Reinforcing ceramic shell

Discussion in 'Investment casting Ceramic shell method' started by MikeMike, Nov 22, 2023.

  1. MikeMike

    MikeMike Copper

    I cast small silicon bronzes in the UK, previously using ammonia-cured binder, now Wexcoat. I usually have some cracking the shell on burnout, and while this has been repaired with a further coat of slurry I would like to minimise it to minimise the risk of shell failure. I've tried using fibre-glass in the final coat but find this difficult, messy and unsatisfactory. Has anyone had experience of any type of microfibre addition to the slurry? Or any other method of strengthening shells?
  2. mytwhyt

    mytwhyt Silver

    I don't do shell casting, but fiber blanket and sodium silicate might be a better choice than fiberglass for patching cracks.. Much higher temperature tolerance. Water cleanup on any spills much easier than cleaning up resin..
  3. ddmckee54

    ddmckee54 Silver Banner Member


    If you go to the Lost PLA Casting forum on this site, there's a thread there called "Just Cool". In that video he appears to wrap a fine wire around the shell, I assume that was to reinforce the shell? Is that kind of what you're looking for?

  4. TanyaJen

    TanyaJen Lead

    going to ask a similar question of trying to reinforce a specific area of the mold, using ceramic shell, and i use fiber blanket, (which i swear by now, feel you have more control then the wire) i then, usually bury in sand before the pour. what im looking for, is some option to fill negative areas in the mold on the underside, where the sand does not fill.

    (imagine, i am casting a upside down bowl, and the inside is hollow.) i would like to fill it with sand, just in case of flashing, or break, but... gravity does not allow. looking for ideas. currently thinking of mixing a plaster/sand mix to fill it? or maybe ?ludo? (chunks of already used plaster and sand or shell from previous molds,) to fill in most of the negative space, and then fill the rest with the plaster sand mixture? any ideas? im a little concerned about pouring plaster/sand on my ceramic shell. i would then have to preheat it with the plaster/sand in place, which would also be relatively weak after the heat.
    had a pour of a bust, and the negative space filled with an extra, unexpected, 30 pounds of metal. trying to avoid doing that again. looking for a cheep, but reliable material to put inside to strengthen it up. or fill entirely. i may just go extra with the fiber blanket, but i would feel better if i could also trap some sand in there. or something.

    any ideas, or experience with that would be appreciated.
  5. ddmckee54

    ddmckee54 Silver Banner Member

    You've probably already thought of this, but I don't imagine that there is any possibility of tipping the mold in the sand to eliminate the negative areas? Using your upside down bowl analogy, tip the bowl just enough to let gravity fill the area with sand.

  6. TanyaJen

    TanyaJen Lead

    a thought.... could try, it feels... counterintuitive. like... tip it, put as much as i can in there, and then set it straight.... some pours out, but... still more then nothing. yeah. thought about that. i feel like... there has to be a better way. even, flipping, it, filling the bowl, putting, maybe a kiln shelf on it, then flipping it over entirely, and then burying it... but... also feel like there should be a better way. i am also concerned about moving it much, with that much extra weight attached. which is why i was looking at ludo, or plaster/sand, leaning toward the airy mix.
  7. Al2O3

    Al2O3 Administrator Staff Member Banner Member

    It's a similar issue I encounter in lost foam casting and preferential positioning in the mold to avoid this is always part of my gating and feed system strategy.........but, sometimes it's unavoidable or other feed system and mold concerns take priority.

    Why not just use sodium silicate bonded sand? It could be catalized or CO2 activated. Very inexpensive, quick cure, doesn't have to be baked, but will probably stand up to your preheat temp depending. Like making cores, you just need to use a mixture that will shake out satisfactorily and not become rock hard.

    Tobho Mott likes this.
  8. TanyaJen

    TanyaJen Lead

    i like the sand idea. i have used straight sand for molds, and even have dont lost wax sand molds, which i love, and you can mold them up in an hour, and then melt it out the next day, and then cast maybe the next day.

    i think it would work, my reservation still reside in, adding the extra weight to the inside of a shell mold, and having to movie it around from preheat to sand pit, to cast. i think the sand would probably clean up the easiest, so i appreciate that.

    i currently am running the pour, but not the molding, so i am getting the molds after they have been invested. i would probably arrange pour spouts differently to prevent this issue, and will be making this recommendation, but have a few lined up that i have to deal with.

    thank you everyone for the suggestions
  9. ddmckee54

    ddmckee54 Silver Banner Member

    Sorry about that, I thought you were asking about the molding end. If you have to deal with the shells you've got, your ceramic blanket idea is about your best option.
  10. TanyaJen

    TanyaJen Lead


    revisiting this a little bit, i was thinking of trying out resin sand again for some of the pieces coming up. looking for any help, as the new studio does not have the set up for mixing sand, like my last. so i can still do the tried, and true, mix in a 5 gallon bucket, and such. im interested in a adjusted mixture that i might use for the filler so that its easier to remove? i have always just followed the recipe.

    im kind of asking all over, i am asking about the molding end, but also working with some of the molds that are already done. so, nothing to be sorry about, and all ideas are welcome and useful. i think i can use your idea for one of these shells, so thats going to work out, thank you.

    i am currently working closer with the artist, and should be able to dissect future pieces differently, to avoid these cavities. only have the last 2 or so that they finished before i got there. fingers crossed, i will be trying the poured sand/silica,(maybe alot of grog, and ludo) and hope for the best.

    thank you again!
  11. Al2O3

    Al2O3 Administrator Staff Member Banner Member

    It's such a broad generalization because there are such a large variety of binding resins.

    Binders can range from professional materials like furan, urethanes, to sodium silicates, and old school traditional materials like boiled linseed oil to even corn starch.....r even clay bound green sand.

    No idea what recipe you are referring to but, the amount of any given binder required for a given core strength will also vary due to sand grain size (finer sands require more due to higher surface area), and the metal being cast. The strength of some binders will be degraded by the high temps of iron while others will become rock hard. The opposite can be true for the same binder in lower melting alloys like aluminum. If by resin you mean epoxy here's a thread to read.

    In the end, you just need to experiment with your recipe and what works for you. This will be difficult if you can't actually expose the binder to casting temepratures yourself. If you search the forum you will find lot's of discussion on cores and binders.

  12. TanyaJen

    TanyaJen Lead

    thanks again for all the input! i appreciate it. work is going slow, so putting together a sand mixer now, and will start playing around with some different recipes. i have used uniset for bronze and iron, but stick with their recipe. might adjust for some of the filler we are doing. also heard good things about ?novaset? which i have not tried yet.

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