Tool Steel Selection Guidelines for High Volume Injection Moulding Tooling Baseline Fabrication
High-volume injection moulding baseline tooling demands martensitic stainless or pre-hardened steels matched to polymer abrasiveness and cycle volume.
Retained Austenite
Meaning
Metastable high-temperature iron phases remain trapped in steel microstructures after incomplete quenching from the austenitizing temperature. When present in mould steels, retained austenite can slowly transform into martensite over time or under mechanical stress, causing dimensional instability. This change can result in the mould parts expanding and distorting, which leads to flashing and binding in high-precision injection tooling.
Microstructural Instability
Uncontrolled phase transformation during steel usage can lead to unexpected tool failure and part defects. Over time, retained austenite will decompose into a larger crystal structure, resulting in a volume increase that alters the critical tolerances of the mould. This transformation can be triggered by the high temperatures and pressures experienced during the moulding of high-performance engineering plastics.
This structural shift is particularly problematic in thin-walled moulding where a variation of even a few micrometres in the core can lead to noticeable wall thickness variation in the finished plastic part.
Mitigation Methods
Thermal processing is the primary tool used to minimize this unstable phase. To reduce the levels of retained austenite, heat treaters perform cryogenic cooling immediately after quenching. This sub-zero exposure forces the completion of the martensitic transformation, ensuring that the tool maintains its shape and size throughout its entire production life.
Wear and Performance
While hardness can sometimes be enhanced by specific phase balances, excessive unstable phases promote uneven wear. The soft nature of retained austenite reduces the scratch resistance of the mould cavity, making it more vulnerable to erosion from filled polymers. By eliminating this phase, toolmakers achieve a more uniform hardness that resists the abrasive flow of reinforced resins.