The material presented in this video are some highlights from NADCA’s online webinar - Defects in Die Casting: Surface Defects and Other Problems. This second course covers defects caused by such things as venting problems, surface defects such as “white surface” and die spray marking. It then goes into the two main types of cracking, leakers, flash and flaking/spalling during machining.
We know the cause of cracks and they are often relatively easy to stop. I was in a plant two weeks ago where intermittent cracks kept occurring and disappearing. When we checked the casting was always cracked, but it was not on the surface it was inside the casting. It was a hot crack, but we'll get more into that later. Cracks are one of the easiest things to fix and one of the most annoying.
You have hot cracks usually sitting in the middle of a place where the heat is coming from both directions, called converging heat flow. These are usually hot cracks and I'll show you those in a minute. Cracks, tears or hot cracks have many causes but are usually partially caused by shrinkage cracks on the surface. If you imagine the area in the previous slide, the one area is frozen, but the other area is not it is part of the rice grains in honey. As soon as you pour it starts to shrink in both directions it opens up the crack.
Most often the casting is stretched in the die as it cools because the die doesn't change dimensions since steel is strong. While the casting is cooling and contracting, the stretching causes cracks in the weakest point often the last place to solidify where the aluminum or zinc hasn't totally frozen or where it is still very weak.
So hot cracks are a localized thermal effect. Shrinkage cracks on the surface occur during solidification, and have a dark surface. so when you break it open, you often see dark marks on the surface. Or it can look like this on the right where you have a lot of shrinkage cracks on the surface. For castings that crack while cooling in the die, the crack will also be at a hot spot, but it will not have a light surface - increase radii.
If we look at a typical hot crack, it looks like that. It is usually not long but it is wide between the distance of the two faces. If you cut it and polish it you see that. So again the crack is not deep but it is wide on the surface. That means it cracked while it was hot. That means this area is most probably a hot area. And if it is a hot area cool it, if it is a cool area warm it.
Mechanical stress can cause cracks when the die opens or the casting ejects (or during slide operations). These are usually sharp cracks. Cracks at the base of long cores or fins, dragging or sticking of projections into one die half may indicate die shift when the dies separate. The factors in die alignment should be checked such as die droop" (I saw that four weeks ago), as the die opened it dropped slightly, worn guide pins, work linkage, worn tie bar bushings, worn shoes under the moving platen, uneven tie bar stress, but the most common is that the casting is in the die too long
Courses for the online education system can be purchased in a variety of ways and vary from purchasing one course to purchasing entire training blocks.
For information on purchasing a downloadable copy of this webinar in its entirety, please visit: diecasting.org/store/detail.aspx?id=WEB228