Researchers at the Fraunhofer Institute for Laser Technology ILT in Aachen, Germany, have developed a new process that assures a longer life for rimless spectacles. In this process a laser is used to form the join between the metal arm and plastic lens.
Liftec is a new process developed by researchers at the Fraunhofer Institute for Laser Technology 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 claimed to be much more stable. ILT engineer Jens Holtkamp describes the process: “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.” Using this process there is no need for the clamp mechanism that would normally be required.
The stability of the metal-plastic bond depends on the shape of the components. To achieve a firmly seated connection between, the researchers prepare the metal part with a bump, groove or drilled hole. Holtkamp explains: “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.”
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, so long as 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, says Holtkamp: “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. 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, Germany, from 13-16 November 2007 (Hall B5, Stand 355).