Casting
Die maintenance in casting: shot counts, PM and tool life
Die maintenance is the planned care that keeps a casting die making good parts: cleaning, inspecting and repairing it at set points in its life, so it does not fail in the middle of a run. The best trigger is the shot counter, not the calendar, because a die wears with every shot, not with every day. Set three levels of PM by shot count, record what you find each time, and you will know when a die needs repair, a rework or retirement long before scrap tells you.
Why a die needs its own kind of PM
A pressure die casting die is a block of hardened tool steel that is heated by molten metal, cooled by water and sprayed with release agent, over and over. In a typical aluminium shop that cycle repeats every minute or so for two or three shifts a day. The die does not care what date it is. It cares how many times it has been filled.
That is why a monthly PM rarely fits a die. In a busy month the die runs far past its safe interval. In a slow month you strip and clean a die that has hardly worked. Run the PM on the shot counter and the work follows the wear.
One word matters here. A shot is one fill of the die. A die with two or four cavities makes two or four castings per shot, but it wears once. If your production report counts castings, divide by the number of cavities before you compare it with a PM interval.
The same applies to wax injection dies in investment casting and to gravity dies: lower loads, longer intervals, same trigger.
How a casting die wears out
Standard references on die casting name a few main ways a die fails. Knowing them tells you what to look for at each PM.
- Wear and erosion. Fast-moving metal scours the die near the gate and in thin sections. You see it as washed-out edges and a gate that slowly grows.
- Heat checking. The die surface heats and cools on every shot. Over time a fine network of surface cracks appears, and the cracks print onto the casting as raised lines.
- Thermal fatigue cracks. After many cycles some cracks grow deeper, often from corners and cooling lines. A deep crack can end a die's life.
- Soldering. Aluminium sticks to the die steel, usually where the die runs hot or the release spray is thin. Parts drag, ejector pins bend and the surface tears.
- Mechanical damage. Bent or seized ejector pins, worn slides and cores, damaged parting lines and blocked vents. These are often the first cause of flash and short fills.
Die life depends on the alloy, the die steel, the metal and die temperatures and the cycle time. Older textbook tables give a maximum of around 100,000 cycles for an aluminium die and around 1,000,000 for zinc. Treat those as a rough order of size, not a target. Your own shot counts and findings are the only numbers that tell you what your dies can do.
Die maintenance in three levels
Most die shops end up with three levels of work. The shot counts below are placeholders: start from your die maker's advice, then tune them from what you find.
- 01
Level 1: every shift, die in the machine
Wipe the parting line, check the release spray pattern and nozzles, clear the vents, look for soldering and flash on the first castings, and check water flow on every cooling line. Ten minutes, done by the operator or setter.
- 02
Level 2: interval PM, die out of the machine
At a set shot count, pull the die to the toolroom. Clean the cavity, polish light soldering, check heat checking against the last record, change worn ejector pins, check slides and cores, and pressure test the cooling circuits.
- 03
Level 3: full overhaul at a larger count
Strip the die fully. Crack test critical areas, repair by welding or insert change where needed, and send the die for stress relief if your die maker or heat treater advises it at this stage. Then decide: carry on, rework or plan a replacement.
- 04
Every level: record and reset
Write down the shot count, what you found, what you changed and who did it. Reset the shots-since-PM count, never the shots-since-new count.
A worked example from a typical Indian die casting shop
Worked example (illustrative)
Take a pressure die casting shop near Chennai that makes aluminium housings for two-wheelers. Die D-14 is a two-cavity die on an 800-tonne machine. It runs about 1,100 shots a day over two shifts. All figures below are illustrative.
Until now the toolroom pulled the die on the first Monday of every month. In a busy month of 26 working days that meant about 28,600 shots between PMs. In a slow month of 9 days it meant about 9,900. Flash and soldering always showed up in the last week of the busy months.
The plant switches to shot-based PM. The production report says 2,200 castings a day, which is 1,100 shots, because the die has two cavities. The team sets Level 2 at every 15,000 shots and Level 3 at every 60,000 shots, and keeps the every-shift check as it is.
| Item | Working | Result |
|---|---|---|
| Shots per day | 2,200 castings / 2 cavities | 1,100 shots |
| Level 2 interval | 15,000 shots / 1,100 a day | About every 14 working days |
| Level 3 interval | 60,000 shots / 1,100 a day | About every 55 working days |
| Shots since new today | From the die record | 52,400 |
| Next Level 3 | 60,000 minus 52,400 = 7,600 shots | About 7 working days away, so book the toolroom now |
| Heat checking | Found at the gate at 30,000, larger at 45,000 | Crack test at Level 3 and plan a weld repair |
What changed. The PM now arrives at the same amount of wear every time, so the findings can be compared. The heat checking trend at the gate gives the toolroom a week of warning to plan a weld repair instead of finding a crack after a run of scrap.
Die PM checklist you can copy
Use this as a starting point for one die family. Replace the intervals with your own once you have a few PMs of findings.
| Level and trigger | Check or task | What to record |
|---|---|---|
| Level 1, every shift | Parting line clean, vents clear, spray nozzles working | OK or not OK, who checked |
| Level 1, every shift | Water flow on each cooling line, no leaks | Any line with low or no flow |
| Level 1, every shift | First castings checked for flash, soldering and short fill | Defect seen, action taken |
| Level 2, every 15,000 shots | Cavity cleaned, light soldering polished | Areas polished, photos |
| Level 2, every 15,000 shots | Heat checking compared with the last PM | Location and size, better, same or worse |
| Level 2, every 15,000 shots | Ejector pins, slides and cores checked, worn parts changed | Parts changed, part numbers used |
| Level 2, every 15,000 shots | Cooling circuits pressure tested | Pass or fail per circuit |
| Level 3, every 60,000 shots | Full strip, crack test of critical areas | Cracks found, depth, photos |
| Level 3, every 60,000 shots | Weld repair or insert change, stress relief if advised | Repair done, by whom, heat treat record |
| Every level | Shots since PM reset, shots since new kept | Shot count at PM, next due count |
Tracking tool life and deciding when to retire a die
Tool life is simply the number of shots a die gives before it is no longer worth repairing. You cannot know it on day one. You learn it by keeping two counts for every die, shots since new and shots since last PM, and a short record of what each PM found.
After a few PMs, patterns appear. Heat checking grows faster in one corner. Flash starts at the same count every time. A slide needs attention at every second PM. These patterns let you set better intervals and give you warning before the die fails.
When you face the repair, rework or retire question, look at four things together: how deep the worst crack is, how scrap and flash have trended over the last few thousand shots, how much the next repair will cost, and how many more shots the die still has to make for the programme. A die that needs a large repair just before the part goes out of production is a different decision from one with years of orders left.
Keep the drawings, repair history and approvals on the die's own record, so you can show an auditor what was done to the tool behind any batch.
Common mistakes
- PM on the calendar. A monthly PM is too late in a busy month and wasted in a slow one. Use the shot counter.
- Counting castings as shots. A four-cavity die at 4,000 castings has done 1,000 shots, not 4,000.
- Resetting the wrong counter. Reset shots since PM. Never reset shots since new, or you lose the die's life history.
- No record of findings. If nobody writes down where the heat checking was, nobody can say whether it is getting worse.
- Ignoring the cooling lines. A blocked or leaking line makes the die run hot in one spot, which speeds up soldering and cracking.
- Skipping preheat on start-up. Shooting metal into a cold die is a sharp thermal shock. Follow your die maker's preheat practice.
Sources
- Die casting (die materials, failure modes and typical die life) (opens in a new tab), Wikipedia
- Thermal fatigue (opens in a new tab), Wikipedia
- Tool steel (opens in a new tab), Wikipedia
- Investment casting (opens in a new tab), Wikipedia
- NADCA standards and publications (opens in a new tab), North American Die Casting Association
Frequently asked questions
What is die maintenance?
Die maintenance is the planned care of a casting die: cleaning, inspecting, repairing and recording its condition at set points in its life, so it keeps making good parts and does not fail during a run.
Should die PM be based on shots or on days?
On shots. A die wears with each fill, so a shot count matches the wear. A calendar PM is too late in a busy month and wasted in a slow one.
What is the difference between a shot and a casting?
A shot is one fill of the die. A multi-cavity die makes several castings per shot. Divide castings by the number of cavities to get shots.
How long does a die casting die last?
It depends on the alloy, die steel, temperatures and cycle time. Older textbook tables give around 100,000 cycles for aluminium and 1,000,000 for zinc as a rough maximum. Your own shot counts and findings are the real answer.
What is heat checking on a die?
Heat checking is a network of fine surface cracks caused by heating and cooling on every shot. It shows on castings as raised lines and is a main sign of an ageing die.
How often should a die be stress relieved?
Follow your die maker's or heat treater's advice for your die steel and alloy, and record each stress relief on the die's history with the shot count at which it was done.