Die Casting Die Cooling Line Design
A die casting die fails first where it runs hottest, and how hot it runs is set at the drawing board by the cooling line. This page sets out what the cooling line has to do, what happens to the surface temperature and to the hardness of the steel when the cooling line is made larger, and the rule a controlled test supports. A cooling line close to the surface lowers the peak temperature without raising the stress range the die has to carry.
Why the cooling line sets where the die fails
Through the casting cycle the surface of the die takes heat in when the melt is injected, and gives it up when the part is ejected and the lubricant is sprayed. In a thin section, a finger or a corner the heat has to leave in two directions at once, so the energy that a small volume of steel has to absorb is much higher than the average across the die. Those are the places that fail first, and they are the places a cooling line has to reach. The cooling line is the heat conveyor of the die. If it cannot pull heat away fast enough the surface temperature climbs, the steel softens, and thermal fatigue cracking follows. The size of the cooling line and its distance from the surface decide how much heat it can pull.
A larger cooling line lowers the surface temperature
The source study ran an H13 specimen with a 1.5 in cooling line through its centre, then opened the hole to 1.6, 1.7 and 1.8 in in turn. The thermocouple stayed in place through the whole series, so the readings can be compared directly. Every reading was taken at the same 9 second immersion into molten aluminium held at 1350 F. As the cooling line grew, the peak temperature at the corner fell from 1087 F to 909 F, and the total crack area fell from 108.56 to 35, a reduction of about 68 percent. The minimum temperature fell with it, and that is the part of the result that matters most.
| Item | 1.8 in | 1.7 in | 1.6 in | 1.5 in |
|---|---|---|---|---|
| Maximum temperature, F | 909 | 939 | 1002 | 1087 |
| Minimum temperature, F | 197 | 237 | 326 | 399 |
| Total crack area, x 106 µm2, after 15,000 cycles | 35 | 62.05 | 79.44 | 108.56 |
| Average maximum crack length, x 100 µm, after 15,000 cycles | 8.25 | 10 | 12.25 | 12.5 |
| Hardness at the average maximum crack length, HRC | 35.1 | 34.6 | 34.1 | 31.6 |
Effect of the cooling line diameter on the temperature at the corner of a premium grade H13 specimen, and on the cracking measured after 15,000 cycles in molten aluminium at 1350 F. A longer cooling line gives a lower peak temperature and less cracking, and the minimum temperature falls at the same time.
Hardness across the surface follows the same line
The softening is concentrated near the surface and fades with depth into the steel. The table below gives the hardness profile measured from the corner inward on the same four specimens after the same 15,000 cycles. With the 1.5 in cooling line the corner is down to 24.3 HRC, roughly twenty points below the tempered hardness of the steel, and even 0.2 in in from the corner it has not fully recovered. With the 1.8 in line the corner still holds 30.6 HRC and the soft band is shallower. A harder corner is a corner that resists the plastic strain that starts a crack.
| Distance from the corner, in | 1.8 in | 1.7 in | 1.6 in | 1.5 in |
|---|---|---|---|---|
| 0.01 | 30.6 | 29.4 | 29.3 | 24.3 |
| 0.02 | 33.7 | 31.3 | 30.6 | 25.1 |
| 0.04 | 35.6 | 34.2 | 33.9 | 27.7 |
| 0.06 | 38.6 | 37.3 | 36.3 | 29.2 |
| 0.08 | 40.1 | 38.2 | 36.9 | 29.2 |
| 0.1 | 40.5 | 39.4 | 38.3 | 30 |
| 0.2 | 43.5 | 42.9 | 42 | 34.1 |
Hardness measured from the corner inward on the four specimens, in HRC, after 15,000 cycles. The soft band runs deeper and softer as the cooling line gets smaller.
The design rule the test supports
Making the cooling line larger pushes the whole thermal cycle down instead of widening it. The maximum and the minimum temperature both fall, so the range that drives the stress stays close to where it was. That is what makes the change worth making. The die is moved towards a lower softening temperature without a large increase in the stress it has to carry. The source records a temperature threshold, below which the thermal fatigue damage stays small, and the peak temperature is what has to be brought under it. There is a limit on the other side as well. Put the cooling line too close to the surface and the hoop stress at the cooling line rises, so cracks can start there instead, in the thinnest section of the die.
What this means on the job
The steel in these tests is premium grade H13, the chromium hot work steel covered on the H13 tool steel page and held as die stock on the 1.2344 ESR H13 supply page. Why the grade is used at all is set out on the why H13 steel is used in die casting page. The cracking mechanism itself is on the thermal fatigue cracking page, the softening that starts it on the H13 die softening page, and the loss of hardness at temperature on the hot work softening resistance page. The two properties that set the gradient are on the thermal conductivity page and the thermal expansion page. The other modes that shorten die life are on the die wear and failure page, the parts of the die that carry the heat on the die casting die component materials page, and the way a grade is chosen for a press on the high pressure die casting selection page.
Before you use this as a die design specification
This page is a reference summary of a published study and it is not an Aobo Steel specification. The temperature, cracking and hardness values are reproduced from the source, which tested one grade, one specimen shape and one set of casting conditions. What a production die reaches also depends on the die design, the alloy, the cycle and the cooling circuit that feeds it. Final die design is confirmed on the job.
Source, Effect of Design Factors on Thermal Fatigue Cracking of Die Casting Dies, D. Schwam, J. F. Wallace and S. Birceanu, Case Western Reserve University, final technical report to the U.S. Department of Energy, Award No. DE-FC07-00ID138486, October 2004.