centrifuge casting and torch

[paulg100]

OK after running a small high end satalite lab for 3 years and sending my metal wax-ups to our main lab in another city for casting, i cant hit the quality of fits i need and have decided its time to sort out on site casting faciilities.

(Was hoping to go the cad/cam route for metal but this still seems a pipe dream for now)

partly due to financial reasons and because i consistantly hear its still the most accurate method, im gonna go the centrifuge and torch route.

My goal is to get as close to 40um as possible with my marginal fits. It may take me a while to dial in but once its done its done.

So my question is, is anyone still using this method and do you have any major Do's and Dont's (apart from flinging molton metal over the room ).?

any advice much appreciated.

 

[TheLabGuy]

-The way gold prices are I'd get a casting well/drum to secure the centrifuge too so you can collect your scrap easily.

 

[NicelyMKV]

I use a galvanized wash tub with a broken arm mounted down in it. Harris torch with multi orifice tip and let it rip I built my own platform so the burnouts were perfect height for quick out and in to the casting cradle.

 

[desertfox384]

Save yourself some burnt metal and call the alloy manufacturer. Ask them some specifics on torch settings and what that particular metal should look like before you let it go...

 

[[email protected]]

Go for it, get a "kerr" broken arm casting centrifuge from harris dental supplies online (i got that one, is a quality unit). bought a renfert casting torch, bit pricey but I believe that cheaper torches can cause drama with casting ie tips melting etc, not too sure how true that is but it never hurts to stick to the well known brands as you only buy it once.
The most important thing about casting imo is to follow the instructions for the investment materials very carefully as this will affect your fit more than anything else. (just from my personal experiences to date).

Had to get a shroud made to go around the casting machine but was all mounted on a low benchtop and i have had some pretty good predictable castings from that set up.

 

[paulg100]

Sounds great so far, thanks

I guess with all the modern cad/cam stuff around these days, these older techniques can be easily forgotten, but hopefully i can start getting back to some real precision fits soon.

I hopefully will get the centrifuge from our main lab. Its as an older unit that has hardly ever been used, due to the health and safety/idot proof factor.

Sits in a well that goes into the top of a bench so sounds good as long as it still works (think the springs may be broken atm).

Ill check out the renfert torches.

"Harris torch with multi orifice tip"

Ooo err. :eyebrows:

 

[TheLabGuy]

I'd suggest getting gauges for your gas and oxygen as well. That way you can know exactly what percentages you're heating your metal with. The high percentage palladiums can be a migraine if you don't.

 

[Al.]

I never thought of casting as old school.
I think it is still the #1 way most labs make pfms.

I would recomend buying the entire casting well cabinet. Mine is metal and heavy so it dosent move if you bolt it to the corner of the wall. No extra counter space for mine just the casting well cabinet.

All you do is bolt it to the wall and bolt your casting machine to the bottom into the predrilled holes.
Bought mine in 1978 and its still doing the job perfect. Im using my 2nd casting machine in as many years.
Takes less than 1 minute per casting.

All you have to dial in is your liquid to water ratio for fits.

I go by the glow for precious and semi metals. The second it quits swirling and is ready to get bright white is when I quickly take my ring out of the oven and sling the metal.
For NP when it slums and starts to move in a semi fluid state I sling it.
I use double the O2 for np as semi or precious. Semi or pd needs to be a bit hotter and since it is lighter needs a bit more metal to push it all in. Also for pd I keep the thin spots .3 or above (prefer above) or you may get short castings.
For pd I leave a slight metal band or thicker or you may have short margins, then I cut it back.
Also for splints esp for pd you for sure need runner bar with 8 ga sprues on the runner bar that connects to the former to keep the porosity of the pontics or splint and in the bottom of the runner bar. HN isnt as sensitave to porosity for me but still I sprue the same for bridges.

For NP you always need a larger button it is a light metal and need the weight to get complete casting, Never reuse your NP buttons !!! Trash them.

For singles I always use a runner bar but I put the runner bar directly on the former, no sprues from the bar to the former. And for singles I use the 8 ga wax sprues for runner bars but splints I use the plastic runner bars for stability of the wax. For larger bridges I use gc resin pattern to reconnect the cut connections.

I never used gloves but for the rings in the back of the furnace you have to grab them fairly quick or youll singe the hairs on your fingers. Done that alot but never burned myself.

I bet best and most consistant resuts especially for splints with slower 2 stage burnouts.

 

[paulg100]

"I never thought of casting as old school.
I think it is still the #1 way most labs make pfms."

Nah your just getting old , thanks for all the info.

No what i really meant was the centrifuge and torch method.

I assumed most labs use the newer automatic casting machines these days.
We have in london for as long as i can remember now.

Think the first auto heraus one we bought had a camera to tell when the metal was ready but that never really worked.

I think its a semi auto one they use now.

Me dad said he never put the centrifugal in to proper use because it wasnt idot proof enough for a larger commercial lab. You have to consider these things when you have lots of apprentices running about

 

[NicelyMKV]

I learned on a big old green induction machine. We cast gold crowns using the broken arm machine. We had two people who could cast with the torch and I was one of them. We closed the partial dept and the owner decided the induction was too much trouble to keep going. We moved everything to the torch andntjatsnhowni have done it the last fifteen yrs. Torch is not idiot proof though. A properly dialed in induction can be to a point. If I was a biiiiiig lab and was going to have constant possible new individuals casting etc I might look more into the induction machines. Just depends on the quality of your help.

 

[Wyolab]

Bego's web-site has a bunch of online classes on spruing and casting. Al is right the liquid to water ratio will be one of the most important things affecting fit, and spruing all your patterns is important in avoiding many other issues. Carefully read the recommended parameters with each type of metal for burnout info. Here is the Mornin' Perk link from Bego Bego USA - Morn Perk On-Demand Library

Quintessence Books Introduction to Metal Ceramic Technology has a great chapter on Spruing, Investing and Casting also.

Here's some troubleshooting tips from "Fundamentals of Fixed Prosthodontics"
A large nodule on the casting indicates an air bubble.
Multiple random nodules on the casting result from inadequate vacuum when mixing investment.
Nodules on the underside of the casting only indicate prolonged vibration after pouring investment.
Shrink-spot or porosity where the sprue attaches indicates a sprue that is either to bulky or too long or thin, or a button that is too small.
Random porosity indicates contamination of the wax pattern, or loose investment particles in the casting.
Fins indicate a dropped ring, rapid heating of an undercured investment, or excessive casting force.
Short rounded margins with a rounded or lumpy button means alloy was not hot enough, or insufficient casting force.
Short rounded margins with a crisp button show that the pattern was too far from the end of the ring, or incomplete burnout of the wax.
Black rough castings mean that your burnout temps are too high.

 

[Mike2]

We use this method and it is not too complicated. As stated get gauges and call manufacturer. We use Microstar HS investment as Jensen offers the best support IMO. Weighing the liquid and water will let you really dial it in and I have found other investments that are cheaper but you get what you pay for. As far as old school goes, when the new and modern crowns match the longevity and wear of crowns I made 20+ years ago, I will be out of business and retired(hopefully),plenty of Dr.'s still just want the old standard GOLD or PFM'S. My 2cents

 

[paulg100]

Seems like some good info here, thanks all. :usa2:

Im sure it will save me a ton of time in R&D so ill be keeping this info to hand when im set up and ready to go.

Took me a while to dial in my press powder / liquid ratios (also stored liquid in a fridge to get some kinda temp controll)

The marginal fits im getting with pressed after waxing and fitting down under mag are awesome, so if i can replicate this with metal than ill be a happy man.

 

[DCP1977]

Paul, I recently got a new oxy/prop torch. It was a pain to find one (BOC only do a 700mm neck),but I ended up getting one from EngWeld. I'll post the part numbers if you need em.

 

[paulg100]

yes please

 

[Al.]

Everytime I try a different metal I have to make some test copings to get the correct lq to water ratios.

You may want a notebook to keep all the different ratios for each metal.
When you get it right you still may need to change the ratios from winter to summer. Some of that is the humidity in the air also. You may need to shorten your mixing time in the summer.

But Im not telling you anything new its the same as emax, just casting instead of pressing.

Its important to mark your trailing edge on your ring and put the ring in the furnace facing the correct way. I usually make my margins facing the trailing edge so when the metal goes in it flows to them easy not having to fight the centrifical force. I usually lean all the copings towards the trailing edge.
Not the spoke like way you arrange emax on the sprue former.

 

[paulg100]

"When you get it right you still may need to change the ratios from winter to summer"

Yep ill be keeping me liquids for metal investment in a fridge also to get consistent control over that.

Probably gonna go for GC's fuji vest platinum. Not to worried about cutting a few $ here or there a pack. Not running a production set-up now, so im more concerned with the fit quality and smoothness of casting.

Thats what i used to cast me Straumann Platinum test piece with a few years back, came out lovely.

Rep came in the other day with an investment that was 1/3 the price of the ivoclar press vest speed I'm using for emax. My straight answer was no thanks. Not worth the head aches when I'm already getting the results i need.

Pressing Emax is still peanuts in material cost, compared to what it was costing me to mill.

Cast Metal vs cad/cam milled will be the same. (although ive had not luck with cad/cam metal, still a long way from cast results as far as i can see)

 

[disturbed]

"When you get it right you still may need to change the ratios from winter to summer"

Yep ill be keeping me liquids for metal investment in a fridge also to get consistent control over that.

keeping it in the fridge won't help with what al is talking about. It's the humidity levels in the air that change in the spring and fall. Putting your liquid in the fridge only extends the life of your liquid, doesn't really do very much for controlling expansion. If you see a shift in the way your metal fits over the next few months don't blame the invest.. blame the change in weather, note the ratio you were using in the summer cause you will have to go back to it next year, slightly adjust if you have problems and you should be good till spring.

I had to do this in the higher altitude areas but at sea level and closer to the equator I find it is no longer ness, because the weather does not change much, from winter to summer, where I am now.

always heat the crucible before you add the metal.. If you have a heavy carbon buildup in your crucible make sure you change it out for a fresh one. Keep your copings out of the heat zone.(towards the outer edge of the ring) these are basics but I have seen some people not doing these simple steps and having porosity issues....

good luck!

 

[paulg100]

"Putting your liquid in the fridge only extends the life of your liquid, doesn't really do very much for controlling expansion"

It seems to have made a noticable difference in the expansion consistancy of my press fits with ivoclar press vest speed, plus it gives me more working time due to the lower temp.

Maybe its an investment thing or maybe im imagaining it!

but thanks for the info.

 

[disturbed]

"Putting your liquid in the fridge only extends the life of your liquid, doesn't really do very much for controlling expansion"

It seems to have made a noticable difference in the expansion consistancy of my press fits with ivoclar press vest speed, plus it gives me more working time due to the lower temp.

Maybe its an investment thing or maybe im imagaining it!

but thanks for the info.

it does increase invest working time..