Meaning
Resistance to indentation and deformation determines the durability of the cavity and core surfaces. Measuring tool steel hardness is usually done on the Rockwell C scale to confirm the effectiveness of the heat treatment. Higher hardness levels protect the mould from the abrasive effects of glass fibres and the high pressures of injection.
However, extremely hard steel can be brittle and prone to cracking under impact.
Wear Resistance
Sustaining a high-quality surface finish over millions of shots requires a material that does not scratch or erode. Increased tool steel hardness helps the mould maintain its sharp edges and polished finish.
Compressive Strength
Resisting the force of the clamping and injection phases is another function of the material’s temper. If the tool steel hardness is too low, the parting lines will eventually deform, leading to flash. A hardness of around fifty Rockwell C is common for many injection moulding applications.
Machinability Tradeoff
Cutting and finishing the steel becomes much harder as its resistance to deformation increases. Most moulds are machined in a softer state and then heat-treated to reach the final tool steel hardness. Any final adjustments must then be made using grinding or electrical discharge machining.
Surface treatments like nitriding can further increase the hardness of the outer layer without affecting the core properties. Hardness testing ensures that the tool can withstand the rigours of high-volume production.
