P20 Steel for Injection Molds

What P20 is

P20 is a chromium-molybdenum plastic mould steel supplied pre-hardened at roughly 30 HRC. The AISI designation maps to DIN/EN 1.2311. It arrives ready to machine, which removes the hardening step, the distortion that comes with it, and the schedule risk of sending a block out for heat treatment in the middle of a tool build.

That convenience is why P20 is the default for short and medium production tools. It machines well, takes a good polish, welds acceptably and is available in the plate sizes most mould bases need. For an unfilled resin and a campaign in the hundreds of thousands, it is usually the correct and least expensive answer.

It is not a universal answer. The same 30 HRC that makes it easy to machine makes it soft against glass fibre, and it offers no corrosion protection against PVC or aggressive flame retardants. Reading the resin before choosing the block is the whole decision.

Hardness, and what it does not buy you

Thirty HRC is enough to resist indentation from a normal ejection load and enough to hold a polish through a reasonable campaign. It is not enough to resist abrasive wear at a gate carrying glass-filled melt, and it is not enough to survive a long run without maintenance.

What hardness does not buy is toughness or polishability. A harder block is not automatically a better mould: it machines more slowly, cracks more readily at a sharp internal corner, and takes a finer polish only if the steel is clean. P20 sits at a deliberate compromise - soft enough to cut quickly, hard enough to hold a cavity, and clean enough to polish to a cosmetic standard.

The failure mode to watch is not bulk wear but local wear. The gate, the vent land and any narrow restriction see the most velocity, and that is where a P20 tool shows its age first. A replaceable gate insert is cheaper than re-cutting a cavity, and it is the standard mitigation on a P20 tool that may see abrasive resin later.

Corrosion and polishability

P20 has low corrosion resistance. It is a chromium-molybdenum steel, not a stainless grade, and it will rust in a humid tool room or in a water line that is left wet between runs. The practical consequences are rust on the cavity during storage, corrosion pits that print through on the part, and cooling channels that scale and lose heat transfer.

The mitigation is discipline rather than a different steel: dry the tool after every run, apply a protective film to the cavity, keep the cooling circuit treated, and avoid leaving the mould closed and wet. Where the resin itself releases corrosive products - PVC is the obvious case - P20 is the wrong choice and a stainless grade is the right one.

Polishability is good for a cosmetic surface. The steel is homogeneous enough to take a fine finish, and a textured cavity holds detail well. It is not an optical-grade steel: for a clear lens or a mirror surface, a stainless grade with a cleaner inclusion structure is the normal specification. The distinction matters because a P20 cavity polished to its limit can still show a faint haze that a customer reads as a defect.

Surface treatments extend the life of a P20 cavity without changing the block. Nitriding hardens the surface and improves wear resistance at the gate and the vent land; chrome plating improves corrosion resistance and release. Both are decisions to make before the first trial, because a nitrided cavity is harder to correct afterwards.

By production volume

Under roughly 5,000 shots, a P20 tool is often more than the programme needs, and aluminium or a soft steel can be the faster route to samples. Between 5,000 and 100,000 shots, P20 is in its element: enough hardness to hold the cavity, fast to build, and inexpensive to correct.

Between 100,000 and 500,000 shots, P20 is still viable for an unfilled resin, provided the gate and vents are maintained and the tool is protected between runs. Above that range, the maintenance burden starts to outweigh the initial saving, and a harder or stainless grade becomes the lower total cost.

Volume is not the only variable. A 50,000-shot tool in a glass-filled resin can wear faster than a 500,000-shot tool in ABS. The campaign length has to be read together with the resin and the appearance requirement, not on its own.

By resin

P20 is a good match for unfilled commodity and engineering resins: ABS, polystyrene, polypropylene, polyethylene, acrylic and unfilled polycarbonate in a short campaign. None of these carry abrasive filler, and none release a corrosive product at normal processing temperature.

Polycarbonate is a qualified match. PC wants a mould at 80–110 °C, and a P20 block can run that temperature for a short campaign, but the tool has to be designed for it and the steel has to hold its hardness at temperature through the run. For production PC, a hardened grade is the safer choice.

Glass-filled resins such as PA+GF are the wrong match. The fibre erodes the gate and the vent land, and a 30 HRC block will show wash within a production campaign. PVC and some flame-retardant grades are also the wrong match, because the corrosion risk arrives before the wear risk.

Optical and medical parts are a qualified mismatch. P20 can produce a cosmetic part, but a clear lens or a part that has to be sterilised usually needs a stainless grade for polish and corrosion. The resin and the part function together decide whether P20 is enough.

Heat treatment, welding and repair

P20 is supplied pre-hardened, so the normal tool does not see a hardening cycle at all. That is its main advantage. If a section has to be harder - a gate insert, a wear pad - the usual route is to machine it from a through-hardening grade and harden it separately rather than to re-harden the whole block.

Welding is straightforward with a procedure. Preheat the area, match the filler to the base chemistry, weld with a controlled heat input, and let it cool slowly. The heat-affected zone is softer than the surrounding steel, so a weld on a polished face leaves a faint halo after polishing. For a cosmetic cavity, that halo is a real constraint: plan the repair where it will not be seen, or accept a re-polish and a possible spot visible under grazing light.

Repair by welding is common at the gate and at a damaged parting line. A gate insert is the better long-term answer when the damage is repeated, because the wear item is then a small block that can be replaced without touching the cavity.

Nitriding after machining is a common upgrade for a P20 tool that will see light abrasion. The case is thin and hard, and it improves wear resistance at the gate and vent. It also makes subsequent correction harder, so the cavity should be dimensionally settled before nitriding.

Surface treatment and coatings

Nitriding is the standard treatment for a P20 cavity that needs more surface wear resistance. It produces a hard case without changing the bulk hardness, and it is well suited to gates, vent lands and ejection surfaces. The trade is that the treated surface is harder to polish and to correct.

Chrome plating improves corrosion resistance and release, and is common on a cavity that runs a slightly aggressive resin or that has to be stored between campaigns. It adds a process step and a lead time, and it can chip at a sharp edge, so the cavity geometry should be reviewed before plating.

PVD coatings are an option for wear at a specific location, but they are usually reserved for a hardened substrate. On a 30 HRC block the coating can fail by substrate deformation rather than by wear, which defeats the purpose. For a P20 tool, nitriding is usually the more sensible first step.

None of these treatments replace the basic decision. A coating on the wrong steel is a delay, not a fix.

Cost per shot vs cost of steel

P20 is the cheapest of the production-capable mould steels, and the difference in block price between P20 and a hardened grade is small compared with the cost of the machining and the heat treatment that a harder grade requires. On a short campaign, P20 is usually the lowest total cost because it avoids the hardening step entirely.

The cost comparison changes with volume and resin. A P20 tool in an unfilled resin has a low cost per shot and a predictable maintenance cost. The same tool in a glass-filled resin has a low purchase cost and a high maintenance cost, and the gate insert replacement can erase the saving within a campaign.

The honest framing is cost per accepted part over the campaign, including maintenance and downtime. P20 wins that calculation for the unfilled, short-to-medium run. It loses it when the resin is abrasive, the surface is optical, or the campaign is long enough that a harder tool would not need the same attention.

Common selection mistakes

• Using P20 for a glass-filled production tool. The gate washes out, the vent land wears, and the tool needs repair before the campaign is finished. The saving on the block is smaller than the cost of the first gate insert.
• Specifying P20 for PVC or an aggressive flame-retardant grade. Corrosion, not wear, is the failure mode, and P20 has no corrosion protection. A stainless grade is the correct answer.
• Expecting an optical polish. P20 polishes to a good cosmetic standard but not to a clear-lens standard. A part that has to be transparent needs a cleaner steel.
• Welding a visible face and calling it done. The heat-affected zone polishes differently, and the repair shows under grazing light. Plan the repair location before the weld, not after.

What to send us

To recommend a block, send the 3D part or a drawing, the resin grade including any glass or mineral loading, the annual volume and expected campaign length, the appearance requirement, and the number of cavities. If the tool already exists, send the current steel and the failure you are seeing - gate wash, corrosion, cracking or polish loss - because the failure mode usually names the correct replacement.