Meaning
Material resistance measurements indicate the ability of a metal to withstand indentation and wear during high-pressure injection moulding or extrusion cycles. Tool steel hardness hrc utilizes the Rockwell C scale to quantify the tempered state of an alloy, determining its durability and susceptibility to cracking. A diamond cone is pressed into the steel under a specific load, and the depth of the penetration is measured to provide a numerical value.
For plastic moulds, this value typically ranges from thirty to sixty depending on the application and the type of resin being processed. Proper hardness is necessary to maintain the tight tolerances and surface finish of the moulded parts over millions of cycles.
Scale Utility
Precision in selecting the right hardness level balances the need for wear resistance with the requirement for toughness. A very hard steel at sixty hrc will resist the abrasive effects of glass-filled resins but may be prone to brittle failure or chipping. Conversely, a softer steel at thirty-five hrc is easier to machine and more resistant to impact but will wear down faster at the parting lines.
Moulders must specify the hardness based on the expected life of the tool and the abrasiveness of the polymer.
Heat Treatment
Thermal processing is used to achieve the desired mechanical properties after the initial machining of the tool is complete. The steel is heated to a high temperature and then quenched and tempered to lock in the crystalline structure. This process must be carefully controlled to prevent warping or internal stresses that could lead to failure during production.
Each alloy has a specific heat treatment profile that determines its final hardness and stability.
Wear Resistance
Surface longevity is the direct result of maintaining the correct hardness throughout the production life of the mould. As the polymer melt flows into the cavity, it creates friction that can erode the steel over time. High hardness levels prevent this erosion from changing the dimensions of the part or creating flash at the seams.
Regular inspection of the tool surface helps in identifying when a mould needs to be re-hardened or repaired to maintain quality standards.
