Re: Lost Foam,refractory coating,etc.


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Posted by bpfink on October 12, 1999 at 21:18:43:

In Reply to: Lost Foam,refractory coating,etc. posted by Xandor on October 12, 1999 at 19:57:49:

I would think the hollow core with loose sand casting you mentioned would be dependent by the success of a chill factor in order to work. The metal surface would have to gain some inflexibility to hold it's shape in order to not shift the sand segments. Too hot metal and the sand core could start to float, too cool and it simply would not fill. So good luck but sure sounds uncontrollably risky to me.

As the pieces get larger, and the wall thickness gets more massive the problems would either surface or increase in appearance. Surface detail and mass would be the telltale indicators.

I would keep the walls of the pouring cup large to hold the most metal needed to both fill the piece but also have a reservior to keep any gas burps or slow metal drops in check before the final fill.

I would also help the flow by slowly putting a red hot welding rod or copper stick down the sprue to melt it just before pouring to start the opening of the flow cavity and if needed reheat it several times to get that opening going.

Not sure about this mention...

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

though it may be true, I would not reach the same resoning.

You also mentioned:

"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."

I should surmise that more sprues will not help the flow if they are solid prior to the pour so no gas can escape... but more will help if they have melted enough for an air channel to let the gas escape... so do a pre melt with that hot rod and get a jump on it.

And you can also do vents in the sand with either

1. soda straws put end to end, or

if it is green sand or has any bonding at all (via moisture, etc.) just pack a welding rod to the styrofoam depth and then remove it at the last minute. (If this rod is copper or aluminum you can also get the end red hot and get a semi styro melt in the mold prior to the pull / removal of it.

I would tend to agree with your cohorts at San Diego U that a bonded sand would give much more control to your results and with these other mentions it should not slow the air / gas escape at all.

(Take a bonded sand test... cast as a small sand block in a paper toilet tube core, let set up, put one end in your mouth and see if you can easily still blow air through it? Do the same with your non bonded sand... and then spit.)

As for a BELOW sprue.... it would only tend to trap the gas all the more as the only way to vent is via both the sand and the sprue... and a longer burping could really disrupt the surface of the metal all the more... and 'wash' in more sand from the mold and inti the metal body.

But what do I know for sure... I like the more controlled and always bonded with something systems.

PS. The graphite and alcohol is usually painted or sprayed on the surface of a bonded mold and then a match torch flames the liquid off before the cast.

To put it on the styrofoam and set it in sand and then pour??? Sounds like you just put a bunch of graphite residue in and added some explosive fuel to a potential pouring. I personally do not even intend to try that.

Good luck on all this.

Try to imagine yourself the hot metal and doing all the things with what you are confronting it with. Make it a game and see if you can get around the hurtles and succeed, or do so without any surface damage and shrinkage from extreme stress.

Be careful; it's a ball park out there.



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