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IndustryArena Forum > MetalWorking > Casting Metals > How to Investment Castings
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  1. #1
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    How to Investment Castings

    I've been reading through the forums on investment castings but i'd like to ask a few questions on how to do it.

    I work for an aluminum foundry and CNC shop and im actually a pattern maker, we do greensand, airset, shell core, permanent and die casting so i'v been around and worked with them all aside from investment. If you're interested in seeing some of my company's work and setup you can check out my youtube channel, i've posted some videos over the basics so far. Im the third generation of a family owned Aluminum foundry and pattern shop.

    dzuari's Channel - YouTube

    Im currently designing the pattern and corebox for a fairly complex part, and im afraid their is just to much negative draft to get the mold and boxes to pull and the next variation of the part i know will not be able to be molded so im starting to look towards lost wax.

    So, what im looking to do specifically is take the part and dip it in a slurry and get a hardened shell around it.

    Now we buy in bulk shellcore silica sand which has a heat activated resin in it(500°F) and we also buy reclaimed straight silica sand for airset then mix it with an air activator. Would i be able to mix either of these sands with a investment plaster to get what im looking for?

    Ive also been looking on some jewelry supply sites and have ran across these products

    Gold,Silver, or Brass Casting Investment

    If im reading those products correctly, they are sand already mixed with the plaster, just add water?

  2. #2
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    I've never done it but I have seen it on TV (that makes me an expert, right?). IIRC, they did a couple dips of just the plaster to get a nice smooth layer on the wax, then they had a slightly thicker plaster they dipped it in and then sprinkled the sand on top of the wet plaster. Repeat several times to build up the required thickness.

    Here you go: [ame=http://www.youtube.com/watch?v=BX8w-GUPz1w]Investment Casting Process - YouTube[/ame]

    Matt

  3. #3
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    These are two distinct processes

    You can't mix and match the ingredients and techniques and hope it will still work. The dental investment consists of silica flour and gypsum plaster. This is not a "dip" process - plaster doesn't work that way. You add water, mix thoroughly, put it in a vacuum bell jar to eliminate air bubbles, pour it over your pattern to fill a stainless steel flask, and when it's set, you put the flask in a kiln to burn out the wax. Then you use a centrifugal casting machine or vacuum casting setup to overcome the surface tension of the typically small amount of metal you're casting. If you're doing this on a larger scale, you don't have the problem with surface tension, so you can fill the mold normally. If you're mixing your own investment, you can use sand instead of silica flour. The important thing is to burn out the mold thoroughly before doing that, since any unburned wax residues will turn your mold into something resembling a small volcano.

    The ceramic shell method is different. It relies on colloidal silica mixtures, mixed with fused silica powder, and special graded refractory particles called "stucco" (but not the same stuff you build houses from). You dip the wax pattern, with its gates and vents securely attached, into a tank of "slurry" that is kept permanently agitated with a timer/mixer so it doesn't settle out. Then you coat it with some of the finest stucco, "shake and bake" style, and let it dry. Then you repeat the process, over and over again, gradually increasing the size of the stucco particles you use, until you have a substantial shell built up, at least half an inch thick everywhere. This shell is then dried thoroughly, then put in a kiln to melt out the wax. A torch can also be used to dewax; it's a bit less critical to remove every trace of wax, since the shell is more porous than the investment, but it's not a bad idea anyway. The shells are then set upright in a bed of dry sand and poured full of metal.

    Andrew Werby
    ComputerSculpture.com — Home Page for Discount Hardware & Software

  4. #4
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    Silicone rubber is forgiving and will allow stripping negative draft in airset.

  5. #5
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    Quote Originally Posted by slowlearner View Post
    Silicone rubber is forgiving and will allow stripping negative draft in airset.
    thats one of the reasons we are looking into doing it

  6. #6
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    The coating is a VERY special mix. It used to take weeks to do because you had to wait for the moisture to evaporate between every layer (or invest in a LOT of equipment). In the last few years they have come up with a formulation that you can do two coats per day. We used this in a casting class at the Indianapolis Museum of Art. After putting on 10+ coats over a week (over wax patterns), we used a blow torch (weed killer) to melt out the wax from each of the forms. Next the patterns had to be fired in a VERY high temperature oven (this one is used for Roku pottery) to completely burn off any impurities and to fire the ceramic. If you don't take this step, you will fracture the coating. Soon after the firing (as soon as possible, so it is still hot) you can pour the aluminum. If the coating is thick enough it does not need to be supported, but you would be better off to surround it in sand to ensure you don't get a blowout. Here are a series of pictures making parts at the arts center:

    Wax pattern ready for dip:



    First layer of dip:



    Fully dipped (10+ layers):



    Other patterns dipped (in process, not yet done):



    Burning out the wax from multiple patterns (catching and recycling the wax):



    The bucket has water in it, and we used a torch to melt the wax:



    After most of the wax was out we placed the patterns in the Roku furnace for firing:



    Everybody tucked in:



    And everybody TOASTY (note even the carbon deposits from the weed gun are burned off)!!!:



    Bronze poured in to this mold:



    Pour cooling:



    Shattering off the shell:



    Details emerging, my part in aluminum:



    In that last shot you can see the detail that is captured by this method (you can see fingerprints that I had left in the wax being transfered to the aluminum). That "finger" is about 1/2" around. Also you can see the multiple layers of ceramic. It takes MANY layers with progressively larger and larger aggregate to build up the surface. It can't be done in one big layer (at least not without a large investment).

    This is obviously the highest labor/lowest invest way to do it. If you are interested I may be able to get information on the ceramic material and where to buy it. As I recall we had to order about $500 to get a minimum order, and it would be enough to do many patterns. I believe you can also break up and reuse the ceramic that was fired as the aggregate for new models - you then just need to purchase more of the slurry.

    This process took us several weeks to do but it was done at our convenience. The pattern making was done one night per week, and once the coating started someone came in every day for a week. The pour was all done on one day (bronze and aluminum). If you want to see more pix, they are available here (I did a control arm in an expoxy/sand form as well):

    http://public.fotki.com/mcphill/metal-casting-1/
    CAD, CAM, Scanning, Modelling, Machining and more. http://www.mcpii.com/3dservices.html

  7. #7
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    Quote Originally Posted by mcphill View Post
    The coating is a VERY special mix. It used to take weeks to do because you had to wait for the moisture to evaporate between every layer (or invest in a LOT of equipment). In the last few years they have come up with a formulation that you can do two coats per day. We used this in a casting class at the Indianapolis Museum of Art. After putting on 10+ coats over a week (over wax patterns), we used a blow torch (weed killer) to melt out the wax from each of the forms. Next the patterns had to be fired in a VERY high temperature oven (this one is used for Roku pottery) to completely burn off any impurities and to fire the ceramic. If you don't take this step, you will fracture the coating. Soon after the firing (as soon as possible, so it is still hot) you can pour the aluminum. If the coating is thick enough it does not need to be supported, but you would be better off to surround it in sand to ensure you don't get a blowout. Here are a series of pictures making parts at the arts center:


    In that last shot you can see the detail that is captured by this method (you can see fingerprints that I had left in the wax being transfered to the aluminum). That "finger" is about 1/2" around. Also you can see the multiple layers of ceramic. It takes MANY layers with progressively larger and larger aggregate to build up the surface. It can't be done in one big layer (at least not without a large investment).

    This is obviously the highest labor/lowest invest way to do it. If you are interested I may be able to get information on the ceramic material and where to buy it. As I recall we had to order about $500 to get a minimum order, and it would be enough to do many patterns. I believe you can also break up and reuse the ceramic that was fired as the aggregate for new models - you then just need to purchase more of the slurry.

    This process took us several weeks to do but it was done at our convenience. The pattern making was done one night per week, and once the coating started someone came in every day for a week. The pour was all done on one day (bronze and aluminum). If you want to see more pix, they are available here (I did a control arm in an expoxy/sand form as well):

    Metal Casting | mcphill | Fotki.com, photo and video sharing made easy.
    This exactly what im looking for, i'd greatly appreciate it if you could tell my the stuff you used for the slurry.

    I can buy everything i need to do this in a production setting, make the fire furnaces, slurry tanks, vacuum tanks.

    I have to go right now, iv looked through a lot of your photos right now but once i get back ill look through them all and watch the videos that are attached.

    Thank you

  8. #8
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    I think the videos are just metal pouring, so you won't learn anything from those. I was actually out of town the week that the coating was done so I don't have any pix or video of the in-between steps. I will look in to my e-mail to see if I can find the brand name on the slurry system...
    CAD, CAM, Scanning, Modelling, Machining and more. http://www.mcpii.com/3dservices.html

  9. #9
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    CAD, CAM, Scanning, Modelling, Machining and more. http://www.mcpii.com/3dservices.html

  10. #10
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    Quote Originally Posted by mcphill View Post
    Thank you, from what i have been reading, first you mix up the slurry and keep it agitated, vacuum out the air in the mixture, dip the model in the slurry and let it dry then repeat till you have enough coatings, then melt the wax and fire, then pour the casting. did i miss anything?

    Do you/can you vacuum the model once you have dipped it in the slurry?

    I've read that a lot of times your first dipping is in a different substance, something that adheres to wax we and holds extremely fine detail, did you guys use 2 different types of slurry? and did you spray or coat the wax to get it to stick better?

  11. #11
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    The process you listed is pretty much it.

    We didn't vacuum, but the first slurry was made thinner (in a bucket aside from the main slurry tank). You can brush it on if needed, but for industrial stuff you probably aren't too worried about super fine detail anyway. The first coat or two were just slurry. After that you add/sprinkle aggregate on to the slurry after you dip - this makes the build up go much faster. The more layers you go, the larger your aggregate size can be.

    Have not heard back on the actual stuff we used, will keep you posted if/when I hear something. I am sure the manufacturers at the links above would be happy to help you if you have more specific questions!
    CAD, CAM, Scanning, Modelling, Machining and more. http://www.mcpii.com/3dservices.html

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