problme never seen before

[yaser]

after i made glaze for p.f.m bridge every theng was perfict and the bridge is very good then i leave it to get cools,after that i take the bridge in my hand an put it on the cast an i leave it on the table.but after 30 minutes i note some cracks on the bridge,and after 15 minutes there is more cracks show up on the porceline. please giv me explain for this. thanks..

 

[corona]

are the cracks verticle or horizontal???? is this the first time it has happened ??? have you changed metal, porcelain, technique, lab tech , etc ... ??

 

[user name]

Are the cracks verticle or angled/horizontal? Most likely a framework design flaw-inadequete support that exagerates any CTE issues.

 

[yaser]

its horizontal and this is the 2nd time .i change the metal but i change it about 2 months ago an no problem but today i faced this problem.but why the cracks shown after allmost 1 hour?

 

[yaser]

its horizontal and this is the 2nd time .i change the metal but i change it about 2 months ago an no problem but today i faced this problem.but why the cracks shown after allmost 1 hour?.the metal an porceline must be stable.right

 

[Mark Jackson]

its horizontal and this is the 2nd time .i change the metal but i change it about 2 months ago an no problem but today i faced this problem.but why the cracks shown after allmost 1 hour?.the metal an porceline must be stable.right

I have seen it so many times over the years, especially on bridges. A small misalignment of CTE, combined with poor frame design, "round the corner" bridges etc, and the margin for error just goes down. It's another reason, I love monolithic restorations so much!

 

[yaser]

i have seen this problem in a single unit alsow and the metal was smooth without any sharp corners

 

[doug]

This appears to be a CTE issue. Just curious, why you would change alloys if you were having success?

 

[Mark Jackson]

i have seen this problem in a single unit alsow and the metal was smooth without any sharp corners

The more narrow the range of CTE on your ceramic, the higher the quality of the metal you must use. Vita for example used to have a very narrow window, and a very small list of approved alloys. If you tried to use an "equivalent" alloy, you better add lots of fresh metal to every casting and have perfect frame design.

The problem could also be cross contamination with YZ ceramics if you use a wet tray. It only takes a few grains of ceramic to change the who CTE. Toss the button, start with fresh alloy and a clean ceramic department.

I promise you. it's related to CTE and frame design.

 

[user name]

Design your frameworks so you have an evenly layered thickness of porcelain. Especially watch the M&D incisal corners on anteriors. Build up mamelons in the metal, and dont have any areas with more than 1.5mm of ceramic.

What alloy and porcelain are you using?

 

[yaser]

thank u mark

 

[yaser]

i use degodent kiss ceramic and ivoclar 4 all metal

 

[user name]

Awesome ceramic choice, but with that alloy make sure you use the NE Bonder.

 

[yaser]

realy...thanks user name..i'll doit

 

[doug]

In the words of a great technician, Ken Gutherie: "Round is your Friend" in frame design.

 

[corona]

sounds like CTE like mark said ..... or could be that bonder that user name mentioned . the bonder never entered my mind .... user name ... what is the function of the bonder ??? other than the obvious .... does it have secondary uses ???

also yaser ... have you tried to slow cool your bridges ??? that often helps when your CTE is close to being out of range . I would slow cool 3 to 5 minutes . good luck .

 

[yaser]

the cool time in the furnace is 4 minutes and i leav the work on the furnace tray until the furnace become coold so i keep the work atleest 15 minutes
thanks...

 

[user name]

The big problem with close CTEs isnt the closeness, its the rate of each comopnents cooling. Imagine CTE graphed on a chart. As long as the lines for both porc. and alloy dont cross, no problem. You just have to make sure the quicker cooling porcelain doesnt drop faster than the alloy; that would be a spike in the chart where the lines would cross. Use wire pegs, honey comb tray. A pontic can be a heat-sink, so dont wrap it completely with porc. Leave a small lingual band of metal (or hide it under the saddle) to relieve the stress during cooling. make sure youre using the correct firing chart. Page 35 in your manual: table 3. There are different temps for NP alloys. Make life easier...switch alloys.