Optics are driving innovations for various products such as ophthalmic and automotive where ultra-precision moulding is needed to replicate such polymer optics (aspherical lenses, LED car headlights, etc.). The highest market share in the precise moulding production lies in the injection moulding. The inserts used in the injection moulding as well as the mould itself are preferably made of steel, because of two important properties the material possesses:
- high durability; a steel mould insert lasts longer and is more resistant to damage and wear than most materials used for moulding. This helps decreasing the cost of manufacturing products deriving from moulding.
- uniform thermal expansion; the temperature of the injected material, i.e. polymer, is uniformly distributed inside the entire mould. This has a decisive impact on the end product in form of high surface quality, high shape accuracy and the absence of defects.
The production of steel mould inserts with high surface quality (roughness of a few nanometres) and high form accuracy (sub-micrometre level) is, however, far from trivial. Currently, the injection moulding industry invests in expensive steel mould inserts or uses non-ferrous ones with inferior properties. The main reason behind the high costs of steel mould inserts lie not only in the high resources, energy and labour costs, or in finding the adequate steel alloy for the required application, but also in the many steps that are required in machining high standard steel mould inserts. Those steps start by rough machining (turning or milling) the steel, followed by up to five steps comprising hard turning, lapping and manual polishing.
UP STING aims at tackling former mentioned challenges by drastically reducing the number of steps needed in the production of high quality steel mould inserts with optical surface finish, by testing novel steel alloys or non-metallic substitutes and by automating the polishing procedure currently achieved manually.
The project is constituted of two approaches:
- Approach 1 is based upon preliminary work of the Fraunhofer IPT in high precision turning of steel on a Ultra Precise turning lathe with Cubic Boron Nitride (CBN) tools to obtain a surface roughness Ra = 40 nm and a form accuracy PV < 2 μm, followed by fine polishing using an automated robot polisher to achieve an optical surface with an Ra < 10 nm and PV < 1 μm. For the current state of the art in high precision machining of steel using CBN tools, the surface quality of the machined surface has, at best, a roughness of Ra = 100 nm. The machined surface must undergo at least four steps of lapping and polishing in order to achieve an optical surface quality. Therefore, Approach 1 differs from the current state of the art not only by a drastic reduction of the steps needed for high precision machining from more than five to a maximum of two, but also by replacing the manual polishing procedure with a fully automated one. Different process parameters, as well as new steel alloys will be also tested for optimized machining and polishing results.
- Approach 2 is highly innovative and involves direct turning of steel with diamond tools while applying an electrical voltage between the steel work piece and the diamond tool. The reason behind applying an electrical voltage is to inhibit the chemical reaction that occurs when machining steel with a diamond tool and, respectively, causes catastrophic wear to the diamond tool. In Approach 2, both, the chemical reaction taking place during machining and the effects of applying an electrical voltage, will be modelled using computational simulations. Subsequently, the simulations will be complemented by a series of tests involving machining various steel alloys with diamond tools while applying a defined electrical voltage based on the simulation results. This approach is highly efficient, because it reduces the multiple steps of machining steel moulds to one without the need for polishing, while still obtaining a high surface quality and form accuracy. The approach is not only limited for turning, but can be also deployed for milling.
- Fraunhofer-Institut für Produktionstechnologie IPT
- Innolite GmbH
- Mateck GmbH
- Protoshop Oy
- VTT Technical Research Centre of Finland
M. Eng. Daniel De Simone
Fraunhofer Institute for Production Technology IPT
Phone +49 241 8904-279
Fax +49 241 8904-6279
Dipl.-Ing. (FH) Dorothee Weisser
Phone +49 721 608-26150
Fax +49 721 608-25456