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Casting Foundry & Lathe
Sand Casting

This page is on sand casting. This is how you turn patterns into objects of aluminum. Molten aluminum is poured into a "green sand" mold, to reproduce the the pattern. See the "First Casting" section for an example of a flask (mold) that has been poured, opened and the casting removed from the sand.

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Not a great pic, but you can see a couple of molding flasks (cope & drag) as well as molding boards, and some ingots. Notice the color inside the cope? When the finish was still wet on the inside I sprinkled a little clean silica sand on the inside. This provides a very rough surface. Never had a cope "fall out". In fact it takes considerable force to dislodge the geeensand.


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These two patterns where cast at the same time. They are the carriage apron and the compound swivel. This gives you an Idea of what can be done with green sand molds. Note the two sprues, the two gates and the large runner between the sprues. The greensand mold was poured from only one sprue, both the pouring sprue and the other sprue or riser, are to provide metal to the casting as it cools. The large runner is so the aluminum stays hot as it travels from the pouring sprue to the other pattern. Neither casting had any hit of a shrink cavity.



GREENSAND

Casting sand or greensand is home made and is composed of silica sand, Bentonite clay, and water. Fireclay could also be used but is not as good a bentonite, and require more work to mull or recondition the sand for its next use. Also Fireclay has the property of vitrification when it gets hot. In another words it fuses together. This can be a problem in that after the flask (mold) is opened the sand in contact with the aluminum must often be thrown away. Fireclay is great in refractory mix but not in greensand. I used about 90% sand, 10% bentonite, and around 7% water by weight.

The amount of clay to use is arbitrary and depends on the clay you use. There are many kinds of clay, kinds of bentonite, and kinds of fireclay. The kind you find and use will determine the ratio of clay to use. With fire clay you may need to use up to 25% clay, with bentonite it could be as low as 5%. I used 10% bentonite, and it works well, but it might be a little to much. The important thing is the less clay you use the better, because the sand will have better natural venting qualities.

Water is also important, or I should say the less water in your greensand the better. When adding water (tempering) do so only a little at a time until the sand seems just barley right, then stop, it will improve with age (in an hour or so it will greatly improve). Tempered green sand should feel slightly damp, a squeezed hand full should clump firmly together, not stick to hands much, and the squeezed clump can be snapped in half without crumbling.

FLASKS

Flask and molding boards are the tools of the trade. The flask is like a box made of four boards for sides, and it has (the box) no top or bottom. A bottom board is what the completed flask will sit on. The flask and greensand are what is used to make a mold out of a pattern, that molten metal can be poured into.

A flask generaly consists of two halves, the bottom half or drag, and the top half or cope. Some sort of provision is provided so the two halfs will closely mate, match, or can be put together in only one way. This is to insure a that the two half are always perfectly registered with each other.

In the simplest, eaisest making of a mold, you would use a beding technique. Heres how it would go: you set down a bottom board, lightly dust with parting powder, set the drag on top, riddle insome sand, ram it up, riddle in some more greensand, ram, repeat until drag is full. Strike off the surface with a striaght board until level. dust lightly with parting dust. Press in the pattern (beding it). light dust with parting, set in sprue pin. Set the cope half in place and riddle and ram sand until full. Strike off the top, rap the sprue pin until loose then remove. Open the two half of the flask (lift the cope straight up and set aside on its side). Rap and remove the pattern, cut a gate from the pattern to the sprue, cut the top sprue openning into a bevel, blow out loose sand, close up the mold (put the cope carefuly back in place). Then pour moltem metal into it (aluminum). Wait about and hour open up shake out and you have a done it!

MOLDING TOOLS

In addition to greensand, flask and molding boards you need some tools to complete the job.

A striking board - just a straight board to strike of the top.

Sprue pins - Round dowels an inch or to longer than the height of the cope.

Gate cutter - something to cut gates between the pattern cavity and the sprue hole. A peice of curved sheet metal works very well.

Vent wire - a small diameter straight wire to push into the sand to make little vents to assist the gases (mostly water vapour) in escaping the mold.

Blower - some low volume air source to blow out any loose sand in the cavity left by the pattern, a turkey baster works very well. (looks kinda like a foot long eye dropper).

Parting powder - a dust, powdery substance that prevents the pattern, and flasks from sticking to the sand and to each other. I use 200 mesh (grit) silica sand, its like flour or powdered sugar.

Rammer - some kind of tool for ramming the sand into the flasks. I turned one on the wood lathe. It has a bevel on one end and is flat on the other.

Riddle - a wood framed screen, either square of round with 1/4" mesh screen. Used to break up clumps of greensand. You riddle the sand into the flasks then ram it to pack it.

Other tools might be different sizes of sprue pins and gate cutters, a rapper (I use a 8" 1/4" rod) to loosen patterns, small trowel to carve out the bevel on the sprue openning (I found a great little set oil art painters use), Swabbing tools (small soft brush) if you think it is needed, anything else that you find helpful.

Cope and Drag tips.

When you make your flasks, the cope and drags, be sure to apply a couple of coats of polyurathane finish. When these have dried, apply another coat but only to the insides, before it dries sprinkle it heavily with clean sand, and allow to dry, this provides a very rough sandpaper like surface and helps a lot to keep the sand from slipping or falling out. This should be in addition to 1/4" to 3/8" ribs glued to the inside.


Molding sand and Casting tips.

Swabbing - Gingery mentions swabbing around the pattern before drawing the pattern from the mold, and swabing any cut gates, sprues, etc. In my green sand (clay, sand, water) this just caused more problems than it helped. In fact it didn't help at all. I just mold, cut, blow out any loose sand, close up and pour, works fine no problems. It (swabbing) may however be required depending on the sand you use.

Green sand - By all means if you can find it use Bentonite clay for your green sand. Never use fireclay in green sand unless you have no other choice. The problem is fireclay will vitrify (fuse together like in firing a ceramic pot) and the green sand around the casting will have to be thrown out. Bentonite clay will not fuse, vitrify, at aluminum casting temps. In fact it will not fuse at any temp. It will however melt, if it gets very high temps around 3000 degrees F.

NO, NO, NO Kitty litter - contrary to what some web sites say it is crap and a waste of time, your results will suffer, would be much better to use the time to find better materials, clays. Kitty litter is great for cats, and for traction on icy roads, and to soak up oil, but not much else. Never ever us kitty litter in a refractory mix. I could be used for greensand, but your results will greatly suffer.