Troble Shooting

Cracks

Problem / Question
Cervical or Incisal Horizontal Cracks in Pontics

Cause (1)
The CTE of the metal is too high.
Solution / Answer (1)
If the CTE of the metal is outside the range of 13. 4 - 14.5 *10-6 [°C]-1 on the 25-500°C scale you must change metals. (For non-precious the range is reduced to 13.9 - 14.1*10-6 [°C]-1 on the 25-500°C scale).
Cause (2)
The Porcelain is not bonding well to the metal.
Solution / Answer (2)
To increase the bonding strength of metal to opaque, sandblast the metal with alumina.
To increase the bonding strength of metal to opaque, clean ultrasonically with acetone or alcohol.
If you are using powdered opaque be sure to wash-bake at 980°C.
Be sure that the paste opaque is slightly shiny. The hold time at High Temp is 1min. w/o vacuum.
Be sure that you are using a metal with sufficient oxide formers like Zn and Sn.
Cause (3)
The porcelain is too thick.
Solution / Answer (3)
Adjust the shape of the framework to keep the porcelain thickness below 2 mm at incisal edges, in occlusal cusps and under pontics.
Cause (4)
The opaque baking temperature is too high.
Solution / Answer (4)
If the second opaque bake is too shiny, lightly sandblast the surface with alumina.
Cause (5)
Bubbles are coming from the metal framework.
Solution / Answer (5)
Degas according to the manufacturer's instructions to prevent bubbles from forming in the opaque. If you see bulges from small un-popped bubbles in the opaque, strip it and degas again.


Problem / Question
Areas of crowns and bridges where the porcelain is thin are getting cracks.
Cause (1)
The metal CTE does not match that of the porcelain.
Solution / Answer (1)
If the CTE of the metal is outside the range of 13. 4 - 14.5 *10-6 [°C]-1 on the 25-500°C scale you must change metals. (For non-precious the range is reduced to 13.9 - 14.1*10-6 [°C]-1 on the 25-500°C scale).
Cause (2)
The High Temp of the first body bake is too low.
Solution / Answer (2)
Adjust the high temperature so that the surface is slightly shiny.


Problem / Question
Cracks appear in pre-soldered areas

Cause (1)
There is porosity in the soldered area.
Solution / Answer (1)
Solder it again more carefully.
Cause (2)
Residual flux was not removed.
Solution / Answer (2)
Use stripper acid in the ultrasonic cleaner.


Problem / Question
Cracks in post-soldered areas

Cause (1)
The framework is so thin in the soldered are that it is distorted by shrinking solder.
Solution / Answer (1)
Make the framework thick enough so that shrinking solder does not distort it.
Cause (2)
The gap to be soldered is so wide that the framework distorts when the solder shrinks.
Solution / Answer (2)
Avoid large gaps.
Cause (3)
Flux has caused a chemical change in the porcelain.
Solution / Answer (3)
Do not use excessive flux. Immediately clean off flux well. Noritake Flux-P is recommended.
Cause (4)
The CTE of the metal does not match that of the porcelain.
Solution / Answer (4)
If the CTE of the metal is outside the range of 13. 4 - 14.5 *10-6 [°C]-1 on the 25-500°C scale you must change metals. (For non-precious the range is reduced to 13.9 - 14.1*10-6 [°C]-1 on the 25-500°C scale).