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RUBBER SILVER BRACELETS : We normally wind the machine 3 times before flinging the metal.So, I think I should stick to smaller waxes in the small flasks,and investigate a larger machine, or vacuum casting. I knowa jewelry shop that does production 5" x 8" flasks, usingvacuum casting, pulling a vacuum on a large chamber, andthen turning a valve to "connect" the chamber to the flask.The flasks they use are filled with holes on the side. Theyheat the metal in a crucible in an oven, pull the flask outof the burn-out oven, get it situated, and then simultaneouslypour the metal and throw the "vacuum" switch.Anyway, I'm looking for solutions. I know it's possibleto make some faultless castings. The bracelets I'm makinguse about 5 oz of silver.Roger, Hard to say what the problem is without seeing the piece, but I don'tthink the problem is the machine. While vacuum casting is most widely usedfor larger flasks and trees, you should be able to get a good cast usingcentrifugal. If the void in the piece contains investment, then yourinstructor could be correct. The investment is weak and breaks off when themetal hits it and gets trapped in the metal creating a void. Less water willgive you a stronger mold. For larger pieces use a water to powder ratio of38/100 as opposed to 40/100 for finer castings. If the void is black roughand jagged, then it sounds to me like the silver was over heated - easy to doif you're torch melting. If the void is smooth and rounded on each side, thenyou are getting an incomplete fill. The silver is freezing before the mold isfilled, in this case raise the flask temp 50-100f.
RUBBER SILVER BRACELETS : -----------== Posted via Deja News, The Discussion Network ==---------- Search, Read, Discuss, or Start Your OwnI teach casting, and have used a centrifuge for years, as well asoccasional forays into vac-assist and other similar evils. 3-4 windssounds about right for most of the commonly available commercialcentrifuge rigs out there. I use an old Kerr Centrifico with a rebuiltarm to cast cans that are 5" dia by 6" long, with really good results. (Irebuilt the arm to allow it to cradle the bigger flasks, and balance theextra weight, not to add more swing radius to it.) The flasks it'sthrowing now are almost 200% heavier than it's original maximum rating,but it's still having no problems, even in bronze, which is notoriouslyviscious. I've used most of the other centrifuge rigs that are out there,and I can't think of one that is really "underpowered", even when pushedto it's max weight. Without seeing the faults, I can't say exactly what'sgoing on, but of the list of possible suspects, "machine underpowered" isso far down that I've never even thought of it until today. If in doubt,try winding it 4 times, and see what happens. (BTW, it's a coil spring inthe hub.)
RUBBER SILVER BRACELETS : Big production shops use vac-assist rigs for a variety of reasons. One ofthe biggest being that the vac rigs were specifically designed forhigh-volume production. The smallest of those flanged, vented vac-assistflasks that anybody makes is the same size as the *largest* flask that theoriginal Kerr Centrifico would throw. Since treeing,investing, castingand clipping are expensive, it makes sense to smash as many objects intoone flask as you can, which is why the production people go for the bigvac systems: in the long run, they're cheaper. But only if you can putout enough waxes to fill the cans.Otherwise, you wind up wasting a lot of investment.machine in our class is adequate to the task of a half-a-bracelet involvingthe pouring of 4-5 oz. of silver.Since I'm still getting cavities, and since the instructor watched over methe second time when I was mixing the investment, I don't know whatto do to improve my results. Every wax involves considerable time.The first wax cost me a weekend, though I did make a rubber moldfrom the wax before casting it. The second wax just took a few hours,and I'm learning, so ... now I just have 9 oz. of silver, ready for mynext project.
RUBBER SILVER BRACELETS : I also did a silver bell, a hemisphere 3" in diameter, also about 1/4 poundof silver. Better results, but still a few cavities.I'm curious ... if I was taking one of your classes, what would you tellme ?? Is it reasonable to expect "perfect" castings, e.g. one minisculecavity, using this kind of equipment ?? I really would like to learnhow to do this.I have a handle for a bread-knife, ready to go, but since I have aday's work in it, I'm very reluctant to cast it, until ironing out thewrinkles in the process. I suppose I'll just "do rings" in the smallflasks, or maybe grab one of my rough bracelets and do it"as is", without any extra carving, to try the instructor's mostrecent suggestion, to use less water in the investment.For the bracelet in question, I used 192 cc of water, 16 oz. ofinvestment, a 2 1/2" x 4" flask, when mixing the investment which,when cast, resulted in the cavities.
RUBBER SILVER BRACELETS : Without seeing exactly what faults you're getting, it's pretty wellimpossible to say what the problem is. The only *typical* problem withinvestment is when it's too thin, and then you get bits of it breaking offinside the mold, or the whole top of the mold blows out when the metalhits it. (Pretty spectacular, that.) If you stay with the mixingguidelines, you'll do OK most of the time, unless there's something*really* weird in your water. (try bottled.)Again, without seeing your pieces, I have no real way to say what's goingon. From the sizes you describe, and the sorts of things you're talkingabout, I surmise that you're casting things that are pretty *thick*. Thiscould be the problem: to get good castings of *thick* sections, you haveto have sprues that are even thicker. Otherwise, you get shrinkage pitsin the metal. This could be what's getting you. The sprues you use mustbe thicker than the thickest part of your wax. Remember that metalshrinks as it solidifies. The idea is that you want the sprues to behotter than the piece itself, so that they feed metal into it as it coolsand shrinks. (That's why they need to be bigger.)In a perfect world, the button at the end of the flask is bigger still,and feeds the sprues.... Ultimately, you *will* get porosity *somewhere*,but it should be out in the sprues where you can just chop it off.
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