PFMs sitting up on occlusal after casting

[illumi]

We have been scanning and designing our wax frames using 3shape and while the wax is fitting nicely after milling we have a continuous problem of the frames sitting up on the occlusal after casting. When we adjust the cement spacer in 3shape to compensate we end up loose side walls and have instead taken to adding rubber spacer on the occlusal before scanning. It seems like an odd thing to have to do and I am wondering if our investing and casting procedure is the problem. We are using Talladiums 1700 in individual packages, bench set 15 minutes and into a hot oven for 45 minutes then cast. We mix on a slow speed mixer and are casting white noble metals.

Anyone have any troubleshooting tips?

 

[JMN]

Verify your investment mix ratios. Sounds like your investment may be incorrectly expanding for the metal compostion you are casting. First guess is that the water/liquid ratio may he out of whack with using too much water, if you're using phosphate bonded that is.

 

[corona]

could it be the wax ? is it made to burn out quickly ?

 

[illumi]

Thank you for the tips! I have a few test crowns going up, some Bego investment to trial on the way to see if it is more forgiving, and a couple different wax block to trial later this week.

 

[Kaypan]

One possibility; There are many types of millable wax, from soft to hard. If you use a fairly soft wax, it will be somewhat forgiving when you seat it on the cast. We saw this on milled wax to be pressed: seats perfectly in wax but sits high on the occlusal(in relation to spots or "pikes" created by drill compensation). Instead of adjusting the spacer, try to adjust your drill compensation/drill radius. You can also change between "old" and "new" drill compensation(where the new one offsets more horizontal, and the old compensates more vertical. I usually choose the best compensation on a case to case basis). Takes some trying and failing to get the spacer/drill comp./ivestment ratio perfectly tuned in.

 

[Hary]

X2 Investment powder liquid ratio
are you casting non precious aloy? if yes try to put less water in the liquid ratio.

 

[illumi]

Thank you Kaypan. Good to know on the new drill compensation vs old. Two great points. Surprisingly we tried a few pucks that were a bit more brittle with less of the fibers that seem to be added to millable pucks and the fit using rapid burnout improved considerably. We were using a very rigid fibery wax. We never had any chipping problems and were able to mill very thin margins with it but the fit was always in need of improvement. Yesterday we used the same ratios, investment and .stl files and only changed the wax type we ended up with a much better fit with the new wax. Still fine tuning to make it perfect and will do some testing with the burr compensation but surprised there was a measurable improvement just switching wax.

 

[illumi]

Just a follow up. Adjusting the burr compensation size and strategy on steep bicuspids or very hilly preps has proved very useful. We are maintaining tight sidewalls and margin with more cement space only where it is needed on the peaks. Thanks again.