At the 100,000-part volume, the usual setup is a 718H cavity (pre-hardened, 33-38 HRC), with S136 hardened or H13 used only for the wearing positions such as slides, core pullers and sprue bushings. If the resin is PVC, flame-retardant ABS or POM, the cavity cannot be downgraded: go straight to S136 hardened to 48-52 HRC. What really decides the grade is four variables - resin corrosiveness, glass-fibre content, appearance grade, and tooling lead time. The volume figure only decides whether you keep a spare cavity.

Quick answer: for 100,000 parts, ordinary ABS or PP cosmetic parts use a 718H cavity (33-38 HRC); transparent or high-gloss parts use NAK80 (38-42 HRC); halogenated flame-retardant, PVC and POM use hardened S136 (48-52 HRC); glass-filled resins need H13 or nitrided S136 at the gate and on slides. P20 (28-32 HRC) is only suitable for ordinary parts under 100,000 shots with no corrosive resin and no glass fibre.

Hardness and shot counts of the common grades

  • P20 (1.2311): pre-hardened, 28-32 HRC. Average polishability, cheap, fast delivery. Typically 100,000 to 300,000 shots, enough for ordinary ABS housings and internal structural parts. The catch: once the client reorders, the tool is essentially at end of life.
  • 718H (1.2738): pre-hardened 33-38 HRC. This is the most widely used cavity grade for home appliance and consumer electronics housings. Hardness is sufficient, it polishes well enough for everything below a mirror finish, machines well, and needs no heat treatment, so there is no quench distortion. Typically 300,000 to 500,000 shots.
  • NAK80: age-hardening steel, 38-42 HRC. Its strength is polishing, making it the first choice for transparent and high-gloss no-paint parts. It costs noticeably more than 718H and has lower toughness, so avoid it for large slides.
  • S136 / 2316: 2316 is a pre-hardened corrosion-resistant steel at 30-34 HRC; S136 is supplied annealed and reaches 48-52 HRC after hardening and tempering. Corrosion resistant, capable of a mirror finish, long life - at the cost of heat treatment, three to five extra days of machining, and quench distortion to manage.
  • H13 (SKD61, 8407): hot-work die steel, 45-52 HRC after hardening, with the best toughness of the group. Used where there is impact and glass-fibre erosion: long core pullers, large slides, gate inserts.

Selection map: ordinary parts under 100,000 shots use P20; ordinary parts in long-run production use 718H; transparent and high-gloss parts use NAK80; corrosive resins use hardened S136 or 2316; glass-filled wear positions use nitrided H13. Harder is not better - as hardness rises, toughness falls, and thin-walled inserts and sharp corners become prone to chipping.

The four variables beyond volume

Corrosiveness. PVC, POM, and ABS or PC with halogenated flame retardants release acidic compounds during injection. A pre-hardened cavity starts pitting and blackening after only tens of thousands of shots, and no amount of polishing brings it back. For these resins we always recommend hardened S136 or 2316 for the cavity. Do not save money here: one cavity costs far less than replacing a tool and stopping the line.

Glass-fibre content. Above 15 percent glass fibre, the wear rate at the gate, sprue bushing, runner corners and slide guide surfaces is several times that of ordinary resin. The approach is to keep 718H for the cavity but switch the sprue bushing and slide inserts to H13 or hardened S136 with surface nitriding, with the nitrided layer held at 0.05 to 0.15 mm. A deeper nitrided case becomes brittle and the sharp corners simply chip off.

Appearance grade. If the client wants a mirror finish, transparency or no-paint high gloss, the steel has to move up with it. Polishing 718H to a mirror finish becomes increasingly difficult, and inherent inclusions in the steel show through on the part. NAK80 and ESR-remelted S136 behave noticeably better. Conversely, using NAK80 for a textured or matte part is a waste.

Lead time and spare cavities. The 100,000 parts a client quotes are usually a two-to-three-year cumulative volume, not one delivery. At a 25-second cycle and four cavities per shot, roughly 15,000 parts a day means the run is finished in under ten days. What actually determines tool life is whether the client reorders in the following years.

Three mistakes we keep running into

Mistake 1: the drawing only says "718". The supplier buys to the lowest price, measured hardness drops to 28 HRC, and the gate collapses after a few hundred thousand shots. The correct wording states grade, supply condition and hardness range, for example "718H, pre-hardened, 33-38 HRC". Where possible, call out 1.2738 directly. Grade systems can follow GB/T 1299, but for acceptance, testing hardness is the most practical route: one measurement when the cavity arrives, a matter of minutes.

Mistake 2: wire-cutting after hardening S136. In steel hardened to about 50 HRC, wire EDM releases stress and the slide mating faces drift out of tolerance. Our practice is to leave 0.3 to 0.5 mm of stock in rough machining, then finish-machine and wire-cut after hardening and tempering have stabilised the part. Too little stock cannot be recovered in finishing; too much means long machining after hardening.

Mistake 3: mixing mold base and cavity grades in one note. An ordinary S50C mold base is fine, but many people take the shortcut of writing a generic "mold steel" in the title block, and the machine shop then uses the cheapest material across the board. The mold base, cavity, inserts and slides must be specified separately.

The tool-life budget has to be spent locally. When a tool reaches its shot limit, the failures usually are not in the cavity body but at the gate, slide guide surfaces, ejector pin holes and shut-off faces. Upgrading those positions specifically is more cost-effective than converting the whole cavity to hardened steel.

Hertz Industrial Design (Dongguan, China) works across product appearance design, structural design, material selection and CMF, and more than 320 of its designs have reached mass production. Website: www.hezidesign.com.

Originally published on the Hertz Industrial Design website (hezidesign.com), Design & Craft column.