored the use of various composite materials but quickly realised that they were not compatible with traditional coach building techniques. Drawing on experience in other industries, its engineers identified the use of adhesives as an alternative solution. Further investigation revealed that not only could adhesives be used with current materials but they would also provide even greater scope for improving construction methods.

Adhesives were used on the old, established Elddis designs but they did not contribute to its structural integrity. This is a field however in which Henkel has considerable expertise. The company has developed very successful structural solutions for the aircraft and automotive sectors and saw the leisure vehicle market as a natural extension.

Henkel knew its products could improve the quality and strength of caravan construction and was keen to introduce Elddis to its way of thinking. In fact Henkel's initial sales call was perfectly timed as Elddis were unsatisfied with the lack of re-engineering and lack of creative solutions from other providers. "A very simple example of adhesive strength given to us by Henkel during our first meeting got us hooked!" Beel added.

All it took were four Post-It notes and a staple gun. Two notes were stapled on non-adhesive sides and the other two simply stuck together. The stapled version pulled apart instantly whilst the bonded alternative held firm.

Henkel's enthusiasm for the project was clear from the start. Beel continued: "Five Henkel experts in structural bonding technology came to our first meeting. I've dealt with development projects like this most of my working life and this level of commitment so early in the process is quite exceptional."

A strong working relationship between the companies was swiftly established and together they embarked on a three year development process to create the world's first fully-bonded caravan.
The best Henkel structural bonding adhesive for the job was identified early in the development process. Very high strength products used to bond aircraft wings were rejected by Henkel in preference for one that has been designed for wind turbine blade bonding. This would provide the elasticity needed to accommodate the forces resulting in vibration and from the caravan or motor home travelling across uneven road surfaces. The product chosen for the structural bond was Macroplast UK 1366 B10/UK 5452. This is a solvent-free, two-component structural adhesive based on polyurethane. By mixing both components in a weight ratio of 4.7:1, a hard elastic structural adhesive is formed through chemical reaction. It achieves handling strength in 50 minutes and after curing the product does not show any measurable change in volume.

All process development was conducted on site at Consett and the first phase involved bonding an Elddis Xplore 302 single axle, two-berth caravan with the Macroplast adhesive. At this stage the panels were simply butt joined using the adhesive in place of screw fixings. Test vehicle "The bonded caravan was then thrown around the car park and over kerbs and we just couldn't break it," Beel confirmed. "In fact we still have that original model and it's currently acting as a test vehicle for our harness supplier." This first success gave Elddis the confidence to continue the development process and the next step was to consider joint design to provide an even stronger and watertight structure. More brainstorming sessions ensued and the result was a keyway joint which effectively increases the bond area by 63%, creating a structure that is at least 10 times stronger than the original. A prototype of a new caravan was constructed
whereby all walls and the roof, floor and chassis were bonded together with Henkel Macroplast; all mating surfaces featured the new tongue and groove joint. It is similar to a Monocoque car
design and as a result the stress is spread across the entire structure so the force at any given point is very low.

The Macroplast product chosen can in fact withstand around 20N per square millimetre of applied force to the bonded structure. This is complemented by a second environmentally friendly product with a strength of 2N per square millimetre for applications such as bonding a vent into the side of the caravan. Elddis had already adopted a bonded floor to chassis design on its motor home range but the Henkel development also resulted in a new prototype with bonded roof to walls.

Less structural support The strong bonded construction transformed flimsy panels into stiff structural components which made many of the original, timber support structures redundant.

By removing these timbers, Elddis has been able to reduce the weight of its vehicles even more. Indeed this is an important tenet of the company's environmental policy. It is committed to this process to reduce the fuel required for each journey travelled to improve overall fuel efficiency.

Henkel nevertheless encouraged Elddis to retain some timber elements as the moisture content of wood makes it an excellent bonding substrate and the material remains strong across its entire
length. The bonded structure has however enabled Elddis to consider its design holistically, replacing aluminium with the stronger GRP (glass reinforced plastic), whilst still reducing overall weight.

Henkel also recommended silane modified polymers to bring exterior fittings in line with the new process. This eliminated the need for even more screws and improved the aesthetics of the caravan. Terostat MS 939, a gun-grade, single component adhesive and sealant was chosen for the job. This product cures by reaction with moisture to form an elastic material; skin formation and curing times are dependent on ambient humidity and temperature and also joint depth. This popular Henkel product is particularly sag-resistant, giving a high positioning tack. It adheres extremely well to many substrates without the need for a primer and is compatible with most paint systems. UV resistance makes it particularly suitable for caravan manufacture and it is therefore used to bond the roof skin in the vehicle and for both interior and exterior seam and joint sealing. As the geometry of some parts makes them difficult to hold in place, Henkel advised the use of Terostat MS 500, a new hot dispensed silane modified polyether, originally formulated for the photovoltaic market. At application temperature, this single component adhesive has a low viscosity and is easy to apply. During cooling a high position tack occurs.

The first application for which Terostat MS 500 has proved ideal is bonding the awning rail to the main structure. In an early test, a fully extended awning was exposed to very high winds on a hill near the company's factory in Consett. Whilst the awning material was ripped from its fixings, the awning rail itself held firm. The next step was to road test the fullybonded
caravan design up to and way beyond BS EN1645 Part 1, the master standard for caravan construction. For this purpose Elddis took a newly built Avante 646 to a number of proving grounds including the Ford track at Lommel in Belgium, a site also used to test the durability of cars, trucks, military and commercial vehicles. Tests subject the vehicles to all the worst conditions they are likely to encounter including pot holes, kerbs, cobble stones as well as high speed motorway driving.

Benchmark

The bonded Avante completed 90 circuits of this proving ground to achieve 'a pass' but again, Elddis wanted more. The durability standard was created as a benchmark for the screw fixed structure that is still the predominant method of construction in the industry. Elddis already knew its design was stronger and therefore needed to create its own measure of performance.

The fully loaded caravan travelled 27,000 miles on the Lommel track and the only casualty was the towing vehicle. Indeed, in order to complete the test, it had to be water cooled after every three laps. The Henkel bonded Avante 646 remained as good as new and, today, is owned by one of the team involved in the project and shows no sign of its punishing ordeal.

A rain simulator at Millbrook in Bedford then put the bonded structure through tests that mimicked annual rain fall and also subjected it to temperature extremes of -40ºC to +50ºC. No sign of
water ingress could be seen but that was insufficient proof for Elddis, the only caravan manufacturer to be certified to the ISO 9001 manufacturing standard.

Engineers then completely dismantled the structure to confirm all elements were dry and there were no joint failures. And on the strength of this Elddis has been able to introduce its 10 year bodyshell warranty. This process also confirmed that the new design is 100% repairable in a standard workshop, an important factor for the aftermarket. Still not content the company provided
a bonded motor home to Motor Industry Research Association (Mira) to undertake its own, fully independent endurance test process. The vehicle again passed with flying colours. Despite wearing out tyres and suspension, no signs of wear and tear were found with the construction.

In a bid to find the absolute limits of durability the company returned to Lommel with another test caravan. This time it was overloaded by an extreme 500kg for another simulated 27,000 mile
test. Yet again the caravan survived and remains an immaculate example in the demonstration fleet. Less can be said about the new 4x4 tow vehicle which needed to be retired from service!
With the design well and truly proven the Elddis SoLiD concept was put into production. SoLiD is the acronym the company has chosen for its stronger, lighter and drier method of construction.
But it hasn't just opted to build some models this way, it has adopted the process across its entire range of touring caravans and coach-built motor homes, such is its fervent belief in the benefits of Henkel structural bonding.

Getting the entire workforce on side was another task. Again pre-conceptions of a mechanically fixed structure over one that is chemically bonded had to be overcome. "From the test results the management team knew SoLiD was the way to go but those who were not directly involved in the development process remained to be convinced," added Elddis' managing director Rob Quine.

Training

In depth product training was therefore an essential part of process implementation and now all employees have completed three training courses on site with Henkel.

It was considered very important from a cultural point of view that everyone at Elddis had complete faith in this new method of construction. The management knew this approach had worked when
they were inundated with design requests and ideas from the manufacturing operatives; all with the aim of improving bonding techniques and eliminating compromises in assembly.

Every department was given a macro plan to drive through the new manufacturing process. Investment in new equipment was considerable. Dedicated dispensing equipment was a vital ingredient
to ensure the correct amount of adhesive is applied to achieve the joint elasticity required to absorb the forces. It also ensures the correct mixing of the adhesive and its optimal storage. With the help of Henkel's equipment specialist Steve Ginger, Elddis specified standard pneumatic guns and other off-the-shelf dispensing equipment that was modified by Henkel to suit the various applications.

The introduction of the hot dispensed Terostat MS 500 also required additional attention as initially cartridges had to be heated and the product applied from a hot melt gun. The equipment team subsequently supplied a heated drum pump with user friendly gun applicator that allowed bulk adhesive to be used, saving money in the process. Additional applications to awning bonding are currently being explored for this adhesive.

Another significant consideration for Elddis when adopting any new process was waste minimisation. The company recovers 95% of its waste via its own recycling stations. For example aluminium tubes and steel cans are segregated, crushed and sold to generate revenue. This applies to all the containers in which the Henkel glues are supplied. Once the glue has dried they follow the same recovery streams.

The reduction in the use of timber supports through structural bonding makes the company less reliant on the supply of wood, although all of its timber is from a sustainable source. Any remaining waste including sawdust, around 2.5% of the total, is sold to another manufacturer as fuel for its processes or used to produce MDF.

Cost neutral

"We didn't set out to achieve manufacturing economies by changing to a bonded structure," explained Alan Beel. "The process is fundamentally the same as before except that we bond instead of screw fix and I would say the changeover has been cost-neutral." Clearly though, the business case for the bonded structure is the creation of a major differentiator between Elddis and its competitors. And it is already paying dividends. SoLiD was officially launched in Autumn 2012 and has already won awards both in the UK and internationally.

Elddis reports that it had its best ever order intake at the main industry exhibition at the NEC last October 2012 and even in a fairly flat market caravan sales are above target.

There is much more in the pipeline however now that Elddis has taken this giant leap in construction design. By early 2013 Elddis will have developed the manufacturing process of all its vehicles to remove every external screw fixing. And naturally the company is also well underway with the development of next generation products that take full advantage of the design freedom that structural bonding provides.

Henkel know-how is essential to these forward plans. "Our working partnership has been better than anyone expected," concluded Beel. "I have worked with many suppliers throughout my 30 years in industry and Henkel is by far the best. Their support has been unbelievable."

Author
Paul Gay

Related Companies

Elddis Transport (Consett) Ltd
Henkel Loctite Adhesives Ltd

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