Injection Molding Materials & Mold Steel Guide

Every injection molding project contains two material decisions, and they are made at different times by different people. The first is the plastic: which resin meets the part’s mechanical, thermal, chemical and regulatory requirements at acceptable cost. The second is the mold steel: which grade gives the tool life, polishability and corrosion resistance the program needs. Getting the plastic wrong is a redesign; getting the steel wrong is a delayed tool and premature wear.

Key takeaways

  • Choose resin from requirements first, cost second. A cheaper resin that fails a temperature or chemical test is not cheaper.
  • Shrinkage varies by resin and grade — it directly determines what tolerances the tool can hold.
  • P20 covers most general-purpose molds up to roughly 300,000 cycles.
  • NAK80 buys polishability and dimensional stability for optical and high-cavitation work.
  • S136 stainless is the correct choice whenever corrosion or cleanroom-compatible tooling matters.
  • Aluminum tooling is a legitimate choice for low volume and prototyping, not a compromise.

Choosing the resin: requirements before price

Work through five questions in order: what temperature will the part see in service, what chemicals will it contact, what load and impact does it carry, does it need to be transparent or food-contact or medical grade, and what surface finish is expected? Those answers eliminate most of the resin catalogue before cost enters the conversation.

Common families and where they win: ABS for housings and general consumer parts; PC where impact strength and clarity matter; PP for living hinges and chemical resistance; PA (nylon) for structural and wear parts; POM for precision mechanical components; PBT and PET for electrical parts; TPU and TPE where flexibility is required. Our guide to choosing the right resin for injection molding works through each family with property comparisons.

Shrinkage: the property that sets your tolerances

Every resin shrinks as it cools, and the amount differs by polymer, grade, wall thickness and gate location. Semi-crystalline resins such as PP, PA and POM shrink far more than amorphous resins such as ABS, PC and PS — and shrink differently in flow and cross-flow directions, which is why parts warp.

This is why the resin decision must be made before the tool is cut. A tool built for ABS tolerances will not hold them in glass-filled PA. Ask your supplier for the shrinkage value they intend to build to, and confirm the resin grade in writing.

PA66 vs POM: the precision-parts comparison

These two resins dominate precision mechanical components, and buyers frequently treat them as interchangeable. They are not. PA66 offers higher strength and temperature resistance and absorbs moisture, which changes its dimensions in service. POM offers lower friction, better dimensional stability and superior machinability and creep resistance, but lower temperature ceiling and poorer adhesion for overmolding.

Our PA66 vs POM comparison puts the numbers side by side — tensile strength, moisture absorption, shrinkage, wear behaviour, cost — and gives the selection rule for gears, housings, clips and fluid-contact parts.

Mold steel: matching the grade to the program

Steel choice is a life-cycle decision. Under-specify and the tool wears out mid-program; over-specify and you pay for hardness the project never uses.

P20 is pre-hardened to 28–32 HRC, machines easily and is the default for general-purpose molds up to about 300,000 cycles. NAK80 at 38–42 HRC polishes to a mirror finish and holds dimensions well — the right call for optical parts and high-cavitation tools, good to roughly 500,000–1,000,000 cycles. S136 (1.2083) stainless at 48–52 HRC resists corrosion and is standard for medical, food-contact and chemically aggressive molding, with a life beyond one million cycles when maintained properly.

Our P20 vs NAK80 vs S136 comparison covers hardness, machinability, polishability, corrosion resistance, cost and expected life for each grade.

Aluminum vs steel tooling

Aluminum molds machine four to five times faster than steel, which makes them genuinely competitive for prototypes, bridge tooling and low-volume runs where the part count will not justify a steel tool. They also cool faster, shortening cycle time. The limit is life and wear resistance — aluminum is not the right answer for a program expecting hundreds of thousands of cycles in glass-filled material.

The cost crossover depends on volume, cavitation and material abrasiveness. See aluminum vs steel injection molds: understanding the cost difference.

Medical and regulated applications

Medical device plastics add a regulatory layer: material traceability, biocompatibility documentation, and in many cases cleanroom-compatible tooling and validated processes. Resins are typically chosen from established medical grades of PC, ABS, PSU, PEEK, PP and PE, with the decision driven by sterilization method — autoclave, EtO or gamma — as much as by mechanical requirements.

Our medical device plastics material selection guide organises the choice around sterilization compatibility and regulatory documentation.

Start here: the full materials library

Not sure which resin or steel you need?

Tell us the part’s service conditions and target volume. We will recommend a resin and a mold steel grade, and explain the trade-off at each alternative — before anything is cut. Talk to an engineer.