porosity in gold bridge

[lonestarlab]

Even with indirect spruing and the sprue on the pontic as big as the Earth ,I still get porosity.What would happen if on a 3 unit br. the pontic had no sprue . thanks for any advice

 

[Jo Chen]

Lowering your burn out temp would be one option to try. Are you casting too hot? If you are casting with a torch, cast as soon as the oxide layer on the molten metal breaks up. Try a lost head sprue. Are your sprues too long, too short?

 

[user name]

Would be helpful to see a pic of exactly what youre dealing with. If the porosity is right at the sprue connection, its likely your sprue technique or over heated metal or mould. If the porosity is more through out the the casting, Id look at your crucible. Make sure its surface is very well glazed. If it isnt, just melt a button of your old alloy in it and let it cool. Pop it out and look at the underside. If the crucible isnt glazed or sealed, youll see bubbles in the metal. Those can get into the casting and will look like porosity but can be distributed unevenly through ouy your units. Toss your used alloy into the refining scrap and start with a new crucible and fresh alloy. I like the glaze I can get on harvest Dentals crucible best. Crank the heat up on your torch and after you get everything evenly hot, really heat the snot out of the area your alloy will pool. Get it a completely glass surface.

 

[the_prez3]

I use a normal sized sprue, and build a reservoir near the heavy crown or pontic. Keep your reservoir about the thickness of the sprue away from the crown. When you cast the ring, pull it out of the furnace for a minute or two and cool it a little prior to casting.


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[ps2thtec]

Vents Vents Vents
And lower your oxygen pressure to 6lbs.

 

[charles007]

Use 2 reservoir sprues on heavy crowns and pontics . Sprue thickest cusps.. The last gold cr. that could have been a nightmare was a lower molar, 4.5 DWT.. I used Ivoclar's 6 ga. reservoir sprues without any porosity.. When your good, your good.. lol
Minimum turns on your casting machine.. 2 to 2.5 .. minimum burnout, don't over heat alloy, good spruing techniques and you can cast anything, with a little luck... lol

 

[kimba]

I posted recently regarding porosity in large crowns and recieved a tonne of helpfull replies . we seemed to have fixed our problem by lowering final ring temp by taking it out of thre furnace one minute before casting with the furnace temp at 700 celcius.

we are in the process of lowering the furnace temp to get a more scientific/ predictable outcome.

The more I read about casting techniques the more I realize what a black art it is.

All the theories have merit and can be argued for and against , but just by looking at all the different responses on this site alone , it seems that through trial and error you will come up with a combination of sprue design ,burnout , casting temp and casting procedure that will work for you.

 

[Gdentallab]

i know it's against all what we studied and most of the education, but sprues on the pontic makes porosity , so why not eliminate the sprues on the pontics? it really works fine , even for full bridge with a large number of pontics.

 

[user name]

I have to dissagree with you Gdentallab. Sprues on a pontic dont create porosity. In a specific situation...it could. That would be if your pontic were in the heat-center of the ring. Its not 'black art'. If your pontic isnt able to draw hot alloy from a reservoir, then the pontic becomes the res for the adjacent units. The best solution for success would be to NOT use an oval ring.Its almost impossible to get the center of a bridge out of the heat center if you use an oval. Use a larger round ring and keep the reservoirs in the middle and the bridge closer to the wall; and like ps2thtech said...VENTS!

 

[charles007]

Good point user name,,,,,, I've been tilting my wax patterns for so may years I totaly forgot to write that.... Same theory when spruing emax,, out of the thermal zone..