Getting started with shell - Can I fix this?

Discussion in 'Investment casting Ceramic shell method' started by 0maha, Aug 1, 2022.

  1. 0maha

    0maha Silver

    After too much time and wasted material working with block, I decided to give shell a try.

    A big part of my inspiration was the "boiling out wax" thread. That looked really promising.

    So I gave it a try. Haven't cast anything yet, but I've got a couple of shells ready.

    I used six layers of suspendaslurry + silica sand.

    After the boil out, one of the shells showed a few cracks. The other looked good. I patched the cracks with some dope made from suspendaslurry and sand, and it looked good. But I decided to fill that shell with water to see if it was tight, and it's got leaks. The problem is the leaks are inside the shell where they are pretty hard to get at. Good news is the other shell holds water.

    My thought is to dip that one once or twice more to see if that fixes the cracks. Otherwise, I think that one's a gonner. Any thoughts? Will one more dip save the shell?

    Here's a video of my boil out:
  2. Monty

    Monty Silver

    Hi Jeff,

    Not uncommon for shell to crack during burnout. Usually you patch it with satanite. Bury the shell in a bucket of sand when you pour to stop any leaks. I've never done the boil out method though.
    0maha likes this.
  3. Mantrid

    Mantrid Silver

    Alot of time goes into making the shell so no point in risking it fail. Dipping it a couple more times is much less hassle han redoing the shells from scratch
    0maha likes this.
  4. Monty

    Monty Silver

    He already boiled out the wax. If there are cracks in the shell it's going to fill up with slurry when he dips it.
    0maha likes this.
  5. Mantrid

    Mantrid Silver

    thick slurry painted on with a sprinkling of stucco. but fire the shell first to eliminate all residues of wax
    0maha likes this.
  6. Amazing vid thanks for sharing, definitely want to try this.

    A couple of questions (for anyone who's done this or wants to weigh in)

    Why put the shell with cup up, would down not be better or it doesn't make a difference? Just thinking that it might help to drain wax out quicker?

    Assume no point to drying out the shell totally before boiling out?

    How long would you fire/torch this for afterwards before pouring? what temp?

    Also wonder if another pot/container on top that catches the steam, then putting the shell in that one on a grate over the water might help with cracking? If they are caused by rolling of the water/knocking on the edges/stand etc?

    I've seen online videos of drilling 'gas vents' (holes basically) in the shell before firing/burn out to help with cracking, not quite sure how it works but if the pattern/sprue system can accommodate, could try this?
  7. Ditto this advice, pack sand around tightly and leaks won't go very far...still annoying to have a 'flash' to work off but often if not in detailed place, is simple enough to do.

    Look forward to seeing the result
  8. Monty

    Monty Silver

    Wax expands a lot when it heats up. Think how much it shrinks when solidifying in reverse. I was taught to fire it hard and fast. This way the outer portion of the wax melts and runs out, leaving room for the rest to expand. You place the cup down to help this happen. This helps minimize cracking of the shell. If you have a bunch of thin sections that are blocked by thick sections, then drilling holes lets the liquid out.

    When you boil it out, the wax will want to float to the top, so you would place it cup up. Other than that, I have no experience with boil out method.
  9. 0maha

    0maha Silver

    My assumption is that the wax being lighter than water, you want to give it the chance to float up and out of the shell.

    Not sure. I boiled mine approx 24 hours after the final layer of slurry.

    We just poured my shells. I took them down to a local art foundry, and we ran them through their standard process. Which is to say, we fired them in their "flash fire kiln", just like they do on their own (non-boiled-out) shells. Even after the boiling, a little bit of residual wax dripped out of mine.

    After firing, we packed my shells in sand, along with a bunch of their own, then gave them a once over with a propane weed burner before pouring. All that said, I don't know the actual shell temp by the time we poured.

    I'm a total newbie here, but my opinion is that you want to get maximum temperature to the shell, uniformly, and melt out the wax as fast as you can. The name of the game is to get it melting all throughout the shell, and give it a way to escape, before it expands and cracks.

    This first test casting seems to have gone really well. The shells held up fine. (It was actually kind of satisfying that neither of my shells leaked out during casting, while a couple of "house shells" from the foundry did leak. Hah!)

    I'll also say that knocking the shell off is about 1000x easier than trying to extract these castings from block method. Night and day.

    I'm heading off to CNC the castings now. That process always tells the truth. I'm interested to see if there is any difference in shrinkage between my block method and this shell method. One thing that became clear today is that shell cools off way, way faster than block. Makes sense that it would be that way. Since my parts need a fair degree of dimensional accuracy, I need to figure out if anything about this new process means I need to make adjustments.

    I'll try to get some pics up later.
  10. 0maha

    0maha Silver

    Here's a pic. This shows the detail retention through the entire process:

    3D Model -> 3D Print -> RTV Mold -> Wax Pattern -> Shell -> Boil out -> Casting -> Media blast


    I spent a lot of time trying to work out a way to do these using block (plus vacuum assist) method because I wanted high levels of surface detail. Based on this result, I don't think that is necessary. I don't see any appreciable loss of detail using shell + boil out. In fact, given that it is so much easier ensuring no air bubbles during investment using shell vs block, you could probably say the surface finish is better this way.

    Also, I CNC'd four castings last night, and can't see that there is any appreciable difference in shrinkage. There might be just a tiny amount more with shell. I previously did some shrinkage tests with block and came up with 2.75%. If the shell is shrinking more than that, it's like 2.85% at most. Not enough to bother with for now.

    Just boiled out two more shells, and have a third one in process (four layers of slurry to go). I should be able to get those cast next week and we'll see if the results repeat themselves.

    The guy at the foundry suggested that there wasn't any need for me to pre-dewax my shells before taking them down to him. Why not just let him use his gas fired flash kiln to dewax? Main reason to use boil out at this point is wax management. He uses a different wax than I do, and it's so easy to reclaim it using the boil out. Boil out allows me to keep my wax separate from his, and I think it also keeps it cleaner.

    Pretty happy with this process at this point.
    Tobho Mott likes this.
  11. Been reading up on wax expansion and so on, all makes sense now thanks :)

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