Screw Coating for Optical Applications
Plastic components for optical applications, e.g. for eyeglasses or cameras, must satisfy extremely high quality requirements. If the melt adheres to the screw during processing, the residence time of the material is extended. This subjects the moulding compound to greater thermal stress and degradation that leads to undesirable discolouration. Some of the discoloured material is picked up by the melt stream and is carried into the injection moulded component. This can result in part defects such as streaks and spots.
To prevent such problems from the very beginning, adhesion of the melt to the screw must be reduced. For this purpose, the Institute for Surface Technology (Institut für Oberflächentechnik/IOT) at the RWTH Aachen has developed a number of coating systems. At the Institute for Plastics Processing (Institut für Kunststoffverarbeitung/IKV), optical components were injection moulded using screws with various coatings and subsequently examined. The best results were obtained with a ceramic coating system containing chromium-aluminium-oxygen-nitrogen (CrAlON).
Using a screw coated completely with CrAlON, the IKV moulded plastic lenses from a cycloolefinic copolymer (COC). A camera-based optical inspection system examined these parts for optical defects. Compared to uncoated screws, the number of spots produced during processing was reduced significantly.
The arc ion plating PVD process, where the coating material is vapourised by means of an arc and ionised to a great extent, was employed to coat the screws. The material vapour subsequently condenses on the substrate. With the aid of this process, screws several metres in length can be coated.