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Thread: Hello and a Question on getting a smooth pour

  1. #1

    Hello and a Question on getting a smooth pour

    Hello Everyone,

    I've been sculpting for over a decade now (where does the time go?) and I thought it would be really fun to learn how to cast my own sculptures.

    Actually it's been a load of fun and I haven't even had a good pour yet!!! There's just something about watching metal melt that's really cool... hot... whatever.

    I tried my first serious pour the other day. I made a two part plaster of paris sculpture of a robot head. I dried the plaster out over 3 days and it felt dry to the touch. I did not put it in a oven.

    The resultant sculpture looks cool for what it is but is NOT the texture I want going forward. The plaster was nice and smooth on the inside, but the casting had a texture that can be best described as zombie skin. I'm putting a picture in this post.

    Can someone tell me what I'm doing wrong? Am I not heating the metal enough? Should I be heating up the plaster?

    IMG_1950.jpg

    Thanks for you advice!!!

    Arthur

  2. #2
    Your problem is the plaster. Even if you put it in the oven and heated the heck out of it you wod still have the same problem. The reason is that p.o.p. has chemically bound water in its makeup, and it gets driven out when you pour the metal in. Hence, the extremely rough texture. Search for lost wax or lost foam coatings on here. Uncalcined p.o.p. has very few uses in foundry work.
    Vade Libram Harenae.

  3. #3
    Quote Originally Posted by OddDuck View Post
    Your problem is the plaster. Even if you put it in the oven and heated the heck out of it you wod still have the same problem. The reason is that p.o.p. has chemically bound water in its makeup, and it gets driven out when you pour the metal in. Hence, the extremely rough texture. Search for lost wax or lost foam coatings on here. Uncalcined p.o.p. has very few uses in foundry work.
    Hi Duck,

    Thanks for that. I haven't had much luck with green sand casting either and I can't get the detail out of it that's I'd like. What do people use for bronze molds that require high detail?

    I've been reading up on lost wax, foam, pla but I've not really found a good answer.

    Thanks,

    Arthur

  4. #4
    Administrator Site Admin
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    I use a mix of one part plaster to two parts silica sand. Then I burn it out at between 1000 F and 1200 F. You must burn out plaster investment molds at at least 900 F to get the bound moisture out. Small molds I burn out for 24 hours.(9 inches diameter X 12 inches high). Large molds (16 inches dia. X 23 inches hight) I burn out for four days.

    This is for bronze. I don't cast aluminum,

    Richard
    When I die, Heaven can wait—I want to go to McMaster-Carr.

  5. #5
    Senior Member Tobho Mott's Avatar
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    One of our members, Rasper (edit - who snuck a post in while I was typing this), casts his own work too, he uses block investment that is made of a mix of plaster of Paris and sand or brick dust I believe. Because of that chemically bound water, molds like this need to be burned out in a kiln before you pour them. The sand/brick dust aka grog makes the plaster porous enough to allow gases to escape the mold without ruining the finish on the casting. Many people prefer to use and have more success with commercially available investment mixes over homemade plaster based recipes. The other common option is ceramic shell molding, for which the slurry the waxes get repeatedly dipped into, as far as I know, is even harder to make DiY-style than the plaster-based investments. I mostly do sand casting and occasionally some lost foam, so any lost wax knowledge I have is second hand... if you search the forum for "investment" and/or "ceramic shell," you should be able to find some brand names. Whatever type of mold you end uo going wih, I don't think you will get the results you want without a burnout kiln to prepare the mold. And probably some trial and error. But don't let that stop you, it can be done!

    What went wrong when you tried sand casting? If your sculpt is shaped in a way that lends itself.easily to sand molding, it should be possible to cast multiple copies faster and cheaper with sand casting than with lost wax, but it takes some time and experience to get your sand right for best results, and best results with sand casting likely won't be quite as good as best results with lost wax. Lost wax is most common for sculptural casting but there are some who use sand casting such as Scott Nelles. But he has to design his sculptures carefully to avoid undercuts etc., so that they can be molded in sand.

    Good luck, you definitely came to the right place to get this figured out!

    Jeff
    Tobho had learned to work Valyrian steel at the forges of Qohor as a boy. Only a man who knew the spells could take old weapons and forge them anew.

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  6. #6
    Hi Jeff,

    Thanks for that information.

    I bought green sand and my first cast which was just a test clearly had way to much water in it.

    The second time I tried I could not get the sand cast to take the detail of my mold. Did you see the picture of what I tried to cast above? Do you think that would lend itself to sand casting.

    And recommendations on how much water should go into the sand? Am I supposed to let the sand dry out at all before I pour?

    Thanks,

    Arthur

    - - - Updated - - -

    Hi Richard,

    Thanks for that. How are you doing the burn out. Do you have an electric kiln?

    Regards,

    Arthur

  7. #7
    Location helps us help you???

    If you are in timbuktu, sand. If you are here in the states near an R&R location, you need to seriously look at ceramic shell. Plaster works well for some, but these days, ceramic shell or sand is the only way to go. Ceramic shell is not as expensive as you would think and the results are perfection every time. I did my first shell bronze piece this year and it didn't look like your alien at all. I think people tend to shy away from shell because of the equipment. Weed burners work well for dewax and kilns are a dime a dozen on craigslist for burnout. A 5gallon pail of slurry and the 2 bags of silica last forever. IF I had to guess to shell a small sculpture might cost me at the most 10bucks worth of investment. The wax is around 75% recoverable and reusable and bronze is $6 a pound for commercial grade stuff. Check out some of the videos on my youtube page, it shows just how easy this ceramic shell thing is. I tried sand, but the sand in my area of texas sucks so I had no option but to go down the shell road. Now that I have this shell thing working well, I'm hooked.

    Good luck, getting your sculpture into a wax positive is key here. I still use some clay, but I am forcing myself to sculpt directly in wax and so far it's worth it.
    WWW.TheHomeFoundry.org
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  8. #8
    whoops.. you need a little link.. sorry https://youtu.be/weaBphjDROQ
    WWW.TheHomeFoundry.org
    Visit me: WWW.HandcraftedLanterns.com
    "Sometimes by losing a battle you find a new way to win the war"
    -- Donald Trump --

  9. #9
    Administrator Site Admin
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    How are you doing the burn out. Do you have an electric kiln?
    I made a lift-off kiln using a 55 gallon drum wrapped with 2 inches of ceramic wool and covered with sheet metal.



    This is the base. I use a 1/2 inch Reil propane burner in the hole at the red arrow. The yellow arrow points to the end of the flame tunnel.




    Regarding investment materials.

    My mentor has done over 1000 block investment castings and hundreds of ceramic shell investments. He says he uses shell only for small castings, that block is better for large items. Most of his investments are rather large.



    He uses a World War 2 Army half-track to drag them out of the foundry to the breakout area.



    He has a lift off kiln similar to mine.



    He has a serious foundry there.

    Richard
    When I die, Heaven can wait—I want to go to McMaster-Carr.

  10. #10
    Administrator Site Admin
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    Regarding surface finish. Pour temperature has an effect. About 2000 F is as much as plaster can stand. I heat my metal to 2150 F. I figure I lose 150 degrees by the time I pour it.

    Richard
    When I die, Heaven can wait—I want to go to McMaster-Carr.

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