Mold Steel for Injection Molding

No. Hardness buys resistance to wear and to indentation, not resistance to every failure mode. A harder block is more expensive to machine, harder to polish and more prone to cracking at a sharp internal corner or under a rough EDM recast layer. For a short cosmetic campaign in an unfilled resin, a pre-hardened grade at roughly 30 HRC with good polishability will outlast a badly heat-treated 50 HRC tool because it will not crack.

Avoid the softest pre-hardened grades on the gate and the vent land, which is where the glass hits first. A 30 HRC P20-type block will wash out at the gate long before the cavity shows wear. Move to a hardened grade such as H13 at 48-52 HRC, or a wear-resistant stainless if the resin also releases corrosive products, and expect the gate insert to be the part you replace.

PVC releases hydrogen chloride as it degrades, and that combines with moisture to form hydrochloric acid inside the tool. A standard pre-hardened steel will corrode in the water lines and on the cavity surface, and the corrosion pits print through on the part. A martensitic stainless such as S136 or 2316, hardened and polished, is the normal answer.

When the campaign is short, the geometry is simple, the resin is unfilled and the part is not optical. Aluminium machines quickly, so a bridge tool can be cut while a steel tool would still be in roughing, and that is its real advantage: time to first samples rather than cost per shot. It is the wrong call for abrasive fillers, high clamp pressures or a tool that has to survive more than a short pilot run.

The 3D part, the resin grade including any glass or mineral loading, the annual volume and the expected campaign length, the appearance requirement, and the number of cavities. With those five we can answer with a grade, a hardness, a surface treatment and the shot life we would expect to run to - and say which of the five drove the answer.