Choosing the optimum steel grade at the design stage

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The mouldmaker is primarily interested in the machinability of the steel, its polishability, heat treatment and surface treatment properties. The moulder is looking for a mould with good wear and corrosion resistance, high compressive strength etc. The steel types most commonly used are pre-hardened mould and holder steel, through-hardening mould steel and corrosion-resistant mould steel.

Pre-hardened mould and holder steel types

These steels are mostly used for large moulds, moulds with low demands on wear resistance, extrusion dies and high-strength holder plates. The steel are delivered in the hardened and tempered condition, usually within the 270–400 Brinell range. No heat treatment is necessary before the mould is put into use. The surface hardness can be increased in many cases. Pre-hardened mould steel is generally used for large moulds and for moulds with moderate production runs.

Among others, Uddeholm offers the Mirrax 40, a remelted (ESR), pre-hardened stainless steel supplied at a hardness of 40 HRc, offering mould users lower maintenance and production costs. The grade will maintain its finish on moulds that are stored or used in humid environments, without requiring special protection. It also resists corrosion, ensuring consistent thermal conductivity for the entire life of the mould. These steel types are mostly used for long productions runs, to resist abrasion from certain moulding materials, to counter high closing or injection pressures and for high-pressure processes like compression moulding.

Through-hardened steel for long production runs

The steel is delivered in the soft annealed condition. It is usually rough-machined, stress-relieved, semi-finish machined, hardened and tempered to the required hardness and then finish-machined, ground as well as often polished or photo-etched.

All winners on stage. Organised by the Fraunhofer Institute for Production Technology and the WZL tooling laboratory at RWTH Aachen University, the "Toolmaker of the Year" award is given annually to the top toolmaking shops in Germany, selected by a jury. (Source: Susanne Schröder)

Through-hardened steel, used for cavity and core inserts, is usually placed in holder blocks of pre-hardened steel, such as Uddeholm Holdax, Uddeholm RoyAlloy or Uddeholm Ramax HH. By using through-hardened mould or cavity inserts, e.g., in the 48–60 Rockwell C range, one obtains better wear resistance, resistance to deformation and indentation and better polishability. Better wear resistance is especially important when filled or reinforced plastic materials are used. Resistance to deformation and indentation in the cavity, gate areas and parting lines helps to maintain part quality. Better polishability is important when high surface finish is required on the moulded part.

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