Core for ceramic shell (Suspendaslurry)

Discussion in 'Investment casting Ceramic shell method' started by DHSC, Jul 12, 2023.

  1. DHSC

    DHSC Lead

    (first post here)

    Hello all,

    I'm attempting a ceramic shell cast that requires a core. Tried it without and it fragmented on burnout.

    Would a sodium silicate core survive the burnout/vitrification temperature?

    If not, anyone know any commonly available materials or methods from which I could make a core to put inside a wax or PLA pattern and then go through the suspendaslurry process?

    My first thought is maybe attempt a 3D printed core box to hold, dry, and burnout a suspendaslurry/sand core similar to a sodium silicate core.

    Thanks for any input!
  2. Mantrid

    Mantrid Silver

    If you are able to weld I would just leave a fairly large hole in the wax and allow the ceramic shell to flow inside to crate the core. Then weld a piece in place to close the hole after casting. If you dont weld then Ive used a core of plaster, sand and broken up ceramic shell. Use metal core pin through the wax to support the core.
  3. DHSC

    DHSC Lead

    Thanks for the reply! I'm a little late in replying back, but I've been up to other stuff...

    I didn't include enough detail for anyone in my question - I needed a non-straight, non-uniform hole approximately 6" long and 1/4" diameter through a piece that has undercuts preventing a simple 2 part sand mold.

    What I ended up doing was a baked sodium silicate two part sand mold using two PLA patterns that were melted out with the undercut features retained in the sand. I put them back together over an epoxy/sand core.

    Worked reasonably well, I'll go for a final piece in bronze "soon" after I refine my mold models a bit.

    This thread is maybe in the wrong place on the forum now that it's a "Baked sodium silicate sand mold with epoxy resin core", but I'm still interested if anyone knows any material to make a strong pre-made core that can survive ceramic shell firing and still be removed by some reasonable method. I read "sintered ceramic cores" when I do searches, but that doesn't sound too workable in my modest home foundry.

    Details that might help someone or someone might have some improvements to suggest:

    Sodium silicate sand weight ratio 10g sand to 1g sodium silicate. I used Teton Foundry Supply Super-Silicate Foundry-Grade Sodium silicate. Sand was sifted play sand. Hand mulled it and hand packed it (not rammed) over PLA patterns with integral PLA indexing features, bottom, and walls (Used Blender for the mesh model and imported into Fusion 360 to assemble the box and features). Baked the core at 350 F to dry it and then 450F to melt out the PLA. Left carbon dusting behind, I dripped out most of the PLA and removed and baked a couple hours more. Apparently baking drives out the water, different from CO2 gassing which is a chemical reaction. Seems the CO2 gassing will leave sand that doesn't survive high temps, making it ideal for cores, but not so ideal for baking out PLA. Didn't test that theory though. Also read that adding 1% by weight(to sand) magnesium carbonate powder will help avoid creating a glassy silicate at bronze and higher pouring temperatures. Haven't tested that either yet, I did the test piece in aluminum. I'll try the magnesium carbonate when I pour bronze.

    Sand molds were very strong and sat for weeks before I got around to pouring. I glued the halves together (with epoxy sand core inside) with more sodium silicate and let dry. No leaks on pouring aluminum, but my halves weren't very flat so I got some bad flashing. Also pretty poor gating in my PLA pattern that I'll fix. Sand molds popped right off with a hammer tap and undercut features were nicely reproduced. Sand/silicate picked out easily.

    For the core I used 3.5 g sand to 1 g of 5 minute epoxy. Maybe a bit much epoxy, but it worked. Might have been able to use a CO2 gassed sodium silicate core, but I think the epoxy might hold up better over such a long span. On pouring, the epoxy degraded pretty nicely and I used a wire to ram out the leftovers.
  4. MikeMike

    MikeMike Copper

    For small (2.5kg finished) pieces such as human and animal figures I've used a 3mm stainless wire armature with fire cement applied to make a core. The core is built up in layers, heating each one to cement it. The cement needs to be pressed and smeared on to make a good bond. Give the finished core a good blast to really set it, You can drill it easily for pins. It is inert (I confirmed with the manufacturers), is light and so can be left in situ. You will probably loose a couple of layers skin off your thumb pads. If you try this, could you let me know how you get on?

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