P20, 718 or 738? Choosing the steel grade for a plastic mold base

Most mold base enquiries we receive name a grade already — usually P20. That is a reasonable default, and for a large share of plastic molds it is the right answer. But the grade drives hardness, machinability, polishability and price, so it is worth two minutes of thought before the drawing is frozen.

S50C (1.1730) is a plain carbon tool steel, supplied unhardened. It machines easily and costs the least. It is the right choice for plates that are structural rather than working — backing plates, spacer plates, plates that will be bolted to something else and never see a polished cavity. It is not a cavity material.

P20 (1.2311 / 1.2312) is pre-hardened at the mill, typically in the low-30s HRC. That means the plate arrives already hard enough for a large share of production molds and does not need heat treatment afterwards — which keeps the base flat, avoids distortion, and saves a week in the schedule. For most injection mold bases going into general-purpose production, P20 is the sensible default.

718 and 718H sit a step up from P20 in strength and cleanliness. When a mold is expected to run a lot of cycles, or the part has deep ribs and thin cores that see real stress, 718 is the safer call. 718H arrives at a higher pre-hardened level than P20, so pockets and cores hold their dimensions better under load.

738 and 738H are similar in role, with a slightly different balance of toughness and hardness. Where a buyer's own standard already specifies 738, we machine to it — the important thing is that we machine all four grades in-house, so switching grade does not mean switching supplier.

When hardness has to go higher, grades like 4Cr13, 420H and S136H are the ones we machine for corrosion resistance or for molds that need hardening after machining — for example PVC or other corrosive melts, or transparent parts that demand a higher polish. These are not pre-hardened at the same level as P20, so the working faces are hardened in a later step. Plan that step into the lead time.

Forged P20 is available where core strength matters more than the pattern cost — for thick plates and heavy sections, a forged blank gives a denser, more uniform structure than a rolled one.

What to put on the drawing. Name the grade and the hardness range you expect. If you have a house standard, say so — many buyers do, and it removes a whole round of questions. If you are unsure, tell us the plastic, the expected annual volume and which faces will be polished; we will suggest a grade and confirm it before quoting rather than after.

One practical note: the grade you pick changes the price. Pre-hardened P20 costs more per kilogram than S50C, and hardened grades add a heat-treatment step. Naming the grade up front means the quotation you receive is the quotation you can approve.