Comparative Die/Mould Prices
Pressure diecasting tooling is made from high quality tool steels with often complex shapes machined to close tolerances. They are inevitably costly to produce. Dies with side cores and stepped parting lines inevitably cost much more to manufacture. However the casting design features they enable often repay the extra cost many times over in reductions in secondary operations.
The price of zinc alloy diecasting dies usually falls in the range €3,000 to €100,000. Small unit tools with flat parting lines and no slides coming towards the bottom end of the range and large multicavity dies with slides towards the top. The initial cost of dies to be used for any diecasting alloy, zinc or any other, is practically identical. Plastic injection moulds are also of similar cost.
The price of tooling, both dies and press trim tools, is dependent to a great degree on their quality. The quality of the tooling required to make a casting depends on the design of that casting and the qualities required of it. Designs which utilise such attributes as the low draft angles available with zinc in order to avoid subsequent machining operations, will inevitably require well finished dies. The attainment of flash free diecasting demands dies to be made with great precision and carefully maintained.
In an ideal world specifiers would always aim to purchase high quality tooling, but for lower quantities of castings the cost of the tooling will have significant impact on the overall cost of producing the parts and cheaper tooling is indicated. In such cases it is probably best to err on the safe side with casting design eg use more generous draft angles and wall sections, even if this means performing some secondary operations to achieve the required final geometry.
Although dies are made from high quality, engineered tool steel, the injection of molten metal and the subsequent rapid cooling induces thermal shock and cyclic thermal stresses which cause deterioration and ultimately failure of the die steel. The rate of deterioration is a function of the quality of the die steel and quality of heat treatment of the die, the total amount of heat transferred from the molten metal to the die and the associated temperature increase. The high-speed flow of molten metal through the die can cause die steel erosion, called die wash. The amount of erosion on one shot is infinitesimal, but the accumulation over many shots must be recognized. The most severe erosion occurs at locations of high metal flow velocity, sudden changes in the direction of metal flow, and at irregular die features and die sections, where these locations cannot be provided with sufficient cooling. Often, these factors can be mitigated in the design of the product and the die through early die-caster consultation. The most obvious visual effects of die degeneration on the casting are loss of sharp definition of the more intricate details, deterioration of surface quality, and thickening of some sections. As the die degenerates, it must be removed from production periodically and repaired; ultimately, the die cavities, and eventually the entire die, may need to be replaced. Zinc alloys are cast at lower temperatures than other die-casting alloys and hence reduce the thermal fatigue stresses in the die steel that are the major cause of their eventual failure. Consequently zinc alloys offer greater die life and lower maintenance costs than other die-casting alloys. The die life achieved with zinc alloys is equivalent to or better than those for injection moulds used for glass fibre reinforced plastics, because the abrasive nature of these materials more than compensates for the lower thermal stresses they impart to the mould.
Whilst it is not possible to accurately predict die life for any given die design, it is safe to allow for a minimum of 500,000 shots with any of the zinc alloys featured in this publication. There are plenty of examples of dies producing around 2 million shots. A typical die life might be around 1 million shots. Dies that produce aluminium and magnesium diecastings have much shorter lives, typically around 200,000 for aluminium and 400,000 for magnesium. When parts will be required in excess of these numbers the die costs will need to be factored in for these materials when making comparisons with zinc alloy diecastings. Replacing dies during a production run can sometimes lead to delays with resultant late deliveries. Also dies that are reaching the end of their life are more prone to produce unsatisfactory castings necessitating rework. These are the hidden costs of shorter die life, which are largely avoided by specifying zinc alloy.
It is usual for most features of die cavities to be produced using the spark erosion process. The surface finish produced by this method is evenly rough and not unattractive. Frequently this die surface finish is left on the tool and is reproduced on the diecasting (although this is not possible in low draft angle situations, as it would result in castings sticking in the die). However where long die life is needed the rough die surface resulting from spark erosion must be totally removed. This is because spark erosion induces large internal stresses in the surface, which markedly reduces the fatigue life of the material and can lead to early failure of the die.
Productivity from Tooling
Only parts made in very high quantities are likely to require more than one die, because the production rates and die life experienced with zinc alloys are very high.
When very high production rates are demanded in order to fulfil market demands zinc diecastings will often be able to meet the requirement from a single die when alternative materials may need two or at least a die with more individual cavities. Whilst this factor will not affect overall cost it does increase initial costs and introduces quality assurance complications.