Surface finish

A high-gloss surface finish may be achieved with proper molding conditions and polished mold cavities. High-gloss polished cavities require careful handling and protection during processing. Mold maintenance needs more frequent attention. Great care should be exercised when removing high-gloss parts from the mold to avoid scratches. The table below provides an indication of the price index for the commonly used surface finish classes according ISO 1302. Price index for various surface finishing classes.

For low gloss, semi-matt or matt surface finishes, the tool cavity needs treatment to obtain fine to very fine textured structures. A matt surface is obtained by vapor blasting techniques. Basic steel roughness should be N3 or better (ra < 0.1 mm). Textured part surfaces have a special visual and haptic appearance, e.g. soft touch. Compared to other surface treatments, textures are relatively cheap. Their popularity is based on: Appearance (wood grain or leather) Functionality, e.g. anti-slip Masking of molding defects Main texturing techniques are: Photochemical etching EDM Engraving Brushing Laser engraving When high quality of textures are expected use a low alloy tool steel with a limited carbon content (< 0.45%). If nitriding is necessary, texturing should precede it. After long periods of use the mold surface deteriorates due to wear. Use of glass fibers will increase abrasion. Frequent checks of the surface condition are recommended. Also differences in mold deposit tendency of materials can cause changes to the (local) mattness of parts, making cleaning at regular times necessary with some materials e.g. old PC/ABS formulations. Semi-crystalline thermoplastics are often less scratch resistant when very fine textures are used. Because of their good flow properties, the mold reproduction is better than that of amorphous thermoplastics. Micro-scopic ridges at the part surface may be easily damaged with a finger nail. For untextured surfaces generally a minimum of 0.5 deg draft per side is recommended although there are exceptions when less may be acceptable. Polishing in draw line or using special surface treatments can achieve this. For textured sidewalls use an additional 0.4 deg draft per 0.1mm depth of texture is recommended.