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It would be good to come up with an easy way to melt down failed castings and produce new bar stock that can be used for new castings.

Basically just need to melt them down in the furnace then pour into a cylindrical mould, maybe a bit of steel tube with a cap welded over one end would work? The metal inside would shrink as it cools so would automatically separate from the steel.

This is 3 spherical sections nested together. Hopefully we'll get 3 inseparable aluminium parts.

Will make the plaster mix quite wet to try to eliminate trapped air in between the spheres. Also will only hold it at 700 degrees for about 5 minutes, instead of an hour.

Plan here is to use an M10 bolt model (oversized by 10% in x/y and 6% in z).

Improve on last time by:

1. Make a new flask that is nice and big
2. Give the plaster over 24 hours to dry
3. Embed a thermocouple inside the part so that we can measure the actual temperature inside
4. Let it air-cool instead of quenching it, and try to see if there is still powdered material inside, whether there are cracks in the plaster, etc.

So it occurred to me that if the material is going runny inside the furnace, and if we need to add a head of material to allow it to flow down into the part, why do we need to bother printing the part out of bronze PLA at all?

We could just print the part in PLA, glue it to a piece of aluminium, and then stick the whole lot in the furnace. PLA burns off, aluminium runs down into the cavity, and you've got a solid cast metal part with none of the hassle of pouring molten metal, and with the metal not getting exposed to oxygen.

So I've glued a piece of PLA to a piece of brass (I didn't have a convenient-shaped piece of Aluminium), and potted inside charcoal plaster like before, and then I'll run the "sintering" temperature profile, except probably with an extra hour or two at the end at a temperature hot enough to melt brass (maybe 1100 deg. C?).

The part is a piece from a puzzle I've designed, so don't look at it too hard if you think there's a chance you might want to solve it :)

The most efficient method I've found so far:

1. Buy a bag of barbecue charcoal
2. Buy a kitchen food blender
3. Chuck a little bit of charcoal in the blender (not too much or it overloads the motor -- or maybe my blender is just too weak)
4. Grind it in the blender for maybe 30 seconds
5. Empty the contents of the blender through a sieve into a big pot
6. Refill the blender with fresh charcoal
7. While blending the second load for 30 seconds or so, shake the sieve contents around to get the powder into the big pot
8. Tip the remaining sieve contents into a secondary pot
9. Tip the contents of the blender through the sieve into the big pot
10. Tip the contents of the secondary pot into the blender, and top up the blender with extra charcoal chunks
11. Repeat from step 7 until you have processed enough charcoal.

Will use the same profile as in #14, but with an extra couple of hours between 150 and 400.

The model is a finger ring with a 10mm spanner hole in it.

It's not inconceivable that we could produce cast versions of "print-in-place" parts by designing a sprue structure that holds the parts still without touching each other, then casting it, then cutting the sprue off.

The professional FDM-and-sinter printers include a "debind" step in between printing and sintering, where the binder is removed but the metal powder somehow retains its shape.

I soaked a bronze-PLA part in dichloromethane for an hour, and the DCM did an excellent job of dissolving out the PLA, but the bronze powder didn't retain its shape, so this isn't really looking viable.

Printed the 10mm socket model at 100% scale.

Initial mass: 5.3g
Final mass: 3.8g (-28%)

Looks good, but hollow at the top.

Pour the plaster, leave it to sit until it is solidified, then try this temperature profile:

1. Ramp 150 2h00m
2. Hold 3h00m
3. Ramp 400 4h00m
4. Ramp 700 2h00m
5. Hold 1h00m
6. Heater off

Idea here is that we have it spend a significantly longer time at lower temperatures, to dry out the plaster gently and stop it from cracking. And we don't need such a high max. temperature if we're only melting aluminium.

Same as in #9, but peaking at 865 instead of 700, so as to sinter the bronze from the print.

Just curious what will happen.

Ideal case is we get good surface detail from the bronze, and good fill in the centre from the aluminium.

So for this sinter I wanted to try something that would produce a "usable" part even if it was hollow at the top 50% or something. I made a M10 bolt with 17mm head, and sintered with the head at the bottom, with the idea that even if the top half of the thread is hollow, I can just cut that off and then I have a usable bolt that is only half as long.

It didn't work, but I did a few things differently this time.

1. I only gave the plaster about 12 hours to set before starting the heating
2. The "flask" was only half as tall as it needed to be, so the extra height was made up with gaffa tape, which was removed before heating
3. I left the lid of the furnace open below 300 degrees C, to allow gases to escape more easily

I noticed boiling/sizzling noises in the furnace for the first few hours, so I think there was still liquid water being boiled out of the plaster. I'm not sure if this created cracks or something which allowed oxygen inside the plaster.

The final part was 100% hollow, and even the walls weren't completely filled in. It broke into two parts, and was nowhere near usable.

Same part from #7 but printed 2% oversize in all axes, and using a little "sprue" model to try to increase the hydraulic pressure inside the mould.

Haven't yet decided how I'll keep the part from falling over inside the flask.

This is another try at #2, but instead of just using another socket for a head of material, I'm going to use a large solid cylinder.

Socket printed 10% oversize in x/y and 6% in z.

I printed the 10mm socket twice, and stuck them together with a soldering iron, with the idea that the "top" one is a head of extra material that the "bottom" one can draw from to get 100% solid.

I printed both sockets oversize by 10% in x/y plane and 6% in z.

I didn't measure the final mass of the 2 combined sockets. Initial mass was 13.4g of the 2 combined (6.7g each), and the final mass of the "good" socket was 6.3g. It still was like 50% hollow, however.

Possibly using a socket for header material doesn't work very well because it has a high surface-area-to-volume ratio, and the bronze seems to stick to the walls of the plaster. Perhaps try an inverted funnel shape next time?