High Noble Gold Porosity.

[TheLabGuy]

Alright, I've waxed this twice.....cantilever bridge, full cast, high noble Gold.....keep getting porosity. Should I sprue a different way?

Any and all suggestions are welcome!!!!!

 

[DeVreugd]

Alright, I've waxed this twice.....cantilever bridge, full cast, high noble Gold.....keep getting porosity. Should I sprue a different way?

Any and all suggestions are welcome!!!!!

1) Try one sprue into interproximal
2) Use lots of chill vents
3) Try fewer turns on the casting arm if using broken arm casting machine

Russ

 

[TheLabGuy]

1) Try one sprue into interproximal
2) Use lots of chill vents
3) Try fewer turns on the casting arm if using broken arm casting machine

Russ

Will do, and I'll take a picture before I cast, since it is apparent I have nothing else better to do on a friday night......lol Thanks Russ, and hey brother, those Steeley's are beginning to run by my house, yours?

 

[kcdt]

I just read something from Bego that said surface roughness and porosity can indicate to much force slinging the alloy. I'd go with Russ' suggestions, he definitely walks the talk.

 

[TheLabGuy]

I'd go with Russ' suggestions, he definitely walks the talk.

Agree totally.
I'm curious, why is that Russ? Less force lets the air escape easily?

Here's the pics....

 

[charles007]

Rob,
Don't use more than 2.5 to 3 turns on you casting machine, even on an old one, I bet you are using 4 turns or more?
Since this is probably a rare occurence, calibrate your burnout oven . If you are like me ,I haven't checked mine in years. Also look at the botton of the sprue button, is it razor sharp? = over heating . Check lbs on your propane and oxy to see if its been accidently changed. I know we all know these techniques, but worth checking.......... crap happens
I use the Renfert pear shape sprues, on molars that are very heavy, I sprue both lingual cusps to prevent porosity....... Very rare that I use 2 sprues. Flair out the connection of the sprue to the cusp. Also place the patterns on the outside wall, out of the heat zone...

Charles

 

[sixonice]

Looking at your pictures, the button looks enormous. Do you weigh your wax-up's (with sprues) and multiply that number (in grams) by the specific gravity of your alloy? The large button is pulling heat away from your reservoir sprues. Using that formula will ensure you cast just the right amount of alloy and end up with a very thin button (thinner than the reservoirs on the sprues). You want the reservoirs to cool last and have any porosity on the underside of them.

 

[bdg]

Along the lines of what sixonice said
Take one of your preformed sprues and cut it so you just have the resevoir and the lead that you attach to the wax pattern.
(Write that weight down until you commit it to memory)
Then when you weigh your wax pattern - add the amount of the "1/2" sprue and do the equation to get the alloy amount.

Then when you sprue (Using a regular sprue - not the one you cut to get the weight) - make sure the resevoir is in the thermal zone of the casting ring

When you cast you will have no button, just the resevoir cooling last in the thermal zone.

This way nothing is fighting for the gold in the resevoir only the pattern.
Basically if you have a button, it is working against what you try to accomplish w/ a resevoir.

 

[DeVreugd]

Agree totally.
I'm curious, why is that Russ? Less force lets the air escape easily?

Here's the pics....

Rob,
Sorry I didn't catch this sooner. You want to place the chill vents (12 or 14 gage) onto the thickest parts of the buccal/lingual area. If they are thick wax -up's, use many. Minimal turns on the casting arm will reduce turbulence which reduces gases when you release for casting. The higher specific gravity of the alloy, less force is needed. Also, make sure it's a smooth release, not jerky.

Russ

 

[TheLabGuy]

Rob,
I bet you are using 4 turns or more?
Charles

Yep Four.....I went to 2.5 this time....

Minimal turns on the casting arm will reduce turbulence which reduces gases when you release for casting. The higher specific gravity of the alloy, less force is needed. Also, make sure it's a smooth release, not jerky.
Russ

Totally makes sense, thanks a bunch.

Do you weigh your wax-up's (with sprues) and multiply that number (in grams) by the specific gravity of your alloy?

I've heard of this, but for full cast gold? Maybe I'm in the dark here, does anybody else do this?

 

[aidihra]

I agree with some of the other posters, you should use chill vents in the heavily waxed up areas. I don't see any problem with the size of your button, it looks fine to me.

 

[Al.]

I agree with Six, your reserviors are WAY too small for that casting and your button is too big. Reverse it.
If you get porosity again on that single sprue, make your own reserviors with rope wax but make them as large as your casting, twice the size of the prefabed ones you used.
Ill bet you a steak dinner Im right.

 

[sixonice]

I agree with some of the other posters, you should use chill vents in the heavily waxed up areas. I don't see any problem with the size of your button, it looks fine to me.

The button is too large. It is acting as a "heat sink" cooling last, and counter-acting the job the reservoirs on the primary sprues are supposed to be doing.

 

[sixonice]

I agree with Six, your reserviors are WAY too small for that casting and your button is too big. Reverse it.
If you get porosity again on that single sprue, make your own reserviors with rope wax but make them as large as your casting, twice the size of the prefabed ones you used.
Ill bet you a steak dinner Im right.

You summed it up even better!

 

[Al.]

Six where did you get your handle from? Is that hockey? Six on Ice?

 

[sixonice]

Six where did you get your handle from? Is that hockey? Six on Ice?

Six pack of beer (your choice!) on ice! Your idea would work also though!

 

[TheLabGuy]

Okay, it's done, It's going out.....I think a combination of larger resovoirs, chill vents, and less turns made this case turn out. Thanks a bunch for everyone who contributed.

 

[charles007]

Rob,
Great to see that you finished that case, very nice........

Rob, its not that I disagree with venting, just takes extra time, I use to vent most of my cases many years ago.. I don't anymore ...... On your case, I would have used two sprues on the linguals of your molar for sure.....I forgot the molar was a very heavy wing pontic = easy to get porosity. For me, the heavier or larger the molar, the easier it is to get porosity. I'm talking about molars that might weight 3 to 4.5 dwt. I used to use Ivoclar's red 6g reservior sprues, looks like yours, but I would still see porosity on large molars sometimes. What I learned is these big fat sprues don't always keep you from getting porosity. Large molars, or large pontics may need 2 sprues. Now I use a smaller reservior sprue (Renfert Geo Wax casting sprues, (pear shape) and flair the wax connection on the lingual cusp, I think the flairing of wax keeps me from getting porosity. I never have porosity problems.......
Your sprues are ok !!

 

[bdg]

I've heard of this, but for full cast gold? Maybe I'm in the dark here, does anybody else do this?

Works for all castings - Its a method to determine the amount of alloy regardless of type.
All castings will have porosity because of the shrinkage that occurs during cooling. All you can do is control where the porosity finally ends up at.
The alloy amount is just one of those control factors.

Just curious - When you cast the final one, did you still have a button?

Casting multiple units was a thorn in my side for a long time
I eventually went back to basics
I use an 8 gauge runner bar
10 gauge feeds (around 6mm in length) to each pattern
and on a thick pontic I run another sprue lead from the pontic down to the runner (18 gauge)
Oh - and all my type III castings I still use a gypsum bound investment
The castings are spot on every time.

Everyone I work with laughs at me - says they have quicker methods -
Heat it and it will cast, all for one investments etc...
Im all for change and new methods when it works- but sometimes I just chuckle as they are 'RE'waxing that bridge due tomorrow morning

 

[TheLabGuy]

Just curious - When you cast the final one, did you still have a button?

Yes I did.....This bridge weighed out at 8.2 grams. What a pain in the keester, but hey, I learned something, a few things actually, and if it was easy everyone would be able to do it....right?