Brass castings freezing short.

[Humph7ey]

Hi Guys, long time. Since my last project making oil pumps for a vintage Austin, I am attempting to make the elliptical bezels for Hobson Telegage fuel gauges.
I am using Mansbond oil tempered sand and the bezel sections are 6.5mm wide 90deg triangular section 85mm high and 45mm wide. I have made a mould for four of these.
There are four 2.5mm dia vents to the rear of each one. The sprue is 30mm dia arranged centrally with 10mm feeders to each part.
I tried doing four at once then when that did not work, tried two which also failed.
I think the brass is hot enough as a steel stirrer rod smokes when exposed to the air.
Is it possibly just a temperature thing, or do I need larger risers?

 

[Lakc]

With that thin a cross section I can see why you have troubles. If its freezing, its simply shedding too much heat before it fills the form, and the only fix for that is more/fatter risers and feeds. Larger mass of molten metal as close as possible behind the feed. If you had one fat riser feeding a circle around the part say 3/4" square with a gate every 30-45 degrees would be my first guess. Sand isnt the best medium for this thin stuff, most likely it was die cast originally.

 

[Humph7ey]

I am hiring a laser temperature gun over the weekend and will get the metal as hot as possible and try your suggestion.
The originals were pressed sheet and I tried that with no success either.
The next think is to get them cnc milled. A pantograph engraver might work also.
I might try leaving the sand in the sun.

 

[jasonh]

I agree- the metal is losing too much heat to the sand and is freezing up before it reaches the extremities of the mold.

Additional ideas:

1) Use an elevated sprue to get the metal's hydrostatic pressure up. ie- get a coffee can, cut out top and bottom. Fill with sand. Cut a sprue in the center and position it over the sprue of the main mold. Extra sprue height == higher pressure.

2) Pre-heat the sand. In industry they probably use ovens, but a blow torch used on the sand before you close up might take the chill off and keep the metal fluid longer, as you say - just leaving it in the sun would be better than nothing.

 

[aonemarine]

Try cutting a circular runner all the way around the part about a half inch deep then cut in multiple gates to the part. If that doesnt work then switch to a bronzer style flask. I would love to see some pictures...

 

[Humph7ey]

Well, that did not work either. Would it be better to try the lost wax process? I have no idea how that works, but is the mould heated and the molten brass poured with the mould still hot? I have a brass French clock with fine casting on it with no sign of sprue marks.

 

[aonemarine]

Lost wax is all ways my first choice for ornimental thin castings.

 

[Lakc]

Pics at this point would be most helpful.

 

[kf2qd]

An open face mold. Less mold in contact with the molten bronze so it will flow better.

 

[aonemarine]

An open face mold. Less mold in contact with the molten bronze so it will flow better.

But no head pressure.....

 

[Humph7ey]

I am giving up on sand casting. There is only so many times I can make up the mould only to see it wasted again. I learned a lot.
Next step will be lost wax process. Sorry about the pics, but there is not much to see anyway. Thanks for the help.