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Edge gating technique

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vtsteam:
Sorry to be so late with answers to a couple of questions above.

Jason, people have tried removing the foam pre-casting via solvents and via burnout. It just adds an extra step and time. For aluminum, at least, it is possible for me to get good results without doing that.

I have only one (failed) trial with iron so I can't say with that. From what I believe happened, the problem for me was the coating broke up before solidification -- but I was trying Satanite as an investment, rather than my usual gypsum based wallboard compound coating, or Plaster of Paris (for aluminum or zinc), both of which I thought might calcine at iron heat. No proof of that either since I did not try them, and I try to avoid definite statements about things I haven't actually experienced!

There are professional investments for iron, but I typically avoid most professional products in favor of home brew and readily available inexpensive or free ingredients, where possible. Just a personal quirk. If I can't do something homebrew style, I move on to something else that does work that way. For the record I certainly did not exhaust all the possibilities on the one failed iron pour. But since I get good results with greensand with iron, I didn't have the interest level to explore DIY lost foam iron. Maybe some day I will revisit.

re. blue/pink extruded polystyrene foam, and white expanded bead polystyrene foam. I've used both extensively in building model airplanes for years, and so I'm very familiar with them. As you say CI, expanded bead (white) foam is used extensively in iron lost foam casting. But that is because it can be molded itself -- generally using steam to expand the beads in the mold. Surface finish can actually be controlled quite well -- as can density. Bead size temperature, pressure and many other factors mean it isn't necessarily going to have a rough or beaded surface like a cheap ice cooler, packaging, or a coffee cup.

When a DIYer uses it -- it will generally be cut to shape, and will come from a rougher or, bigger bead, or lighter density source, and depending on how it is cut (knife, saw, hot wire, needle cutter) will vary in surface finish by all of these factors. But it would be an over-generalization to say white foam will have a worse surface finish than extruded foam, for casting purposes.

I agree, CI, that the fumes from lost foam casting are objectionable, period. It also impregnates the sand (which does not have clay or moisture in it, as greensand does). I hate that smell, so my use is only occasional, outdoors, and for small parts, if and only if I can't do what I want using greensand molding.

Or by fabrication. Even though I like casting, I also enjoy fabrication. Particularly brazing for small steel parts -- which not a lot of people seem to do these days. It works quite well for cast iron, too.

As far as machining allowances go, that's different from shrinkage allowance. Shrinkage allowance in applies to all of a casting. Machining allowance applies only to surfaces that will be machined. If you like castings in old engines, most of the surfaces of many parts will be left in as-cast condition. So you only need to apply shrinkage allowance to those surfaces. Where you will machine, you add the machining allowance. Of course you guys know this, but sometimes it's possible to forget in a general discussion about pros and cons.

In my experience, lost foam casting with extruded foam leaves a finer as-cast surface finish on aluminum, than greensand casting. This can also mean that a machined surface may need less of an allowance to get down to clean shiny metal. Really it's a judgment call to determine machining allowances for a pattern. That will be based on personal experience with what your own castings are coming out like in a particular material, and with a particular casting method.

As a dyed-in-the-wool simple DIY hobby caster, I use only scrap aluminum and scrap iron, and I do not heat treat aluminum. I also wait for aluminum molds to cool for at least an hour and allow at least 5 hours or more for iron before breaking out. I don't douse aluminum parts with water to cool them, as I see so many do on YouTube. I have never experience aluminum or iron part warpage due to rapid cooling.

I have occasionally experienced chill in cast iron, and in that case have heat treated in the furnace and slow cooled it to anneal it, and this has generally worked out well.

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