Dear Daniel and Peter,
Certainly you are both right in that carving wax has a much higher
melting temperature. The higher melting temperature is not the only
issue however and neither is the thermal expansion. I am including a
spreadsheet with this email but I am not sure how the ganoskin
website deals with this type of thing. This spreadsheet is a result
of a research project that I did this year for my presentation at the
2003 Santa Fe Symposium that dealt with various defect cause and
effect relationships. At 160oF, the thermal expansion of injection
wax is close to twice that of carving wax. The difference is that
the viscosity of injection wax is around 283 centipoise or about half
the viscosity of motor oil as compared to the carving wax which is
still solid. The injection wax simply expands right out of the
cavity if the central sprue wax is proper and is already melted
away.
The temperature of the investment material will not exceed 100oC
(212oF), if there is still water present in the investment material.
What will happen is that the steam pressure will increase quite
dramatically when the oven temperature reaches 150oC (300oF). As you
can see, carving wax is not viscous at all at the boiling temperature
of the water in the investment. Besides any damage that is done by
the thermal expansion of the carving wax, the steam gets between the
surface of the carving wax and the surface of the investment mold.
This steam erodes the mold cavity and can in some cases can cause
large pieces of investment to become unattached from the mold cavity.
As the wax does finally melt, these investment chunks and pieces
drop into the mold cavity and become investment inclusions and at
the very least become a very rough surface on your castings when the
molten metal enters the mold cavity and this investment floats to the
surface.
While extending a lower temperature at the begining of the burnout
cycle will help, I might suggest an even lower temperature than 300oF
for a starting temperature if you must use gypsum bonded investment.
Maybe 250oF would be a good place to start instead in order to cause
less stress on the investment. There was a very interesting paper
done just this year at the Jewelry Symposium in St. Petersburg,
Russia by a professor from Rice University in Houston, Texas. This
paper was on wax elimination during the burnout cycle. I am not sure
if it will be published in the USA or not. In any event the research
indicated that trying to rush a burnout to the higher temperatures
actually did harm to the elimination of the carbon residue from wax.
The paper’s evidence indicated that it would be better to slow down
the burnout at the lower temperatures and introduce as much oxygen
as was feasible during that time so that all of the components of the
wax could be combusted as a unit, rather than burning off the
volitiles first and leaving carbon alone to be removed at the higher
temperatures. Rushing the burnout through the lower temperatures
tends to leave lots of carbon. Residual carbon in the flasks causes
additional gas related defects to your castings. I’ll get off this
subject, I guess I got on a roll. His presentation in Russia was a
lot more interesting to me than my own.
Personally, I don’t have the faith in gypsum bonded investment to
cast my carving wax pieces. I agree with Daniel that if you have
time, make an RTV mold and cast the injection wax copies. If you
don’t have time there are new high strength, high speed investment
materials that are soon to hit the market that work quite well for
this application. They are much stronger and do not suffer the same
effects of gypsum bonded investment when casting carving wax. You
can cast with them in a standard flask or flaskless. I am using this
stuff now and have been for quite a while. Daniel has a secret but I
will let him tell you about it in his own time. Right Daniel?
Best Regards,
J. Tyler Teague
JETT Research
Machinable Carving Wax Example 3D Printing Wax Example Standard
Injection Wax Example Plastic Containing Injection Wax Example
Hardness (Shore D scale) 55 Hardness (Shore D scale) 55 Hardness
(Shore D scale) 44 Hardness (Shore D scale) ND
Specific Gravity 0.92 Specific Gravity 1.25 Specific Gravity 0.94
Specific Gravity 0.980
Flash Point 575oF Flash Point 347oF Flash Point 465-F Flash Point ND
Softening Point 226oF Softening Point 215oF Softening Point 158oF
Softening Point 164oF
Ash Content 0.003% - .015% Ash Content 0.015% Ash Content 0.003% -
.015% Ash Content 0.004%
Viscosity 150oF (CPS) Solid Viscosity 150oF (CPS) Solid Viscosity
150oF (CPS) 420 - 430 Viscosity 150oF (CPS) ND
Viscosity 160oF (CPS) Solid Viscosity 160oF (CPS) Solid Viscosity
160oF (CPS) 283 - 300 Viscosity 160oF (CPS) 810 - 960
Viscosity 170oF (CPS) Solid Viscosity 170oF (CPS) Solid Viscosity
170oF (CPS) 227 - 240 Viscosity 170oF (CPS) 777
Viscosity 180oF (CPS) Solid Viscosity 180oF (CPS) Solid Viscosity
180oF (CPS) 200 Viscosity 180oF (CPS) 510 - 635
Viscosity 190oF (CPS) Solid Viscosity 190oF (CPS) Solid Viscosity
190oF (CPS) 160 Viscosity 190oF (CPS) ND
Viscosity 200oF (CPS) Solid Viscosity 200oF (CPS) >10,000 Viscosity
200oF (CPS) 130 Viscosity 200oF (CPS) 360
Viscosity 220oF (CPS) Solid Viscosity 220oF (CPS) 32.00 Viscosity
220oF (CPS) ND Viscosity 220oF (CPS) ND
Viscosity 230oF (CPS) Solid Viscosity 230oF (CPS) 20.50 Viscosity
230oF (CPS) ND Viscosity 230oF (CPS) ND
Viscosity 240oF (CPS) 7100.00 Viscosity 240oF (CPS) 15.50 Viscosity
240oF (CPS) ND Viscosity 240oF (CPS) ND
Viscosity 250oF (CPS) 3350.00 Viscosity 250oF (CPS) 15.00 Viscosity
250oF (CPS) ND Viscosity 250oF (CPS) ND
Volumetric Expansion 120oF 2.500% Volumetric Expansion 120oF 1.500%
Volumetric Expansion 120oF 3.4% Volumetric Expansion 120oF 3.30%
Volumetric Expansion 140oF 4.300% Volumetric Expansion 140oF 2.800%
Volumetric Expansion 140oF 8.9% Volumetric Expansion 140oF 8.60%
Volumetric Expansion 160oF 6.200% Volumetric Expansion 160oF 4.200%
Volumetric Expansion 160oF 10.6% Volumetric Expansion 160oF 12.80%
Volumetric Expansion 180oF ND Volumetric Expansion 180oF ND Volumetric
Expansion 180oF ND Volumetric Expansion 180oF ND
Volumetric Expansion 200oF ND Volumetric Expansion 200oF ND Volumetric
Expansion 200oF ND Volumetric Expansion 200oF ND