Plastic mould steel
Mold steels for plastics are tool steels for injection molds and compression molds. They must machine and polish well, hold tight tolerances, and often be corrosion resistant — depending on the plastic and environment.
Physical-Chemical Mechanism of Action
For mold steel, it's not the maximum hardness that matters, but the surface. Every cavity gets polished, and every inclusion in the steel turns into a small crater that shows up on every molded part. That's why clean steel and a fine, uniform structure matter more than the last few points of hardness. The material is usually supplied already , so it's pre-finished, meaning no heat treatment will mess with the dimensions after milling.
Metallurgical Relevance & Welding Risks
Repairs are done in the quenched and tempered state, and that's where the trouble starts: the weld and the end up with a different structure than the surrounding area. isn't visible at first, but it shows up during polishing because the hard spot wears slower and stays as a shadow or a step. With aggressive plastics containing chlorine or fluorine, the resistance to also decides how long the mold lasts.
Workshop Practice & Action Points
Work out the damaged area cleanly down to bare metal, , then fill with matching filler in thin passes. After that, or temper at the quench and temper temperature, and only then grind and polish. The polish is the final check: anything that shows up as a shadow is a structure difference and has to be reworked.