Mould Engineering
P20, H13 or S136? A Practical Mould Steel Selection Guide for Injection Mould Buyers
Which mould steel fits your resin, volume and finish? A practical guide to P20, 738H, H13 and S136 from a working mould shop - plus three buyer questions that reveal real capability.
Choosing mould steel is one of the three decisions that determine a tooling project outcome - the other two being the tooling source and the part design itself. This guide explains the steel families we quote at D-TOP MOLD, what each one is actually good for, and how three buyer questions reveal whether a supplier can handle demanding applications.
Pre-Hardened Steels: P20, 738H (1.2738), 1.2311, 1.2312
These are the workhorses of general-purpose injection moulding. They arrive pre-hardened (typically around 30-36 HRC), so no heat treatment is needed after machining - which keeps cost down and avoids distortion. Use them for interior trim parts, housings, and short-to-medium production runs. They machine faster than hardened grades, which also helps compress lead times.
Through-Hardened Steel: H13 (1.2344) at 52-54 HRC
When a tool runs glass-filled or abrasive compounds, or needs to survive millions of shots, H13 hardened to 52-54 HRC is the reliable choice. The higher hardness resists wear on gates, ribs and sliding features. The trade-off is cost and machining time, so we recommend it where the resin or the production volume genuinely demands it - not as a default upsell.
Stainless and Mirror-Polishing Grades: S136 (1.2083 ESR) and NAK80
For medical, food-contact, optical or corrosion-sensitive applications, S136 (1.2083 ESR) and NAK80 deliver both corrosion resistance and the ability to hold a polished surface. These are also the grades behind high-gloss finishes - more on that in the FAQ below.
Do Not Forget the Mould Base
A stainless core in a rusting base defeats the purpose. Our standard base material is S45C; where anti-rust is a real requirement - moisture-heavy PET work, corrosive compounds - we quote 2085 stainless bases so the whole tool matches the application.
Three Questions That Reveal Real Capability
Question 1: Can you do high-gloss mirror polishing? Our answer: yes - with S136 hardened steel we reach SPI A-2 and higher optical-grade finishes, suitable for transparent parts and parts destined for painting. If a supplier answers with vague "very good polishing" and no steel grade, keep asking.
Question 2: How do you handle corrosive flame-retardant resins such as 5VA-rated PPE? Our answer: a full stainless configuration - 1.2083 cores and cavities in a 2085 stainless mould base, plus insulation plates and heaters so the mould stays rust-free and thermally stable. Partial stainless is where rust problems start.
Question 3: Does your quote include the temperature controller? Ours includes the built-in hot runner system and nozzles, but not the external temperature controller - stated up front so the price is comparable and transparent. You can use your existing controller, or we quote YUDO or cost-effective domestic units separately. Suppliers who hide line items in vague "complete system" pricing make comparison impossible.
How to Actually Decide
Match the steel to three variables: resin (abrasive? corrosive?), volume (how many shots?), and surface finish (texture, gloss, transparency). A reliable supplier proposes the steel with reasons tied to those variables - P20/738H for general work, H13 for wear, S136/NAK80 for corrosion and gloss - and puts it in writing on the quotation.
What You Get From Us
Every mould we build carries a fixed inspection chain - steel hardness verification, electrode measurement, core and cavity dimension measurement, final sample measurement - and machining-only inserts ship with a complete CMM report verified against your 3D tolerances. T1 machine time and base material are included in the tooling price, moulds carry a 300,000-shot warranty, and in-house trial presses from 80T to 450T let you see real shots before shipment.
Send your 3D model (STEP/IGS/X_T) and annual volume, and the steel recommendation comes back with DFM feedback and a transparent quotation.