NeoTech, there is a machining allowance, but it wasn't enough. I would like to avoid taking off 6mm, however through better molding practices, if I can.
Andrew, I think you're right it would have been better to orient vertically, but I'd have needed a deeper drag. I do think more head on the pouring basin would have helped. I'm thinking maybe ramming softer might also relieve the gas, and reducing sand moisture, maybe. Also, there was no crud -- just a depression and bubbles, so feeding and gas/moisture relief were the problem.
However, look at Ironman's castings -- amazing stuff. Big wide plates, letter plates, muffler parts, etc, with super finish, 2 simple gates (like the ones I used/imitated) very low head -- copes are quite shallow, shallower than mine, simple small sprues, the plates oriented horizontally, and they come out finish quality, with no shrink depressions, no crud, no bubbles, crisp lettering. So it can be done. I'm just not doing it right.
MetalCaster, hello, welcome to the forum. Please introduce yourself, and tell us about your melting rig and projects. Do you cast in iron?
re. your suggestions/questions -- I used a 3000 degree rated clay/graphite crucible.
re. burner: the burner doesn't impinge directly on the crucible. The flame swirls around the furnace barrel all the way to the lid.
re tongs: the lifting tongs fit the crucible properly and it is lifted at the proper point in the bilge. (Please see my earlier construction thread re. "Sawed off cupola" for all of these details.)
re. firebrick: The hard firebrick I used has very good insulating qualities since it is 4" thick. Thickness counts in insulation -- read about thickness in relation to R values in insiulation if interested. I believe this furnace probably does nearly as well as some of the 2" kaowool foundries, and I'm sure it would compete with castable refractory furnaces of lesser thickness for insulation value -- there's also packed sand/clay with airspace in this build. This furnace does not get hot to the touch on the outside for a very long time after a melt is underway, and retains heat after it is over overnight.
Soft firebrick is holding up perfectly in the lid -- it has a barrier of about an inch of sand/fireclay under. Materials are not good or bad per se. -- the devil is in the details. Be careful of generalizations, like saying soft firebrick is unsuitable for use in an iron melting furnace, Specific cases, application and actual experience can contradict a lot of the commonly repeated axioms on the internet.
re. mold venting: my moulds were vented, as I mentioned earlier.
re. core venting: I wasn't using a core.
re. gating references: I have a lot of foundry books illustrating a multitude of gating and risering schemes, methods, and theory but if you watch Ironman's youtube videos, you will see perfect flat simple castings in iron with the simplest double or single gates. We aren't casting a V-8 engine block here.
re. crucible misuse via overtemp, or wrong metal rating -- it's possible the final temp was too high, as I mentioned above. I think there are many other possible factors, too, though. Oxidizing or reducing atmosphere, corrosive flux, etc. And as mentioned also earlier, the reason for the crack and mishapen upper portion was that the crucible got glued to the furnace wall and had to be pried off. That might also have been partly because of overheating. My bigger concern is crucible thinning. If the crucible is not up to par, then okay, but if it's a result of a bad furnace atmosphere, or the sodium carbonate I used, or over temp, then those are things I can, and would like to correct.
Again welcome, and please introduce yourself. A fellow metal caster wold be a great addition!