Injection mold steel selection: match the steel to the wear mechanism

Start with the wear mechanism, not the trade name

Most wrong steel choices come from treating "wear resistance" as one property. Two mechanisms decide the answer, and they point in different directions.

Abrasive wear is mechanical: hard fillers — glass fibre, carbon fibre, some mineral fillers — slide across a softer steel surface. It appears first where flow is fastest and where the melt changes direction: gates, runners, deflector surfaces, and nozzles. The countermeasure is a harder matrix.

Corrosive wear is chemical: the melt attacks the steel itself. PVC is the classic case, and flame-retardant compounds are the common modern one, because the additives that suppress combustion are aggressive to tool steel. The countermeasure is chromium, not hardness.

A tool can need both. Glass-filled flame-retardant PA6/66 is the standard example, and the list of grades that satisfy both requirements is much shorter than the list that satisfies either one alone.

Wear is also local. It concentrates where flow accelerates or changes direction, so the practical answer is often to put the harder material only where the work happens — a harder insert at the gate, the runner, or a deflector surface — instead of hardening the whole cavity. The same logic applies where steel parts slide against each other, such as slides, lifters, and ejector systems: make the component that is more expensive to replace the harder of the two.

The four families you will actually be quoted

Prehardened is not "cheap steel". The nickel-bearing 1.2738 grades are delivered at the same 290–330 HB as smaller plate and keep uniform strength through sections up to roughly 600 mm. What you save is the heat-treat cycle, not the metallurgy.

Corrosion resistance is a maintenance issue before it is a wear issue. Rust roughens a cooling channel and insulates it, so heat transfer — and cycle time — drifts for the rest of the tool's life.

Hardness follows filler content, then volume

For filled resins, the filler content sets a floor on cavity hardness — these are industry targets used when selecting mold steel for engineering plastics, not absolute rules.

At equal volume fraction, glass and carbon fibre wear a cavity similarly. Geometry matters as much as content: a lower filler percentage can still wear badly in a thin gate or at high injection speed, because wear tracks local flow rate rather than the global recipe.

Volume then decides whether prehardened steel is enough. It is the correct answer for prototype validation, bridge production, and low-to-medium volume on unfilled commodity resins: no heat-treat cycle, late changes still practical at the bench, straightforward to weld or modify. The decision flips with a filled or flame-retardant resin, a multi-year program, an optical or high-gloss finish that must survive the tool's life, or tolerances tight enough that a worn cavity drifts out of specification between maintenance windows.

The choice also shows up in the schedule, not only in the tool price. A prehardened cavity skips the hardening step, the distortion risk that comes with it, and the wait for a heat-treat slot, which is why this family is usually attached to a shorter lead time and to late engineering changes staying practical. Hardening after rough machining inserts a process step between machining and final finishing, and changes after that point are expensive. When two mold quotes differ mainly on steel, they usually differ on schedule as well.

Where finishing, geometry, and coatings override the grade

Polishability, texture, and photo-etching are steel properties, not just finishing effort. Fine, uniform microstructure and low inclusion content polish to a higher gloss and hold a texture more evenly, which is why remelted premium grades appear on lenses, light guides, and textured visible surfaces. Pick the finishing route before the steel, because photo-etching, laser texturing, and high-gloss polishing all have preferred grades.

EDM leaves a recast layer, and how it behaves depends on the grade: on some steels it is soft relative to the substrate and has to be removed before the surface will perform, on others it is thin and polishes out quickly. That difference changes finishing time, so it belongs in the quote discussion.

Thin cores and deep ribs are the exception to "harder is better". Their limiting factor is manufacturing, not wear. A softer, more ductile steel that survives machining is the better choice for a 0.8 mm core — use steel as hard as necessary, not as hard as possible.

• Nitriding raises surface hardness while keeping most of the core toughness, and it is available on prehardened grades — a common way to make a 1.2738 tool viable for a moderately abrasive job. It changes dimensions slightly, so plan it before final finishing.
• PVD coatings such as TiCN, CrN, and WC/C reduce abrasive wear and can improve release and flow. They are the standard answer above roughly 60% fibre content, and they also make sense at lower filler content where geometry focuses flow into one small area.
• Corrosion-resistant coatings supplement a stainless substrate; they do not replace one.

What to write into the mold quote

Normalise competing offers on steel by asking for six items instead of a trade name:

• Material number and standard — for example 1.2738 to ISO 4957 / EN 10027 numbering, not only "P20".
• Delivery condition and hardness — prehardened as supplied, or final hardness after hardening, stated in HB or HRC.
• Which part gets which steel — cavity, core, and inserts can differ, and so can plates, mold base, and wear components.
• Heat treatment and surface treatment scope — who performs it, at what point in the sequence, and whether it is included.
• Expected tool life and the assumption behind it — shots, resin, and cavity count.
• Replacement policy for wear parts — interchangeable inserts, gate area, and hot-runner components.