Although both kinds of plaster are made with the same ingredients, I like to use them for different things. I generally keep two types of plaster on hand in the studio, Pottery Plaster and Plaster of Paris. If not, click here: Unsubscribe Me and you will be promptly removed! (but I hope you don't) Ideally, you want to get an experienced instructor as well.I hope you are still interested in hearing from me. If you are not familar with the processes get comprehensive guide books and study them closely before attempting a project. The excess metal rises into the plenum below the roof and is later removed as flash.ĭealing with molten metal is not an area in which you want to "wing it". For example, one trick for creating fine structures that end in a curved surface is you make the mold open at the top and then build a secondary "roof" that sits a few hundreths over the upper surface of the mold. There are many tricks to casting that take a long time to learn and master. When plaster casting metals the plaster has to be reinforced with silica and kiln dryed. Production automotive aluminum castings are die cast using high-pressure equipment, a totally different beast than plaster casting. You said a "typical" heat sink, which I imagined to be a relatively small, electronics-type heat sink. Too bad I did not have digital camera yet when I did it. Took 3 hr from cold to bring 100 lb AL to pouring temp. I poured the Al, salvaged from automotive AL sand castings, at about 1400 deg F Melted it in a stainless steel pot made from 8" SS pipe, in a coal-fired furnace with air blown by a bicycle driving an old Buffalo blower. The plaster set so damn fast I could barely get it mixed. After that, the molds were too fragile to take from the oven in one pieceĮventually settled on about half plaster half sand and 1-2% chopped glass fiber, a type sold wet so it dispersed well in the plaster mix Risers also tell you, while you pour, when all your molds are full.Īs for the plaster, I had to burn wax out of my molds as well as moisture, used 900 deg F for several hours. I doubt your molds could be made porous enough to vent without risers. An experienced founder, which I ain't, could tell you how to do that. You could do all this with cavities formed in plaster by a metal master, if your open-top mold were then inverted on a flat piece of plaster, EXCEPT the risering. Top of part at least 2" below top of mold. Top corner of all fins on each part connected to one riser. Well connects via a gate to a corner of each part base. I would arrange several mold cavities around a central pouring well. I do not think that pouring metal into an open-top mold with the fins on bottom ad base up would work, because force of flowing metal would break some of the fins in the mold, the base would end up porous and slaggy and not-flat Alternatively, you could tilt the patterns in the mold so as to have one corner of all fins high, then tie those corners together with a bar that would have to be cut off the finished casting. I like the idea of making pattern a double of the finished part ,so that you do not have to vent air from fins individually. I made a few parts from clay master to plaster-supported rubber mold to wax pattern to plaster mold to aluminum casting. 356 is a good common alloy and a good source is auto cylinder heads. scrap like aluminium extrusions aren't going to do the job. When pouring something with fine detail or thin walls you need a good alloy designed for casting. The mold should be HOT when poured, straight from your bake is ideal. yes, plaster should crumble when abused like that, that's what the sand is for. How you bake it depends on the thickness but i always did it at around 450☌ for a minimum of 2 hours. Plaster should be mixed something like 1:1 with good clean silica sand. i've only done plaster mold casting with multipart molds and because of all the seams always had good venting.Ĭouple of things you may or may not know already: you may still have too much moisture gassing off and blocking the flow. Never tried to pour something like a heatsink but can't really see what the problem is if you did a slow gradual pour and didn't just dump your whole load on there and expect it to fill the mold.
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