spruing FGC techniques

[kimba]

Hi all ,

I am looking for some suggestions/techniques in regards to sprue design for thick gold crowns. I have inconsistant porosity problems in thick crowns. Any comments ideas diagrams of sprue design are welcome and much appreciated.

Thanks

 

[PDC]

What is the percentage of Au is in your alloy and what is your temp before casting?

 

[kimba]

69% Au for high gold

2% Au 34% pd 30% ag 30% in for economy alloy

I useally cast at 850 celcius with my bonding alloys

I have the same inconsistencies for both alloys. That is porosity but not always and only in thick crowns

 

[shane williams]

For larger gold crowns we have large sprues with a bigger ball on it. I think before we found the larger sprues we used two smaller ones. Good communication with the metal finisher who is casting the ring is important too. I tend to only place the single large gold crown in a ring by itself.

 

[kimba]

The sprue size on the last one today was 3.5mm sprue with 7.5 mm reservoir with a second slighly smaller on the other cusp

 

[user name]

Make sure you weigh your pattern and calculate! If you are having a button left over, it can draw alloy away from the reservoir during cooling. I burn out at 850c for 30 minutes then drop the temp to 950 before I cast. Dont over wind the machine. high gold on a centrifico I use 2 turns max. Make sure your used gold is CLEAN. Keep the reservoir in the middle of the ring. Clean out the tip of your torch and have it properly adjusted. It doesnt take much heat...dont get the gasses in the flame on the alloy and dont over heat it. Hot metal expands alot...and shrinks alot too.

 

[desertfox384]

To be safe on heavy crowns I make the reservoir at least as heavy as the crown I'm casting. One single sprue with a reservoir is what I do for most. Keep the crown as close to the wall of the ring as you can and 1/4" of investment over the top of the crown. Make sure you double check your oven settings, if its set too hot this will cause problems, and so will overheating your metal.

 

[doug]

I've had more porosity problems from too much winding on the casting arm than heat.

 

[kimba]

Thanks to all. Had'nt thougt about overwinding. Any other thoughts still welcome

 

[ztech]

The best advice ever given to me in casting alloy is that the last area to cool will have porosity. Using this axiom you view the patterns in two ways. Thick areas will tend to stay hot longer and the center of the ring will stay hot longer. Thick crowns usually have porosity in and around the sprue area. This is caused because the mass of alloy in the crown moves the heat center toward it and away from the reservoir and into the area of the sprue. The way to stop this is to increase the reservoir size larger than the mass of the crown to be cast. I have cut my sprues into the different segments and weighed them. Knowing the weight of the unaltered reservoir portion I then weight the wax pattern. I make the reservoir 20-30% larger by weight and when I place it in the ring I make sure that the reservoir is closest to the center of the ring. If the porosity is not localized to the sprue area but is all throughout the casting then you have a different porosity problem and it is probably in the melting techniques.

 

[ps2thtec]

Similar to your sprue I use a 8mm reservoir with 8ga feeder, but I dip the ball afew times to make it larger . I get a lot of FCC that weigh over 2.5dwt. Sprue to a thick cusp and I also use a small ga vent off the opposite cusp.
Using Nat gas/ oxygen torch I set the ox pressure at 5lbs. And two winds. When casting a HN ceramic alloy I use 20lbs. And three winds.

 

[cheadlemick]

The reservoir should always be at least as thick as the thickest part of the casting! Keep the pattern above the thermal zone so the pattern cools last drawing alloy from the still molten reservoir!

 

[PDC]

69% Au for high gold

2% Au 34% pd 30% ag 30% in for economy alloy

I useally cast at 850 celcius with my bonding alloys

I have the same inconsistencies for both alloys. That is porosity but not always and only in thick crowns

For your C&B 69% Au: I would use 2 turns on the casting machine. I would burn out at around 800-815 C for about 45 minutes and then lower the temp to 537 C before casting. This metal has a much lower melting point and the ring needs to be cooler so the metal can cool faster. This prevents the casting from staying hot for so long and acting kind of like a button trying to cool.

We used to use Beauty Cast (Whip Mix) years ago with a low burnout and never had porosity problems.
I think you will find this will solve the issue...the reservoir is still a good idea on very heavy crowns.

Hope this helps.

 

[SiKBOY]

Your burn out temp is too high. Burn out at 850-900 and then drop the temp to 650 And let it sit for halfer at 650. Then cast.

 

[user name]

Make sure you weigh your pattern and calculate! If you are having a button left over, it can draw alloy away from the reservoir during cooling. I burn out at 850c for 30 minutes then drop the temp to 950 before I cast. Dont over wind the machine. high gold on a centrifico I use 2 turns max. Make sure your used gold is CLEAN. Keep the reservoir in the middle of the ring. Clean out the tip of your torch and have it properly adjusted. It doesnt take much heat...dont get the gasses in the flame on the alloy and dont over heat it. Hot metal expands alot...and shrinks alot too.

My numbers were screwy...I burn out at 850c; when I said I drop the temp to 950...thats fahrenheit. That would be 510c.

 

[EJADA]

I have read this thread and just want to say the advice on this topic is excellent. There are many things to consider which many of us veterans take for granted. I feel you all have covered the important things to be considered very well. I wanted to add my two bits but you all did great.

 

[cheadlemick]

You could use Howes colour scale to assess the mould temp!

 

[Al.]

Ditto on the reservoir size.
The prefabed ones usually are not large enough.
I make my own with balled up rope wax for each crown. Make it as large as the crown.
That means I usually always have a really large reservoir but my porosity is almost always on the reservoir.
I attach a short 10 ga spruce, then attach the reservoir, then attach a short 8 ga spruce to the reservoir to attach it to the former.

 

[kimba]

No the porosity is localized, not all through. I dont think I we are overheating the metal, but I will duoble check over the next few weeks. I am thinking more and more it is positioning and reservoir size. Thanks for input

 

[kimba]

Thasnks , I gathered you meant that. Am using Bego xl they recomend 700c so I will start there and work down as I need to. Thank you