Chamfering Sand (Or How Not to Crack Under Pressure)

Discussion in 'Sand Casting' started by Melterskelter, Oct 21, 2021.

  1. Melterskelter

    Melterskelter Gold Banner Member

    Many of you know that I have repeatedly cast a couple of patterns that utilize sand cores bound usually with silicate and occasionally with epoxy to cast prism-shaped iron straight edges. The cores weigh 3 or 6 pounds depending on which mold is being cast and are shaped like a rectangular board with triangular-in-cross section lumps on them. Here is a six-pound core.
    Sand Chamfer5.JPG
    And here is the resulting cavity in the casting. 26 Casting3.JPG 26 Casting1.JPG 26 Casting4.JPG

    One problem I have had is too frequently (30%?) of the time I found defects in the casting surface from broken-off chunks of mold sand. The problem was that it was often difficult to conclusively find the origin of the sand. I was sure it was not there due to inadequate care in blowing out the mold after extracting the pattern and I was very gentle and careful when dropping the core into place.

    However, once that core was in, I could not inspect the mold nor blow it out any more. I was reluctant to pull the cores out after inserting them as they themselves are fragile and I try to minimize any manipulation of the mold so as to avoid damaging it.

    Eventually, I came to suspect the source of errant sand was from the the very edge of the mold impression where the coreprint intersected the main pattern.

    So, I installed a 45 degree chamfer piece on the core printing board at the intersection of that board and the pattern itself.
    Sand Chamfer1.JPG Sand Chamfer2.JPG

    That results in a nice 1/4" wide by 45deg chamfer in the sand.
    Sand Chamfer7.JPG

    I have cast this setup a couple times since and have had absolutely pristine results in the castings. There is no way to tell for sure that the problem is solved, but I think it is likely. I'll be pouring one today and give feedback tomorrow. I am pretty sure what was going on was two-fold. First, that hard unchamfered edge was fragile and as the main pattern was drawn the edge may have developed micro-cracks that were invisible and in some cases they were visible and patted back or blown out. And then, the edge was further cracked when I placed the core as the slightests irregularity in the core print would put point pressure on the sand and result in failure.

    Of course, the downside is the resulting ridge of iron at the part line. This is is easily and quickly trimmed off with my portable band saw. But it does add a cleanup step. That is worth in my book it if I can reduce casting failures. Hate those!

    In retrospect, I know it violates principles to have sand corners that are unchamfered. But I also know that many patterns pull out of the dand with a sharp edge where they intersect the part line. The addition of the core to the part line surface was different than closing a mold though, as there are more likely slight discrepancies in contour of the core/print intersection vs the "mutually-made" intersection of cope and drag.

    Anyway, I thought I would pass this experience along in the spirit of FWIW.

    Denis
     

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  2. Melterskelter

    Melterskelter Gold Banner Member

    And more follow-up on sand chamfering. 30 minutes ago I dug yet another casting completely free of any loose sand out of the flask. I am pretty well conviced of the benefit in this particular setup of making sure the sand core print does not have a sharp edge. I suspect the principal has more general application. (Probably I am the last one to figure this out ;-) )

    Denis
     

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