How to Select Steel for Plastic Injection Molds

The steel used in a plastic injection mold determines how the mold performs, how much it costs to maintain, the quality of every part it produces, and how long it stays in service. A well-matched grade maintains tolerance under pressure, resists wear and corrosion during the molding process, and keeps polishing and machining time under control. The wrong grade shows up later as sink marks, cracked cavities, rusted cooling channels, or a mold that wears out long before the production run is finished.

The economics reinforce the point. Raw tool steel typically accounts for only 10 to 20 percent of the total cost of a finished mold, with the remaining 80 percent allocated to machining, polishing, and assembly. Choosing a cheaper steel to save on raw materials rarely pays off, because the savings are small compared to the labor already invested in the mold and the cost of downtime if the mold fails early. The steel has to be fit for purpose, which means matching its properties to the plastic, the production volume, and the surface finish the part demands.

What a Mold Steel Has to Do

Before comparing grades, it helps to be clear about the demands a mold steel must meet, because each requirement traces back to a specific failure that the wrong material would cause.

Machinability matters because most of a mold’s cost comes from cutting and drilling complex cavities. A grade that machines cleanly shortens that work and lowers the manufacturing bill, which is one reason prehardened steels supplied ready to cut are so widely used.

Polishability decides whether the mold can produce transparent or high-gloss parts. Lenses, optical components, and cosmetic surfaces require a mirror finish, and that finish requires steel with a uniform structure, high surface hardness, and a high degree of cleanliness. Non-metallic inclusions appear as pits and streaks that no amount of polishing can remove, so mold-quality steels for these jobs are refined to keep inclusion levels low.

Wear and abrasion resistance becomes critical when the plastic itself is abrasive. Glass fiber, mineral fillers, and other reinforcements scour the flow path and the gate, and a soft steel erodes there first.

Corrosion resistance is required for plastics that decompose during molding. PVC and flame-retardant ABS release hydrogen chloride and hydrogen fluoride as they break down, and these gases attack ordinary steel, pitting the cavity and etching the cooling channels. Molds for these plastics need a corrosion-resistant grade, or they degrade from the inside.

Toughness and core strength keep the mold intact under clamping pressure and thermal cycling. High compressive loads can cause an undersized cavity to sink, and the repeated heating and cooling of each cycle induce thermal fatigue that cracks brittle steel. The grade needs sufficient core strength to hold its shape and enough toughness to withstand cycling without failing.

Dimensional stability favors steels that need little or no finishing after heat treatment. Any grade that has to be hardened after machining risks distortion, so prehardened steels, which arrive at working hardness and go straight into service, avoid that risk entirely.

The Main Families of Mold Steel

Prehardened Medium-Carbon Steels: The P20 Family

For general-purpose molds, the P20 family covers the large majority of what the industry uses. P20 and its European equivalents DIN 1.2738, 1.2311, and 1.2312 are supplied prehardened at roughly 30-36 HRC and around 300 HB. That prehardened condition is the whole point. The steel can be machined and put directly into service without a high-temperature hardening step, which removes the distortion that heat treatment would otherwise introduce into a finished cavity. P20 suits large machine-cut molds, automotive parts such as dashboards and bumpers, and standard consumer goods. Where surface wear later becomes a problem, the mold can be carburized, flame hardened, or nitrided to add a harder skin. P21, an aluminum bearing precipitation hardening grade also supplied prehardened at 32 to 36 HRC, is the choice when polishability and critical finishes matter more than anything else in this class.

Low-Carbon Hubbing Steels: P2, P4, P6

A smaller group of grades, P2, P4, and P6, has very low carbon content, between 0.05 and 0.15 percent, and is supplied soft and annealed. The low carbon lets the cavity be formed by cold hubbing, pressing a hardened master hub into the soft blank rather than machining the cavity out. Once the cavity is formed, the mold is carburized to a hard case of 58-61 HRC over a tough, shock-resistant core. Among these, P4 offers the highest hardenability and wear resistance, making it well-suited for high-pressure injection and compression molds where surface wear is the main concern.

Stainless and Corrosion-Resistant Steels

When the plastic is corrosive, or the shop environment leaves cooling channels prone to rust, martensitic stainless grades are the answer. AISI 420, DIN 1.2083, is the most common. Its roughly 13 percent chromium gives a strong combination of corrosion resistance, abrasion resistance, and mirror polishability, and it is typically hardened to 46 to 52 HRC. Where heat-treatment distortion is a concern, prehardened stainless grades such as DIN 1.2316 and the 414 type offer rust resistance without the hardening step, which is why they are common in PVC molding and in medical and optical work.

Hot-Work and High-Hardness Tool Steels: H13, S7

For high-volume production or molds that require high toughness and heat resistance, hot-work tool steels are used in plastic molding rather than die casting. H13 is heavily alloyed, tough, and strong in the core. It heat-treats to 48 to 54 HRC and nitrides exceptionally well, reaching a surface hardness of 65 to 70 HRC that withstands severe abrasive wear. S7 is the grade to choose when shock resistance and strength are the priority. It reaches about 58 HRC and polishes well enough for clear plastics.

Wear-Resistant and Powder Metallurgy Steels: A2, D2, P/M Grades

When the plastic is heavily filled with abrasive material such as fiberglass or ceramic powder, standard mold steels erode too fast to be economical. The high-carbon, high-chromium cold work steels A2 y D2 provide extreme abrasion resistance and are commonly used for small inserts, wear points, and cavities that run abrasive compounds. Powder metallurgy grades go further. Elmax, ASP-23, and CPM 440V carry an ultra-fine, evenly distributed carbide structure that combines high wear resistance with toughness that conventional melting cannot match. Elmax, a P/M stainless, adds corrosion resistance and excellent polishability on top of its wear resistance, which suits high-wear electronic and IC molds.

Matching the Grade to the Job

By Production Volume

Production volume sets the baseline. Molds built for very high output, the SPI Class 101 range above a million cycles, need high hardness carbon and tool steels, with H13, 420 stainless, or A2 and D2 in the wear areas. Medium to high-volume molds, Class 102 and 103, are where prehardened P20 and DIN 1.2738 earn their place, because they combine adequate life with low cost and no risk of heat treatment. For prototypes and short runs of a few thousand parts at low molding pressure, high-strength aluminum grades such as 7075-T6 and Alcoa QC-10, or even 3D-printed polymer tooling, are far more economical and conduct heat better than steel.

By Plastic Material

The plastic being molded further narrows the choice. Standard thermoplastics such as polyethylene, polypropylene, and ABS perform well in P20 and similar medium-carbon grades. Corrosive plastics, including PVC, fluoroplastics, and flame-retardant ABS, require stainless grades such as 420, 414, DIN 1.2083, and DIN 1.2316 to resist the acidic gases they release; otherwise, the cavity will pit and etch. Abrasive and reinforced plastics, such as glass-filled POM and polycarbonate, call for high-hardness tool steels like D2 and A2, nitrided H13, or powder-metallurgy grades.

By Surface Finish

The finish the part demands can override the other factors. A high-loss or mirror finish for lenses, optical parts, and clear acrylics depends on steel free of inclusions, which means mold quality grades refined by electroslag remelting or vacuum arc remelting. P21, 420 stainless, and specialized age-hardening or maraging steels give the best results here. Textured and etched surfaces have the opposite sensitivity. Chemical photo etching requires a highly homogeneous structure so the acid bites evenly, and heavily segregated steels or free-machining grades with high sulfur, such as DIN 1.2312, should be avoided where uniform texture matters, because their internal variation shows up as uneven etching.

Working Through the Decision

No single grade is best for every mold. The right choice comes from answering the same three questions in order: how many parts the mold must make, which plastic it will run, and what finish the part requires. Answer those and the field usually narrows to two or three grades, at which point machinability, availability, and cost decide the rest.

Most of the grades in this decision are stocked by Aobo Steel. Our plastic mold steel range runs from prehardened P20 and 420 stainless for general and corrosion-resistant molds to H13, S7, A2, and D2 for high-wear and high-volume work. For cross references between AISI, DIN, and JIS designations, our tool steel equivalent grade finder covers the full mapping.