How to Cast Parts

Warning!!

Activities on these pages about foundry work are very dangerous. The chances of having a fire, causing injuries, or even dying are very good if you are not careful. I am not a professional foundryman, and will not attempt to disclose all hazards. If you choose to try this hobby, every precaution must be taken to be safe. Remember, a hobby is only fun until you're standing in the middle of the street with your family watching your house burn.


I'm going to give a little explanation of the basics of casting parts using a green sand mold. Everything will be based on the way I cast parts, and the graphics are going to be simple. This explanation is by no means all-inclusive. In building the equipment that I have, I have used several different techniques to cast the parts. And from what I have read, there are a lot of other techniques.

A Few Analogies

In order to make this a little clearer, here are a couple of analogies. Casting parts is something like using baking pans. A muffin pan is a type of mold, but it only shapes the bottom of a muffin. An even better example is a mold that my grandmother had. It was a two piece pan in the shape of a lamb. The cake batter is placed in the bottom half and the top is put on. When the cake is baked, the batter rises to fill the pans, and the air escapes through holes in the top. This results in a cake in the shape of a lamb.

Another, possible better, analogy comes from my youth. We used to make Popsicles® using a plastic mold. We'd take a flavored drink like Kool-Aid® or Hawaiian Punch® and pour it into the mold. After freezing them in the refrigerator they would pop out of the mold. The big difference here is that the water froze at 32ºF (0ºC), and aluminum freezes at about 1200ºF (650ºC).

Green Sand for Molding

The first question is "What is green sand?" Green sand is a mixture of silica sand and a clay for a binder. My molding sand has 25 pounds (11.3 kilograms) of fire clay and one pound (0.45 kilograms) of corn starch added to each 100 pounds (45.4 kilograms) of sand. The clay coats the sand to bind it, and the corn starch helps the sand absorb moisture. To make the molding sand, the sand, clay, and starch are mixed together while they are dry. After it is mixed well, water is added carefully. This has been the most difficult part of the process for me. If the sand is too dry it won't mold well, and if it's too wet the excess moisture will create steam that will displace the metal. See this picture for an example.

There are other types of molding sand that can be used. One is called K-Bond and was developed at Kent State University. It calls for Bentone to be mixed with the silica sand, and adding a two-cycle engine oil and rubbing alcohol. This is the type of molding sand I'm going to make when I'm ready to change.

Flasks

Two views of a flask

The first thing that you need to make a mold is a flask. A flask is basically a two piece box without a top or bottom. The top half is called the cope and the bottom is called the drag. The cope has a peg installed on the end that will fit into a socket on the drag. This helps to line up the halves when the mold is put back together. That sentence will make sense in a little bit.

Flasks can be made of several types of material. They can be made from metals like steel or aluminum, plastic, or wood. In my shop I use 1"x4" (19 mm x 89mm) pine for all my flasks. The inside of the flask has to be sealed to prevent it drawing moisture from the molding sand. Either a clear sealer from a spray can or a varnish will work. The cope has to have a rib installed to keep the sand from falling out when the mold is taken apart. The rib is shown in blue, and it is actually inside the flask.

Patterns

Pattern

First a word on patterns. Patterns are made out of any material that can be formed to the shape desired, and can be drawn out of the molding sand to leave a cavity. Wood is a convenient material to use, but it also needs to be sealed before use. Other materials are plastic or plaster of Paris. Some molding methods even use wax and foam.

When the mold is made, the flask must be taken apart to remove the pattern. The line between the flask halves is called the parting line. The pattern (usually) will have a parting line. All vertical surfaces below the parting line must be tapered in at about 4º to allow the pattern to be drawn from the mold. All of the corners on the pattern, either inside or outside, must be rounded. Outside corners can be sanded round, and fillets can be formed on inside corners with wood putty or auto body filler.

When a pattern is made its size must be increased approximately 1/4" per 12" of length (6.4 mm per 305mm). This is because the aluminum expands when it is melted. When it fills the cavity in the liquid state it fills it completely. The metal will then shrink as it freezes.

Making the Mold

Three views showing the sequence for molding

To make a mold, the drag and pattern are placed upside-down on a molding board [Figure #1]. The top surface of the molding board is the parting line. The flask is filled with sand, and the sand is rammed in around the pattern. The sand does not have to be rammed real hard, but the sand must end up being firm enough to hold the shape of the pattern. This is done until the drag is completely filled. Then the sand must be vented with a wire. A wire is pushed into the sand at fairly small intervals. I usually make vent holes at about every 1" (254. mm) in each direction. The vent holes allow the air in the cavity, and the steam generated, to escape through the sand.

After venting, some loose sand is sprinkled on the mold and a bottom board is placed on top [Figure #2]. Then the drag must be rolled over and the molding board removed [Figure #3]. The parting line is exposed now, and it must have parting dust sprinkled on it. Parting dust is a powder that will not absorb water, so it forms a barrier between the halves of the flask. This will allow the flask to be opened. If parting dust is not used, the sand would become one big block.

Next three views of mold being made

After dusting the sand in the cope with parting dust, the cope is put on the drag. Then a sprue pin is placed inside the mold [Figure #4]. The sprue pin will leave a hole in the sand in the cope called the sprue. The metal will be poured into the sprue. After the cope and sprue pin are in place, the cope is rammed full of sand [Figure #5], and the sand is vented. When the venting is finished the sprue pin is rotated to taper the sprue [Figure #6].

Next three views of mold

Next the flask is opened and the cope is placed behind the drag [Figure #7]. The pattern must be removed from the sand. A screw is screwed into the pattern and is rapped with a rapper. I used a large open-end box-end wrench for a rapper until I made one from 10 gage (0.135" thick, 3.4 mm) material at work. The pattern is rapped in all directions horizontally. This is done to slightly increase the size of the cavity and to compact the vertical surfaces of the cavity. Then the pattern is removed by pulling straight up with the screw. This must be done carefully to keep from damaging the cavity. After the pattern is out a runner called a gate is cut from the sprue to the cavity [Figure #8]. Now the inside of the mold is inspected for loose sand and the two parts of the flask are moved to where they will be poured and are put together [Figure #9]. The mold is now complete. For a picture of an open, finished mold, click here. For a picture of an closed, finished mold, click here.

Crucibles

Three views of a crucible

Now we need something to melt the aluminum in. I have heard of people using cast iron pans, with and without the handles. Another possibility is a length of pipe with a plate welded on one end. I make my crucibles out of 10 gage (0.135" thick, 3.4 mm) black iron at work. I cut the pieces and roll up the side. Then I weld the seam, and weld the bottom on, using a MIG welder (Metal-Inert Gas, or wire-feed). After welding it I check for any suspect spots, and wash them in with a TIG welder (Tungsten-Inert Gas, or heli-arc). I form the spout by heating the area with an acetylene torch, placing the end against a vise, and using a hammer to form the spout.

Another tool needed is a pair of tongs. These are made of steel bar stock and open like a pair of scissors. The tongs are formed by heating and bending the steel, and then the pieces are riveted together. Two pieces of steel strap are rolled to fit the crucible and are welded to the bar stock. Pictures of me using my tongs are on this page, or you can click these pictures: Picture 1, Picture 2, and Picture 3.

Furnaces

Section view of a charcoal furnace

The next thing we need is a furnace to melt the aluminum in. The first one I made uses charcoal. It consists of a metal five-gallon bucket with a 2" (51 mm) thick refractory lining. The refractory is similar to concrete, but it will withstand the high temperature necessary to melt the aluminum. There is a lid that has a larger diameter than the bucket, and it has a vent in the center to let the gases and fumes out of the furnace. Unfortunately, some of the heat is lost also. The furnace has a tuyere (pronounced "tweer") to allow an air blast to enter the furnace.

Gas-fired furnace

A gas-fired furnace is more complex than the charcoal furnace is. It is still basically a bucket with a refractory lining and a lined lid. The gas, whether natural gas or propane, is delivered through the burner, along with an air blast from the blower. On this type of furnace the crucible must be placed on a plinth. This is a stand made out of refractory, and it has channels on the bottom. If a crucible breaks, the plinth, and the drain hole in the base of the furnace, will allow the spilled metal to drain out of the furnace and onto the sand bed below. A solenoid valve installed in the gas piping and wired to the blower is required to stop the gas in case of a power failure. This will shut the gas flow off as a safety measure.

Electric furnace

An electric furnace is similar to the other types of furnaces, except that the heat comes from an electric resistance heating element. The element is attached to a control, such as the control for a heating element in a stove.

Pros and Cons of Different Furnaces

This material is from my experience, as well as from what I have read.

The charcoal furnace is fairly safe, as home foundry furnaces go. The temperatures attained are not so high that a safety-minded amateur can't handle it. Charcoal is also available almost everywhere. The cons are that it is a messy way to melt aluminum and must be done outside or with very good ventilation. Also, if the air blast is too high it can act like a cutting torch and melt a hole in the crucible.

I've used my electric furnace only occasionally, mostly because it is so slow. With a charcoal furnace it takes about half an hour to melt about 110 cubic inches (1.80 liters) of aluminum, but the electric furnace takes about an hour. The electric furnace can also be used for melting the wax out of lost-wax molds, some heat treating, and carburizing steel. I've heard that some people think the electric furnace can be operated without constant supervision, but I don't feel that's right. And of course there is the risk of electrocution if you're not careful.

My gas-fired furnace is built, but I need to get a 100 gallon (379 liter) propane tank before I can install the refractory and fire it. From what I've read the temperatures attainable are close to 3000ºF (1650ºC). This is almost high enough to melt steel. On the other hand, it is clean, and can be used on metals such as brass, bronze, copper, and even iron.

This concludes my little primer. If you do choose to try this hobby, make sure that you do it safely!

Updated July 13, 2002


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