Another weekend it's not raining: another attempt at the skull ashtray...
I spent some time drying out about half of my molding sand a bit. Maybe a bit too much, but I had become convinced it was too damp. And I decided to gate in at a thick section on the other side of it, closer to this thin parts that did not fill the first time.
I also added another riser, near where the gate came in the first time I tried casting it. To act as a vent so that gases (ie. steam) could not be trapped inside the mold and also hopefully to feed any shrinkage, since the reason I gated in there last time was because I thought it seemed like the thickest part the furthest from the thinnest parts. Molds need to be designed with directional shrinkage in mind - thin sections freeze first and they shrink as they do so, drawing in molten metal from still-molten thicker sections. The trick is to have an even thicker sacrificial "riser" to take that shrink, and it needs to be attached to the last section(s) to freeze. Despite moving the sprue, I still wanted some hot metal there to feed the thick part on the far side from the new sprue location, hence the extra riser. Once the risers, the highest parts in the mold, began to fill, I even planned to stop pouring into the sprue and finish topping up the mold through that far riser to heat it up a little more. Maybe that was all overkill, but I thought it might help.
The sand I dried out did not seem very strong, so after the first layer of sand in the mold I began adding handfuls of the other half of my sand that was still a little more moist along with the dry stuff, I think I this helped but the sand was still kind of weak at the parting line in the drag (bottom half of the mold, where the skull's face and the runner connecting the sprue to the gates was molded). I did not use just the dry sand right up against the pattern in the cope (top half, which only contained the risers, sprue, and gates) I used a mix of the two so that turned out a little better.
But the drag had more loose sand coming off the parting line in the drag than I had hoped to see when I flipped it over... It's possible I just didn't ram it up quite as well as the first time - I finally found my aluminum rammer, and I was a little nervous that I'd chip the pattern if I rammed as hard as I did with the improvised wooden one I used last time.
I had a little trouble connecting the far side riser to the mold cavity, some sand broke off, but nothing disastrous as it was all in the cope and the part was all in the drag. Forgot to take any more molding pix due to fiddling around fixing that though.
Here are the molds, spiderproofed and waiting. Spiders can turn into steam explosions when they crawl in and get trapped in a mold full of molten metal, and steam explosions can chuck molten metal at your face, which will ruin your whole day and probably your casting too...
#12 crucible loaded up in the furnace...
Crucible is full of wheelium (backyard metal caster's lingo for aluminum car wheel alloy, usually A356, a great general purpose a casting alloy) blobs I retrieved from a campfire I lot in my backyard last summer to break down some large aluminum scrap into crucible sized bites. And you can the runner from the last attempt poking up there too. Piled loosely, nothing wedged in tight - things expand when they get hot, and crucibles are expensive and more fragile than flexible... Seemed like it began melting before the furnace was even hot enough to start the oil drip, I must be getting the hang of running it on propane or something! I didn't even have to relight it once this time... :). And I even remembered to lay out my homemade flux, a eutectic of sodium chloride and potassium chloride. Helps separate out the dross from the clean metal at skim time, and improves the flow of the metal. I made sure to pour hotter this time to ensura a complete fill of the thin sections.
It filled! Woohoo!
Not quite as nice and clean as the good parts of the first attempt, but it looks pretty darn good in this pic I'd say... :) Just a tiny bit of crustiness you can see here if you look really close, such as around the top of the eye sockets and the little divot in the hinge of the jaw must have had some sand break off in the mold somehow. I blame myself for drying out my sand too much. You can also see where I broke some sand joining the riser near his teeth to the rest of the mold cavity. That extra metal in the cope side only touches about 2mm of the back of the ashtray, should be easy to clean off.
Gating details. The sprue is the skinny tapered one. The pouring basin on top of it is oval shaped to prevent whirlpooling which would suck air into the mold.
Broken sand on the drag side... :( Including that jaw divot again in the second pic.
I'll have to do some grinding and some covering up of grinding marks to fix this one at the top of his head:
Jaw divot flaw close-up, I will leave as is, I think.
Risers didn't feed much, but there's no shrink anywhere I can see, so I guess they fed enough...
Next one'll be perfect... :)
edit - OMG, I just sold it; I wasn't even sure I would try to, but somebody who I had shown a pic of the first one that did not fill just asked me if I could make one for her son who is a smoker. I gave her a reduced price because that little divot in the jaw got filled in... Not that I have a regular price; I'm only a hobbyist after all... but I do have a hobby budget to not exceed if I can help it. This helps. 2nd sale ever, woo-hoo! Hopefully it'll remind the recipient of the grim fate that awaits all smokers, and will inspire him to quit. I myself am off the combustion for 3 years as of tomorrow; for me, e-cigs were the way out. I still vape, but at least it has kept me from smoking some 30,000 cigarettes to date...
edit 2 - made the exchange this morning, she seems very happy with it! I asked, she isn't giving it as a grim reminder of the smoker's fate; her son just likes skulls and is a smoker.