Engineering Properties

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Vibratory Finishes

A vibratory finish is commonly given to zinc alloy diecastings, it serves to even up the reflectivity of the surface and remove any sharp edges remaining after the runners and any overflows have been removed. A variety of surface appearances can be achieved varying from bright to matt by altering the materials used in the vibratory bowl or trough. Bright finishes are achieved by burnishing with such material as stainless steel balls or shapes or with walnut shell and this involves little or no metal removal. Metal removal is achieved utilising plastic cones or balls which incorporate abrasive materials, the process being carried out with a continuous flow of water-based media that serves to accelerate the process and remove the detritus. The coarser the abrasive content of the plastic cones the duller the surface finish and the faster the metal removal rate, all else being equal. Obviously, the faster the process the lower the cost.

As far as the casting designer is concerned the main concerns that should be addressed are the potential for the finishing materials to become stuck in the casting, (so called “lodgement”) and allowing for the surface impacts that inevitably happen when castings are handled en masse. In the latter case this means ensuring that critical dimensions are not prone to be damaged by the collision of one casting on another, this is usually achieved by ensuring that critical features are shielded from impacts by non-critical ones. Minimising resulting minor surface dents on non-critical features is the business of the diecaster who needs to avoid overloading the vibratory equipment and ensure that the handling is as gentle as is practical. The avoidance of lodgement is a subject for discussion between designer and diecaster.

It should be noted that vibratory finishing is not generally useful for removing flash, as this will often tend to be bent over rather than removed. Flash can be removed by thermal deburring or cryogenic bead blasting but these processes are not widely available and not universally applicable. Therefore the best answer is to avoid producing it or to ensure that it is reliably removed by a clipping operation.

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