Die Life Planning for Die Casting Dies
How many shots a die will give is the first question most purchasers ask, and the tooling section of the NADCA standards answers it with a method rather than a number. The method rates each area of the casting for what it has to do and for the life expected of it, turns the pair into an inserting plan, and records each insert so that its history stays on file. This page sets out the two rating scales, the plan and the identification matrix they feed, the seven measures the standard lists for adding die life, and the grading used to decide when an area of the die is spent.
Why the answer is a plan and not a number
Die life is a function of many factors. The commercial section of the standards lists part design, the configuration of the part in the die, the quality expected of the part, the type of tool steel used for the die, the heat treatment of the die and the alloy being cast. Even when the die caster makes every effort to extend life, an early failure is still possible, and an unusually long life is possible as well. The tooling section puts the same point another way. Aluminum and copper dies wear out because the metal being cast is aggressive and is cast at high temperature. What a customer and a die caster can control is where that wear lands and how cheaply it is repaired, and that is why the standard sets out a way of defining the critical areas of the casting before tool design begins.
The two rating scales
The guideline pairs two ratings. One describes what an area of the part has to do. The other describes the life expected of it, expressed in shots. An area is graded on both, and the pair decides where an insert is worth planning and how early that insert should be replaced.
| Part consideration | What the class means | Estimated die life | Shots |
|---|---|---|---|
| Class A | Critical to function and to cosmetics | Class 1 | Less than 10,000 |
| Class B | Cosmetic, no function | Class 2 | 10,000 to 25,000 |
| Class C | Critical to function | Class 3 | 25,000 to 50,000 |
| Class D | Not critical but functional | Class 4 | 50,000 to 100,000 |
| Class E | No function | Class 5 | More than 100,000 |
The part consideration classes and the estimated die life classes published in the guideline for increasing die life. They are read together, so a cosmetic area expected to run past a hundred thousand shots and a cosmetic area expected to run for twenty thousand are not planned the same way.
What the plan produces
With the ratings done, the next step is an inserting plan for the tool steel construction, and the last step is the matrix that communicates replacement needs and carries the history of each insert. The source prints two examples of that matrix, one as it looks at the start of a new project and one as it may look after the first year of production. The sample part used with the method labels its areas in the same language as the rating table, separating sharp corners, cosmetic surfaces, nonfunctional surfaces, a sealing surface, a mounting surface, an O-ring surface and a radius.
The standard is explicit that this is a suggested approach and one of several possible. Die casting dies do wear out. Laying the plan out at the start of a project allows an insert to be replaced before it fails and production is lost, and the matrix is only useful when it is written for the actual tool construction and the actual annual usage, part by part. The same reasoning sits behind the die casting die component materials, where each part of the die is listed with the material and hardness it carries.
Seven measures that add die life
The guideline lists the measures that can be taken before tooling starts, and it prices each one in the currency that a customer cares about. Three of them change the tool or the part and cost money. One changes nothing but the part design. The last of them is about the hardness a die loses as it works.
| Measure | What the source says it costs |
|---|---|
| Redesign the part to remove or reduce sharp internal corners and features that promote early cracking of the tool steel | No tooling cost, but it is a change to the part design |
| Use special tool steels in the areas where high wear is expected | Increases tool cost |
| Insert the areas of the cavity blocks so they can be replaced more economically after wear has occurred | May increase tool cost and brings replacement forward |
| Shoot blast the surface of the tool steel to help reduce heat checking and cracking | Adds to tool cost, and it leaves a surface texture on the die cast part |
| Add a vibratory, shot blast or deburring operation to the part | Added part cost |
| Add a machining operation to remove heat checking or cracking in the areas that are critical on the part | Adds to part cost |
| Reclaim the surface hardness when it drops from the 40s HRC into the high 30s HRC | Needs a reheat treatment route for the die, and it is stated as an aim rather than a certainty |
The measures listed in the guideline for increasing die life, with the cost each one carries as the source states it.
Deciding when an area is spent
The failure mode the grading exists for is heat checking, which the tooling section also calls thermal fatigue cracking. A die in service develops small cracks that form a network and larger leading cracks, and both matter to tool life. The published grading uses two photographic scales, one for the network and one for the leading cracks, each running from 1 to 10. The die surface is compared against both, graded on each, and the two gradings are added. The combined reading is the degree of heat checking.
| Combined heat check rating | What the source says to do |
|---|---|
| Grading the network cracks, 1 to 10 | Compare the die surface against the photographic scale for the small cracks that form a network |
| Grading the leading cracks, 1 to 10 | Compare it against the scale for the larger leading cracks, then add the two gradings |
| A combined rating of six | The point at which a die making a critical surface, one that is to be polished or chrome plated, might be stopped |
| A combined rating above 14 | The limit given for surfaces that are not seen by the user of the finished product, which can run while the rating climbs toward it |
How the published heat check grading is applied, and the two points at which the source says a die might be stopped for a given class of surface. The comparison photographs in the source figure are reproduced there by permission of Uddeholm and are not reproduced here.
The scale has a second use. It gives a concrete basis for comparing one tool with another and for relating the condition of a die to the number of shots it has run, which is what turns a verbal report on die condition into a number that can be put next to a replacement cost. The cracking itself, and what drives it, is set out on the thermal fatigue cracking page.
Who repairs what
A die life plan only holds if the maintenance side of the contract matches it. The commercial section sets out the usual split. Minor maintenance is run-to-run maintenance of a serviceable die to keep production going, and it is normally the die caster who provides it. Major repair and replacement is normally the purchaser’s, and it covers replacing or rebuilding an entire die cavity, a die section or a complex core slide that makes up a significant percentage of the casting detail, along with major die resurfacing. The die replacement cost is sometimes asked to be amortized into the piece price instead.
Two details decide whether the arrangement works. The rapid wear components are frequently replaced by the die caster, and ownership of those components usually stays with the die caster, so the purchaser should read the maintenance practice rather than assume it. And where a customer expects something specific, such as a stress relief after a certain number of shots, the die caster has to know before the quote so that it can be included in the costs. Die preheating practice, gating design and die temperature control are named as particularly important to long die life, which is the subject of the cooling line design page. The stress relief and the white layer left by EDM both sit in the specifications the same standards place on the steel and its treatment, which are set out on the NADCA 229 acceptance page.
Where to go next
The grade that carries the longest runs in aluminium and magnesium is on the H13 tool steel page, and the choice of grade against each cast metal on the die casting die steel selection page. Where a die needs a core or an insert in a different material is on the core and insert selection page, and how a die actually fails on the die wear and failure page. The hardness a die loses in service, and the reheat treatment that brings it back, are on the H13 die softening page, the structure behind it on the carbide coarsening page, and the heat treatment of the hot work grades on the hot work heat treatment page. The toughness that decides how much cracking a die can take before it spalls is on the impact toughness chart.
Before you use this as a die life guarantee
This page is a reference summary of published sources and it is not an Aobo Steel specification, and nothing here is a guaranteed number of shots. The rating scales, the measures and the grading are reproduced from the NADCA product specification standards, which is a copyrighted document published by the North American Die Casting Association, and the grading figure the standard uses belongs to the company credited in it. Die life on a real job depends on the part, the tool construction, the steel, the treatment, the machine and the way the die is run, so the plan is agreed between the purchaser and the die caster and the steel actually supplied is confirmed on the job.
Source, NADCA Product Specification Standards for Die Castings, Tooling for Die Casting and Commercial Practices sections, North American Die Casting Association, 2009.