| Waxing |
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Minimum recommended wax thickness:
0.3-0.4 mm. |
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Design for uniform porcelain
thickness: 1.0 - 2.0mm is considered ideal1,2,3
although today's technicians seem to feel
comfortable with up to 3mm in some areas
of the mouth. |
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Design for structural support.
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Build pontics up sufficiently.
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Keep bridge connectors minimum
of 3mm (incisal-gingival) x2mm (lingual-labial).
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Think Smooth! Sharp edges
raise stresses. And, it's easier to smooth
wax than metal. |
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| Spruing |
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Sprue to the thickest part
of the pattern |
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Sprues should be as thick
(or a little bit thicker) than the thickest
part of the pattern, typically 8-10 ga.
|
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Some mass of metal needs
to be provided to reservoir the casting
during solidification. Use either a
sprue reservoir (direct sprue) or a runner
bar (indirect sprue). Use of only a button
for a reservoir is ok for small castings,
but can be dangerous for larger castings. |
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Keep reservoir close (1-3mm)
from patterns, and use sufficient sprue
length from crucible former to reservoir
to put the pattern within ¼"
of the (sealed) end of the investment. This
should place the reservoir in the heat center
of the ring. |
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Use "chill" vents
on heavy castings. A chill vent is just
a thin piece of rope wax attached to the
pattern on the opposite side of the sprue,
which runs blind into a cooler part of the
ring. |
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We like to keep the reservoir
about the same mass as the casting; then
we cast only enough metal to fill the casting
and reservoir (ie, cast no button). |
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Think Smooth! Sprue with the thought
of smooth metal flow, or the path of least
resistance for metal to fill the pattern.
Avoiding "reverse flow" situations
will enable you to "get away with"
the occasional thin wax-up.
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Investment
Selection
Use a good quality investment. Rough castings
from cheap investment will negate any savings
on materials. |
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Gypsum bonded investments
can be used with gold-silver-copper crown
and bridge alloys, and most alloys designed
for low-fusing porcelain. We like gypsum
bonded investments with our white C&B
alloy J-7 as well. Carbon containing
phosphate bonded, or carbon free phosphate
bonded can also be used with these alloys.
|
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Carbon containing phosphate bonded
investments can be used with gold based
PFM alloys, and PFM alloys with less than
about 55% palladium. These investments
should also be used with the higher melting
white C&B alloys like our JWE and
JWG. Carbon-free phosphate bonded
investments can also be used with these
alloys.
|
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Carbon-free phosphate investments
must be used with PFM alloys higher in
palladium than about 55%. We prefer
carbon-free investments for nickel-chromium
alloy as well. |
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The new rapid burnout investments
can be used with Jensen alloys. Some of
these investments have provided very high
quality castings regardless of burnout technique.
Some, however, provide less smooth castings
when the rapid burnout is used than when
conventional burnout is used. |
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|
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We are asked every day
what investments we use at Jensen. We are
continually trying new investment products!
Here's what we have been using lately: |
|
Whip-Mix Fast-Fire 15
Seems like a nice investment. Smooth, easily
poured, and much improved surface quality
over Powercast. Excellent results with rapid
burnout technique. May be a little inconsistent
from lot to lot. |
|
Bego Bellavest SH
Very fine grained, Consistent fits and plenty
of expansion, even with nickel-chrome. Great
for rapid burnout. |
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Microstar
Nice castings but very difficult
divesting |
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Talladium 1700
Castings seem a bit rough, especially
when rapid burnout is employed. |
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Whip-Mix Powercast
Inconsistent surface quality, especially
with larger rings. |
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| Investing
Tips |
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Check the vacuum level of
your mixer before AND after connecting the
bowl. Before connecting the bowl, the vacuum
gauge should read low, approximately 0-7
inches of mercury. After attaching bowl,
vacuum level should rise to about 29. |
|
Pattern cleaners and debubblizers
should be completely dried prior to investing.
|
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If you choose to "paint"
your patterns with investment, use investment
from the center of the mixing bowl, not
the edge. |
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Be observant of "extremes"
in your investing process. For example,
if you need to use much less Special Liquid
than the manufacturer recommends to get
fit, or if you can't spatulate for the full
recommended time and maintain adequate working
time, there is a good chance something is
wrong. |
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Wait the recommended amount
of time before pulling crucible formers
and/or removing plastic ring forms. |
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| Burnout |
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Burnout furnaces are inherently
inaccurate. Jensen recommended burnout temperatures
are approximate, but should be very close
to what provides you adequate results. If
you find that you are unable to get complete
castings within +/- 25oF
from our recommended temperature, check
the calibration of your furnace or give
us a call. |
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Lower burnout temperatures
generally provide smoother castings and
fewer casting defects. We like to use the
lowest practical burnout temperature with
our alloys. |
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Never exceed 1200°F for
gypsum bonded investments, typically 900oF
hygroscopic, 1200°F
thermal.
|
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We are frequently asked about
recommended burnout rates for our alloys.
The truth is that the alloy does not care
about the burnout rate, provided:
1) The investment
is thoroughly burned out,
2) The investment
is not damaged by the burnout process.
3) The necessary
high temperature is reached.
Follow investment manufacturer's recommendation
for burnout rates, cycles, and hold times.
|
|
Some of the newest fine grain
investments will not burnout completely
at 1200°F. To use these investments
for low temperature gold alloys, best results
may be obtained by raising the temperature
to 1400°F for a half an hour, then dropping
the temperature to 1200°F and allowing
the investment to soak for another hour
prior to casting. |
1Miller,
Lloyd. Tooth Prepartion and the Design
of Metal Substructures, Dental Ceramics,
Proceedings of the First International Symposium
on Ceramics, John McLean ed., 1983.
2McLean,
John W. The Science and Art of Dental
Ceramics, Vol II, p208, Quintessence
Publishing, Chicago IL 1980
3Air Force
Manual 162-6, p462, 1982 |