Jimi,
- ramp 150 degrees/hr, temp 325f, hold 60 min
- ramp 90 degrees/hr, temp 525f, hold 60 min
- ramp 60 degrees/hr, temp 725f, hold 60 min
- ramp 180 degrees/hr, temp 1350f, hold 45 min
- ramp 300 degrees/hr, temp 1000f, hold indefinitely
That seems like an odd burnout cycle to me.
The most glaringly odd part is how short your time at 1350 is.
That’s not enough even for some ordinary wax models to fully burn
out. I’d suggest at least an hour and a half, and better, two or
three hours at 1350. This assumes an accurate controller or
pyrometer, since you don’t want to go over 1350 by much. If you do,
the investment can start to break down, giving very rough surfaces
and oxidation.
The rest of the cycle too, seems odd. The thing to understand is
that between 300 and 400 degrees, quartz/silica undergoes a
structural change, called “quartz inversion”. During this, it expands
at a faster rate than the change in temperature would ordinarily
cause, so especially with larger flasks, one wants to go through this
range fairly slowly. That would give an initial ramp and hold temp at
300 to 325, as you do. Holding there for 60 minutes is only needed
for large flasks. We use 2.5 x 2.5 inch flasks most of the time, and
I find a ten minute hold at that point is enough. Then, as you do,
slowly increase temp during the second ramp. But I’d suggest holding
at 375 to 400, just high enough to pass the quartz inversion, and
holding there long enough for the entire flask to equalize at that
higher temp. I use 20 minutes, you can use more if you like. But
going all the way up to 525 as you do sort of negates the need for a
hold at the end of it at all. The effect, by the way, of going
through the quartz inversion temp range too quickly, is cracking of
the investment, not roughness or surface pitting. But it’s still
worth noting. Other than that key temp range, you can usually just go
all the way up, at a suitable speed, to 1350. If the material being
burned out has some critical temperature range in which it will melt
and flow out of the mold, then holding at that range could reduce
the amount of material that actually has to burn out of the
investment. But I doubt your plastic, even when melted, is going to
be flowing much. I could be wrong on that though. But anyway, you
held for a time at 525, and then hold again at 750? Don’t know why
you’d do that. Something to do with the nature of the plastic? With
wax it would serve no purpose.
But in any case, first try increasing the hold time at 1350. By
quite an amount, I think. That’s the temp where the carbon in the
investment is actually being burned away. The lower temp holds don’t
do much in that regard.
Also, another thing to ask is whether you are using a kiln intended
for burnouts of this sort. Burnout kilns need more air flow /
ventillation in order to work than do, say, enamelling kilns. Oxygen
needs to be making it into the kiln in order to burn off the carbon.
If not enough oxygen is getting in there, the carbon just sits there.
If you are using an enamelling kiln, it might be worth talking to the
manufacturer about the possibility of adding vent holes to increase
air flow through the kiln.
you might also try running a quick test with scrap of the plastic.
Burn it on an open surface (a crucible, charcoal block, or something
of the sort), just to see how completely it burns. You can use an
ordinary torch flame for this. Some plastics, even cooked to death
with a hot torch, still leave an ash residue. These, if burned out
and cast, also have that residue left on the insde of the mold, and
the surfaces can be quite rough as a result. If, though, it burns
pretty completely, without leaving a bunch of discolored ash or
residue, then you should be able to equally burn it out and cast it.
And finally, also pay attention to the ventillation in your burn out
kiln’s environment. Plastics when burned out, can give you some
distinctly nastier fumes than do casting waxes. Depending on the type
of plastic, some of it can be pretty toxic. So be sure the burnout
area is properly ventillated.
Peter