Permanent Mold and Core Materials for Gravity Die Casting
Permanent mold casting, also called gravity die casting, pours metal into a reusable metal mold under gravity. The mold and its cores are the tooling, and what they are made from is settled by four things rather than by one. The published recommendations below set out the choice by casting alloy and by the number of pours the mold has to survive.
The four factors that decide the mold material
Four principal factors affect the selection of a material for a permanent mold or a core. They are the pouring temperature of the metal to be cast, the size of the casting, the number of castings per mold, and the cost of the mold material.
Those four variables are the basis for the three selection tables below. The first two decide how hot the mold runs and how much heat it has to pass, the third decides whether the extra life of a tool steel is worth paying for, and the fourth decides between gray iron, a low-alloy cast steel and a cast tool steel.
Recommended mold materials by casting alloy and number of pours
The table covers molds for small castings up to 25 mm (1 in.) and for medium and large castings up to 915 mm (36 in.). Each row is a casting alloy and each column is the number of pours the mold has to make, so the same alloy moves to a different material as the production run lengthens.
| Casting alloy | Number of pours | ||
|---|---|---|---|
| 1,000 | 10,000 | 100,000 | |
| For small castings (25 mm, or 1 in., maximum dimension) | |||
| Zinc | Gray iron; 1020 steel | Gray iron; 1020 steel | Gray iron; 1020 steel |
| Aluminum, magnesium | Gray iron; 1020 steel | Gray iron; 1020 steel | Gray iron with AISI H14 inserts; 1020 steel |
| Copper | Gray iron | Gray iron | Alloy cast iron |
| Gray iron | Gray iron(a) | Gray iron(a) | Quantity not poured |
| For medium and large-size castings (up to 915 mm, or 36 in., maximum dimension) | |||
| Zinc | Gray iron; AISI H11(b) | Gray iron; AISI H11(b) | Gray iron; AISI H11(b) |
| Aluminum, magnesium | Gray iron | Gray iron | Gray iron; AISI H11 or H14(c) |
| Copper | Alloy cast iron | Alloy cast iron | Alloy cast iron(d) |
| Gray iron | Gray iron(a) | Gray iron(a) | Quantity not poured |
Recommended permanent mold materials, reproduced from Table 1 of Permanent Mold Casting, in ASM Handbook, Volume 15, Casting. Where two materials are given in one cell, the source lists them without an order of preference. (a) Same composition as being poured. (b) AISI H11 is used when polish is required on medium-size castings. (c) For medium-size parts; recommended materials for large parts are gray iron with H11 inserts or solid H11 die steel. (d) For medium-size parts; large parts are not poured in this quantity.
What the mold table says
Gray iron is the most commonly used permanent mold material, and in the small-casting block it carries every alloy up to 100,000 pours. Tool steel enters the table in two places. For small aluminum and magnesium castings the 100,000-pour column adds AISI H14 inserts inside the gray iron, and for small zinc castings AISI H11 is named where a polish has to be held on medium-size castings.
In the medium and large block the pattern repeats more strongly. Large aluminum and magnesium castings call for gray iron with H11 inserts or a solid H11 die steel at 100,000 pours, and zinc castings take gray iron with AISI H11 from 1,000 pours upward. Copper is the exception, because its higher pouring temperature pushes the selection to alloy cast iron and no tool steel is named. Gray iron castings are marked as not poured in the top column.
A plain carbon steel such as 1020 appears in the low-count columns. It is cheap and adequate where the mold does not run hot, and the step up to H11 or H14 is a step up in hot hardness and in resistance to heat checking. That step is bought only where the pour count or the surface finish on the casting demands it.
Recommended core materials
Cores are recommended separately, and the split is by core size rather than by casting size. Materials in these two tables are listed in descending order of preference, so the first name in a cell is the first choice.
| Casting alloy | Recommended core materials(a) |
|---|---|
| Zinc | Sand, plaster, gray iron, 1020 steel |
| Aluminum, magnesium | 1010 or 1020 steel, sand, plaster, H11 die steel or equivalent(b), carbon(c) |
| Copper | Sand, 1020 steel, gray iron, plaster(d), graphite(c) |
| Gray iron | Sand, graphite, carbon, gray iron |
Recommended materials for small cores, below 76 mm (3 in.) in diameter and 255 mm (10 in.) long, reproduced from Table 2 of Permanent Mold Casting, in ASM Handbook, Volume 15, Casting. (a) Materials are listed in descending order of preference. (b) Hardened to 42 to 45 HRC. (c) For use with relatively few pours. (d) For casting of aluminum bronzes.
| Casting alloy | Material for indicated number of pours(a) | ||
|---|---|---|---|
| 1,000 | 10,000 | 100,000 | |
| Zinc | Gray iron, 1020 steel | Gray iron, 1020 steel | Gray iron, 1020 steel |
| Aluminum, magnesium | Gray iron, gray iron with 1020 steel inserts(b), sand, plaster(b) | Gray iron(b), gray iron with 1020 or H11 inserts(b), sand, plaster(b) | Gray iron(b), gray iron with H11 inserts(b), H11 die steel |
| Copper | Sand | Sand | Quantity not poured |
Recommended materials for large cores, above 76 mm (3 in.) in diameter and 255 mm (10 in.) long, reproduced from Table 3 of Permanent Mold Casting, in ASM Handbook, Volume 15, Casting. (a) Materials are listed in descending order of preference. (b) Except for openings with complex shapes, which require expendable sand cores.
The H11 die steel named for small aluminum and magnesium cores is hardened to 42 to 45 HRC, and carbon and graphite are offered only for relatively few pours. For large cores the step to H11 comes at 10,000 pours as inserts and at 100,000 pours as inserts or as a solid H11 die steel. The second footnote to that table keeps expendable sand cores available for openings with complex shapes.
Cast-to-shape molds and inserts
Mold parts can be cast to shape when the castings to be made are not complicated, and cast molds suit any application for which gray iron or AISI H11 die steel is recommended in the core table. They are used most often when production requirements are large and when mold replacement or shipping schedules require several of the same molds.
Full or partial cavity inserts are used to obtain longer mold life or to simplify machining, handling and replacement, and they can also carry venting, cool thin walls or heat part of the cavity. Cast-to-shape gray iron inserts for complex aluminum and magnesium parts run for 5,000 to 20,000 pours, depending on casting complexity.
Machining cost against material cost
Machining the cavity is often a more significant factor in the cost of a mold than the cost of the material it is cut from. With a gray iron mold taken as 1.0, and with heat treatment included where it is required, the published ratios of machining cost for identical molds are as follows.
| Mold material | Relative machining cost |
|---|---|
| Gray iron | 1.0 |
| Cast low-alloy steel | 1.44 |
| Gray iron with tool steel inserts | 1.73 |
| Cast tool steel | 2.16 |
Relative machining cost of identical molds, reproduced from the machining section of Permanent Mold Casting, in ASM Handbook, Volume 15, Casting. Heat treatment is included where it is required.
Permanent mold and core materials, printable PDF
The three published selection tables for permanent molds and cores, with the relative machining cost of each mold material, in one PDF with our contact details.
Grades and stock
The tool steels named in these tables are hot work grades. The general-purpose chromium hot work grade behind the zinc molds and the aluminum and magnesium cores is covered on the H11 tool steel page. Where the mold runs hotter or the run is longer, the higher-alloy grade is set out on the H13 tool steel page.
Cores and slides are frequently nitrided for wear resistance, and the tool steel nitriding guide sets out the process and the case depth that goes with it.
The same selection for pressure die casting is listed on the die casting die component materials page, which gives the component materials and their hardness. The wider list of tool types and the materials normally used for them is on the tool and die materials by tool type page.
Where a permanent mold is replaced by a pressure die, the selection moves to the high pressure die casting die page. Cores and inserts for that process are covered on the die casting cores and inserts page.
Permanent mold and core materials, printable PDF
The three published selection tables for permanent molds and cores, with the relative machining cost of each mold material, in one PDF with our contact details.
Before you use these values
This page is a reference summary of published practice and it is not an Aobo Steel specification. The materials are reproduced from the source tables, and a different casting alloy, a different pour count or a different mold design will move the choice. Final selection is made against the casting drawing and the machine the mold runs on.
Source: ASM Handbook, Volume 15, Casting (9th Edition Metals Handbook), ASM International, 1988.