The Shop > Metal Stuff
Plaster of Paris, Investment, etc.
vtsteam:
So I've been doing some online reading about lost wax casting, investments, homemade and commercial, and am coming across a lot of conflicting information.
First question I had was, what is the calcining temperature of plaster of Paris. You'd think that would be a pretty straightforward figure. Well it isn't. I've seen:
About 600C
About 650C
About 600 F
"It is obtained by heating gypsum or calcium sulfate dihydrate to about 140°C - 180°C. "
"It is necessary to take the investment to the calcining temp (~650°F) at a controlled rate of rise so as to maintain its integrity. Once it it fully calcined it can be raised to 1200°F fairly quickly."
"I would strongly suggest that you obtain some true casting investment rather than trying to use homemade stuff. Plaster of Paris cannot withstand the high temperatures (1200°F/650°C) needed to get a clean burnout."
"The book I use as my guide says that 48hours burnout at 600 deg C should be enough more than that is just overkill."
Okay, what the heck?
Is this like a bunch of blindfolded people feeling up an elephant for the first time and trying to describe what they imagine the animal is?
What is THE calcining temperature of Plaster of Paris? And please cite your reference.
Pretty please?
mattinker:
My source was the "Fondrie Godard", a Parisien foundry who now work for the musé Rodin amongst. My girl friend of some twenty years ago worked for the Stanford in Paris program and one of the things she organised annually was a visit to the Godard Foundry,so I got myself in! At the time, there wasn't all the conflicting material available on the Internet. It was hard to find concrete information as there is a lot of mystique around lost wax. I got the 600°C from the horses mouth! The founders that I met and spoke to about the process were adamant about soaking the mold in heat at 600°C to make sure that there was no water left. One of the differences between sculpture and mechanical parts is the thickness of the casting. Sculpture is usually made as thin as possible to keep the cost of material as low as possible, the 600°C preheat allows the molten metal to flow more easily. If the mold is left to cool, it will reabsorb humidity from the air.
"It is obtained by heating gypsum or calcium sulfate dihydrate to about 140°C - 180°C. " This applies to the transformation of Gypsum into plaster. The rock is heated and crushed to form plaster, this is an irreversible process, once it has been turned into plaster, it cannot be recycled back into reusable plaster.
I started working with plaster over forty five years ago, first of all casting and running moldings, later here in Paris I did a lot of building work in Plaster. The home of plaster used a lot of plaster in places where cement is used else where. One can still buy "coarse" plaster which is used to set window frames and beams, ordinary plaster and dental or casting plaster. Investment plasere needs to be pure plaster with a grog added, brick dust or silica sand.
It is about twenty years since I did any lost wax casting, the preheating kiln being a big limitation. I have a project to build one in a 200 liter oil drum, I hope to get this done before I retire in three years time! I sill have sculpture in mind to cast!
Regards, Matthew
tom osselton:
I have been surfing tonight and have found this site he did his research masters degree thesis on this subject. he talks about ash content, shrinkage and burnout times varying on the size of the casting.
http://www.calumdouglas.ch/metal-casting/
This next site is using abs plastic but states that it is under proper ventilation as it is toxic!
http://jason-webb.info/2012/11/lost-abs-experiment-with-3d-printed-objects-and-aluminum-casting/#burnout
This is the utube video that I saw originaly he says he has 20 years experience casting metal and fires his plaster mold after a couple hours of setting and it seems to go fine, he says because of the sand content.
Heres a pdf for HYDROCAL®Gypsum Cements
http://www.usg.com/content/dam/USG_Marketing_Communications/united_states/product_promotional_materials/finished_assets/hydrocal-gypsum-cements-application-en-IG538.pdf
vtsteam:
Thanks Matt, great to hear about your sculpture casting experience in France. That must have been really interesting to be a part of.
And Tom thanks for the references that adds even more "takes" on investment casting. Obviously there is a huge range of opinion and procedure.
Now let me clarify my question, because I haven't seen in any of the above information the actual answer to it. Yes there is lots of information about investment casting practice, but that isn't what I want to know.
I want to know at what specific temperature CaSO4·2H2O calcines, that is, loses its bound water. Not recipes for heating molds with plaster(s).
And 140C to 180C mentioned above (if correct) is kind of a big range for a straightforward chemical reaction process, I think it's probably more specific.
Once I understand what the calcining temperature of plaster actually is, then I can move on to understanding other parts of the molding process, and truly WHY differing heat schedules are used by different people. But first things first. Lets get the basics down.
I do truly appreciate the responses, btw!
ps. I gotta say though -- the guy who did his master's degree thesis on investment casting -- that is one of the most complex, cost ineffective means of producing a rough aluminum casting with scabby finish I have ever seen! I can straight sand cast a better looking part in a half hour with a simple charcoal furnace. Not trying to brag, here, just, that kind of complexity is absolutely unnecessary -- a 3D printer, 49% fused alumina content, 24 hours of fuel for kilning, ten risers and a tooled flask, sheesh!
awemawson:
Steve, I'm wondering if the range of values quoted is because in the real world the Plaster of Paris perhaps varies in composition, and the reaction is a bit more complex than the simple CaSO4 .2H2O -> CaSO4 + (free) 2H2O implies.
With a well regulated lab oven I suppose a series of experiments would fairly easily determine the value for a specific sample but it wouldn't necessarily be true for all sample of 'Plaster of Paris'
I used to set my kiln at 650 deg C, but what the temperature actually was, and whether it needed to be so high I have no idea !
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