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Thread: Best Investment for Special Conditions

  1. #1

    Best Investment for Special Conditions

    Dear AlloyAvenue forums,

    I'm new here! I don't have too much experience in casting, but I'm looking to get into it more. It's something that really fascinates me and that I really enjoy.

    Anyhow, I have a project right now that involves casting a metal part. I've tried different methods and so far have not had much success. So first let me say that I'm trying to cast a metal part based off of a 3D print. So far, I'be just been using a 50/50 Plaster of Paris to Fine Sand mixture as an investment. I started off using a printed part made of PLA and attempted to try to burn it out of the mold as seems to be standard practice. This unfortunately did not work because I do not have access to a kiln. I used different methods, but just simply was not able to heat up the mold hot enough.

    Now, my new plan is to cast using PVA. If you don't know, PVA is a water soluble plastic. My plan initially was to make a mold using the PoP sand mixture, then simply submerge it in water and watch the PVA dissolve away. This also did not work. Since the plaster/sand is rather brittle -- especially when soaked in water -- it began to flake apart, ruining my mold. It also doesn't help that as the PVA dissolves, it soaks up water which causes it to expand. This cracks the plaster even more.

    So, my basic question is: what do you reccoment to use in place of my plaster. It needs to be capable of being submerged after it sets without altering its strength whatsoever, also needs to be relatively strong, resistant to cracking/chipping, and not overly expensive. I need about a gallon of the stuff (when all mixed up) and hope to not spend more than $20.

    Thanks guys for any insight you can give. Also, before you mention it, using a kiln is not an option, I DO NOT have any access whatsoever to a kiln. Thanks!

  2. #2
    Got a photo of the part??? and where are you?

    Welcome btw.
    Jason
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  3. #3
    Senior Member machinemaker's Avatar
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    Welcome.

    Usually a casting or molding plaster will not fall apart if soaked in water, are you sure that you are using real plaster of paris? I buy 50# bags of molding plaster from a sheet rock / stucco supplier for about $15 and I can leave it in water and nothing happens.
    kent
    Kent
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  4. #4
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    I regularly soak plaster molds in water before pouring wax into them. I have never had plaster deteriorate from soaking. Granted, those molds are 100% plaster and not a plaster/sand mix.

    After you do get the pattern out of your mold, how do you plan to burn out the investment without a kiln? If you are pouring brass or bronze you will need to heat them to at least 900 F. to remove the bound water. I understand that you can use a kitchen oven to burn an investment out at 550 F. and pour aluminum successfully. (This is second-hand knowledge as I don't pour aluminum.) If you were to pour brass into an investment burned out this way you would have a volcano of molten metal caused by the released steam. (Don't ask me how I know.)

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

  5. #5
    Without knowing where you are... I was able to score a kiln for just $25 recently. It only needed a little work to get it ready again. Check Craigslist and low ball sellers. People often give these away just to get the space these take back!

    Do you have a furnace yet? How are you melting your metal? Can the part be burned out it that?

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    Visit me: WWW.HandcraftedLanterns.com
    "Sometimes by losing a battle you find a new way to win the war"
    -- Donald Trump --

  6. #6
    Thanks for all the responses so fast, guys!

    First let me say that a kiln is seriously not an option. I actually did check craigslist, there are none near where I live. I've also contacted various schools and art centers near me, some had a kiln and wouldn't let me use it, and some didn't even have one. So a kiln is out of the question.

    I'll probably dry out the mold when I'm done dissolving the plastic either using a gas grill or a large gas torch which I have. I tried using both of these to burn out the plastic, but unfortunately neither one was hot enough to burn it out.

    However, as I write this, my mold is still sitting in water and the plaster is crumbling horribly. It's pretty much to the point where it won't be usable anymore. I think the main reason that it's cracking is because of the expansion of the PVA as it dissolves. So, my question is this: If I were to make a mold out of regular Plaster of Paris (without sand mixed in), then soak it in water to dissolve the part inside, then use an oven to dry out the mold, do you guys think it would crumble into bits at any point in the process? I can experiment with this in smaller scale too just simply using various PVA parts. If it does crack, what do you recommend for an investment, or have any of you even used other investments?

    Thanks again for all the replies so quickly!

  7. #7
    Moderator DavidF's Avatar
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    PVA filament does not work well for investment casting. It leaves too much ash behind when burned out, and you can soak the mold for years and never was it out.
    Might want to try the burn out in a charcoal foundry. Keep in mind though that a 4" dia flask will need 8 hours of burn out time.

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  8. #8
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    Sigilwig444, im not sure if you read raspers comment above but i will second what he says. You REALLY, REALLY should think twice about pouring molten metal into a plaster mould without removing all the water. If you are unable to heat your mould enough to burn out PLA then you are certainly not getting it hot enough to drive out the bound water. This is different to just drying it out.

    If there is any water left in the plaster the hot metal will cause the water to turn to steam, expanding 1700 times its size instantly. At best you will get a molten metal shower, at worst it can expload.

    If it doesnt matter what material your metal part needs to be made from you might want to look at using a low melting alloy such as bismuth or tin based alloys. These can be cast into special silicone rubber moulds and would save you having to risk an insufficient burn out of a plaster mould.

    Just a thought.

  9. #9
    Quote Originally Posted by Artopsy View Post
    Sigilwig444, im not sure if you read raspers comment above but i will second what he says. You REALLY, REALLY should think twice about pouring molten metal into a plaster mould without removing all the water. If you are unable to heat your mould enough to burn out PLA then you are certainly not getting it hot enough to drive out the bound water. This is different to just drying it out.

    If there is any water left in the plaster the hot metal will cause the water to turn to steam, expanding 1700 times its size instantly. At best you will get a molten metal shower, at worst it can expload.

    If it doesnt matter what material your metal part needs to be made from you might want to look at using a low melting alloy such as bismuth or tin based alloys. These can be cast into special silicone rubber moulds and would save you having to risk an insufficient burn out of a plaster mould.

    Just a thought.
    This is a fantastic idea. It seems that casting using plaster and aluminum is just simply probably not going to work for me. I will look into purchasing some molding silicone. Thanks for the input.

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