We left off on Step 5, packing sand into the casting box. I came back a different day to finish.
Step 6: Gently remove the pipe from the fill hole. Pick up the upper half of the box and flip it over.
Step 7: Inspect the clay. We can see here that they jeep's tire fell apart on disassembly. It turns out there is a small undercut in the plaster mold which caught the clay and ripped it apart.
Step 7b: I did extensively trim the jeep mold in hopes the draft angles would work out, then repacked the second box. But because I missed that undercut the second one didn't work either. I decided to scrap plans for the jeep in the first round so I could at least get one out.
Step 8: Remove the plaster pieces and reassemble the box. Melt scrap aluminum in a foundry (I used Northern Arizona University's foundry) and pour it into the feed hole (bottom). The metal will bubble up through the vent hole (top) and then you know it's ready. Let it cool for at least a half hour.
Step 9: After the exposed aluminum is cool enough to touch, start digging the sand out. Remember that the aluminum will be HOT inside the sand, so avoid touching it below the very top 1/4"/5mm. After you've removed the sand on top, use foundry tongs to remove the casting from its box.
Step 10: Clean the sand out of the casting and break or saw off the feed tube and vent tube. Throw any aluminum waste back into the scrap pile for the next casting. Below is a photo of the other side. I was thankfully able to make the edges thicker by scraping out some sand in step 7. This will give me a little heavier-duty mold with more material to work with.
Next steps: I will use the angle grinder to clean up this casting's backside and make a good shelf for bolts to go through. Then I'm going to put the front side on a belt sander to make it nice and smooth. The flat side there has to mate up to a steel plate, so I'm guessing the smoother it is, the better the molding process will go. Stay tuned!