Lost Foam,refractory coating,etc.


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Posted by Xandor on October 12, 1999 at 19:57:49:


I have had a certain amount of success using this process for both aluminum and bronze, including hollow cast (loose sand core) with (high density) styrofoam as well as urethane foam.

I find, however, now that I'm trying a larger scale (approx 1 foot^3) that I am getting problems with porosity, voids, and incomplete surfaces.

The smaller pieces have been quite simple to set up, without any gating to speak of: just the pouring cup sitting atop a foam runner. Although I thought it was weird the first time I did this and it worked, I have subsequently theorized that:

1. The foam nearly instantaneously evaporates, nearly amounting to the equivalent of a void.

2. The incoming metal is of a sufficiently higher density than the sand such that the sand does not collapse inwards.

3. The gas escapes primarily through the sand itself (I'm using dry, unbonded) and is probably not in a tremendous quantity to begin with given the relatively small mass of the foam.

I'm not certain what caused the kind of "scarring" I saw on my larger piece (a two-foot high torso), but I know I was pushing my luck. At this point I think I will try some sprues so that the metal enters the foam at a few places, and I would also like to use some kind of "permeable refractory shell" that I have seen references to. The latter seems to be a difficult puppy to locate, but I'm wondering if anyone has actually tried the alcohol/graphite mixture referred to in this forum. Also, am I mistaken about the gas escaping from the sand and should I actually be using vents? Some people here (San Diego State) think I should be using bonded sand (for safety?), but I think that would increase the venting difficulties. Also, might it be better to create a sprue so that the metal pours from BELOW the foam rather than pouring in from above?

Very interested in anyone's comments!

- Xandor

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