Miniature Die Casting Die Materials and Hardness

Miniature die casting makes small zinc and zinc aluminium parts on machines that cycle up to around a hundred times a minute, holding dimensions under a thousandth of an inch. The die is not a pair of blocks. It is a set of slides, each one a die and a shank, and every component in that set has its own material and its own hardness. This page gives the component list with the material and hardness published for it in the NADCA tooling standards, sets out which of those grades are hot work and which are not, and explains what the different hardness levels are there to do.

How a miniature die is assembled

A die and a shank together make a slide. A four slide die therefore has four of each, arranged around the casting, and the standard’s own illustration of the assembly shows the die head on each slide, a core pin block, a stationary and a shank mounted ejector plate, and the runner system fed from beneath. The machine is programmed to move the slides in sequence. After the die is sprayed with lubricant the top slide comes in, then the bottom slide, then the left and right slides, and the metal is injected from beneath on the hot chamber principle. After a short solidification the slides cycle out in the reverse order and the casting is stripped and blown clear.

Ejection is one of the two choices the standard points to. An ejector plate strips the casting from the die and leaves no mark on it, while standard ejector pins are also common practice and tend to leave marks on the casting. The choice is made at the design stage rather than in the tool room. On cavities, single cavity dies are preferred so that every part is identical, which matters where automatic assembly equipment is downstream, and multiple cavity dies are used where volume demands it.

Component material and hardness

The table below is the published list for a typical miniature die. It is short, and the pattern in it is worth reading before the individual values. H-13 appears three times at three different hardness levels, so the same steel is being asked to do three different jobs. The two components that take the closing load are S-7. The die head is held harder than anything else in the set, apart from the surface of the ejector pins.

Construction materials and hardness requirements for typical miniature die components, as published in the miniature die casting section of the NADCA product specification standards. The source prints the hardness values on the Rockwell C scale, and the material designations are the American grade designations.

The three stocks in the list

Two of the three materials in that table are tool steels in ordinary service everywhere else, and one is a different animal. H-13 is the 5 percent chromium hot work steel used across die casting, forging and extrusion, and it is the grade that faces the molten metal in this die. S-7 is a shock resisting grade, formulated for tools that take blows rather than heat, which is why it appears on the shank and the ejector plate where the slide closes against the machine. T-1 is a tungsten type high speed steel, a cutting tool steel by origin, and it is carrying the die head here because the impression is small and has to hold its shape.

The tool steels named in the miniature die component list and the role each one takes in the assembly.

Why the inserts run softer than the shank

The inserts and subinserts are held at 46 to 48 HRC and the shank and ejector plate at 54 to 56 HRC, and the gap between them is deliberate rather than a mistake in the list. A hot work steel loses toughness as it is driven harder, and the insert is the component that sees the thermal cycling of the zinc and the die spray. Running it in the upper forties keeps the toughness that resists heat checking, and keeps a margin for reworking the impression. The shank and the ejector plate see no molten metal at all, only the closing load, so they can be held harder for wear and for dimensional stability. The same logic is behind the core pins at 50 to 55 HRC hardened through, which is a small diameter part that has to resist wear and bending at once.

The ejector pins are the exception and they are given a case. Held at 65 to 74 HRC on the surface, a pin resists the wear of sliding against the die, while the core of the pin stays soft enough to survive being driven. That combination is only available where the hardness is produced by a surface treatment rather than by hardening the whole pin through. The trade off is between the two properties, and it is set out on the impact toughness chart and explained for a hot work steel on the H13 die softening page.

Grades and stock

Of the three steels in the component list, the H-13 line is the one we produce and hold, in forged bar and plate, and it covers the inserts, the core pins and the ejector pin stock on the list. The H-13 page carries the analysis and the supply forms, and the alternative for a die that is proving crack sensitive is on the H11 page. S-7 is a stock line as well, because the shock resisting grades are wanted in the same tool rooms, and its hardness range and treatment are on the S-7 tool steel page and the S-7 hardness chart. T-1 is a tungsten high speed steel rather than a hot work steel, it sits outside our standard stock range, and where a die head is specified in T-1 the block is normally sourced to the drawing.

Where to go next

The hot work grades themselves are described on the H13 tool steel page, and the materials used in the components of a conventional die casting die are listed on the die casting die component materials page. Grade choice by cast metal is on the die casting die steel selection page and the core and insert choice on the core and insert selection page. The zinc side of the process, which is where miniature die casting mostly sits, is covered on the zinc and magnesium die materials page. How these dies actually fail is on the thermal fatigue cracking page, and how a grade is confirmed against its certificate on the quality verification page.

Before you use this as a die specification

This page is a reference summary of a published source and it is not an Aobo Steel specification. The component list, the materials and the hardness values are reproduced from the NADCA product specification standards, which is a copyrighted document published by the North American Die Casting Association, and the reasoning offered around the numbers is general tool steel practice rather than a requirement of that document. Hardness windows on a working die are set by the die builder against the part, the machine and the run, and the grade actually supplied is confirmed on the job.

Source, NADCA Product Specification Standards for Die Castings, Engineering and Design section, Miniature Die Casting, North American Die Casting Association, 2009.