Advice on sprueing needed

Discussion in 'Lost wax casting' started by david wendelken, May 2, 2021.

  1. I've been casting of and on (mostly off) for some years, but my castings were all fairly large and clunky.

    I'm now able to produce much more delicate models and I'm looking forward to casting them.

    I would really like some advice on how you would go about sprueing this piece.

    It would be really helpful for folks to make a copy of this photo and mark "UP" where they think "up" should be when the piece is oriented in the flask and the metal is about to be poured in and place arrows where they would attach the sprues. And if you think the dimensions are too small, I would be glad to know what's the smallest size you think will work (and for which dimension).

    Hopefully your good advice will save me hours and hours of experimentation!

    The piece is 50mm long (just under 2 inches). It's 24mm wide at its widest and 2mm thick. along the central stem. The depressions (or cells) in the piece are 0.5mm deep and are on both sides. The "wires" separating the cells are 0.2mm wide and the outer edge wires are 0.3mm wide.

    The piece below is a draft printing from my 3d printer, it will be printed in the Formlabs wax/resin mixture for casting and the various nubbin artifacts the supports leave will be cleaned up on that version.

    Thanks in advance for your help!
  2. I'm learning how to create closed tubes along a drawn line in the Rhino design software so I can build in some sprueing to the design. That way, I'm not trying to attach wax sprues with my fumble fingers. I think I want to add little sockets onto the ends of the printed sprues (on the side away from the earring) so I can just slide the wax sprues into the socket and apply a smidge of heat to seal the wax within. If I can make the sockets curvilinear instead of with right angles and pay attention to the angle the metal would be passing thru the sockets, I think they might act as mini buttons to feed in a bit of molten metal to counteract shrinkage. Well, that's the plan.

    But aside from the 3d design software, I still need to decide where to attach the sprues to the piece, and that's where you come in.

    A couple of hours after you or I make our best estimate of where the sprues should go, I'll be able to print a batch of wax/resin models to cast that will look a bit like this. (Except they'll be purple.)
    earring build platform ready to go.png
  3. DavidF

    DavidF Administrator Staff Member Banner Member

    Typically you would gate into the bottom of the part and vent out the top so the mold cavity fills bottom up. This helps to flush out any ash or debri from the mold.
    I assume you have a vacuum casting set up??
    You might want to consider adding tabs/ears to your models so you have a spot to attach the in gates to that can be easily sawn off later...
  4. I've never tried doing it that way, but that makes sense. I'll give it a try.

    I do. I can vacuum the investment to debubbleize it and also to assist the metal into filling the mold. But how does that work with the metal filling from the bottom? Wouldn't the metal be fighting the vacuum pressure when it travels upwards?

    And it's got twice the distance to travel, so wouldn't it cool down more before it gets into the mold?

    I'm definitely going to do that. I was hoping for some clear-cut guidance on where to put them, by having someone draw arrows on the photo of the piece where they think the tabs should go.
  5. DavidF

    DavidF Administrator Staff Member Banner Member

    The investment is porous kinda like a sponge in a way so there really is not a direct path one way. As the lower section is filled the vacuum just finds another way to draw through.
    I use perforated flanged flasks and find it takes minimal gating and venting. So much so that I only need a very small ingate in most cases...
  6. Wow, that's a whole lot less sprueing into the piece than I would ever have thought would work!

    I don't have a set-up that will work with perforated flasks, mine just pulls downward.

    I'll give simple, small tabs a try, then.
  7. What kind of vacuum pressure should a vacuum unit produce when you're pouring the metal?
  8. DavidF

    DavidF Administrator Staff Member Banner Member

    Atleast 28"
  9. Thanks! I never had a manual and wasn't sure.
  10. Stayed up late to add the tabs and start the print. First print with castable wax resin!

    first wax print screenshot.png
  11. Had to get my garage re-wired and replace some equipment that was failing, but I'm trying out some of my earrings tonight in the first casting that actually has a chance of working. :)

    I'll post results tomorrow.

    In the meantime, I have another set of pieces I want to cast in bronze or silver. The dimensions given are in millimeters. The thinnest parts of the shield are 1mm thick and the thickest are 2.2mm thick. If need be I can adjust those a bit but I would prefer to succeed with these measurements.

    Where and how would you sprue them? The backs are all flat so any place on the back will do.

    Probably easiest way to answer would be to download the picture, draw some circles where you would put the sprues on the back, and upload the photo again.


  12. The above drawing is two different pieces from the same line drawing, just varying what's thick and what's thin. I'll be enameling in the thin parts.
    The back on both pieces is flat as a pancake.

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