The Shop > Metal Stuff
Plaster of Paris, Investment, etc.
RussellT:
Hi Steve
The reference you've found is interesting but an important aspect that will have a big influence on the converserion to anhydrous calcium sulphate is how fast the unbound water can be removed.
In the open pan trial it is all removed straight away - not surprising as they are using powdered material. It will take much longer to remove the water vapour from the centre of a block of plaster - which will affect the reaction speed (and temperature) - as demonstrated by the pan with a tiny vent hole method.
Undoubtedly given long enough at 185 deg C all the water will be driven off, but it might be better to do it at a higher temperature as the increased vapour pressure should speed the process.
I am also wondering whether it is necessary to get a thick mould completely dry - if for example parts of a mold were a couple of inches thick then would getting the surface dry to a certain depth be OK. I'm pretty sure it would if the mould is hot as there would be no chance of a steam explosion.
This might explain the variability in the casting procedures - it may be related to mould thicknesses.
Russell
vtsteam:
Andrew total agreement with you that an attachment to theoretical exactitude is foolish. I'm just trying to understand what goes on behind the scenes for my own benefit. I don't like just following other people's recipes, I also want to know what's happening.
I find that if I understand that, I often can discover something new, or different, or when I make a change to a procedure it has a better chance of succeeding rather than being just a shot in the dark. That's the fun in it for me. Understanding what's happening beneath the surface.
I'm basically just thinking aloud but not coming to any practical conclusions here, at least so far. I haven't said what should be done in investment. I'm thinking about what has been done.
Anzaniste, re. babbitt I'm not the person to ask because I haven't done it, someone else who has should say. I have poured lead "ducks" (drafting spline weights) in a reusable plaster of Paris mold made without any heat curing or baking, but simply letting the mold dry for a week -- that was a loooong time ago. I'm not recommending no bake, just saying I did it at the time according to descriptions in a book. Lead has a lower pour temp.
And if people are also pouring aluminum in baked molds, and it has a higher pour temperature than babbitt, I would guess the answer is yes, it can be done.
RussellT total agreement with what you are saying about length of time to drive off water will be dependent on thickness. However We should be careful of whether we are describing the temperature something is subjected to vs the temperature it is at. The temperature vs free water graphs did not show time as a variable. vs thickness since that's dependent on so many different things.
However, we can say that water at 365F (185C) is steam at 300 PSI -- pretty obvious that any free water in the plaster in the size molds we are interested in here would migrate out nearly instantly, since plaster is porous. I don't imagine any free water remains at these temperatures. And they certainly don't drive water inward.
I am talking about plaster at that temperature, not surface temperature of a big block with a gradient inside at lower temperatures, or the temperature a kiln thermostat is set to.
So when people give procedures and temperatures, that's not what I'm talking about. I'm talking about plaster that has arrived at that temperature. And I'm not recommending any procedure at all here.
RussellT:
--- Quote from: vtsteam on August 05, 2014, 09:33:40 AM ---I don't imagine any free water remains at these temperatures.
--- End quote ---
I think some will - as the plaster is heated steam will drive out the air - I don't see what will drive out the last bit of steam. Given time diffusion will substitute air for steam - and to be pedantic it's still only at 15psi - unless you can heat it fast enough to make the pores of the plaster into pressure vessels of their own.
I don't claim that the remaining water will have any significance at all - but I would be cautious about applying results from a powder to a solid plaster.
Russell
vtsteam:
Russell, sorry, I should have said 350 psi potential pressure.
If there is porous resistance, at 185C the pressure will increase internally until the vapor escapes, the greater the resistance the greater the internal pressure. If you say the steam escapes slowly, the resistance is correspondingly high then. Not 15 psi.
Here's the situation: either the plaster matrix is very porous and the pressure never gets to be much more than 15 PSI because the steam all escapes. Or if not very porous, the internal pressure is well above 15 psi. The greater the resistance, the greater the pressure -- until it equalizes by complete release from the plaster.
In practical terms on the scale we're talking about, that shouldn't take long. Plaster is very permeable to steam, and even water, which is far less fluid. Cold water soaks readily into even air dried plaster -- in fact solid plaster of Paris is used to de-water clay in slip casting molds.
I don't think steam at 185C is going to reside long in an inch or two of plaster in a mold for a model engine part. This is what we're talking about. Not casting a life size horse and rider in bronze, right? Or at least that's what I'm interested in.
The speed at which steamcan be released is probably what determines how quickly the mold is heated initially, since building up pressure will cause it to break. That would be a big concern when the plaster still has unbound water at temps approaching and probably somewhat above 100C (since water can exist as a liquid under pressure above that point).
An interesting question (to me) would be seeing pressure curves as the plaster internal temp increases at various rates. In say 2 inches of plaster. And understanding at what internal pressure it breaks the solid.
Not that such information is available to look at, or even necessary to practical casting if you already have a process that works well for you. Just that I like to see stuff like that, and understand things.
It's just fun for me to think about how things work. It's my way of taking apart a clock, like when we were kids.
RussellT:
Hi Steve
I understand completely what you're doing - and I enjoy reading about it.
I've always found that if you understand how and why stuff works then you have a much better chance of remembering it.
As a child did you ever try taking stuff to bits only to be working desparately to get it back together again before your parents came in.
Russell
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