Small FOUNDRY CASTING

This is where I tell you all about foundry work. Yeah right, I don't have all that information, so I'll just tell you what I've done at home. I got the idea for building a foundry when I saw a Stirling engine I wanted to build. Just figured an as cast surface would look more original. So here goes.

I decided to build the furnace out of a 2.5 gal propane tank, and to fit a #6 crucible. Then I needed a burner, The Oliver upwind burner worked to melt aluminum. But it took a long time, 30 to 35 minutes, so I decided to modify it. Second burner was a little better, a Reil type with blower, still needed something. Did shorten the melt time though. The burner I have now is a combination of everything I have read on the net. Thanks all! Well now I'm melting aluminum in 10 to 12 minutes and copper in about 35 minutes. A closer look at My burner and My furnace.

Seems like one thing leads to another, I have no foundry equipment! So I started to build what I needed flasks, hand rammer, sprue cutter, etc.

Plywood, yes it worked just want something more professional. But you need something to start out with. So I had 3 of them, of different sizes. I used until I finished the castings for the aluminum flasks.

Lost foam, it work too, but nasty looking. These didn't last too long because I melted them down. It was an experiment anyway. This was the first time I cast anything in lost foam and loose sand. It looks like I need some more time with this medium.

Ok, this looks better. The sides screw together and are replaceable if melted or broken. I've made the patterns for several sizes, and upsets for the same, in case I need to increase the cope or drag height. the flasks are 2.75" high, upsets are 1.25" and can be stacked.

Now I have the flasks, and I need to start making castings. Oh yeah, patterns first. But this is the foundry page. Setting up a home foundry takes some time. I started this project in January 2006 and still don't have a Stirling/steam engine. I'm going to try to break this up into several subjects involved in patternmaking, molding, casting and machining. So here goes.


Molding Tools


The picture above is of my molding tools. Some home made some modified and some bought. These are what I consider the basics. As far as sand goes, I use olivine sand 140 mesh, premixed, using water as a binder. This is a personal choice, they all work about the same for any kind of home use. Sand should be reconditioned after every use. When squeezed hard in the hand the sand should pack tight, leaving little or no sand on the hand when released. When breaking the sand between the fingers you should almost hear a snap.

I try to keep things simple so if I can't buy it I make it. So when molding I use baby powder as parting, the kind that is all talc and fragrance. I use a flask that will leave at least 1/2" sand from the pattern to the flask. Ram the sand at least as hard as you can squeeze it, in your hand that will somewhat ensure a good mold. Watch the surface porosity, of the castings, you'll soon know how hard to ram the mold. Mounting the patterns on a snapboard will also help in ramming as you will be able to ram the cope harder with out fear of damaging the drag. Ram the mold in layers, leaving loose sand between the layers, until full. I will try to show pictures of the process when I can. The sand and how well you ram it is probably the most important part of the foundry business. The best pattern and the best alloy will not correct a dry sand or soft ram. Unless, of course, you don't mind spending a lot of time cleaning flash off a bad casting.


Now here goes I'm going to try to explain how to ram a good mold. My molding experience is limited, just what I've been told by molders I worked with and a few hours in front of a molding machine. And a course on casting practices, a semester in high school metal shop, I spent all my time in the foundry... go figure.
Molding

I ram the flask in layers, start by riddling a layer of loose sand over the pattern then
adding more unriddled sand on top to fill the mold about halfway. This works
for small flasks, mine are 7X8X2.75 deep. I start by ramming lightly increasing the
force as I go.
The drag mold should over filled and rammed tight by then. Scrape the mold smooth.
Two methods can be used for the bottom board. One make the bottom board larger than the
flask, or make the board smaller, all around, by about 1/4" that way you won't have to fill the
mold all the way, saving yourself some work, and some sand.
Obviously I made my bottom board bigger, but my flasks are small. If you do make them smaller
bed the board in some loose sand and use a rubber mallet pound it down tight. If done this way
make sure you don't hit the flask when ramming the cope, you'll knock the flask loose from the sand.

Of course before you get to this point a lot of other things have to happen before any of the above can happen. Patterns have to be made, calculations for gating, although not required, should be done. You can just cut a gating system and have it work the first time. I've found this doesn't happen very often, without some experience and a bit of luck. There are a lot of variables that go into casting a good part and just getting one that "will do". So I guess now would be the time to talk about How to Gate a Pattern.
Melting and Pouring

I've covered most of the processes up to the actual melting and pouring of the metal. At times this seems like this should be the easiest part of the whole thing. But this can be the point at which you'll get a workable casting, work of art or scrap. Now that the pattern is made, type of metal to be used, all the gating figured and the mold rammed, all you have to do is pour some metal down a hole, right? Maybe, but what about the temperature of the melt, outside temperature/humidity, gasses in the melt, pouring time, temperature drop while moving the crucible the furnace to the mold. All of these can effect the quality of the casting, most of these are of little use to the hobby foundry. As your skills improve and you demand more from the castings you make, paying attention to these areas will yield higher quality castings. Keeping notes on these variables should allow you to achieve similar quality castings in varying conditions.

Temperature of the melt is easly controlled thru the use a pyrometer. An electronic device inserted into the metal and measures the temp.it can be homemade using a digital voltmeter that has a temp option or just by one off the shelf.
Outside temp/humidity you have no control over. Keeping records of the successful pours will help in getting good castings regardless of these variables.
Gasses in the melt, you can add chemicals to you melt to eliminate gasses. For the most part this would not be required if your metal is poured into ingots first, effectivly removing most if not all gassing problems.
Pouring time, how long does it take you to pour your mold. for th average hobby foundry if it takes longer than 3 to 4 seconds to pour a mold than it taking too long. things ot look at would be gating size or you ability to keep the sprue filled during the pour. Faster is better.
Temperature drop, even a 50 degree drop in temperature can effect the outcome of the casting. Keep the molds close to the furnace, bring the temp of the melt above the pouring temp and work quickly enough to get the job done without being careless.
Always UNDER CONSTRUCTION

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