I have tried a number of times to make a plaque using the lost wax process. All seems to be good until the pour. I can make a ceramic shell, with no or minimal cracks. I do this my maintaining the was and ceramic at ~35C. I then allow to cool to room temperature before placing in an over at 250C to shock-melt the wax. I can then also reuse the wax. The shell has a pouring funnel, and a significant number of vents which I hope would be sufficient to assist the movement of molten alloy and displacement of air. I heat the shell in a kiln and soak at 750C. The shell is clean and appears good quality, no hint of residual carbon after the pour. The shell sits on a layer sand which I hoped would assist in some heat retention after removal from the kiln. I am using LM25 alloy, though have had similar results with pure aluminium. A thermocouple suggests the alloy is perhaps hotter that ideal, around 800+C before the poor. I remove the ceramic shell from the kiln. It is on a steel tray so I can extract the shell from the kiln without drama. I then skim the aluminium whilst the flask is still in the furnace and transfer to a pouring "ironwork". I do the pour as quickly as I can and it appear to low well. The result suggests a too quick cooling leaving to holes. The thickness of the shell at its minimum is ~5mm. I'm wondering if I should leave the flask in the kiln and pour directly to this. Something I'm not keen on doing. Any ideas would be appreciated
Hello Mike, welcome to the forum. I don't do shell but I do plaques in sand. Is that the sprue in the middle ? Are those nubs on the back for vents? To get a good pour the metal needs to advance from one end to the other in a steady front. Like a wave on the beach. That is accomplished by slightly raising the side farthest from the sprue. Here's my usual plaque pattern, sprue in the corner with a full knife gate. If you need a vent, put it in the far corner.
I think that orienting the plaque part of the mold horizontally is causing the metal to spread out in all directions from the bottom of the sprue, then come back together surrounding air bubbles. Maybe if you sprue and shell the plaque on its edge, it will fill without needing nearly so many big vents. I am not a shell guy though, so take that with a grain of salt. Some of our members do use shell molds and can probably make some better suggestions where to put vents and gates. When I sand cast plaques, I mold them horizontally due to the nature of my molding flasks, but I pour with the mold slightly tilted so that the sprue and gates are at the low end in order to prevent exactly this sort of thing. Edit - ie. what fishbonz just said Jeff
Welcome Mike. Bonz and Jeff are on the money. I was close on a couple of plaques last week, but no cigar. Tipping the molds would have made the difference. Those are 12" square and at the minimum, 1/4" thick. It's hard to tell from your pictures but yours look pretty thin. Pete
Many thanks for all the replies.They're greatly appreciated. I hadn't thought that maintaining a upwards flow would be so important, though I can see that trapped air is a probable cause. The shell ought to be a little porous from some added flock which I hoped would prevent the inclusion of air pockets. I'm having another go soon and taking photos as I go along.
I thought I would post a series of photos of my process and the results The cente pillar was added so I could hold the plaque when adding the ceramic. I did make a handle at just one end but the wax wasn't man enough to take the load and it cracked! The flow has been good. Two things of note: 1) The reult is heavily pitted in the thinner areas. These are quite deep pits ~1mm 2) The centre is darker and more oxidised. I presume this was because this took longer to cool down due to the mass of the pillar in the middle. The alloy also seems sucked away from the front face. Bext time I might add the ceramic to the front side, and then to the read to keep it flat and remove the need for a 'handle'.
