Zinc alloy diecastings have a number of advantages in this area. Good design freedom allows the stylist a reasonably free rein. Surface sinks are a much smaller problem for castings than for injection mouldings. Features can be sharply defined if necessary and surfaces can be very smooth or textured when required. These qualities can be maintained over long production runs because of the extended tool life available without such problems as heat checking. For many if not all functional applications an as cast finish may prove perfectly acceptable. Frequently however the casting is given a low cost vibratory finish, which simply abrades or burnishes the surface depending on the media used and serves to even up its reflectivity and remove any staining. The appearance of castings finished in this way is quite attractive but they will become duller over time. The surface quality desired for a new casting should be established at an early stage of its design because avoidance of surface blemishes demands special attention to gate size and position.
Zinc diecastings can be finish coated by a wide variety of techniques, which can confer almost any desired appearance on them, further information is given in the section devoted to this subject. The finishing process to be used will often have an effect on the component geometry chosen. For instance castings to be giving gloss or reflective coatings should not have large flat surfaces. Those to be coated by a rack rather than mass finishing process will need features that allow it to be readily attached to the racks. Castings finished by processes requiring immersion in liquid need a geometry that drains rapidly. Special attention should be paid to this for castings to be mass finished because of their random orientation on removal from the liquid.
The positioning of the die parting line has a major effect on the appearance of the casting. It must be remembered that complete removal of parting line witnesses and gate and overflow scars is expensive and time consuming. However by carefully positioning such features they become either less noticeable or more readily removed. In some cases it is better not to try to hide the parting line but instead to accentuate it with a bead. This raised feature tends to be preferentially abraded during a simple vibratory finishing operation and hence leads rapidly to a casting without sharp edges.