The Virtual Foundry The Virtual Foundry Sintering Refractory Ballast - Al2O3 - (1kg)

THE VIRTUAL FOUNDRY AL203 SINTERING REFRACTORY BALLAST

PRINT, DEBIND, AND SINTER AT HOME, IN THE LAB, OR IN YOUR MAKERSPACE

Al203 (Aluminum Oxide) is made for use as refractory ballast when firing Copper and Bronze Virtual Foundry Filamet in an open environment kiln.

WHAT DO I NEED TO DEBIND AND SINTER COPPER AND BRONZE 3D PRINTED PARTS?

• Kiln
• Heat resistant gloves
• Crucible
• Al₂O₃ and Sintering Carbon for Bronze and Copper Filamet

Note: Filamet debinds with only heat so there is no extra debinding equipment needed.

Sintering can be done in any kiln that can reach and hold the sintering temperature.

WHAT IS THE SINTERING PROCESS LIKE?

Two things need to happen to be successful while sintering:

The printed part shape will need to be supported, and oxygen will need to be prevented from reaching the part.

Supporting the shape of your printed part is solved by using a refractory ballast: Al2O3 (for copper and bronze) or Steel Blend (for the steels).

The oxygen exposure part is solved with Sintering Carbon.

If you have the oxygen problem solved with shielding gas and/or vacuum, you can ignore any reference to Sintering Carbon in the instructions.

You will still need to manage part shape support, however, the crucible and Al2O3 or Steel Blend will still be needed.

Simply bury your part in the refractory ballast in a crucible. Then, add Sintering Carbon to the top. Place the crucible in your kiln, set the time and temperature program, press go and you’re all set.

WHAT KILN OR FURNACE SHOULD I USE FOR DEBINDING AND SINTERING?

The Virtual Foundry Filamet can be Dedbinded and Sintered in any kiln so long as the kiln is programmable and can hold the sinter temperature of your desired material for the required duration.

The same kiln you use for sintering will be used for debinding. The Virtual Foundry Filamet debinds using only heat, so this process will happen on the front end of the heat cycle at lower temperatures. No special equipment or chemicals are required for debinding.

Kilns are available for sintering The Virtual Foundry Copper, Bronze, and Steel Filamet

HOW MUCH DO SINTERED PARTS SHRINK?

You have some control over how much green parts will shrink during the Debind and Sinter process. You can shrink the parts 7% and get about 80-85% density. If you add enough heat and time for the parts to shrink by 20%, your density will be in the low 90’s. The less they shrink, the less the shape will change. The X and Y axes shrink fairly uniformly. The Z axis will have slightly more shrink due to gravity. Circles and holes keep their shape well.

A good rule of thumb when starting is out is 5% isostatic shrink for Copper and Bronze, 10% isostatic shrink for the steels. 

INSTRUCTIONS FOR DEBINDING AND SINTERING COPPER AND BRONZE FILAMET

Items Needed: 

• Kiln / Sintering Furnace
• Refractory Container (Crucible)
• Sintering Refractory Ballast: AI₂O₃ & Talc
• Sintering Carbon

Pack:

BC1: Place AI₂O₃ refractory in the crucible
BC2: Bury the print in the AI₂O₃, centered in the crucible
BC3: Tamp down and pat the sides of the crucible
BC4: Part should be surrounded by refractory
BC5: Keep at least 15mm between the part and the crucible walls and top of refractory
BC6: Put the crucible in the kiln

Debind:

BC7: Ramp furnace at a rate of 55.6°C (100°F) per hour to 482°C (900°F)
BC8: Hold at 482°C (900°F) for 4 hours*
BC9: Let furnace cool to room temperature

Apply Sintering Carbon:

BC10: Turn off the kiln and unplug it to cut the power
BC11: Remove the part and refractory from the crucible
BC12: Place Talc refractory in the now empty crucible
BC13: Bury the part in the Talc, centered in the crucible, leaving at least 25mm empty at the top of the crucible
BC14: Tamp down and pat the sides of the crucible
BC15: Part should be surrounded by refractory
BC16: Keep at least 15mm between the part and the crucible walls and top of refractory
BC17: Fill up the whole 25mm of space on the top with the Sintering Carbon
BC18: If possible, place a cover over the crucible – Don’t seal it (the cover can be tool wrap, ceramic or kiln paper. It is used to preserve Sintering Carbon.)
BC19: Put the crucible back in the kiln

Sinter:

BC20: Ramp furnace at a rate of 111.1°C (200°F) per hour to the Sinter Temp (chart below)
BC21: Hold at the Sinter Temp for 5 hours

Cool Down:

BC22: Program ends – let furnace cool to room temp from Sinter Temp

*Hold times listed are ideal for a part that is less than a 50mm cube. Hold longer for larger or very thick parts and/or larger crucibles.
Experiment with hold times if the parts are not sintered correctly.

GENERAL SETTINGS AND CRUCIBLE COMPATIBILITY CHART:

	Crucible Type
	Alumina	Graphite	Stainless Steel	Refractory	Debind Temp	Ramp Time	Sub Sinter Temp	Sinter Temp
Bronze	Recommended	Good	Good	Al2O3 & Talc Sintering Carbon	482°C (900°F)	7.635 Hours (Step BC20)	–	885°C (1625°F) (Step BC20)
Copper	Recommended	Good	Good	Al2O3 & Talc Sintering Carbon	482°C (900°F)	9.46 Hours (Step BC20)	–	1052°C (1925°F) (Step BC20)
316L	Recommended	Good	Not Recommended	Steel Blend Sintering Carbon	427°C (800°F)		593°C (1100°F) (Step S14)	1260°C (2300°F) (Step S15)
Inconel	Recommended	Good	Not Recommended	Steel Blend Sintering Carbon	427°C (800°F)		593°C (1100°F) (Step S14)	1260°C (2300°F) (Step S15)
17-4	Recommended	Good	Not Recommended	Steel Blend Sintering Carbon	427°C (800°F)		593°C (1100°F) (Step S14)	1232°C (2250°F) (Step S15)

SANDING AND POLISHING YOUR GREEN PRINT

Manipulating prints: When exposed to heat, Virtual Foundry metal filament becomes clay-like. It can be carved, and re-sculpted, pieces can be added and seams smoothed. Soldering irons or wood-burning tools work well for this. For best results, use a tip that won’t be used for soldering/wood-burning and set the temperature to 200-235°C (392-455°F).

How to avoid melting when sanding metal filament: Important! Constant movement to different areas of the part is necessary when sanding to avoid unintentional melting. Experimenting is worthwhile.

Needle file: To make print lines vanish, sand the surface even. The loose particles from sanding are smashed into the print line gaps with the heat from the friction, fixing them in place. This step is complete once the entire print’s surface is smooth and even.

Sandpaper or 3M Radial Disc: Start with 120 grit sandpaper or 80 grit 3M Radial Disc, and go over every part of the print. The matte surface will become shiny as finer grits are used. Complete the entire surface of the print before moving to the next grit. The Virtual Foundry recommends using 4 grits with 3M and 6 or 7 grits with sandpaper. A nice shine can be achieved with less, but the mirror shine comes closer to the 7, ending around 3000 grit. After sanding, rub the print down with some flannel or a sunshine cloth to clean off loose particles. A mirror shine should be evident at this phase, even before the last step.

Sewn Buffing Wheel and Zam: Place sewn buff on a rotary tool, then liberally apply zam to the buff and to your print. The print will melt if it gets too hot, so it is critical to keep the buffer moving and continue to apply zam liberally. It may be useful to practice this step on a simple print or a “failed print.”

If you will be sintering your print: Polishing before sintering is not necessary. Post-sinter, the print will behave as the metal it’s made of – file it, weld it, polish it.