Anyone who wears glasses knows the problem all too well: You only need to bump into something or pull a sweater over your head without paying attention and the damage is done. Rimless eyewear is particularly prone to such accidents, which can loosen the metal arms. A possible solution is LIFTEC, a new process developed by researchers at the Fraunhofer Institute for Laser Technology ILT in Aachen, for which a patent application has been filed. Plastic-metal bonds produced using this process – for instance between the lens and arm of a pair of spectacles – are much more stable than before. ILT engineer Jens Holtkamp describes the process as follows: “We use a laser to heat the end of the metal pin that attaches the arm to the lens. The laser beam penetrates the transparent plastic lens without damaging it. When the light meets the metal pin, it heats it to a temperature higher than the melting point of the plastic. The heat radiated by the metal pin melts the surrounding material, and mechanical pressure is applied to push the metal part into the plastic. As the joint cools down, a positive bond is formed between the two parts.” There is no more need for the clamp mechanism, similar to that of an earring, formerly used to mount the lens to the arm. The stability of the metal-plastic bond depends on the shape of the components. To achieve a firmly seated connection between metal and plastic, the researchers prepare the metal part – such as the pin in a spectacle frame – with a bump, or a groove, or a drill-hole. “We also use a pyrometer to measure the heat radiated by the pin. This enables us to precisely regulate the joining temperature in accordance with the type of material, ensuring that the components are not overheated, and are thus exposed to the minimum of stress,” the expert explains. As well as joining plastics to metals, the new process can join plastics to ceramic materials. It is also capable of joining two different types of plastic, on condition that one of the two components has a higher melting point than the other, for instance an epoxy resin or Teflon. LIFTEC offers a multitude of options: “In the case of eyewear, the new process gives designers greater scope to exercise their creativity – they can attach the arms at any point in a variety of different 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.” The new process is being presented at the Productronica trade show in Munich from November 13 to 16 (Hall B5, Stand 355).
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