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Casting a plastic model


#1

For the past several days, we have been attempting to burn-out low
density polyethylene, but to no avail. The main problem we have
encountered is substantial surface-pitting. Based on some of the
articles we have read, we think we have identified the culprit as
either carbon build-up inside the investment, thermal expansion, or
some sort of chemical reaction between the molten plastic and
investment. Does anyone know of a way to eliminate this
surface-pitting or a way to test what, exactly, is going on inside
our molds?

Some basic info;

The last burn-out cycle went as follows;

  1. ramp 150 degrees/hr, temp 325f, hold 60 min
  2. ramp 90 degrees/hr, temp 525f, hold 60 min
  3. ramp 60 degrees/hr, temp 725f, hold 60 min
  4. ramp 180 degrees/hr, temp 1350f, hold 45 min
  5. ramp 300 degrees/hr, temp 1000f, hold indefinitely

argentium silver temp was 1775f at cast vacuum cast the polyethylene
we are using is approximately 3 yrs old, but shows no signs of
oxidation and is still sufficiently pliable

More available upon request.

Thanks,
Jimi


#2

Jimi,

  1. ramp 150 degrees/hr, temp 325f, hold 60 min
  2. ramp 90 degrees/hr, temp 525f, hold 60 min
  3. ramp 60 degrees/hr, temp 725f, hold 60 min
  4. ramp 180 degrees/hr, temp 1350f, hold 45 min
  5. 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


#3

Try increasing the time at max temp (1350) to make sure all the
residue is burnt out, maybe up to 1.5 hours. Depending on the size of
the flask, 45 minutes might not be long enough. I always give it an
hour even with wax, better safe than sorry and since I burnout
overnight, a little extra time isn’t a concern.

Before casting, tap the flask, open end down and then gently blow
compressed air inside to evacuate any ash. This is a good idea when
burning out any non wax item, casting wax is formulated to produce
little or no ash.

Good luck!
Harry
www.harryhamilldesigns.com


#4

Thermal expansion is your main culprit but the other two could
contribute to problems as well. The way to avoid the problem is to
make a RTV silicone rubber mold of the item and then inject and cast
a wax. I have never had good results using plastics as models
sometimes fair but never good. I know there are some polymers used
in metal mold injection models that work ok in small size pieces but
bulk polyethylene has way too high a softening point and it just
trashes the mold surfaces from the over pressure generated by the
thermal expansion of the polymer.

Jim

James Binnion
@James_Binnion
James Binnion Metal Arts


360-756-6550


#5

Hi Jimi: It is possible that you are using a polyethylene not
designed for burnout. Contact Zero-D Products for their metal mold
beads. This is a polyethylene used by many companies because it is
trouble free. Call 800-382-3271

Craig


#6

Hi Peter,

Re the crystobolalite-- quartz-- inversion. My research has shown
that the temp. is more along the lines of 1100 F.

We depended on this expansion when casting crowns to accommodate for
shrinkage thus insuring a perfect fit.

Andy


#7

Hi,

I’ve had a little experience with casting plastic models for toy
developers and movie props. What I discovered after a few major
failures is that the mass of the plastic relative to the amout of
investment is important. With a wax burnout the wax becomes liquid
and runs out of the flask and very little wax is left burn out at the
higher temperatures. With plastics there is no liquid state so the
entire mass of plastic just cooks in the investment and gives off a
lot of gas in the process. When I tried to cast larger pieces like,
say, a dagger handle I would remove the flask from the oven only to
find that the investment had completely broken down to a crumbling
powder! With smaller objects like a button I had no problems. What
I’m getting at is if you use a flask that is too small for a large
plastic model the gas build up destroys the surface of mold cavity.
Try using a much larger flask than seems necessary. Also try to find
out the ignition temp of the plastic you are using and hold a ramp at
just below it for at least 2 hours to dissapate the gasses before you
run up the temperatures. You might also hold your final temp (1350)
longer.

Hope this helps,
Rick
www.zxprops.com


#8

Hi Jimi, I don’t know how much LDPE you are trying to evacuate, but
your hold time at 1350F is woefully insufficient. Three to four hours
at top burnout temp should do. Don’t worry about thermal expansion or
ash with LDPE, but do yourself a favor and increase the investment’s
green strength by waiting at least 2 hours after investing to start
the burnout. Also, there is no problem ramping at 240F per hour to
quicken the cycle to give you more hold time at the relevant
temperatures. In this case those would be 300-350, 700 and most
importantly 1350.

Good Luck,
Jason Borgstahl
Casting House


#9

Andy,

Re the crystobolalite-- quartz-- inversion. My research has
shown that the temp. is more along the lines of 1100 F. We depended
on this expansion when casting crowns to accommodate for shrinkage
thus insuring a perfect fit. 

Are you sure we’re talking about the same effect? I could of course
easily have been mistaken in my understanding of that effect, but I
am pretty sure I’ve read somewhere that the silica itself changes at
the lower temp range I mentioned. And crystobalite investment is a
different animal, I thought, than the usual jewelry investments.
It’s designed to give a much higher level of expansion to the mold
for exactly the purpose you describe. I recall needing to put flask
liners in the flasks when using the crystobalite investments, in
order to provide some padding and keep the investment from cracking
itself due to that expansion. It does occur to me that perhaps the
slow stages in burning out through the 300 to 375 range might be
intended just to allow more of the wax to drain out of the mold
before it starts to actually burn, thus reducing carbon buildup in
the investment. But that’s not the way I was taught it.

Thanks for the note. I’ll now have to go do some reading up on the
subject.

Cheers
Peter


#10

Hi Everyone,

I’m jumping in a little late here but I think I have some
that will help. We did a comprehensive research project
on this subject that was published at Santa Fe Symposium this year
(please contact me off-line if you would like a copy of the paper).
Basically, the conclusion was that the biggest issue in casting
models generated through the various CAD/CAM methods is thermal
expansion of these patterns in the early stages of burnout. For
example, a photopolymer model will expand significantly (up to .012
in our research) well into the 600 to 700 degrees F range. While all
of this is happening the investment is expanding as well, and in the
case of gypsum based investment it is weakening as the curve
proceeds. All of this spells disaster for designs that have deep
setting holes with tiny cores of investment or sharp edges that
propagate cracks in the investment. Bottom line: ash is not the
problem to chase. It is present to some degree, but it is rarely
responsible for catastrophic failures in casting of CAD/CAM
materials.

My belief is that many of the failures we see in casting these days
are a combination of wild CAD designs that do not take into account
whether they are castable and the expanding materials that are
becoming common place in our industry. We have to figuratively put
ourselves inside the flask and think about expanding models pressing
against soft investment and the design features that are most
vulnerable to this phenomenon. No burn out curve in the world will
save you from this. Your alternatives are to cast with minimally
expanding materials that melt at very low temperatures such as
injection wax and thermoplastics (e.g. Solidscape material) or
design to accommodate the expansion, (e.g. add radii to your sharp
edges and avoid deep setting holes and thin walls of investment
between features).

Hope this helps!

Teresa Frye
TechForm Advanced Casting Technology


#11

Hi Peter,

A quick look around, primarily at the Kerr site for Satin Cast 20 a
very commonly used investment, shows that it is indeed a crystobalite
investment. I’m certain that all higher heat --over 600F melting
point-- investments are made with it. (It gets its name from San
Cristobal NM, if I recall correctly.) The expansion–the quartz
inversion-- happens in them all. We lined our flasks to foster a 3
dimensional expansion of the mold cavity, which compensated for the
usual casting shrinkage of the metal. Without this cushion/liner, the
expansion occurred at the ends of the flask.

When I moved out to Seattle in '84 (and last made crowns), they were
marketing a system of removable flasks. After the investment set up,
the flexible plastic flask was removed (and reused) leaving a chunk
or plaster cylinder which was free to expand in all directions.

I found that the quartz inversion happens at 1100F or so.

Nice weather today, huh?

Andy


#12

I’d like to thank everyone who replied to my post. Your information
has proven most helpful, as I had my first good cast with this LDPE
stuff yesterday.

My earlier complications arose from the thermal expansion of the LDPE
exerting enough force on the walls of the invested mold to crumble
the investment. My models were relatively large (8mm-ish diameter),
and the plastic simply expanded too quickly with the investment
already in a weakened condition. It’s all good now, and my models
have never been smoother!