Thursday, June 21, 2012

Out of the Slurry Tank and into the Wax Burnout Kiln

Our new ceramic shell slurry tank.

The Shopping list:

We mounted the motor in a steel box for protection against slurry splashes.  The shaft should enter the tank at an angle off center to keep the slurry from osilating too much.
We mounted the percentage timer next to the motor with an outdoor switch cover over the controls.

We had success with minimal settling with a timer setting of 4 minutes on and 4 minutes off.  The slurry got warm from the friction but not bad enough to cause evaporation.  The only time it got hot was when we mixed new batchs of slurry for a 2hr continuous cycle.  The shells ended up being very strong, had excellent detail and had minimal cracking thru dewaxing.

 The first two coats we use a thicker primary slurry mix using very fine silica powder in the colloidal silica based slurry and a fine grade silica sand for the stucco.
 We check the viscosity of the slurry using a Zann cup pictured in the bottom left of the photo.
 After the second coat we move to a courser grit sand for the stucco and a courser powder in the slurry.
The slurry mix is also thinned dramatically so as to allow the shells to develop the pores needed for the shells to breathe.
More pores in the shell layers equals less necessary venting as with standard investment procedures.
 I gate many pieces using direct gating theory.  The sprues feed directly to the pattern and also act as vents, but this is reliant on proper metal feed rate too not over feed so the piece burps air pressure through the stream of metal.  I look for a steady flow as I cast
Fourth dip: wrap with fiberglass strand
The final dip is done without a sand coat so as to seal the shells
6-8 dips for large pieces
4-6 for small and medium size pieces.

Also, the weight of the metal to cast plays a role in determining shell thickness.
 A student of mine doing the terrible job of grinding off the top of their cups, preparing for burnout.
We really need to make a filtered cup grinder.  Can you see it?  I can.  Humboldt State Sculpture had one, so shall I.
 A cup ground down  to separate the outside shell from the inside shell of the cup.  This part of the cup should not be there, but can be used for other many things such as:
  • shell repairs,
  • another cup if one is lost,
  • a tiny crucible,
  • thrown into a crucible full of molten aluminum to stop the metal from oscillating in the furnace, etc.
 Shells suspended in the wax burnout kiln...
 Actually a high-tech piece of equipment!

The Shopping List:
  • 90,000 BTU forced air gas burner
  • 1 1/4 55 gal steel drums:     (1 for the fire chamber and 1/4 for the water troff to catch the melted wax.)
  • Angle iron for legs
  • Steel sheets for baffles
  • 4' steel bar
  • Enough kaowool to line the barrel
  • Ceramic buttons and bailing wire to tie back the wool.

A view at night.  It always looks more dramatic this way.

 The shells are pulled out and loaded (cup up) into another kiln (in our case an electric) and heated to 1550 F and held at that temp for 2 hours.  This makes the shells stronger and burns out the rest of the stubborn and problematic carbon residue.

A closeup photo of one of my shells after the final kiln firing.  Can you see the crack that needs to be filled?

Something I will do next time to cut down on the shell cracking as a result of wax expansion in the burnout kiln.  I will go full blast with the burner right away, no babying up in temp, straight to red.  This flash melts the wax layer that is up against the shell before it has time to expand and cause a crack. 
 Using kiln cement to patch holes and cracks.
 Back in the kiln to be heated to 900F for the aluminum casting extravaganza.

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