Hairline fractures on devesting

[JMN]

Had a 3 unit bridge (29-30-31) come out of the investment today with crack thru the occlusial of 31. It was fine in wax, invested in Fujivest II 150gm ring w/3 singles, bench set for 20 min before going in oven, 20min burnout, 1 hr benchset cool. The other copings and other bridge abutment were fine.
Sorry I don't have any pics, don't have a camera at the lab.
Our lead had never seen or heard of this before either, he's 30+ years in.

Any ideas?

 

[ps2thtec]

Why do you only burnout for 20 min? A ring that size would be in the oven at least 1.5 hrs here.
Can't say I've seen that before. How thick was the occlusal? Maybe too thin?

 

[JMN]

It's a phosphate bound investment, so it self heats to ~ 200 F before we put it in. The oven temp was 820 C.
We've historically had good success with those timings. We've pushed to 15/15/cast/quench before if there was a real time issue with nary a problem. Oh, it's a belle de st claire/kerr ovoid ring too, not cylindrical. The thickness was about 3 tenths mm(is that 300 microns?),usually if they're too thin, what we've seen is, they just develop a "hole". This crack was avout 3mm long buccal to lingual, .15mm wide about 1.5mm from the connector.

 

[ps2thtec]

I'm never in that big of a hurry luckily. Still think thin (thickness) may be . Sometimes well never know.

 

[2thm8kr]

I've used fujivest II forever. I burnout no less than 50 mins. for a ring that size. Same bench set as you. Check your investment liquid. If it has crystals at the bottom of of the bottle, chuck it. That's the only time I get hairline cracks with fujivest.

 

[Tayebdental]

I think, 1- the burout is unconventional 2- what alloy are you using? 3- the ratio of old alloy to new alloy, old alloy should not exceed more than 40% of tota alloy, the result will be less homogenous alloy with difference in physical property that could cause cracking in the thickest area of bridge. (Specificity semi precious alloy) I hope this could be helpfull.

 

[JMN]

I think, 1- the burout is unconventional 2- what alloy are you using? 3- the ratio of old alloy to new alloy, old alloy should not exceed more than 40% of tota alloy, the result will be less homogenous alloy with difference in physical property that could cause cracking in the thickest area of bridge. (Specificity semi precious alloy) I hope this could be helpfull.

That makes some sense, it's Superior by jensen, 62.5Paladium, 24.5 Silver.

Thanks.

 

[ztech]

If the cracks edges are jagged and the edges look like sand granules the most likely cause is a cooling tear. It would usually occur with quenching but if you have highly variable thickness it can occur also. Rounded or smooth areas in the tear would indicate a burnout or alloy heating problem. Without photos I couldn't venture a guess as to which but. As for the burnout, I have used the short burnout on all sizes of rings with wax patterns using with Fujivest II and have never had a problem but the shortest time I have ventured is 35 min.

 

[JMN]

I've used fujivest II forever. I burnout no less than 50 mins. for a ring that size. Same bench set as you. Check your investment liquid. If it has crystals at the bottom of of the bottle, chuck it. That's the only time I get hairline cracks with fujivest.

Brand new btl, 3 days open, no crystals. Recast it today, had a thin/weak spot in same place, a dip that wasn't in the wax. Weird. but it worked.

Gremilins.

 

[GAP]

We've pushed to 15/15/cast/quench


Quenching too early can cause steam tears

 

[JMN]

This one wasn't quenched, never quench bridges, can cause warping. We rarely quench anything, just a full cast once in a while if doc says "pt will be here in 45 min, where is it" when he wrote deliver tomorrow kind of stuff.


Edit: When we do quench it's only after the metal no longer glows at all when in a totally dark room.

 

[JMN]

If the cracks edges are jagged and the edges look like sand granules the most likely cause is a cooling tear. It would usually occur with quenching but if you have highly variable thickness it can occur also. Rounded or smooth areas in the tear would indicate a burnout or alloy heating problem. Without photos I couldn't venture a guess as to which but. As for the burnout, I have used the short burnout on all sizes of rings with wax patterns using with Fujivest II and have never had a problem but the shortest time I have ventured is 35 min.

It was quite jagged. And it was right about 1mm from where the sidewall of the coping ended and it was the wall the connector was on. This wasn't quenched, it bench sat 90min.

Edit: When I came on the scene they had cast after 30 min, but we've since pushed it incrementally.