If you're not familiar with a charcoal foundry there are several good instructables and the books that originally got me started in metal castings by David Gingery (see link below) If you enjoy this instructable please consider voting for it in the Black & Decker Fix and Repair contest.
Step 1: Making the Mold
Once the original parts have been prepped, fill the bottom half of the mold with packed green sand. Then coat the original parts with baby powder and set into the bottom half of the mold. Push the parts in with the 3/8" steel axle in place and pointing up. Press into the sand until the axle is just buried. The top half of the steering knuckle, including the tie rod connection, should be above the sand. leave enough room between the two parts for a 3/4" pipe that will create the fill tunnel.
Before you start the top half of the mold coat the bottom half with baby powder, to help with separation of the two halves. Next place the top half of the mold box in place and begin filling and packing with sand. Now you can loosen up and remove the pipe.
Finally, you need to carefully separate the two halves. If you're successful, then remove the original plastic knuckles from the bottom half of the mold, being careful not to collapse the sand or disturb the axle. The steel axle will stay in place and become permanently fixed to the new aluminum steering knuckles.
You need to cut small channels from the fill hole to the two steering knuckle molds, in order to give the molten aluminum a clear path. Now you can place the top half back to complete the mold.
Step 2: Fire Up the Foundry.
I used a 6" piece of 3" schedule 40 piped with a pipe cap welded on one end to hold the aluminum inside of the foundry.
Allow the charcoal to get going until you have a raging jet of fire coming from the foundry then place the pipe with the aluminum into the foundry and add charcoal around the pipe. Place the lid and wait until the aluminum melts.
Step 3: Time to Cast
Pour in the aluminum until it fills the riser. Allow ample time to cool before trying to remove.
Step 4: Claim Your New Parts
You should have two steering knuckles with the embedded steel axles connected to the riser. I used a hack saw to separate the steering knuckles from the riser and a grinder to knock down the rough edges that form where the seam between the two halves of the mold meet. Then just drill a hole for the tie rods and you're ready to go.