Plastic and metallic components are manufactured with various surface finishes all from shiny and glossy to functional surfaces of different appearances. In this brochure we will inform about the factors that have the biggest impact on the polishability of tool steels and give recommendations on how to obtain the required surface finish on moulds, dies, punches and metallic components/ parts. The most common defects are shown in the Uddeholm brochure “Defect Chart and Hints for High Gloss Polishing of Steel Surfaces” Depending on the application and requirements we can distinguish between two types of surface finishing methods; high gloss polishing and functional polishing.
High gloss polishing
Tools for plastic moulding do require a high surface finish especially when extreme transparency and/or high gloss are aimed for. In such cases it is of utmost importance to choose a proper tool material and establish a suitable surface preparation technique. To achieve a reflective surface with mirror finish the preparation process must involve several grinding and diamond polishing steps and these have to be performed in a clean workplace. The use of proper working tools facilitates the process a lot. High surface finish reduces the risk of local corrosion and fracture or cracking due to temporary over loading or pure fatigue. The tool surface finish may also have an impact on productivity as in the case of injection moulding. Here, the release forces of the plastic component from the tool steel surface are dependant on the adhesion properties of the polymer to the mould surface. An improved smoothness of the tool surface may lead to higher release forces and eventually to sticking phenomena, which partly can be overcome by an optimal choice of tool steel and preparation strategy.
Most cold work applications do not need high gloss polished tool surfaces, but it is always advantageous to create functional surfaces for a prolonged tool life. In forming operations where lubricants are involved a preparation strategy may consist of removing larger peak formations on the surface and preserving a controlled depth of valleys as lubrication pockets, which then will contribute to a reduced friction during forming. However, it is always important to consider the final tool steel surface quality in relation to the application. If a high quality surface coating is going to be applied, then it is always recommended to perform high gloss polishing of the tool surface before the coating process.