S136 Steel for Injection Molds

What S136 is

S136 is a martensitic stainless plastic mould steel corresponding to DIN/EN 1.2083. It is supplied soft at roughly 30 HRC for machining and polishing, then hardened to around 50–52 HRC for production. Its chromium content gives it high corrosion resistance, and its clean structure takes an optical polish.

Those two properties together define its place: a cavity that has to stay clear and bright while running a resin that would corrode an ordinary steel. PVC, medical parts, clear polycarbonate and optical components are the classic applications.

It is not the hardest grade available and it is not the most wear-resistant. Where the resin is heavily glass-filled and corrosion is not the issue, a hot-work grade such as H13 is the better answer. S136 is bought for corrosion plus polish, and it should be judged on those two.

Hardness, and what it does not buy you

At 50–52 HRC after hardening, S136 holds a polished cavity through a long production run. The hardness supports the surface: a soft stainless cavity polishes beautifully and then loses that finish as it wears. Hardening is what turns the polish into a production surface.

It does not buy the wear resistance of a hot-work grade. Stainless steels trade some abrasion resistance for corrosion resistance, and a glass-filled melt will still wear the gate. For a glass-filled PVC or a filled flame-retardant grade, the answer may be a stainless grade with a replaceable gate insert rather than a single block.

It does not buy freedom from corrosion either. Stainless is a relative term. Chlorides, stagnant water and a contaminated circuit can pit S136, and a mirror surface makes those pits visible immediately. Passivation after machining and repair is part of running the tool, not an optional extra.

Hardness also does not buy toughness. A hardened stainless cavity is more sensitive to a sharp corner and to an unrepaired EDM layer than a pre-hardened block, so the design and the finishing sequence have to account for it.

Corrosion and polishability

This is the pair that justifies the grade. The chromium content gives S136 high corrosion resistance, enough for PVC, for medical parts that are cleaned aggressively, and for a tool that sits in a humid climate between campaigns. It is the reason a PVC mould in S136 does not produce the rust-stained parts that a P20 mould produces after a few months.

It is not immunity. Chlorides are the weakness: a water line fed with high-chloride water, a cleaner left on the cavity, or a stagnant circuit will pit the surface. Passivation restores the passive oxide layer after machining, welding or polishing, and it should be part of the finishing sequence rather than a repair step.

Polishability is excellent. The steel takes an optical finish, which makes it the normal choice for a clear lens, a light guide or a medical device surface. That polish is also the reason the corrosion behaviour matters so much: a rust bloom on an optical cavity is not a cosmetic inconvenience, it is a scrapped surface.

The practical routine is to dry and protect the cavity after every run, keep the cooling water treated and monitored, and avoid leaving the mould closed with residual moisture. A stainless tool that is stored wet can still fail, and the repair of an optical surface is expensive.

By production volume

For a short run, S136 can be used in the soft condition, which makes it a fast route to a polished sample cavity. That is a reasonable bridge for a cosmetic or optical part, provided the tool is not expected to run a long campaign.

For production, harden it. A hardened S136 cavity at 50–52 HRC holds the polish and the dimension through a long run, and the corrosion resistance means the maintenance interval is longer than for a standard steel. Above roughly 500,000 shots it is a natural choice for a corrosive or optical part.

Volume is not the deciding variable when corrosion is present. A 50,000-shot PVC tool may still need S136, because a cheaper steel would corrode before it wore out. Read the resin first.

By resin

S136 is the standard choice for PVC and CPVC, where the resin releases corrosive products and a standard steel would rust. It is also the standard for medical parts, where cleaning and sterilization put the cavity in contact with aggressive media.

Clear and optical PC is a strong match: the steel polishes to an optical surface and resists the corrosion that a wet medical environment can cause. Pigmented PC also runs well, though a pre-hardened cosmetic grade may be enough where the surface is not optical.

Glass-filled resins such as PA+GF are a qualified match. S136 gives corrosion resistance and a good polish, but it is not the most wear-resistant grade, so a replaceable gate insert is the sensible design. Where the resin is filled but not corrosive, H13 is the better wear answer.

Unfilled commodity resins such as ABS do not need S136. A pre-hardened cosmetic grade is cheaper and sufficient, unless the tool also has to resist a corrosive additive or a aggressive cleaning regime.

Heat treatment, welding and repair

S136 is supplied soft and hardened after machining to 50–52 HRC. The hardening cycle has to be followed by tempering, and the tool has to allow for distortion. Finish polishing happens after hardening, which is why the process plan sequences the work around the heat treatment rather than treating it as a final step.

Welding is possible with a preheat and a matching stainless filler. The heat-affected zone loses some corrosion resistance, so the repaired area should be passivated after polishing. On an optical surface, a weld will usually show, and a replaceable insert is a better design than a repair.

Repair at the gate is the common case. A stainless gate insert, hardened and passivated, can be replaced without welding the optical cavity, and that protects the surface that the grade was bought for.

EDM surfaces should be dressed or stress-relieved before service. The recast layer is a corrosion site as well as a crack starter, which matters more on a stainless cavity than on a standard steel because the passivation depends on a clean surface.

Surface treatment and coatings

Passivation is the treatment that belongs on every S136 cavity after machining, polishing, welding or repair. It restores the passive oxide layer and is the difference between a stainless tool that performs and one that pits at the first wet season.

Nitriding can be applied where extra wear resistance is needed, but it changes the polish response and is usually reserved for a gate insert rather than an optical cavity.

PVD coatings are less common on S136 than on H13, because the corrosion resistance already addresses the main failure mode and the coating adds a step that must be stripped before a repair. Where wear and corrosion both matter, a coated insert in a stainless cavity is a reasonable compromise.

Cost per shot vs cost of steel

S136 is more expensive than a pre-hardened grade, and the heat treatment and passivation add cost and schedule. On a PVC or optical part, that premium is the cost of not repairing a corroded or dulled cavity mid-campaign.

Against H13, S136 trades some wear resistance for corrosion resistance. Where the resin is both abrasive and corrosive, the honest answer may be a stainless cavity with a hardened insert at the gate, which costs more than either single grade but avoids the failure mode of both.

The cheapest option is the one that does not need a mid-campaign repair. For a PVC tool, that is usually S136 from the start, not a standard steel with a coating.

Common selection mistakes

• Using S136 soft for a production cavity. The polish is the point of the grade, and a soft cavity will not hold it. Harden it for a production run.
• Assuming stainless means rust-proof. Chlorides, stagnant water and a contaminated circuit will still pit it. Passivate and maintain the circuit.
• Choosing S136 for a glass-filled tool on hardness alone. It is not the most wear-resistant grade. Use a hardened insert at the gate where the fibre hits.
• Skipping passivation after a weld or polish. The repaired area loses corrosion resistance, and that is where the first pit appears.

What to send us

Send the 3D part, the resin grade including any corrosive additive or flame retardant, the annual volume, the surface standard, and the cleaning or sterilization regime if the part is medical. Those last two decide whether S136 is required or whether a pre-hardened cosmetic grade would do the same job for less.