bad metal casting help

[Dushan]

first of all sory for my bad english i´m from Europe
i started working with metal 3 years ago and never had any problem until recently .
noticed some porosity,cracks and bubbles so i changed metal .
now i´m using Kera N metal and Maruvest microfine investing material and all of my castings are ok but from time to time i noticed porosity like on picture.
problem is that i invest 2,3 bridges and other bridges are solid and perfect cast but third is like this one.so porosity is only on one place around sprue position,but the rest is also solid.
these castings are from two different cases,invested with other bridges which are perfect.
sprue used in these cases are 2,5 for wax structure,4 mm for reservoir and afterwards 3,5 mm .final temperature is 950 for about one hour,casted using a machine.
i´m well awear that metal is not class one material but the thing that bugs me is that other bridges and crown are solid and perfect and invested in same time .
so any ideas???

 

[TheLabGuy]

wisdom tooth abutment...hmmm

As for your issues, could be a contaminant in the metal, breakdown in crucible, but looks like it's cooling too fast after casting...are you quenching? What type of metal is this? "casted using a machine"...induction or torch casting?

 

[Dushan]

induction casting,i always wait for self cooling and then start processing it
http://www.eisenbacher.de/en/non-precious-dental-alloy-kera-n.htm about kera n

 

[zahni]

Try a different sprue technic. Check for ZTM Herbert Thiel or Andreas Sabath. On the bridge in the second pic I would set only on sprue, no bar! only for cooling a few retentions on the other side.