Hi all, this is my first post so I hope I get it right.
I seem to have inherited a porosity problem lately and I cant
figure it out. Maybe someone could help me. In the last couple of
years, I have been getting "shrinkage" porosity in some of my
castings. The only thing I can attribute it to is the silicon
copper alloys that I have been using. It seems that when I cast
with "old" customers gold, I dont have any problems- but when I
use new or 50% new gold, I get porosity.
outlined below are the procedures that I follow :
1) waxes are sprued at heavest areas. sprues are short and at
least 3mm thick.
2) Kerr Satin Cast 20 investment mixed and invested at 40/100
ratio. I mix and vacuum the investment in a rubber bowl- pour
into flasks, and vacuum again. Total work time is 8 minutes
3) I dont dewax my flasks before burnout. I use a 12 hour
burnout cycle starting at 300deg F ramped up to 1350deg F and
held for 3 hours. Then I ramp down to 1000deg F and hold for at
least 1 hour prior to casting. My oven is well ventillated and
the temperatures are correct.
4) I use at least a 50% mixture of new gold each time I cast. I
alloy my own metal with either Stuller or United PMR non
5) I am using a Rey centrifugal caster (wound three turns) and
an oxy-acetylene torch. I dont believe I am overheating the gold.
6) after casting, I let the flasks cool down for at least 15
minutes before removing the investment.
I have been using this procedure for about 15 years and until I
started using the non-oxidizing alloys, I didnt have too many
problems. Should I try increasing my flask temp another 100
degrees? This problem usually shows up on medium to heavy weight
high shine pieces. Should I start using vacuum to cast with
instead of the centrifuge? Would it help to coat the waxes with
a wetting agent prior to investing?
Also, when I carve a pattern from carving wax(Kerr or Matt)
sometimes I get bad spots in my castings. It almost appears to be
holes in the wax that the investment seeped into- although I
never see any holes in the wax before investing.
Someone told me that the non-oxidizing alloys love heat and that
I should raise my flask and melting temp more.
Any suggestions or ideas on this would be very helpful. Ken