| Burnout | Investment | Spruing | Waxing |
Burnout: Tips & Techniques
Tip#1
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 +/- 25°F from our recommended temperature, check the calibration of your furnace or give us a call.
Tip#2
Lower burnout temperatures generally provide smoother castings and fewer casting defects. We like to use the lowest practical burnout temperature with our alloys.
Tip#3
Never exceed 1200°F for gypsum bonded investments, typically 900 hygroscopic, 1200 thermal.
Tip#4
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.
Investment: Tips & Techniques
Tip#1
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.
Tip#2
Pattern cleaners and debubblizers should be completely dried prior to investing.
Tip#3
If you choose to "paint" your patterns with investment, use investment from the center of the mixing bowl, not the edge.
Tip#4
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.
Tip#5
Wait the recommended amount of time before pulling crucible formers and/or removing plastic ring forms.
Spruing: Tips & Techniques
Tip#1
Sprue to the thickest part of the pattern.
Tip#2
Sprues should be as thick (or a little bit thicker) than the thickest part of the pattern, typically 8-10 ga.
Tip#3
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.
Tip#4
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.
Tip#5
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.
Tip#6
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).
Tip#7
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.
Waxing: Tips & Techniques
Tip#1
Sprue to the thickest part of the pattern.
Tip#2
Minimum recommended wax thickness: 0.3-0.4 mm.
Tip#3
Design for uniform porcelain thickness: 1.0 - 2.0mm is considered idealalthough today's technicians seem to feel comfortable with up to 3mm in some areas of the mouth. 1,2,3
Tip#4
Design for structural support.
Tip#5
Build pontics up sufficiently.
Tip#6
Keep bridge connectors minimum of 3mm (incisal-gingival) x2mm (lingual-labial).
Tip#7
Think Smooth! Sharp edges raise stresses. And, it's easier to smooth wax than metal.
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