Meaning
Maximum stress level that a mould steel can withstand before it undergoes permanent, plastic deformation. Permanent damage to the tool occurs if the stress from clamping or injection pressure exceeds this limit. Tool steel yield strength determines the durability of the mould and its ability to maintain tight tolerances over millions of cycles.
Material Selection
Different grades of steel offer a trade-off between toughness and yield strength. P20 steel is commonly used for large frames, while H13 or S7 steels are preferred for cavities and cores that face high pressures. Heat treatment processes like quenching and tempering are used to raise the yield point to the required levels.
Stress Calculation
Designers must ensure that the pressure concentrated on small areas, such as shut off lands or core pins, remains well below the yield limit. The calculation includes the safety factor required to account for the dynamic nature of the injection process. If a core pin is too thin, the injection pressure might bend it beyond its elastic limit, causing it to stay bent.
Failure Modes
Permanent deformation often manifests as hobbing, where a hard part of the mould sinks into a softer plate. This creates a permanent depression that leads to flash or dimensional changes in the moulded part. Regular inspection of the mould surfaces is necessary to catch these signs of failure before they result in a complete production stop.
