Time for a little bonding

Written by: Tom Shelley | Published:

Two techniques developed at Fraunhofer Institutes could lead to stronger glasses that are made more quickly and cheaply. Lou Reade reports

Rimless spectacles will benefit from a new joining process, which uses lasers to form a better bond between the metal arm and plastic lens.
However, it is likely that many other applications in which metals are joined to plastics might benefit from the technique – such as parts for mobile phones or components in PVC window frames.
The process, Liftec, was developed by researchers at the Fraunhofer Institute for Laser Technology (ILT) in Aachen, Germany, and unveiled at the recent Productronica exhibition.
A laser heats the end of the metal pin that attaches the arm to the lens. The beam penetrates the transparent plastic lens without damaging it and heats the metal pin to a temperature above the melting point of the plastic.
“The heat radiated by the pin melts the surrounding material and mechanical pressure is applied to push the metal part into the plastic,” says ILT engineer Jens Holtkamp. “As the joint cools, a positive bond is formed between the two parts.”
There is no need for the traditional clamp mechanism that is normally used to mount the lens to the arm.
The stability of the bond depends on the shape of the components. To achieve a firm connection between the two materials, the metal part was prepared with a pin, groove or drill-hole.
By using a pyrometer to measure the heat radiated by the pin, the researchers could control the temperature to ensure that components were not overheated – and therefore exposed to the minimum of stress.
The process can also join plastics to ceramic materials, or join two different types of plastics – as long as the components have different melting points.
“In the case of eyewear, the process gives designers greater scope to exercise their creativity – they can attach the arms at any point, in a variety of ways,” says Holtkamp. “Other possible applications include the hinges on cell phones, or the joining of PVC window panes or wall panels to metal frames to form a stable, impermeable unit.”
A patent application has been filed.

Faster glasses
Fraunhofer’s expertise in spectacles also extends to the lenses. Researchers at the Fraunhofer Institute for Mechanics of Materials (IWM) in Freiburg have used hot embossing to make glass lenses. They claim it is 10 times faster – and far cheaper – than the traditional method of precision moulding.
Again, the breakthrough could be transferred to other areas, because devices such as cameras, car headlamps and optical sensors for machine control also use glass lenses.
“We were able to cut the cycle time to less than one minute,” says Peter Manns, who leads the IWM research team. “This saves production time and is cheaper than precision moulding. The cost of the lenses drops to less than 30%, at comparable quality.”
Lenses are moulded on both sides. Each surface is of high optical quality and there is no need to post-process the lenses, says Manns.
In precision moulding, the glass is heated with the forming die and pressed into shape, and then cools while still under pressure in the die. This takes about 10 minutes. In hot embossing, the glass is raised to a higher temperature, but the dies are around 10ºC lower – allowing controlled heat exchange during embossing.
But there is a careful balance to be made regarding cooling speed. “If the glass is cooled too quickly, thermal stresses occur and the quality is diminished,” says Manns. “If the glass cools too slowly – because the die is too hot – the moulding process takes too long, which pushes up the costs.”
The method has been tested on a laboratory scale. The next step is to scale it up to a pilot plant.

Liftec process, which uses lasers to join dissimilar materials, such as metals and plastics, could be used on rimless spectacles, mobile phones and PVC window frames

It could also join plastics to ceramics – or two plastics with different melting points

Glass lenses could be made 10 times faster and 70% cheaper by using hot embossing, rather than precision moulding

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