The Shop > Metal Stuff
Plaster of Paris, Investment, etc.
mattinker:
You are describing making plaster, the pre-heating of the mold is not about making plaster, but about preparing the mold for casting the metal. The reason that is given traditionally is that the three molecule of water need to be driven of right into the core of the investment. This is not powdered gypsum, but a mass of plaster and grog. It is altogether probable that a lower temperature can be used, but there is an enormous difference between 600°F and 600°C! There is possibly a question of thermal shock, fluidity also comes into play.
Regards, Matthew
vtsteam:
Wax bake out:
Okay so we have another bunch of conflicting information scattered round the Internet about bake-out times. Some people bake their molds for a "few" hours, some for 24 hours, and some say 48 hours is essential.
Obviously the fuel and regulation needs for 48 hours while maintaining temperatures in the 600-650C range is considerable. So why the variation here?
How long does it take to get wax out of a plaster mold at over 1000F? Well, I imagine that if you subjected a candle to a 1000F atmosphere It wouldn't take but a few moments to go up in a flash. Something else must account for these times.
Maybe you need to increase temperatures of the mold slowly so it doesn't develop excessive stresses and crack, or explode from steam being boiled off in the plaster conversion process.
Well how slowly? I guess that depends on the mold thickness and composition. That warm up should finish around 185C (365F) when the plaster has gone anhydrous. There shouldn't be any steam after that. But maybe even anhydrite can develop stresses if heated too rapidly beyond this point. So maybe that accounts for some time.
Any residual wax in the mold will be molten at 185C though not necessarily burning. Depending on the wax, its autoignition point is about 225C-250C. So the remainder of the wax not poured off probably has absorbed to some extent into the mold, and hasn't been reduced to ash yet. Probably the less absorption the better. Which means the less wax residue after pouring out the pattern, the better.
Because if it absorbs it doesn't burn. No oxygen. Like a candle it needs to wick out of the plaster to reach oxygen. But when it does and burns, is that good for the plaster surface and detail.?I don't imagine it is.
So how to reduce the wax residue. Well, steam it out. And if that can happen, it will be happening at lower temps -- maybe even 100-150C during the plaster conversion process. If you could steam it all out, it wouldn't absorb, and burn later.
Now I have noticed in a few online comments that people recommend drying out a mold first before beginning the casting process. While others recommend the opposite -- keeping the plaster damp, or even soaking it.
Maybe the reason is that by doing that they can create enough steam to repel residual wax before it absorbs at a later stage in the curing process.
vtsteam:
--- Quote from: mattinker on August 04, 2014, 10:04:28 PM ---You are describing making plaster, the pre-heating of the mold is not about making plaster, but about preparing the mold for casting the metal. The reason that is given traditionally is that the three molecule of water need to be driven of right into the core of the investment. This is not powdered gypsum, but a mass of plaster and grog. It is altogether probable that a lower temperature can be used, but there is an enormous difference between 600°F and 600°C! There is possibly a question of thermal shock, fluidity also comes into play.
Regards, Matthew
--- End quote ---
I am not describing making plaster. Yes I know there is a difference between 600F and 600C. I did not say a lower temperature can be used to cast. I said that the chemical reaction completes at a lower temperature.
awemawson:
Steve
I always did three stages:
1/ steam bulk of wax with steam wallpaper stripper
2/ melt any solids left - kiln at about 200 deg C - wax drips onto baking tray and is removed from kiln
3/ burn residue - kiln ramps up to 600-650 deg C and held there for several hours dependant on bulk of mould.
I know that you want accurate parameters for the various stages but at a pragmatic level these three steps work
Anzaniste:
Will a mould made from plaster of Paris be ok to use for casting a Babbitt metal component with a pouring temperature of 450C?
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