Choose mold steel by matching each tool component to the resin, fillers, required part finish, planned production, component size, heat-treatment route and maintenance strategy. A single “premium” grade is not automatically the best answer, and a steel name alone cannot guarantee a fixed number of molding cycles. The practical goal is a tool specification that controls wear, corrosion, polish, dimensional risk and lifetime cost at the locations where each issue matters.
For a useful supplier recommendation, provide the exact resin grade and additives, expected lifetime quantity, cavity count, cosmetic zones, dimensional requirements, molding environment and replacement-insert plan. Then ask the moldmaker to identify the steel, supply condition, heat treatment and hardness for the mold base, cavity, core and high-wear inserts separately.
Start with eight project inputs, not a preferred steel name
- Resin grade and additives: record the supplier grade, glass or mineral content, flame-retardant package, colorant and any corrosive processing by-products. “Nylon” or “PC” alone is not enough.
- Required accepted parts: distinguish a launch forecast from the required tool-life quantity. An initial cycle estimate is approximately accepted parts divided by cavities and expected good-part yield, plus an allowance for trials, start-up and maintenance. It is an estimate, not a mold-life guarantee.
- Part surface: identify optical, gloss, appearance, texture, sealing and low-friction zones on the drawing. The steel cleanliness and the polishing or texturing route must suit those zones.
- Part geometry and local load: thin cores, sharp shutoffs, deep ribs, slides and small inserts can need more toughness or a replaceable design than a large cavity surface.
- Tool size and section thickness: a grade that performs well in a small insert may not harden uniformly or remain dimensionally stable in a large block. Check the manufacturer’s size limits and heat-treatment data.
- Cooling and molding conditions: melt temperature, mold temperature, pressure, gate velocity, cooling-water quality and storage conditions can change wear and corrosion risk.
- Repair plan: define whether polishing, welding, re-machining, coating renewal or insert replacement must be possible at the production site.
- Evidence: require the ordered producer grade or an approved alternative, a material certificate for critical components, the final heat-treatment/hardness record and traceability to the component drawing.
Match the resin and local duty to the failure mode
Abrasive compounds
Glass fibre, mineral and some flame-retardant systems can accelerate wear, especially at gates, runners, shutoffs, slides and sharp changes in flow direction. Do not respond by hardening every plate without analysis. A tougher base material with harder, coated or replaceable local inserts may reduce repair cost and the risk of cracking. The final choice depends on filler type and percentage, flow path, pressure, feature geometry and the planned maintenance interval.
Corrosive processing or storage conditions
PVC and some halogen-containing or chemically aggressive compounds can create a corrosion risk, particularly when processing is unstable or the tool is exposed to moisture. Stainless mold steel may be justified for cavities, cores and cooling-sensitive regions, but the exact grade and heat treatment still determine wear, toughness and polishability. Corrosion-resistant steel also does not remove the need for correct processing, venting, cooling-water control and storage protection.
High-cosmetic and optical surfaces
A mirror-polish callout requires more than a nominal steel family. Steel cleanliness, inclusion content, block homogeneity, machining history, EDM recast removal, heat treatment and the polishing process all affect the result. The final plastic appearance also depends on resin, color, fillers, geometry and molding conditions. Specify a visible-zone boundary and an approved sample instead of assuming that a steel grade by itself guarantees an optical part.
Select steel by mold component
| Tool region and duty | Candidate steel family or producer-grade example | Why it may fit | Limitations and items to verify |
|---|---|---|---|
| Mold base, support plates and moderate-duty large cavities | Prehardened P20-family mold steel; for example, Uddeholm Impax Supreme, sold as AS 718 Supreme in specified Asian markets | Supplied prehardened; avoids the distortion risk and schedule of through-hardening after machining. The manufacturer lists a 290–330 HB supply range for this specific product. | Not inherently corrosion resistant and not a universal answer for severe abrasive wear. Confirm block-size hardness uniformity, polish requirement, exact producer grade and certificate. |
| Cosmetic cavity or core requiring a prehardened machining route | Daido NAK80 | Daido identifies NAK80 as an approximately 40 HRC prehardened plastic-mold steel with machining, polishing, texturing and repair-welding characteristics. | NAK80 is a Daido product name, not a generic substitute for every P20-type grade. Confirm corrosion exposure, weld procedure, achievable finish on the actual geometry and local availability. |
| Corrosion-sensitive or high-polish small and medium inserts and cores | Stainless mold steel; for example, Uddeholm Stavax ESR | Uddeholm describes Stavax ESR as a stainless mold steel combining corrosion and wear resistance with excellent polishability and stability in hardening. | The manufacturer associates this product with W.nr 1.2083/AISI 420, but that does not make every 420-type or supplier trade grade interchangeable. Confirm heat treatment, section size, toughness, polish class and approved alternative rules. |
| High-load, hot or abrasive local inserts | H13-family hot-work tool steel, such as Uddeholm Orvar Supreme, or another producer grade selected for the actual duty | Can provide a useful balance of hot strength, toughness and wear resistance after a controlled heat treatment. Hardened local inserts can be replaced without remaking the entire cavity. | Final hardness must be selected with the steel producer and heat treater; excessive hardness can reduce toughness. Review sharp corners, EDM condition, tempering, coating compatibility and crack risk. Do not attach a guaranteed shot count to “H13” alone. |
| Gates, shutoffs, slides, wear plates and other service items | Replaceable hardened, nitrided or coated inserts chosen for the local contact and lubrication condition | Concentrates premium material and maintenance effort where wear occurs. | A coating cannot compensate for an unsuitable substrate, poor support or incorrect clearance. Specify replacement access, spare quantity and inspection limits. |
Important equivalence note: AISI, DIN/W.nr and supplier trade designations can indicate a useful family relationship, but chemistry limits, remelting route, cleanliness, heat treatment and guaranteed properties may differ. Put the approved producer grade and substitution process on the purchase specification rather than writing “or equivalent” without acceptance criteria.
Hardness is a design variable, not a quality score
Higher hardness can improve resistance to indentation and some wear mechanisms, but it can also increase machining difficulty, repair complexity and sensitivity to cracking. Toughness, compressive strength, steel cleanliness, heat treatment and component geometry are equally important. The correct value is therefore a component-level engineering decision.
For through-hardened components, the quotation should identify the steel producer, incoming condition, heat-treatment sequence, target hardness range, test locations and distortion allowance. For prehardened blocks, confirm the producer’s guaranteed range and whether it remains uniform across the ordered section. A portable hardness reading on one accessible face is useful evidence, but it does not replace material traceability or a documented heat-treatment process.
Do not buy “mold life” as a steel-only promise
Mold life is affected by steel and hardness, but also by tool design, wall support, shutoff angle, alignment, venting, gate velocity, cooling, resin contamination, preventive maintenance and repair quality. A statement such as “H13 lasts one million shots” removes these conditions and is not a defensible purchasing specification.
Instead, define:
- the required accepted-part quantity and expected production window;
- the resin grade, filler percentage and regrind policy;
- critical wear locations and measurable maintenance limits;
- planned inspection intervals and permitted repair methods;
- replaceable inserts and recommended spares;
- the supplier’s responsibility if the tool does not meet the agreed acceptance criteria.
Surface finish and steel selection must be reviewed together
The part drawing should identify the required finish by zone. A polished cosmetic surface, a dry-blasted mold finish and a proprietary etched texture involve different preparation and inspection routes. Review accessibility, draft, EDM stock removal and repair blending before steel is ordered. For the finish callout itself, see the SPI mold finish selection guide.
If a supplier proposes a substitute grade, ask how the change affects polish, etching response, welding, corrosion resistance, heat treatment and the availability of a matching repair insert. A lower material price can be reasonable, but it should not silently transfer finishing or downtime risk to the buyer.
A practical mold-steel review checklist
- Is the full resin grade, filler level, flame-retardant system and color known?
- Is the required accepted-part quantity separated from an optimistic sales forecast?
- Are mold base, cavity, core, slides, gates and replaceable wear inserts specified separately?
- Are cosmetic, optical, sealing and textured zones marked on the drawing?
- Does the proposed grade suit the component size and planned heat-treatment route?
- Are hardness range and measurement locations stated rather than implied?
- Are welding, polishing, coating renewal and spare-insert requirements documented?
- Will material and heat-treatment evidence be included in the tool file?
- Has total cost been compared across purchase price, machining, heat treatment, maintenance, downtime and replacement?
What to send for a supplier recommendation
Send the 3D model and drawing, annual and lifetime demand, target cavity count, resin datasheet, cosmetic specification, expected molding location and any mandatory steel or traceability requirement. During mold design, ask for a component-level steel schedule and the reason behind each selection. Before shipment, connect the certificate and hardness records to the mold testing and validation package.
For a project-specific steel review, request an engineering quotation with the resin and production information above. The recommendation should be confirmed against the selected steel producer’s current technical data and the actual tool geometry.