Re: Aluminum shrinkage?


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Posted by OhilB on April 12, 19100 at 18:03:36:

In Reply to: Aluminum shrinkage? posted by Jack Fisher on April 09, 19100 at 17:30:12:

I'm also relatively new to this topic (1 year) but I have spent some time researching and experimenting. Two types of shrinkage occur. When the aluminium changes from liquid to solid. This is quite a large value but you should not need to worry about it as your mold/sprue design and progressive solidification should replace this loss. The shrinkage that you need to worry about is once the object becomes solid (at ca. 650 deg C) and then cools to room temperature. The average linear coefficient of thermal expansion over the temperature range of 20 to 650 deg C is approximately 26 x 10^-6 per deg C. This results in a contraction of 1.6% when cooling from 650 deg C to room temperature. In theory at least the size of the object is irrelevant. This number is reasonably in line with the 1/8" per foot recommended in another reply. I have performed a number of shrinkage tests and had a range of 1.3% to 2.1%. I suspect that the large value includes some shrinkage during solidification.

I suspect that aluminium is not an appropriate material for your application unless you can totally avoid the ingress of grit/sand. As a minimum I presume each segment has a hardened steel hinge pin/bush and seals linking it to the next segment.

Two hundred sand molding operations sounds like a lot of work. Have you carefully considered all alternative molding alternatives. A metal die might represent a considerable saving in time.

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