Factors which may influence
the service life of dies

Corrosion in dies for die-casting is defined as the damage caused by the constraint which is created between
the steel of the dies and the molten metal with which it comes into contact. The phenomenon depends mainly
on the temperatures which play a fundamental role on the solubility of the various chemical elements.

Corrosion, just like gluing; is governed by the formation of inter-metallic phases (two metallic elements mixed together with precise proportions which enable a “stacking” of the elements of a crystalline structure which differs from the two initial ones). Furthermore, the formation of hot cracks enables, for example, aluminium injected under pressure, to penetrate into these cracks and to grip onto the dies, thereby damaging their operation.

ABRASION

Abrasion is due to the presence of hard particles which, when in contact with the surfaces of the dies, remove and abrade the materials with which they come into contact. The pressure exerted by the material to be molded and its temperature determine the speed of wear. The most critical geometries of the dies are those which create the greatest rubbing, such as continuous changes of section and sharp edges.
It may therefore be seen that a very high surface hardness of the die will be able to overcome these shortcomings. For example, with the PVD coatings, the hardness can reach 2200 HV and more, which is twice as much as that which may be obtained by nitriding: so this will, without doubt, be able to prevent wear.

The repetitive heating and cooling cycles which the dies undergo during the die-casting cycle results in alternating expansion and contraction. The surface in contact with the metal to be molded heats up and, as it expands, it creates compression stresses and, possibly, plastic deformation. When the die is no longer in contact with the molded piece, the hot surfaces are cooled by air and/or atomised liquids; the steel then contracts producing tensile stresses.

The more frequently this expansion and contraction occurs, the greater will be the progressive damage to the surface. In practice, steels tend to be preferred with a high hot elastic limit and a good fatigue resistance, but, above all, the dies must be heated to a uniform temperature of at least 300 °C before putting it into service. This operation has two advantages: reducing fragility to thermal shock and reducing the thermal gradient between the surface and the core, which is the origin of thermal fatigue.

THERMAL SHOCK

The repeated heating and cooling cycles which the dies undergo during die-casting result in alternating expansion and contraction. The surfaces in contact with the metal to be molded heat and increase in volume, thereby producing compression forces. When there is no longer contact with the molded parts the hot surfaces are often cooled suddenly (this is not recommended when the dies are at a temperature of more than 150-200 °C), forcing the steel to contract by tensile forces.

The dies must be heated to a uniform temperature of at least 300°C before putting them into service. This operation has two advantages: reducing the fragility caused by temperature changes and reducing the thermal gradient between the surface and the core, which is the cause of thermal fatigue.