Gold Crown Problem

[awmattson]

Hello all,
I am a dental student and had a somewhat unique problem. We are casting our own gold crowns for a course, and I (like a lot of my classmates) have a hole in the occlusion where the sprue was. In previous years, the students were allowed to have the lab solder the holes for them. We are not afforded this luxury.
I have experience soldering wires and PCBs, and I was wondering if someone could give me some direction. We are using Midas gold alloy. My soldering iron only goes to about 800F, but I just purchased a micro-torch that I think should get hot enough. We have access to flux.

Thank you in advance for any help!

Edit: It is only going to be used on a typodont, so I'm not worried about it holding up intra-orally. The main concern is to have it blend in and look good... We know one class isn't going to make us half as skilled as you guys in this stuff!

 

[DeVreugd]

Hello all,
I am a dental student and had a somewhat unique problem. We are casting our own gold crowns for a course, and I (like a lot of my classmates) have a hole in the occlusion where the sprue was. In previous years, the students were allowed to have the lab solder the holes for them. We are not afforded this luxury.
I have experience soldering wires and PCBs, and I was wondering if someone could give me some direction. We are using Midas gold alloy. My soldering iron only goes to about 800F, but I just purchased a micro-torch that I think should get hot enough. We have access to flux.

Thank you in advance for any help!

Edit: It is only going to be used on a typodont, so I'm not worried about it holding up intra-orally. The main concern is to have it blend in and look good... We know one class isn't going to make us half as skilled as you guys in this stuff!

You and your classmates should evaluate "why" you have "a hole" where you placed the sprue.... re-wax & try again. This is not the time to look for "band-aids"! IMHO

Russ

 

[timbangley]

Use the proper size sprue for the size of the crown, also you could put one or two vent sprues and I agree you should rewax and try again.

 

[awmattson]

OK, I understand the idea about rewaxing and starting over. At this point we don't really have enough time. Being novices and wannabe perfectionists, a full wax-up takes an embarrassing amount of time. Multiply that by 2 and it just isn't an attractive option. As I said, previous classes were allowed to have the lab fix the holes... I'm just curious as to what they did to fix them.

Also, would putting more sprues even fix the problem? It seems (to us) to be more of a temperature issue where maybe the gold in the sprue is cooling too rapidly and sucking some of the gold out of the crown.

And as far as the size of the sprue, we weren't given an option...

Thanks for the responses!

 

[amadent]

as dental student you have been just given a small insight on what happens to alot of techs on weekly sometimes daily basis. you stated remaking it " just isn't an attractive option." we feel the same way, but more times then not that is the best way for the product we are providing our client to be the best it can be.

your hole may not have come from the sprue itself, it way have come from a thin spot in your wax up around the area where the case was sprued

 

[charles007]

You can solder the holes with low fusing post solder.. 590 I think ! over a Bunsen burner with a little flux....
Sprue using a 8 ga sprue and add an reservoir, or 10 ga. Sprue on the thickest part of the crown, such as the lingual cusp. Larger crowns use larger sprues. Where you attach the sprue, flair wax around the area to allow more metal to enter, this will help in preventing porosity.
Don't over heat your alloy.... use 2.5 to 3 turns using a Kerr casting machine.

Post pictures of your casting before you cut off the sprue should this happen again..... Your problems are a simple fix, just need to see your casting.

 

[user name]

Sounds like your sprue didnt have a reservoir, or if it did it wasnt located in the heat-center of the ring. I always say...If you dont have time to do it right the first time, how are you going to have time to do it over. If youre being graded on the crown, do it over or accept the grade. Sounds like you guys need a new teacher.

 

[DMC]

suck back porosity.

Need larger sprue. Move pattern out of center of ring.

You want pattern to cool first and pull alloy from hot center.

Lower temp of ring at casting could also be suggested, but may not be the right answer.

Maybe needed more alloy?

 

[paulg100]

What makes me smile about this is that students are having to spend so much time and energy on these techniques... How long is your course..

Will you even do any of this once you qualify or will all monolithic crowns, be they metal or ceramic, be milled by then???

sad but true.

 

[rkm rdt]

Fill the hole with wax and cover it with gold model paint. However keep in mind that if you do this you can't complain if your lab does this to your rush cases!

Rewax,don't sprue it !

 

[araucaria]

Fill the hole with wax and cover it with gold model paint. However keep in mind that if you do this you can't complain if your lab does this to your rush cases!

Rewax,don't sprue it !

 

[TheLabGuy]

Your problem is that you don't use the specific density for the grams/dwt required to cast your gold crown properly. Wind up your centrifuge no more than 3 turns. Open your pack of gold or whatever alloy, take the weight of your sprued waxed crown, times that by the specific density of your alloy and that is how much alloy you want to melt and throw at your crucible. Anymore you get suck backs and/or porosity. With a heavy crown, you may have to add some little sprue vents off the sides so you don't get porosity. There is a great thread on here somewhere about the same thing.

 

[charles007]

Years ago I use to cast gold crowns for a dental student all the time and was always given so LITTLE gold to use, I didn't know if they would cast or not. They always cast for some reason... and no SuCk back
I've cast crowns and bridges with only half a sprue and no button, and its a trade secret how to do it

 

[DeVreugd]

awmattson:
I (like a lot of my classmates) have a hole in the occlusion where the sprue was."

Why sprue to the occlusion? Is it an inlay, onlay or full cast crown ?

Have you been given any instructions on this procedure? Are you working blindly? Does your School have access to any DVD's or video from the AARD Master Series of Dr. Robert Fadal ? If so, check it out !

I agree, you will probably never do another after you graduate but learn the basics while in school! You never know how this knowledge might lend itself to your future practice.....

 

[AL1]

You probably violated the laws of casting.
You need a reservoir and sprue to the thickest part of the casting also the casting needs to be above the heat zone and the reservoir needs to be in the heat zone.
If you where to draw an x from the top of the ring to the bottom, where it intersects is where the reservoir needs to be, this is the hottest part of the ring.
This allows the crown to cool first and draw the gold towards it from the reservoir.
Also dont use to much or to little gold. To much and you shunt of the cooling with to big a button to little and you dont have enough gold.