Injection Mold Steel Selection: Hardness, Wear, Corrosion

Mold steel is usually chosen on one number: hardness. That single number is the reason tools crack, stain, lose their polish or wear out halfway through a program. Hardness tells you how well a steel resists indentation — it tells you very little about whether the tool will survive the resin you are about to run, at the volume you have promised.

Figure 1. Injection Mold Steel Selection: Hardness, Wear, Corrosion.

This guide sets out the five factors that actually decide steel selection, then compares the grades you will meet in almost every quotation.

1. The Five Selection Factors

### 1.1 Wear resistance

Abrasive fillers are the main wear driver. Glass fiber, mineral filler and carbon fiber act like a grinding paste on the gate, runner and cavity surfaces.

• Unfilled resins (PP, ABS, PS): low wear, standard pre-hardened steel is fine.
• 20–30% glass-filled (PA6-GF30, PBT-GF30, PPS-GF40): high wear. Hardened or nitrided steel at the gate and high-flow areas.
• Highly filled or flame-retardant grades: very high wear. Consider hardened inserts, or a change of gate location to a replaceable insert.

Rule of thumb: the higher the filler content and the higher the melt temperature, the more you need through-hardened steel rather than a pre-hardened one.

### 1.2 Corrosion resistance

Corrosive attack comes from three sources:

• Resin chemistry: PVC, POM, PBT and flame-retardant grades release aggressive by-products.
• Moisture and condensation: water in the cooling channels and humid plant air.
• Mold release agents and cleaning chemicals.

If any of those apply, a stainless or corrosion-resistant grade (S136, 420SS, or a pre-hardened steel with a high chromium content) is not a luxury — it is a maintenance decision. A rusted cavity plate produces surface defects on every part and cannot be polished back indefinitely.

### 1.3 Polishability and surface finish

For a class-A cosmetic part, the steel has to take a mirror finish and hold it through millions of shots.

• Best polishability: S136 (ESR), NAK80, 718H, 738H.
• Poor polishability: high-carbon, high-alloy tool steels with coarse carbides.
• Texture (MT/VDI/YS) parts need consistent hardness and a fine microstructure so the etch is even.

### 1.4 Toughness

Hardness always trades against toughness. Very hard steel resists wear but cracks under impact, sharp corners and thermal cycling.

• Deep cavities, thin ribs, tall cores and sharp internal corners need toughness more than hardness.
• Stress-relieve after rough machining, and specify generous radii — most cracked tools crack at a sharp corner, not in the middle of a plate.

### 1.5 Heat treatment and dimensional stability

• Pre-hardened (delivered around HRC 30–36): no post-machining heat treatment, low distortion, fast delivery. The default for the majority of injection tools.
• Through-hardened (HRC 48–54): maximum wear resistance, better for glass-filled and high-volume work, but requires a heat-treatment step and dimension control afterwards.
• Nitrided or PVD-coated: a hard surface layer over a tougher core. Excellent for gates, slides and lifters, where you want a hard skin without losing core toughness.

2. Grade Comparison Table

3. A Practical Decision Path

Work through these four questions in order:

1. Is the resin corrosive or flame-retardant? If yes → stainless or corrosion-resistant grade (S136, 420SS). Stop here; do not compromise.
2. Is the resin abrasive (glass/mineral filled)? If yes → through-hardened steel at the gate and flow areas, or hardened inserts in a pre-hardened plate.
3. Is the cosmetic requirement class-A or optical? If yes → NAK80 or S136 for polishability, and specify the polishing direction on the drawing.
4. What is the tool life target? Under 100,000 shots → a good pre-hardened steel is usually sufficient. 100,000–500,000 → 718H/738H or hardened inserts at wear points. Over 500,000 → through-hardened steel (H13 class) with proper heat treatment and stress relief.

4. Where Hardness Differences Bite

A 5 HRC difference looks small on a datasheet. In service it is not:

• Wear rate in abrasive service is highly non-linear — a few points of hardness can halve or double gate life.
• Hardness consistency matters as much as the average. A poorly heat-treated plate with 5 HRC variation across its face will polish and texture unevenly.
• Hardness cannot be checked after the tool is polished. Specify the hardness on the drawing, verify it on the steel certificate, and verify it again after heat treatment before final machining.

5. Supporting Specifications to Write on the Drawing

• Steel grade and hardness range — not just "hardened steel".
• Heat treatment process, including stress relief after rough machining.
• Corrosion protection for any non-stainless exposed surface, including storage and shipping.
• Polishing requirement and direction for cosmetic surfaces.
• Dimensional tolerance after heat treatment — heat treatment moves steel, so finish machining must follow.
• Insert strategy — which areas are replaceable, and how.

6. Cost vs Life — Framing the Trade-off

Paying twice for the tool to avoid a mid-program rebuild is almost always the cheaper decision — provided the volume is genuinely there. If the program may be canceled or the design may change, buy a pre-hardened tool and accept the wear.

Figure 2. Quick reference: the key checks and reference values from this guide.

Conclusion

Select mold steel by asking what will destroy the tool: abrasion, corrosion, impact, or thermal fatigue. Hardness is the answer to only the first of those, and it is directly opposed to the answer for the third. Write the grade, the hardness range, the heat treatment and the polishing requirement on the drawing — and verify them on the steel certificate, not on the supplier's good intentions.

If you want a second opinion on a steel selection for a specific resin and volume, our tooling engineers will review the drawing and the resin data sheet together.