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Hello All!
For the last few months or so, I've been reading tons of information about casting aluminum parts at home. I've been slowly collecting all the bits and pieces I will need to make my furnace and crucible. I'm pretty darn excited to get started!
I understand the concept of making your patterns approximately 2% larger (for aluminum) to deal with the shrinkage that occurs when the newly cast part cools. This makes perfect sense to me. But I'm still left with some nagging questions I can't find answer to. So I will simply ask the experts!
My questions:
If you cast a part that has sand cores to allow hollow sections inside your final casting, how do you keep the core from destroying your casting as it cools? Wouldn't the core be non-compressible, and essentially break apart your casting as it cooled around the core?
And if the core does not collapse, and the part somehow safely cools around the core without breaking apart, do you need to re-adjust the 2% shrinkage factor to compensate for a part that ends up larger than expected?
Thanks everyone in advance for your thoughts.
Take care,
David
Nobody has any thoughts on this?
Well, looks like I'm a one man show on this one! HA!
That's OK, because I just got some great info from an engineer who figures this stuff out for a living.
According to this individual, sand cores can actually collapse a little bit as the aluminum cools around it. In fact, he said there are core formula systems the big foundries use to calculate the amount the sand will collapse. But even with this, he said the amount the core will collapse is still very small.
Adding to that, he also said that cores will provide a fair amount of structure to the casting as it cools, effectively leaving you with a larger than expected final part.
So the real answer to my two scenarios is "both." The sand core will collapse a tiny bit, and at the same time, the core will also keep the cooling part from shrinking as far as estimated. He says there's a bit of magic involved in figuring out where and how to calculate for shrinkage when using cores.
Well, I hope that helps! It answered my questions.
Take care all,
David
do not expect high precision from sand casting. shrinkage can vary by alloy and casting thickness.
sand cores i have made using sand and molasses and cooking in a toaster oven leaves a sand core with the strength of a cookie. it is not suppose to be a strong as a brick. a sand core just needs the hold back against the weight of liquid metal as it cools.
the taller the casting the stronger a core needs to be.
ideally the heat from the cooling casting will cook the core binder more so the core will crumble or be easy to break up and remove. the sand and core binder for cast iron would need to be different than one for aluminum. for example cast iron can be hot enough that the sand actually starts to melt (especially if it is not a high temperature rated sand) sand sticking to a cast iron casting can leave a rougher finish. venting or leaving a way for water vapor to escape is more important at higher temperatures. yes there are core materials that can leave a very nice cast finish but chemicals and equipment for this can be expensive.
once had a drop on water in a steel mold i was pouring cast iron into. it exploded like a fireworks explosion with me in the middle with sparks going at least 10 feet in all directions.
How much does quartz (the major component of greensand) shrink over the same temperature range, anyway?
That's actually a funny question, because quartz shrinks suddenly with a phase change (which is particularly troublesome to potters, where the expansion can blow pieces apart on heating, or shiver glazes right off the surface when cooling. Kilns have to go very slowly!) IIRC, this occurs at a temperature where the metal is already solid, so it doesn't matter.
The only case that would matter is a truely solid core, such as a gravity mold / die casting, or you can use steel rods in sand molds as well (Gingery recommends this). The metal forms an as-cast interference fit around the shaft, which probably stretches the hole a bit.
BTW, don't forget that, when a ring expands, it expands in all directions equally. The inner and out diameters grow proportionally.
Tim
Hi RexRod
Sand cores are not for precision. Usually surfaces are machined afterwords if needed.
A Product to check out for cores is "Pepset". I have no info on it. If your core is held in place correctly and a good size Riser and clean gating there should be little problem
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