Casting Aluminium. It is just too light.
When doing my aluminium castings for the model Armstrong cannon, I noticed that the objects in the wax/PLA tree which were closest to the funnel (i.e. the topmost ones) were the ones which were most likely to have significant deficits, and I wondered whether the pressure of the molten metal at that level was the problem. The objects at the bottom of the tree were most likely to be successful.
Well, my friend Stuart Tankard is working on a positive pressure system, and I am working out a negative pressure system, to increase the pressure at the higher levels within the tree.
An interesting number is that for every 1″ / 25.4mm increase in the head of molten aluminium, the increase in pressure forcing the molten aluminium into the casting voids, is only 0.1 psi!!! Bronze, being much more dense (x3 – x4) would be less problematic, but still less than 1psi.
Some casters use a centrifugal system to increase the pressure on the molten aluminium. Frankly, that idea frightens the shit out of me. One episode of molten metal flying around my workshop was enough.
Stuart T is working on a positive pressure system, using approximately 5psi on the melt, to force it into the tree spaces, and he is well along the path of manufacturing the hardware to accomplish that.
I am inclined to use negative pressure to suck the melt down. I already have a vacuum pump, and I think that it might be easier to seal the hot steel cylinder to the silicon gasket which is required. There is a YouTube video on the subject.
(oops. I pasted the wrong VOG video. The one below is the intended one.)
I have ordered some 3mm thick Silicone sheet.
VOG, in the above video, allows the surface of his casting cylinder to cool to 100ºc before pouring the aluminium melt, so the silicone gasket does not burst into flame, and he has had some excellent results. It is casting heresy, and he should be burnt at the stake. But if it works…. hey?! (maybe the core of the casting cylinder is still closer to the molten aluminium temperature of 710ºc?).
So that is the path which I am following. Not exactly. But using the principle. Watch this space for my results.
Actually, molten bronze is my next pour. I doubt that it will require vacuum or pressure.