Holes , small pontic holes , caves into the pontics orinto thick metal supports imake

[Edy]

Hi guys , suddenly i dont know what i changed in my own investment of wax before casting the metal , its sure something small but i cant figure it out , its a new problem i get in my metals , after i cast the bridges i find these annoying holes like caves into the thickness of my pointics or where i make the metal thicker into occlusion of a crown i get the same thing .How i see this.... it start when i see a very small hole , i try to eliminate it but while i start to grind it over and over the hole gets bigger and wider like an empty pool inside the pontic or the thick metal , OH and i get this in the leading sprue , what i mean sprue is the part of metal from which the melted metal passed until reaches the crowns , when i hit with the hammer the bottom of the cast ring to open the investment i see some bars of metal just fall apart form each other with the same kind of small/big hole/s , dont get me wrong the crowns in the end of the construction come out fine , problem is in pontics and thick metals , siiiiiiighhh . I am sure there is some logical reason why this is happening , and thats why i am thankful this site is available , maybe someone can share with me the solution for this, thanks.

 

[Marcusthegladiator CDT]

Porosity... it sucks...
Your definitely doing something different if this was previously uncommon...
Could be a million different things.
Would have too see your waxed sprues on the ring and know what alloy your casting.

 

[Tom Moore]

Sounds like your sprus or spru bar are not in the hottest part of the ring and being the reservoir of molten metal for the bridge to draw from. Make sure the bridge is at the top of the ring covered by about a 1/4 of investment so it cools first and has enough metal in the hottest part of the ring to draw metal from as it cools. Hope this helps.

 

[lcmlabforum]

There is porosity issues - there is a diagram of a cross section of an investment with a diagonal cross-hair marking the center
of the ring when you look at it from the sides. The hottest part has to be where the sprue/reservoir area so that it continues
to stay molten and feed the pattern as the latter chill. However, still would not explain the extra 'bars' of casting - it seems
like there is some major turbulence possibly from a part of the investment that had the porosity, then broke off leading to
insufficient alloy to fee the pattern itself.
Good luck, but should not repeat itself all the time, might be just a single episode/fluke.
LCM

 

[dmonwaxa]

Good advic given above. Also your ring may be too cold and incomplete melt of alloy, molten but not enough to flow properly. Pontics usually require a larger diameter sprue, or more than one. Double check oven temp and heatsoak times. Ensure metal is properly melted, and use larger or multiple sprues on pontics. Also pieces of investment broken off during initial flow may become inclusions within the mould and prevent complete flow of molten alloy.

 

[Al.]

I agree with all the above. Sounds like porosity issues which is usually a spruing and reservoir problem.

I recently started (2 months) casting everything with the Neo Super Cascom, it uses electric, vacuum, argon and compressed air.
Im getting really beautiful dense porous free castings. An amazing difference from the centrifugal and torch castings.

 

[Edy]

Sounds like your sprus or spru bar are not in the hottest part of the ring and being the reservoir of molten metal for the bridge to draw from. Make sure the bridge is at the top of the ring covered by about a 1/4 of investment so it cools first and has enough metal in the hottest part of the ring to draw metal from as it cools. Hope this helps.

i think your right on it , i am covering the crowns with a bit more then 1/4 of investment , will try that , say , u put sprue bar over the pontic too ? because a local tech says he never puts any leading bars over his pontics and he never has holes in them...[even if he has a bridge of 6 crowns with 4 pontics , he only puts 2 sprue bars only on the crowns which are not pontics and his bridge comes 'awesome' as he explained me] , other mate tells me the bars which lead to the crowns maybe too big/thick in diameter which leads to the effect that the melted metal hits the end of the crwons and its goes backwords because i have too much thickness on the bars and that might create the air holes in the metal

 

[Gru]

To the best of my knowledge, thinnest areas of metal cool first and continue to draw alloy from thicker areas until "frozen". The areas of "suck-back" are always in thick areas, right? If so, they are feeding alloy to the thin areas as they cool, but are not being properly re-supplied by the reservoir, sprue or bar. The thick areas must have enough molten alloy flowing into them to replace the alloy being drawn to the thin areas as they cool. Ceramco (I think) showed a really cool vid of this eons ago at lab day where they managed to film the alloy flowing into the investment and then freezing.

 

[MasterCeramist]

Are you getting this with all alloys or a specific one? We have gotten this before but only on non precious.

 

[Edy]

i use only NP lately , 5 mins ago i had another holes like these in a 4 bridge crown where i added support metal into mesial/oclusion , trying to grind the small holes withing this support i grind and i see its gone ,i go sandblast for opaque i start to see another 5-6 pinpoint holes , very small , i go back to grind all fine , sandblast ....again more holes until i just reached the other side , uffff, now i am in trouble , need to redo the bridge from wax

 

[Al.]

Edy never had these fail.
10 sprue to the crowns and pontic, 6 ga plastic runner/reservoir bar, 8 ga sprues from the bar to the sprue former.
Basically its the text book technique.

 

[Al.]

Put the bar directly on the former and often you will get suck backs in the thick areas.

 

[Edy]

Al mate , i do exactly like that , then i have something else doing these holes to me , i really maybe need to lift them more twords the upper end , i put my crowns near the mid ring, that might be my problem , they are still hot in the mid ring . who knows.

 

[Al.]

Maybe you are over heating the metal ?

 

[Edy]

Maybe , what is overheating means for u .. too big flame from torch and melt metal in a few seconds or a normal to small flame torch flame and keep burning metal for like 40 seconds and a bit more ? because i tried both lately and in both i got this things.I am using Verabond II a US manufacturer , supplier advised me to give a good heating flame and melt the metal as fast as i can in few seconds until the metal cylinders start to unite with each other and not letting them get too liquid because i might burn them , so i do like he said and of course i tried to let them get liquid , but with same results.

 

[Al.]

If you think you are doing everything right and you have been using this metal for a long time with no problems till now than I would switch metal.

Could be a manufacturing problem they aren't telling you about. Maybe they lost one of their suppliers or have a bad batch.

They don't recall products they just let us think we are crazy. Sometimes its not our fault. We can go along for years with no problems then all the sudden problems for a month then they go away.

Maybe the material is great 99% of the time but dosent work 1% like large pontics.

I always used Rex 3 for NP for 20+ years and never saw porosity. Very forgiving except when I waxed under .3 thickness. It was so easy and very forgiving for technique inconsistencies.
Pd based Noble alloy on the other hand is sensitive to sprueing technique for suck backs and porosity and taking in gasses from casting.

 

[Al.]

If everything is perfect with this metal for you except large pontics and you want to keep using it than make your large pontics hollow and the problem will be solved.

Hollow pontic meaning scoop a hole in the occlusal and buccal till they meet. Then fill the hole with a mix of opaque and dentin or opaque dentin then procede like normal.

Hollow pontics also will give you a better cte. Williams used to sell them but not any more.

 

[dmonwaxa]

Edy, NP does not melt into one big pool like N, HN or AU. I just slumps because of the oxide layer surrounding the melt. If you're trying to get multiple ingots to flow together which seldom happens if never at all then more han likely you're overheating the metal way way too much. At those high temps and duration you're vaporizing the flux elements in the alloy. Making it more brittle and less flowable. Cut back on the melting temp and time heating the alloy, abserve the slump, shake/wiggle the cruicible arm and observe the metal move. The alloy is not gonna be shiny, just dull and may have some cracking on the surface. ..............SLING!

 

[dmonwaxa]

If everything is perfect with this metal for you except large pontics and you want to keep using it than make your large pontics hollow and the problem will be solved.

Hollow pontic meaning scoop a hole in the occlusal and buccal till they meet. Then fill the hole with a mix of opaque and dentin or opaque dentin then procede like normal.

Hollow pontics also will give you a better cte. Williams used to sell them but not any more.

Al, I dont know if you read correctly; but he has his own technique, he's already making them hollow.

 

[Tayebdental]

Edy,
Great advice above, when casting NP you have to keep the oxide layer srounding the molten metal intact , if that layer is broken you already overheated the metal.

when spruing the Pontic run a thinner sprue opposite the sprue that feed the Pontic all the way to the sprue former( to help get rid of gases). Always cast in the blue zone of the flame for hotter and complete burn. Good luck