This is one of the more confusing threads I have ever read, but none the less very interesting.
Someone named "floppyearedmule" (LOL) started this thread, and then 8 years later ringsnapper picked it up again, with all sorts of other folks apparently doing parallel builds, popping in randomly ?
After a quick read, I have a few comments, as follows:
1. I have followed numerous backyard "lost foam" casting projects, and so I researched lost foam castings (but have not done them myself).
The commercial foundries use polystyrene beads that are expanded into a metal mold via heating/steam.
The coating on the foam pattern has to be permeable enough to pass both liquids from the melting foam, and gas produced by the vaporizing foam.
The defects I have seen in backyard lost foam castings seem to be mainly due to using foam that is not polystyrene (rigid foam so that it can be machined), and perhaps from using a coating that is not appropriately permeable.
If the wrong foam is used, it may produce so much liquid and gas that it won't work regardless of the coating.
2. One guy showed me some lost foam castings, and he did not use any coating on the foam at all, and the castings turned out quite decent.
He just packed the foam in sand.
3. I experimented with sodium silicate cores, and learned what not to do.
I found that 3-4% of sodium silicate is all you need, and any more than that makes the core very difficult to break out after casting.
And putting a sodium silicate core in a bag with CO2 destroys it, which is why many people jack up the percentage of sodium silicate very high.
Sodium silicate sand should be gassed for 5 seconds only, and then no more CO2.
The SS cores I made and put into ziplock bags lasted less than a day on the shelf, regardless of SS percentage.
The SS cores I made at 4% with 5 seconds of CO2 have sat on the shelf and been perfectly usable after a year, and break out easily after casting.
4. Overheating aluminum causes all sorts of defects, such as porosity, and bad surface finish.
I heat my aluminum (356) as fast as possible to pour temperature (which is 1350 F), and then pour immediately.
Never stir aluminum; just skim and pour immediately. I don't use a degasser, and don't need one the way I do it.
1350 F is gererally hot enough to fill most mold cavities, and cool enough to give a smooth surface finish, especially in Petrobond.
5. I would probably bake those cores to be sure to drive off all moisture; perhaps at 250 F?
I generally lightly flame my cores with a gentle propane flame, to do the same.
6. I generally use a runner down the side of thin parts like that, with several gates.
Thin parts seem to fill better with knife gates, which are wider gates with fill the mold faster.
7. I have had molds like that trap air, and ruin the casting with a large air bubble in the casting.
I have since started venting the high points of the mold with small holes poked with a wire.
8. And the best way I have found to stop ss sand from sticking to a corebox (ss sand is very bad about sticking) is to apply blue painter's tape to the interior surface, and then apply a little Johnson's paste wax to the tape every time before you make a core.
There is not much that will stick to 3M blue painter's tape.
These are a few things I have discovered over the years.
Great thread whoever is posting on it !!!
I must say that floppyearedmule wins the prize for best screen name though.
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