Al's porosity

[TheLabGuy]

Hey Al,
I started this thread because you posted some pics of some porosity castings (which I can't seem to find).....I thought about it and have come to a conclusion (suggestion). Try winding up your casting arm an extra round. Wind it up 4 times and I bet you don't get any porosity. Let me know.

[Al.]

Hey Robmeister, the thread is in the misc, section. i dont have problems with porisity anymore, but I did. I know there are quite a few techs that still have problems with it though.

I do wind up my arm 4 times except for when I cast that metal that comes from old railroad tracks. I wind it up 4 1/2 times for that stuff.

When beauty cast was the investment of choice, or really the only thing available at the time, often 4 times and alot of metal would bust the investment.

[TheLabGuy]

When beauty cast was the investment of choice, or really the only thing available at the time, often 4 times and alot of metal would bust the investment.

Been there done that, so what was the problem?

[Pronto]

I use 2.5 for Argent 75 and 3 for 52sf. I do occasionally get porosity at the tip the sprue on pontics. I usually vent the larger pontics.

[AL1]

Every casting will have porosity. So you need to control where it goes.
For gold crowns, take your casting ring and draw an imaginary X from top to bottom, where the lines intersect should be the middle of the ring, thats where your reservoir goes, the crown goes above, this allows the reservoir to be in the hottest portion of the ring allowing the crown to cool first and drawing the gold from the reservoir. the reservoir will then have the porosity not the casting.

[Sanhlabo]

Every casting will have porosity. So you need to control where it goes.
For gold crowns, take your casting ring and draw an imaginary X from top to bottom, where the lines intersect should be the middle of the ring, thats where your reservoir goes, the crown goes above, this allows the reservoir to be in the hottest portion of the ring allowing the crown to cool first and drawing the gold from the reservoir. the reservoir will then have the porosity not the casting.

I am turning 2.5 for designD10 ,and my wax copings are 0.3.I can't get one with out porosity at the edges of the copings(one or more).I used conventional sprue method and also bredent method.But non work.Why?

[charles007]

Rob, if this is a type lll alloy, less is better.......if you remember, Kerr had a Jr. casting machine that didn't have the take off power as the regular one. Used this one in an inhouse lab.. table top, no casting well ! burned holes in my shirts and pants all the time.
To many turns will create turbulence, in my opinion. I use 2.5 to 3 turns on gold.
Bet 2 turns will cast gold..
I find that spruing technique critical, and this is where most porosity comes from. I flare out the sprue onto the pattern, this stops porosity for me. Years ago I cast a 4.5 dwt full cast crown, porosity free....used 2 6g reservoir on the lingual cusps.
My sprue of choice now, Renfer"s pear shape reservoir sprue. On ceramics, I mostly use a tapered 8g, large pontics, use 2 sprues.
Sprue size, location, are key...assuming the alloy is not overheated...colder is better.
Just saying!

[Hal2a]

I use Argedent 52sf and Argenco 62. I find that if you design the sprue system larger than the coping or frame you will be all right. The last spot to cool will get the porosity and thats the thickest portion. If you need extra protection use a chill rod conected to the thick spot.

[Mark Jackson]

I do wind up my arm 4 times except for when I cast that metal that comes from old railroad tracks. I wind it up 4 1/2 times for that stuff.

One of the things about our FDA GMP's that has been invaluable is the equipment reapirs and maintence logs. We found out that once we started keeping track of every little adjustment, repair or calibration, that some of equipment was on LIFE SUPPORT. We seemd to spend more time fixing things, many time initiated by some kind of little technical problem, traced back to equipment.

One of the things we do now, every two years is replace the bearings and spring in our casting machine. When you think of the heat and loads we put on those things, it's amazing we don't have more problems.

If four winds was good three years ago, and you haven't changed the spring out, you may only be getting three winds worth of intial acceration as the spring memory goes away. It might feel just as tight, but it's the first instant of acceleration that makes the quality of the casting.

For what it's worth...

[JohnWilson]

Thats good info Mark,

Although we don't cast as much as we used to given the market trends we did purchase a motorized casting machine. No more springs to wear out, and so far its been so ridiculously trouble free/consistent I can't imagine going back to a broken arm winder again.

[Mark Jackson]

Thats good info Mark,

Although we don't cast as much as we used to given the market trends we did purchase a motorized casting machine. No more springs to wear out, and so far its been so ridiculously trouble free/consistent I can't imagine going back to a broken arm winder again.

We still use one for small cast gold restorations as our big induction unit is too powerful. We could probably reduce the replacement interval by tracking the number of castings and then replacing as needed, but it's just one more thing to record. Easier to do on a calendar basis.

Like you said, though, LDS and YZ have become the new alloys.

[doug]

I can, comfortably, cast my FGC's with only 1.5 windings on my Jelenko casting arm. Less is better when looking to reduce turbulence and porosity.