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? :)
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
