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#1
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casting investment
I've been casting (as an amateur) for at least 20 years. I may have
'missed' 6 or 7 castings in the last 5 years - not bad for a non- professional. However, I don't know exactly what goes on during the last part of the burn out cycle. I use steam to dewax the molds, then use a 300 degree F. in kiln to finish the dewax. When the temp is raised to about 800, all the 'organic' material is eliminated. So far so good. Then the temp is raised to 1250 d. F for about 2 hours before being lowered to casting temp of 900 to 1,000 degrees. Ok, what happens at 1250? I know that the investment I'm using will start to break up at about 1300 degrees, releasing sulphur into the cavity. But why does the cycle need the 1250 degrees? My standard flask is 2.5 inches diameter by 3 inches tall, with no more than 5 ounces of SS. I have done bigger flasks, but I try not to. Charlie |
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#2
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casting investment
On Mon, 25 Sep 2006 18:28:30 -0700, in rec.crafts.jewelry Charlie Leo
wrote: I've been casting (as an amateur) for at least 20 years. I may have 'missed' 6 or 7 castings in the last 5 years - not bad for a non- professional. However, I don't know exactly what goes on during the last part of the burn out cycle. I use steam to dewax the molds, then use a 300 degree F. in kiln to finish the dewax. When the temp is raised to about 800, all the 'organic' material is eliminated. There's the possible flaw. at 800, by far the majority of the organic material will indeed be gone, at least as far as organid compounds. But a small amount of the wax, which didn't flow out during steam dewaxing or later, but instead, soaked into the investment, has by now turned to carbon. You can verify this by stopping the burnout at this point with a scrap test flask, letting it cool, and breaking it apart with a hammer to examine the investment walls in the mold cavity. Usually, you'll find they're not pristene white, but instead, gray. Now, to a certain extent, this can act as a deoxidizer for metal if it's still in the flask, but it can also be a major cause of gas porosity, as well as no-fills, since when it reacts with oxygen, it forms carbon dioxide or carbon monoxide. The influx of molten metal can cause some of it to do exactly that, and combined with the turbulence of the casting operation and all, some will be incorporated into the metal. Not only can it cause gas porosity, but it can also, with some metals, cause the formation of metallic carbide compounds, which show up as hard annoying speck like inclusions in the surface. Some types of white golds are especially prone to this. The other thing that this carbon does is to take up space in the pores in the investment. It simply makes it less gas permiable by clogging it. So that makes it more difficult for the air in the mold cavity to get out of the way of the incoming metal. That's the reason for raising the temp to 1250 or so. it ensures that all the carbon in the mold, not just that visible at outer surfaces, but that close to the inner mold cavities, has been burned away. With that said, it's not always essential to raise the temp that high, at least not with standard gold/silver investments. Since you're steam dewaxing, most of the wax, and therefor the carbon source, is gone, minimizing the problem. And if you raise it just a little higher, and then hold for a more extended time, you can get the same quality of casting. Casters who cast pieces with the diamonds or other stones already set in the wax, often will need to do this, since the stones (especially diamonds) won't always withstand the higher temperatures without damage. So the flask is simply raised to about 900 or 950, no higher, and held there long enough for a complete burnout, and then cast at that same temperature. The hold times at this temp, though, can be quite a bit longer to get a properly clean burnout. So far so good. Then the temp is raised to 1250 d. F for about 2 hours before being lowered to casting temp of 900 to 1,000 degrees. Ok, what happens at 1250? I know that the investment I'm using will start to break up at about 1300 degrees, releasing sulphur into the cavity. But why does the cycle need the 1250 degrees? Only because at that temp, full burnout is fastest and most consistantly assured. That temperature level does not cause any intrinsic change in the investment structure or anything like that. Now, higher temperature investments, such as some dental investments or platinum investments, may need their full high temperature range (often higher than 1250) in order to properly sinter together, to be strong enough to withstand the casting operation. But that's not the case, so far as I know, with the standard sulphate bond investments like Satin Cast 20, or the like. peter |
#3
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casting investment
On Tue, 26 Sep 2006 01:47:56 GMT, "Peter W.. Rowe,"
wrote: So the flask is simply raised to about 900 or 950, no higher, and held there long enough for a complete burnout, and then cast at that same temperature. The hold times at this temp, though, can be quite a bit longer to get a properly clean burnout. Peter - Not only did you give me a good answer, but you may have solved a major problem as well. I do demo casting at our annual Houston Gem & Mineral Society show. We have a programable kiln timer to run the burn-out cycle during the night so we can cast the next morning. Unfortunately we have had a lot of electrical problems within the building that we are using. Even though we have built an uninteruptble power source for the controller timer, the cycle still gets 'knocked' out. If I'm reading you correctly, I can do a steam elimination of the wax and then set a manual controler to 950 degrees and let it run overnight (from 6 pm until 10 am next morning. It wouldn't matter if the electricity was off for a couple of hours during the night - as soon as the power came back on, the controler would bring the temp back to the 950. Show opens at 10 am - we start casting at about 10:30. Thanks much Charlie |
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