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Choosing the Most Suitable Method and Alloy

This is usually a trade off between cost and ability to reproduce the form of the intended diecasting. Where very small numbers of prototypes are required it will often be found that the most suitable approach is to machine the parts from solid billets. However there are two exceptions to this rule, one is when the component’s form is impossible to machine (note: it is possible to spark erode zinc alloys to avoid this problem) and the other when the section of the billet that would be required exceeds about 50mm thick, when it is difficult to ensure that it will be sound. In these cases it will be necessary to make a casting and if the casting needs to have high precision in unmachinable areas then an investment casting is called for, if a plaster mould casting is not available. When larger quantities of parts are needed the investment casting approach becomes more viable in itself and for very complex components will usually be the lowest cost option. The choice between a sand casting or an investment casting depends on the as cast precision required. The extra cost of the investment casting tooling can sometimes be recovered in reduced subsequent machining costs. Remember however that further modified prototypes may be required and that it is easier to modify a pattern for sand casting than a wax injection tool. The previously mentioned proprietary prototype casting methods must be considered as alternatives to investment casting.

An important issue when obtaining prototypes is of course lead-time. It will generally be found that machining from solid offers the quickest option and that the other approaches will involve fairly similar timescales of two to four weeks although much faster times are possible in theory.

The following table is intended as a guide to assist in the selection of the most appropriate prototyping alloy and process to use for any intended zinc alloy diecasting application. Note that the choice is heavily influenced by the tests which the prototype will be subjected to, and hence the properties which are demanded to be similar to the pressure diecastings.

Reference 2

Ductility Min.
Electro- plate Wear Creep Strength Fatigue
Alloys ZP3 & ZP5
Gravity cast in same alloy _ ** _ = ** ** ** **
Machine from billets in same alloy _ _ = = ** ** _ _
Machine from existing diecasting in the same alloy = = = = ** = = =
Gravity cast in ZL12 = _ _ _ + + = =
Gravity cast in ZL27 and HT = = _ N/A + + ** +
Alloys ZP8 & ZP2
Gravity cast from same alloy _ _ _ = ** ** _ _
Machine from billets of same alloy _ _ = = ** ** _ _
Machine from existing diecasting in the same alloy = = = = ** = = =
Gravity cast in ZL12 _ _ _ _ + + + _`
Gravity cast in ZL27 + _ _ N/A + ** + =
Gravity cast in ZL27 and HT = + _ N/A + ** + +

= Prototype should perform approximately the same as diecasting

+ Prototype superior to diecasting

_ Prototype inferior to diecasting

** Properties have not been evaluated for both the prototype and die cast conditions

Reference 21