How to Make Metal Casts and Work with Molten Metal
If you are experienced with metalworking, you may be ready to begin making metal casts using a gas-fired crucible furnace.
Using a Gas-Fired Crucible Furnace to Make Metal Casts
Before lighting the furnace, you must load it with the metal you will melt, which is called the charge. Place a crucible in the furnace and fill it — but do not pack it — with metal. If you pack the crucible too tightly, you might crack the crucible if the charge expands too much when you heat it. Leave the furnace lid open. To light the furnace, first turn on the blower, then open the gas valve partway and adjust the air valve until the flame catches. Close the furnace lid and after five minutes increase both gas and air flow until the flame reaches its maximum heat. You will quickly learn to tell when this point has been reached by listening to the roar of the furnace. Check the color of the flame through the flu opening in the lid. If the flame is yellow, increase the air flow.
When the original charge in the crucible has melted, use tongs to add more metal as needed. Periodically insert the probe of a pyrometer through the flue opening and into the molten metal to determine whether the metal has reached pouring temperature.
Removing a Crucible from a Furnace
When the charge has reached the desired temperature, turn off the furnace. Before you remove the crucible from the furnace, lay the crucible shank across the carbon block in the sandbox so that you have it in place for pouring. Slip the crucible tongs over the crucible, lock its grippers in place, and, in one smooth motion, transfer the crucible from the furnace to the carbon block. Unlock the crucible tongs and store them away from the pouring area.
Skimming Slag from Molten Metal
Using a skimmer, ladle off the slag, or impurities and oxidants, that has risen to the top of the molten metal. Dump the slag on the sand in the sandbox. Plunge a flux pellet to the bottom of the molten metal, pushing it down with the skimmer, and lightly stir the metal. Slag will again rise. Skim and dump it as you did before.
If you are using a bottom-pouring crucible, there may be no need to skim the slag a second time. It is also possible to leave the slag in place when you are pouring from the top, it will act as a protective coating. But in that case your helper should hold back the slag with a metal rod called a slagging bar as you pour the metal.
How To Prepare the Mold for Pouring Metal
Cutting the Sprue and the Riser
To form the sprue — a passage through which you'll pour the metal — measure the depth of the cope, and mark this distance with tape on the side of a section of thin-walled 3/4-inch pipe. Push the pipe straight down into the sand in the cope, in a spot where it will not hit the pattern. Twist the pipe back and forth to cut an opening through the sand, stopping when the tape mark meets the surface of the sand. Use a slick to carve a funnel shape in the sand around the pipe, beveling the edges of the sprue opening. Then slip the pipe out, with the sand core inside, leaving a passageway through which you'll pour the molten metal.
On the other side of the pattern, form the riser opening — the passage into which excess metal will rise — by using a slightly larger diameter pipe to cut through the sand in the cope.
Venting the Mold
Use a 1/16-inch welding rod or a stiff wire (a bicycle spoke is ideal) to poke channels through the sand in the cope, so that hot gases can escape. Push the rod into the sand, stopping about 1/2 inch from the pattern. Make about a dozen vents over the pattern area.
Lift the cope from the drag and set it on edge, off to one side, where it will not be disturbed.
Removing the Pattern
Before lifting the pattern from the drag, firm the pattern edge of a water-based sand mold by moistening it with a molder's bulb or a small brush dipped in water. This will help to keep the sand from collapsing when the pattern is removed. Screw draw pins into the holes in the back of the pattern and lightly tap the pattern to loosen it from the sand. Then gently pull on the draw pins, lifting the pattern straight up and out of the casting cavity.
Cutting the Gates
Using a piece of sheet metal bent into a 1/2-inch-wide U shape, cut a channel, or gate, running from the casting cavity to the riser position. Lift out a bit of sand at a time, forming a gate slightly smaller than the diameter of the riser and slightly shallower than the depth of the casting cavity. Scoop out a similar gate from the casting cavity to the sprue position. For large molds, cut several gates to the sprue and the riser from various parts of the casting cavity. Blow out or tamp down any loose bits of sand in the gates.
Refining the Mold
Rebuild crumbled sections of the casting cavity by adding bits of moist sand, smoothing it into place with molding tools. Tamp down or blow away all loose sand to prevent it from mixing with the molten metal when the metal is poured.
Replace the cope over the drag and set the flask, still on the bottom board, in the sandbox near the furnace. If you are not going to pour the casting immediately, cover the mold to keep dirt from falling into the sprue and riser.
How to Pour Molten Metal
With a helper, lift the crucible shank and hook its safety lock over the lip of the crucible by pushing the latch forward. Lift and tilt the crucible a few inches above the mold, pouring the molten metal quickly and steadily into the sprue, or molten metal passage, into the cast. Stop pouring when the metal nearly reaches the top of the riser. Immediately, while it is still molten, pour all the extra metal into the ingot mold. Leave the casting mold in place until the metal in the sprue and in the riser is hard. To test the newly cast metal for hardness, tap it with the tongs.
How to Remove a Metal Casting
When the metal has hardened, carry the mold to the sand-storage area. Wearing gloves, separate the cope and the drag. Lift out the hot metal casting with tongs. Break apart the sand in the mold, and dump it back into the sand-storage container. Set the casting aside to cool, leaving in place the extrusions that the sprue, riser, and gates formed.
When the casting is completely cool, cut off the gates with a hacksaw and file down the rough areas. Finish the surface as desired.
How to Make a Sand Mold for Casting Metal
Step 1: Position the Pattern
Place the drag upside down on the molding board and center the pattern in the drag, flat side down. Leave enough room between the pattern and the drag for the openings of the riser and the sprue — the passageways through which the molten metal will flow. Dust parting compound evenly over the pattern.
Step 2: Riddle Sand Over the Pattern
Shovel molding sand into a riddle, and sift the sand over the pattern either by shaking the riddle or by pushing the sand through it with your hand. Cover the pattern with sand about 1 inch deep. Then set the riddle aside and use your fingers to tuck the sand around the pattern, working from the sides of the drag in toward the center.
Step 3: Ram the Sand
Shovel sand into the drag until it is almost full, and compress it with a bench rammer. Starting with the peen end of the rammer, push the sand against the sides of the drag, then use the butt end to flatten the sand in the center. Pack the sand firmly, but not so rock-hard that hot gases cannot escape. Repeat until sand is slightly higher than the sides of the drag.
Step 4: Level the Sand in the Drag
Pull a strike-off bar across the top of the sand, using the edges of the drag as a guide. Work the bar back and forth to make the surface of the sand perfectly flat. Then place the bottom board over the drag, sandwiching the first half of the mold between the bottom board and the molding board.
Step 5: Roll the Drag
To turn the drag right side up, grasp the edges of the molding board and the bottom board tightly on each side and flip the drag over. If the drag is especially large or heavy, pull it toward you and roll it over to the opposite side, using the edge of the workbench for leverage. Remove the molding board from the top of the drag, exposing the mold's parting plane and the underside of the pattern.
Step 6: Perfect the Parting Plane
Blow away stray bits of sand with a bellows, and use a trowel or a slick to smooth rough spots or fill in loose areas, especially around the pattern edge. Sprinkle the entire surface lightly with parting compound. For guidance in placing the sprue and the riser, measure the distance from the pattern edges to the sides of the drag.
Place the cope on top of the drag, fitting the pins of the drag through the holes of the cope (inset). Sprinkle parting compound over the pattern back, then repeat steps 2, 3, and 4 to fill the cope.