The unit cost of the casting process is inversely proportional to the rate of production of good castings. Casting rates achieved in practice with zinc alloys vary from less than 200 to more than 4000 shots per hour. This latter figure is only relevant to miniature parts cast using specialist equipment. The maximum casting rate achieved using conventional dies is around 1000 shots/hour.
The productivity of zinc alloy diecasting is higher than those achieved with aluminium or magnesium alloys. Aluminium is normally only diecast via the cold chamber process, which is inherently slower than the hot chamber process used for zinc. Magnesium is also hot chamber diecastable but productivity is lower than for zinc because regular production stoppages are necessary to remove oxide build up from the die. Following stoppages of more than minimal time it is usually necessary to reheat the die by making several scrap castings. Pressure diecasting is generally faster than plastic injection moulding. Injection moulding times are especially sensitive to the materials section thickness. Increasing the wall section of a plastic moulding in order to compete on strength or stiffness terms with a diecasting not only increases the material cost in line with the wall section but also increases the moulding cost to an even greater degree.
For diecasting anything that helps to reduce the shot to shot cycle time will reduce costs. Usually the limiting factor is the time taken after injection of the molten metal for the complete shot including all runners to freeze and cool to a temperature at which it can be ejected from the die without distorting. During a steady casting run the factors which influence this time are chiefly the thickness and geometry of the casting and the runners, the die cooling system, and the layout of the ejector pins. Casting design has the most critical effect on casting rate, thin even wall sections providing the fastest rates, providing that the part is strong and stiff enough to resist the ejection forces required. Higher speeds can be achieved by increasing the die cooling but this is limited by an increasing tendency for cold lapped finishes to occur as the die temperature drops. If surface finish is an issue then the number of “good” castings will begin to fall as die temperature drops below a critical level. However this lowest usable die temperature will vary from casting to casting and from one type of machine to another. Prediction of the lowest usable die temperature is possible using computerised runner and gating techniques.
Uninterrupted production of good castings is also a key requirement to keeping costs down and the things that militate against this are chiefly the reliability of the die and the machine. Probably the most common cause of stoppages is all, or part, of a shot sticking in the die, which not only calls for the machine to be stopped for it to be manually removed but it if the removal takes any appreciable time it may mean that the die will require to be warmed up again by making several reject shots. There are a variety of causes of castings sticking in dies, but good casting and die design and high quality toolmaking will eliminate most.
Automatic vs Semi-Automatic Production
Diecasting machines can operated either semi or fully automatically. In the former case the operator removes the complete shot from the die. In the latter case the diecast shot can be dropped from the die or removed by a robot. Where the casting is dropped from the die it often falls into a quench tank from whence it is removed by a conveyer. The water tends to break the fall and prevent damage to the casting, however there is still a reasonably small fall from the conveyer into a stillage or box to contend with. Therefore the casting designer should make sure that any critical features are not vulnerable to damage during this fall, if necessary by adding protective features to the casting. The decision whether to aim for semi or fully automatic casting is one for the diecaster, but fully automatic is the norm for high volume manufacture because the increased set up costs are more than offset by saving in labour cost and the improvement in product uniformity.
Cost Versus Alternative Materials
Generalized cost comparisons between alternative manufacturing techniques are inherently difficult. The costs of the individual methods and materials vary over time and factory costs are dependent on geographical situation. If cost comparison is difficult, generalized price comparison is impossible because this involves the local supply and demand balance for each of the manufacturing techniques. Nevertheless there follows a generalized rough cost comparator for zinc, aluminium and magnesium diecasting, and plastic injection mouldings. The specific materials included in the comparison were:
- Zinc alloys ZP3, ZP5, ZP2 and ZP8
- Aluminium alloy A380 (equivalent to EN1706 AC46500)
- Magnesium alloy AZ91D
- Injection moulding plastics ABS, PA66, PA66 30%GF, Polycarbonate, Polycarbonate 30%GF, Polypropylene, Acetal Copolymer, Acetal Copolymer 30%GF, Polysulfone, Polysulfone 30%GF