incomplete pour problem

Roy wrote:


Ah, the sand gets reused, then.
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On Wed, 14 Apr 2004 10:57:09 -0700, Offbreed

yes, Greensand weather its water or opiol bonded is reuseable many many times..........probably never wear it out in a home shop environment. When it gets weak in strength, you add more binder and add oil or water and remull it and use it again.
So its important to not put anything that may screw up your sand in the mix. Visit my website: http://www.frugalmachinist.com Opinions expressed are those of my wifes, I had no input whatsoever. Remove "nospam" from email addy.
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IIRC, the "This old house" show was investment casting where the mold is ceramic. Usually around 1/8" thick. The ceramic is fired at something like 2000 degrees, comes out of the oven, and is the pour is done within seconds. I've seen stainless steel done this way, mold is glowing bright orange, pour in the steel and it instantly goes incandescent.
Joel Corwith wrote:

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Yeah, that occurred to me after the previous comment. They also had to melt the wax out and lose it {lost wax technique}.
Joel. phx

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I got a personal guided tour of a high volume investment casting house. Brass, aluminum, stainless, you name it. Pretty nifty process.
I also found the prices qouted on the TOH segment to be pretty outlandish. The shop I toured would do them for about 20% of the price quoted on TOH This would be in quantities of 20 to 50 or so.
Joel Corwith wrote:

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Roy J wrote:

That's because they're just a bunch of slobs who do investment casting. TOH only deals with artistes, who have better marketing -- er -- aesthetic -- skills.

They used plaster of paris, prompting me to do some web research. Apparently it's a good way to do near-investment quality casting in aluminum and bronze, but can't be used with steel or iron because the melt temperature is high enough to fry the plaster before the cast solidifies. Do an Altavista search on +plaster +aluminum +casting or some such for links.
--

Tim Wescott
Wescott Design Services
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wrote:

I memory serves me,,and sometimes it doesn't,, thought they were investment casting those doorknobs,, if that were the case they would have been removing the molds from a burn-out oven after burning off the wax. Bear
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Dies Deambulo wrote:

Dies, I had the privilege of shoveling greensand into a muller as a night job while in high school oh-so-many years ago. The foundry was in back of a pattern shop, and 2-3 nights a week and Saturdays, a top skilled foundryman would work a second job there. He was a magician with sand, an expert marksman, and had the steadiest hands of any person I have known. He could draw a cylinder eight inches tall with less than a half-degree of taper on the sides up out of a mold freehand, and never disturb the sand at all. I would hold my breath while he did some of those draws...
On to some suggestions: 1 Make the sprue larger (1.5"- 2" is not out of the question) 2 Make a big reservoir in the drag (bottom) offset toward the casting 3 Cut wide, thick gates in the cope, (top) from the reservoir to multiple points in the casting. I saw one thin section cast with a gate leading more than half way around the casting. At intervals, feeder gates were cut in the sand from the surrounding gate to the casting. 4 On the farthest corner from the sprue, poke a 3/4 inch diameter riser up through the cope a short distance away from the casting and gate this to the casting. You need the air to get out of there fast! It also serves as a secondary reservoir to reduce shrinkage as the metal cools. 5 Pour as cool as possible. Having said that, the red-almost orange color mentioned in another post is my best memory of what we poured at. The molten metal in the pot will be dull on top, stir it a bit to observe the color. BE SURE THAT EVERYTHING that touches molten aluminum is DRY! 1200 degree steam has a LOT of power to throw metal in the wrong directions [first hand experience talking]. 6 Work the sand fairly dry if using water bonded greensand. Water (overly wet sand) rapidly cools the metal.
The theory is kinda like this; you want a lot of molten metal to move into that hole in the sand very quickly with minimum turbulence. The cooler it is, the smoother your casting is and the slower it flows so the metal freezes quicker, with less shrinkage. BUT,you want the metal hot enough to flow into all corners and pick up the detail, however the trade-off is... at some point the metal gets hot enough to actually shape to and bond to the grains of sand. This gives you a nasty casting covered with silica that will dull your hss tooling very quickly. At even higher temperatures, you alter the alloy and burn off some elements, causing ugly castings.
Some alloys just don't flow quickly. If you are using scrap extrusions for your pour, you may not have as good a luck as with a specific alloy made for casting (no flames on this, please! I use scrap too).
Good luck! When you are through I might be interested in the patterns...
Rex in Portland in the rain, wishing the forge/furnace was running today.
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Wow! Would have loved to see that guy in action. It would make a wonderful piece for the History Channel.
And a hardy thanks to all! I did a perfect casting of the base today! I experimented at night to get a feel for the temperature using a steel rod to check the metal. In one or more of the many suggestions that I read, someone said the temperature would be correct when the aluminum stopped sticking to a small steel rod. That seemed to work for me. I tried bigger vent's than just a small wire and put the sprue directly in the center of the pour so that the metal would only have to make one turn to be in the mold. Don't know if that helped but it seemed to make sense to me. Anyway, bottom line is I got the base for my lathe! And I feel like I learned a lot. Well, maybe a little...
Robert

If time is the 4th dimension, then money is the 5th dimension. We are always concerned about dollars per hour for the space we occupy. - Rod White
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