When designing a casting you design the part, then you design the mold, then you design the tool to form the mold. It helps to think of the plaster mold as a finished product in itself, then think of the tool as a finished product. In order to make the part fill correctly, you must follow Campbells rules for casting, if you want to get into casting it’s the cheat codes and 10 commandments all rolled into one, go read it. I’ve been thinking about mold flow for casting a cylinder for 5+ years while taking long poops or deer hunting or watching rom-coms with my wife so I have a pretty good plan for this, which I shared with Jurrian.
The sprue is where the molten metal enters the mold. You want to think of this like a beer bong. You know how when you do a beer bong right the beer goes down as a solid slug of liquid, and when you do it wrong it gets all mixed with air and foamy? Exactly. Air mixing into molten metal causes little fragments of aluminum oxide skin to form which shows up as porosity which is bad. To avoid this you want to have a smooth pour that fills the sprue completely. By running the sprue down the middle of the part you will get good flow everywhere and also the sprue will stay hot, allowing the mold to continue to fill as the metal cools and shrinks. You always want to have a gooey center to your casting so you don’t get shrinkage voids.
The next feature is a large well in the bottom, this will capture any debris and let the liquid metal settle down before evenly filling the mold from the bottom. You always fill from the bottom so the air gets pushed up. As the mold fills, the molten aluminum will combust deposits on the surface, especially residual burnt plastic, so you need to give that a chance to rise. If the aluminum is above the burning plastic, those gasses will get trapped and cause more flaws. This is why you never ever ever pour straight onto the top of a mold, it just doesn’t work and if the gasses are bad enough it can pop and spatter in your face.
The flow from the well into the actual part itself is very important, you need a single ‘front’ like a wave. Anywhere the aluminum is being exposed to air it will form that oxide skin, so its kinda like when you see lava going into the ocean. If two ‘waves’ come back together the skin will keep them from knitting and you will get weird knit lines which are a significant weakness in the part. If the wave moves fast and in a single smooth motion, the aluminum behind the wave will still be hot enough to incorporate and you won’t have that problem. For the cylinder using the entire base gasket surface makes the most sense.
At the top, the mold will be open-faced. This means the whole top surface will be open to the atmosphere. This is used in submersion casting and is probably how real 2t cylinders are cast in permanent molds. This way the entire top surface becomes a riser that will be cut off and any imperfections get chopped off in machining. It’s a lot more metal waste and machining time, but when you need a very high quality casting this is the best way to do it. Also, this way the mold will burn out upside down and everything will come out. When you burn out the PLA, anywhere it sits it will soak into the plaster almost like oil, and burn when it gets hit with molten aluminum.