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Thread: 4cylndrfurys refractory crucible thread

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    Senior Member 4cylndrfury's Avatar
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    4cylndrfurys refractory crucible thread

    I wanted to create a way to make my own refractory crucibles, so I devised a mold.





    So its about time I got started...heres the base for the mold pattern:


    Its a double walled drinking glass - $3 at the local big box retailer. As it stands its only about 3mm thick, far too thin for a crucible, so I need to thicken it up, but how can I do it quickly and uniformly? Wrap rope around it:


    Now its closer to 9mm at the thinnest, I also need to beef up the bottom, so hunk of 3/8 plywood wil be added and shaped to fit. I will coat all this with some drywall mud, and then hit it with several coats of high gloss enamel so that the mother mold will release from this pattern down the road. Im hoping to end up with a bilge shape at about 1/2" thick for durability.

    and for fun, my new tongs:


    holding my old crucible:


    More pics to come later

    p.s. And here is my first shot at DIY refractory crucibles...crude, but effective...

  2. #2
    You have a nice start there 4cf.

    I keep some old cups as masters for crucibles.
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    Moderator greencheapsk8's Avatar
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    Nice :lol:
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    Senior Member machinemaker's Avatar
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    I was thinking that somewhere I heard of ceramic things being made out of compressed dry clay, then fired. I did not know if the drier the clay, the less shrinkage when drying would equal less stress and cracking while drying and firing. Just a though with your mold in mind. that and I am always a bit impatient to let clay dry slowly to avoid cracking.
    kent

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    Administrator Site Admin Anon's Avatar
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    Yes, that would give less shrinkage (zero during drying if it's completely bone dry when compressed, but I think usually those mixes have a little water in them), but also more porosity. It could potentially make shrinkage more uniform, which is very difficult to do with drying from plastic, and would make the body more tolerant of firing stresses since it's not under residual stress from drying.

    Bonding larger particle sizes, or better yet, a diverse range of particle sizes, will achieve lower shrinkage for the same porosity as bonding only small particles like clay. Obviously you would use a dry clay-based mix, not a pure dry clay, but I want to emphasize that particle size is really important in controlling shrinkage rates.

    The other problem is actually achieving low porosity. To sinter in reasonable conditions, and to be strong enough to survive handling in the green state, the green product needs to have comparatively low porosity, at least compared to the porosity of loose powder (~50% for hard spheres, I don't know what it is for the plate-type particles that clay is made of). It so happens that water is a very effective means of densification for clay--you can envision it like the water displacing air between the particles, then sucking them closer together as the amount of water decreases. (Gross simplification because I don't actually know that much about what's going on microscopically in this process.) For mechanical compaction to be as good, it has to use high pressures, and because of the way most machines work (a ram pressing down into a form, for example), the density will be highest (lowest porosity) next to the ram form and lowest (highest porosity) farther away from it. But in the case of the rammed mold, the "drying shrinkage" part of the process is occurring in the mold during compression, instead of slowly over time as water evaporates, and it's done under compression (duh) so the body can't develop tensile stress from uneven shrinkage.
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    Senior Member 4cylndrfury's Avatar
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    Hmmm, i never thought of ramming clay, I intended to use castable refractory. The only ting is im not sure my furnace would be up to super long burn times for firing using my current burner. I also dont have propane equipment at this time, so i will have to think about that option a bit.

    This was going to get an RTV mother mold backed by a 2 pc pop shell. The RTV will make releasing the green crucible easier without cracking it.

    I suppose getting my hands on a fireclay mix or something that would resemble mullite wouldnt be too hard to source. I may give it a shot once the mold is done.

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    Senior Member 4cylndrfury's Avatar
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    OK, so I got some more work done on the pattern, but not a whole lot. Theres not too many weekends in April around where I live where the meteorologist says theres 0% chance of rain. So, while that means theres definitely gonna be good weather, that also means theres definitely gonna be a 100% chance of yardwork :roll:

    Anyway, heres the update...

    Covered the pattern with lightweight joint compound - the fast drying stuff used to patch cracks and the like in drywall:


    Then I sanded and sanded and sanded and sanded and the refilled a few low spots cuz I sanded too much and then waited a few hours and sanded and sanded ... cycle that another 3 times and, well, you get the idea lol:


    Then I hit it with several coats of primer, a bit of light sanding with some high 600 grit the next day, and then 3 coats of rattle can industrial enamel (same stuff from my burner resto). Wall thickness is 11.5mm, so just a hair shy of 1/2"...good enough for who its for (me :lol


    Probably gonna let that cure for about 48 hours (its only about 55 today) and then actually wet sand that paint to get it as smooth as I can in order to aid with release of both the pattern and the actual green crucibles later. Im going to start building the mothermold from RTV, and I have a big hunk of modeling clay ( i hope its enough ) to build a 2 part RTV mold. I also need to build the jig to center and hold the core. Updates to follow....

  8. #8
    A little hint with regular drywall mud is to use a wet sponge to smooth it AFTER it dries. I works as well as sandpaper, but makes no dust. That's what I'm doing on the inside of my bell pattern. I also use this method on home drywall repairs.

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    Senior Member ragingslab's Avatar
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    Great tip! Seems obvious but I never thought about it.
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    Senior Member 4cylndrfury's Avatar
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    So I am ready to make my mold to take the cup pattern and create the void that will be filled with castable. I was originally going to use RTV to shell the pattern and aid in releasing the green crucible, and fill around the RTV with concrete to create the mold. But that presented challenges as far as creating the a 2 part mold. Then I had an idea...I could mount my pattern to a jig, similar to my pictures posted above. But rather than use concrete and RTV, I would use great stuff minimal expansion foam in a piece of sonotube. After the foam cures, I can remove the pattern, use a jigsaw with a razor blade attachment to split the mold in half. I will even add some alignment pins to the mold halves for a good seal. The foam is also a little spongy, which should help release the grean crucible easily without much damage.

    Heres my issue - great stuff will stick to EVERYTHING...including my mold. Does anyone know a trick to keep great stuff foam from sticking to something? I know for example, when using fiberglass or epoxy resins, you can use gool old carnuba wax as a mold release agent. Does anyone know if this wil work with great stuff? Or a similar waxy or oily agent? I could use a layer of plastic wrap around the cup pattern to release, but then the mold will have all the wrinkles from the wrap in it, and Id like to avoid that if possible.

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