The Home Machinist!

I'm building a 1" scale passenger car and will be needing a lot of windows, so I fabricated a permanent mold for a window frame from three layers of 1/4" aluminum plate and tried a few experimental pours using pot metal and later lead. The first three pours were short runs until the mold heated up, afterwards I consistently got complete castings using the pot metal; however, every one had hot tears in the the locations marked on the the one casting in the attached photo. I switched to lead, thinking that it would be more ductile and less likely to tear but got no complete castings. The geometry of the mold is the problem, but that's what I need; any thoughts on a low temp alloy that may work?
John

You need someplace for the metal to draw in molten metal as it cools. With you pouring into the thinnest section, that section is cooling/ solidifying fastest and the rest of the part as it cools and shrinks has nowhere to get the extra metal to make up for the shrinking as it cools. If you put the fill on the other end you may have better luck. You can also try heating your mold in an oven to a couple to few hundred degrees, that will take trial and error.
I would cap where you are filling the mold now (from the outside so you have a glob of metal at that end) and fill from the opposite end of the part.

Rick, I've been playing around with this all afternoon and I think I have a fundamental design flaw with the solid core. The solid core is not able to burn out and collapse, so the metal tears as it cools and shrinks around it. This wasn't one of my better ideas...going to have to come up with a plan-B.

Your mold design isn't the problem, this is done all the time in the die cast industry.
Look at the bottom left part in the picture, the big blobs along the edges of the part are there for a couple of reasons one being for the part to draw material from when it cools so it doesn't have as much issue stresses induced by shrinkage. Another thing that will help is to preheat the mold and get the part out of the mold just after it has solidified, don't let it cool completely in the mold. This will allow the last of the shrinking to happen without anything constraining the part and causing it to tear.

Hmmm, I'll see if I can modify the mold and give it another try. Thanks Rick.
John

I'm going to get back into making the window frames I need for my passenger car project, I tried to modify the permanent mold but didn't have enough space to add feeders, so before I commit to making a new permanent mold, does anyone have a better approach. I made some window frames for a caboose project, they were made from aluminum cast in PetroBond molds, the window frame and pattern are shown on the left side of the photo, the frame on the right is what I want for the passenger car. I had to pour 4 caboose window frames to get one good casting, and since I'm going to have to make 24 frames in 2 styles for each passenger car, a one for four success ratio is out of the question. Any thoughts before I start on a new permanent mold will be appreciated.

I made a new permanent mold from steel this time and reconfigured the sprue and runners to provide better metal feed to the casting. I gave it a try and got a 99% complete window frame with no hot tears on the first try, the only problem (a big one) was I didn't put any relief on the mold and had a difficult time extracting the casting. I'm going to rework the mold and give it another try tomorrow. Thanks for the tip Rick.
John