AUGUST 2001 COVER FEATURE STORY

Light machining focuses on a clean break

Tom Shelley reports on a fast way of breaking down adhesives and polymers and perhaps fast bonding them as well

Car windscreens will in future be de-bonded for replacement by intense flashes of white light.

Ordinary light turns out to be the fastest, cleanest and most efficient way of breaking down polymer bonds when they are not wanted.

As well as easing the removal of windshields, the technique shows great potential for speeding robotic surgery, medical sterilisation, lithography, bonding and removing graffiti.

The present development comes out of research at University College, Swansea, and subsequent product design work by PDD in Hammersmith. It began when Belron International, who own Autoglass, Carglass and other leading players in automotive windscreen replacement approached Swansea with a view to finding a method of doing away with the need to cut away windscreen seals with knives or cheese wire. Whereas car windscreens used to be held mechanically in rubber seals, they are now firmly bonded to the body of the car. The main reason is the need to provide sufficient strength to withstand the impact of a car driver or passenger striking an airbag in turn supported by the windscreen.

Removing a bonded windscreen by manual cutting is hard and heavy work, with the ever present risk of accidents to operators or the paint work of the vehicle. The task set to the researchers was therefore to find the best way of quickly breaking down the polyurethane adhesive bond between the black fritted outer area of the windscreen glass and the polyurethane seal.

There are many possible ways of doing this. The study undertaken by Swansea and Belron's development company, Automotive Glass Solutions International, has come to the conclusion that the fastest and most efficient method is to use single flashes of white light from a xenon gas filled flash tube. The inspiration was, apparently, the light flash process used to remove human body hair.

When low-pressure xenon gas breaks down under the action of 4,000V in the flash tubes used, it is momentarily nearly as hot as the surface of the sun. 4000J of input energy is converted into 2,400J of light energy in the 350 to 480nm wavelength band (green to blue). The light is concentrated by a linear parabolic reflector onto the windscreen, just above the black frit region round the edge.

The black frit is baked on during the manufacturing process in order to protect the polyurethane bond and sealing bead from degradation by sunlight. Here it serves to absorb the input light energy with maximum efficiency and convert it to heat. Even after a 25% loss caused by reflection at the upper surface of the glass, 1,800J is still absorbed by the frit.

PDD then became involved in taking the idea and turning it into a safe to use product.

Operators have to wear goggles. In addition, the lamp cannot be activated unless it is first pressed down on the glass surface. PDD says that before finalising the design, they used their behavioural research method in order to study how windscreen fitters work, and ask them what form they would like the product to take. The final design has the flash tube hand held, but equipped with a little wheel so that it can be pulled around the edge of the windscreen and flashed from time to time.

A first batch of 30 prototypes has been made for workshop use. The next stage is the development of portable battery operated units for roadside use. In order to provide sufficiently high storage capacity and discharge rates, PDD has plumped for nickel hydride technology. Lithium, the only viable alternative, is considered too un-green in this day of increasing environmental concerns. The battery pack and electronics are mounted on a two wheel trolley unit.



As well as reducing the time required to replace a car windscreen and the risk of accidents or damage to the car, the units have appeal for other purposes. In the removal of windscreens, they are proving to be of interest to: those expecting to be responsible for dismantling cars for recycling, the car repair industry, and fire brigades responsible for removing victims from cars involved in road accidents. In trains, there is interest in both using the flash tubes to de-bond windshields for replacement, and also, possibly, to accelerate the cure process when bonding them in. Light activated adhesive curing is becoming of increasing interest in a wide range of industries, particularly in electronics, and there is always a market for something capable of speeding up the process.

If the flash tube is flashed on a newspaper page, it is sufficiently strong to be able to bleach the ink, now that printers have turned to polymer based inks which do not rub off, instead of traditional mixtures based on oil and carbon powder. This has given rise to the idea of using the flash tubes to remove graffiti by breaking down the polymer based vehicles for the pigments. The only practicable technique available at the moment is to use a combination of environmentally evil chemicals and scrubbing.

Following on from the hair removal technique, flash lamps are being seriously considered as a means of treating human scar tissue and burning out tumours. Flash lamps offer an almost instantaneous means of sterilising surgical instruments and devotees of science fiction films will recall the flash lamps used to burn off superficial skin tissue from humans entering hypothetical, high security, biological research laboratories of sinister purpose. Fiction apart, flash lamps are a faster and more efficient way of inputting light energy than alternatives for lithography, an area of increasing interest in moves beyond silicon chip processing to the mass manufacture of micro and nano mechanisms and machines. More information may be found at www.pdd.co.uk/magic .

Design Pointers

Xenon flash lamps are the most efficient and cost effective means of quickly inputting large amounts of non-contact energy into a work piece

Able to lead to the breaking down of polymer bonds, they have great potential both for breaking down polymers and also accelerating light induced curing

Other applications range from rapid surgery to the removal of grafitti

Other ways of non-contact machining and breaking things up

One of the first methods to be studied in the present investigation was the possible use of lasers. Large lasers are certainly good for delivering energy into a very small region, right on target, but they are an order of magnitude less energy efficient than flash lamps and too dangerous to use hand held. Ultrasonics are good for shattering kidney stones and welding plastics, but require good mechanical contact to deliver their energy. Microwaves are good for injecting heat into foodstuffs and work pieces for heat treatment, but insufficiently localised to be used as a precise machining tool. Hand-held, powered mechanical tools, used on a car windscreen seal, are likely to lead to even more operator and inadvertent vehicle damage than the knives and cheese wire used at present.

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