What Are Plastic Mold and Special Purpose Tool Steels?
Plastic mold and special purpose tool steels are the group of tool steels selected when the tooling must combine toughness, hardness, wear resistance and, in some grades, corrosion resistance rather than optimum cutting performance. Plastic mold steels are intended for the moulds used in injection moulding and extrusion of plastics, while special purpose tool steels are developed for a specific duty such as water quenching, high impact loading or a particular forming process.
The common thread is a balanced property profile: these steels resist impact and thermal cycling without chipping, they hold a polished surface, and they can be machined into large, deep cavities that would be impractical in a higher alloyed cutting tool steel.
How the Group Is Classified
Two families dominate practical selection.
Plastic mold steels such as the alloy mold steel AISI P20 (covered by ASTM A681), supplied in the pre-hardened condition so that the cavity can be machined directly. Chromium bearing mold steels of the 420 stainless type, for example the 1.2083 grade, are used when the plastic is corrosive or when a mirror finish is required.
Special purpose and water-hardened tool steels of the AISI W series (covered by ASTM A686), which are water quenched and contain high iron contents with little or no additional alloying. The small alloying additions that are present improve mechanical properties without the cost penalty of the highly alloyed tool steels.
Water-hardened grades are quenched rapidly in water, which gives a hard surface with good wear resistance at low material cost. Their limitation is size and distortion: they suit small tools rather than heavy blocks, and the quench must be controlled to avoid cracking.
Property Profile of the Family
The table compares the typical behaviour of the three groups rather than absolute figures, since values depend on the specific grade, section size and heat treatment.
| Property | Mold steels (P20 type) | Water-hardened (W type) | Corrosion resistant mold steel (420 type) |
|---|---|---|---|
| Toughness | High | Medium | Medium to high |
| Wear resistance | Medium | High | Medium |
| Corrosion resistance | Low | Low | High |
| Impact strength | High | Medium | Medium |
| Polishability | Very good | Good | Very good |
| Hardness after treatment | Pre-hardened, moderate | High | High |
Mold steels are chosen for their toughness and polishability together with a hardness level that keeps the cavity accurate during long production runs. The special purpose group is chosen where wear resistance or a specific quenching response matters more than dimensional stability in a hardened block.
Low Carbon Mold Steels for Thermoplastic Molding
A typical example of a special purpose mold steel is the low carbon mold steel used for thermoplastic molding. This material does not need exceptional impact resistance, but it does need good wear resistance and the ability to tolerate the heat carried by the melt cycle after cycle. Low carbon content keeps the steel machinable and dimensionally predictable, while the alloying elements provide the wear behaviour and the surface hardness needed for long mould life.
AISI P20 is the most widely used grade in this category. It is supplied pre-hardened, commonly around 30 HRC, so no further hardening is required after the cavity is cut, and it can be nitrided or plated when additional surface wear resistance is needed. Where the plastic releases aggressive compounds, the stainless mold steel alternative avoids rust staining of the moulded part.
Selection and Heat Treatment Notes
Grade selection starts from the plastic being moulded, the production volume and the mould geometry. Abrasive or glass filled plastics push the choice towards higher wear resistance, while corrosive plastics and humid production environments push it towards a chromium bearing stainless mold steel. Large cavity blocks favour pre-hardened mold steel because hardening after machining would distort the cavity.
Water-hardened tool steels follow a different route: the tool is machined soft, austenitized, quenched in water and tempered immediately to relieve quench stresses. Tempering temperature is chosen to trade hardness against toughness, and the quench is kept agitated and controlled to limit distortion. Where dimensional accuracy is critical, the water-hardened family is usually replaced by an oil or air hardening grade.
Frequently Asked Questions
Q: What is the difference between plastic mold steels and special purpose tool steels?
Plastic mold steels are optimised for moulds used in injection moulding and extrusion, while special purpose tool steels are designed for a particular duty such as water hardening or high impact service.
Q: Which grade is normally used for injection moulds?
AISI P20 is the most popular choice because it is supplied pre-hardened, is easy to machine into deep cavities and holds a good polish.
Q: Are plastic mold steels corrosion resistant?
Standard low alloy mold steels such as AISI P20 are not corrosion resistant. Corrosion resistance is provided by the 420 type stainless mold steels, for example the 1.2083 grade, which are chosen for aggressive plastics and humid environments.
Q: What does water-hardened mean?
It describes tool steels that are hardened by quenching in water. They are simple high iron, high carbon steels with few alloying additions, giving low cost and high hardness for small tools.
Q: Can these steels be polished to a mirror finish?
Yes. Mold steels and stainless mold steels are both noted for good polishability, which is essential for the surface quality of moulded plastic parts.
Q: Why are special purpose tool steels cheaper than high alloy tool steels?
Because the alloying content is deliberately kept low. Only small additions are made, enough to improve the mechanical properties without the cost of a heavily alloyed steel.