Why thermal design is important.
The material presented in this video are some highlights from NADCA’s online webinar - Engineering Dies: Thermal Design - Why Bother. This covers WHY the thermal design is important. It introduces the simple, but important variables such as surface area and the amount of heat that needs to be controlled. It has been clearly shown that using excess die spray to control die temperature usually results in lower die life, higher variation in quality day-to-day and increases gas porosity. Thermal design is important but can be simple.
For information on purchasing a downloadable copy of this webinar in its entirety, please visit: diecasting.org/store/detail.aspx?id=WEB320
Transcript from the video highlight,
"Thermals cause a lot of problems in die casting. A lot of problems. More and more, we're seeing that the thermal design of the die is somewhere around 80% of the problems that we're seeing. And it can be anything from a little mama and papa shop out the back with two die casting machines to some of the big guys, some of my customers have got 750 die casting machines, one of them has got over 1000. But it's a consistent problem.
Thermal problems. Shrinkage porosity in thick sections. You can't always stop the shrinkage porosity, but in most cases you can. So if you know you've got a thick section, you can do a quick review of thickness using a CAD or using some of the other simulation packages for thermals. And you can see that there are thick sections and you've got to cull them. So you put the appropriate culling in, that's it. As long as that casting section freezes before the surrounds, then there will be no shrinkage. Shrinkage porosity occurs because aluminum, zinc, magnesium, brass, all shrink as they freeze.
Cold flow in thin areas. We know it's a thin area. We can work out how much heat we need in that die and we can add water cooling or not, or thermal, or change the design of the die so that it's a more thermally isolated, not insulated. We can do that to make a die hotter by the very design of the die. We're going to go through that in later webinars.
For in some cases we put water cooling, other cases we put oil heating, and in other cases we thermally isolate each section of the die so it becomes quickly to heat up and quickly to cool down. It's called a ballerina die.
Cracking and sinks in hot areas. Cracking is mostly a thermal problem. If one section is cooling faster than another, it wants to shrink, it will apply stress on other areas. If they are hot, then those areas will crack because all molten metals, as they get to ... as they just freeze, they have very little strength. I can take a screwdriver and push it through a casting just as it's frozen. So if it's sitting just below the solidus, solid temperature, I can push a screwdriver through the casting. It's very weak.
Sinks in hot areas are just shrinks just near the surface. Often in one die you can get a casting that's cracking in one spot and sinking in another. Which makes it difficult, but it's very, very common unfortunately. So the cracking phase, I'm shrinking in the die get me out sooner. Eject after three seconds, culling in the die or you get hot cracking.
The sink is saying leave me in the die for four seconds, otherwise you're going to get a sink on the surface. So you have this compromise. Over three seconds you get cracking, under four seconds you get sinks. So you eject at 3.5 seconds and some are cracked and some have sinks, and you say gee, this is a difficult pass.
If you had done the thermal design from the beginning, then it wouldn't have been.
To purchase access to this course, visit the NADCA marketplace at www.diecasting.org/store. Purchasing a course through the online education system grants you one year of access to the recording, presentation, support material and test when applicable.
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