Attempt 5: Puzzle part, brass, lost PLA casting

[jes]

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 :)

[jes]

Just need to wait a day or 2 for the plaster to dry.

[jes]

Last time I melted down some brass it produced lots of white powdery stuff and white smoke which I understand to be zinc oxide fumes.

So even if this doesn't work very well, it might be worth trying it again with aluminium in place of the brass.

[jes]

Note to self so I remember: 200g of plaster mix (before water) is way too much. Maybe try 60g each of plaster and charcoal powder.

[jes]

Running this temperature profile for this attempt:

1. Ramp 150 10m
2. Hold 1h15m
3. Ramp 400 3h20m
4. Ramp 1050 4h00m
5. Hold 4h00m
6. Heater off

[jes]

So this didn't go according to plan. The first 9 hours were fine, but when I went out to check on it after 10 hours, I found that the furnace was off and the lights in the workshop had also gone off. The breaker had tripped. I switched it back on and tried to carry on, and it tripped again quite quickly. I tried again and it switched off again, and I tried manually setting the duty cycle to 100% in case it was the switching on/off that was causing trouble, but it still tripped again.

It was at 1050 degrees when I checked on it after 9 hours, and had been on the "hold 1050 for 4 hours" step for 2 minutes. When I gave up on trying to run the furnace it was still at 1000 degrees, so I just let it cool down slowly in hopes that the brass had got molten enough to fill the mould.

The brass had got molten enough to fill the mould, so this "failure" has actually helped in that it has informed us that there is no need to keep it at 1050 degrees for 4 hours, just 1 would do. And probably even less than 1 hour. It only needs to stay hot for long enough for all of the brass to become molten, and then it can start cooling down again.

I opened the lid after a while and found that the inside of the lid was slightly yellow-stained and had some white powdery build-up on it. Presumably this is zinc oxide stuff. It has built up in interesting shapes, almost like little mushrooms growing.

After it cooled down to about 380 degrees I took it out of the furnace and chucked it in a bucket of water. Unfortunately, despite the brass becoming molten as intended, it was not a successful part.

The part had tipped over inside the flask before the plaster had set, and so when the brass flowed downhill, it only filled up half of the mould. That's annoying. The reason is that the brass is much heavier than the PLA so the part is very top-heavy while inside the wet plaster. Not sure on the best way to stop this from happening.

The part cleaned up quite nicely:

Although the surface colour is surprising. It is quite varied and looks coppery in some places and brassy in others.

I cut a corner off and polished it to see what the metal is like inside, and it is clearly solid brass all the way through, so the funny colours are only on the surface:

So once again this attempt was mixed success and failure. We've learnt:

• this method of casting does work and on balance is probably more promising than sintering
• we need to make sure the top-heavy part doesn't fall over inside the wet plaster: either prop it up with some steel wire, or put it in the flask upside down and then flip the flask before putting it in the furnace, or something else
• the furnace can't reliably heat above 1000 degrees without tripping the breaker (won't be a problem for aluminium)
• it doesn't need to stay hot for very long with the casting process, as long as the metal has all become liquid that's all that's required
• the surface detail is better at the "bottom" than at the "top", presumably because the pressure is greater at the bottom while the metal is liquid; we could put a sprue between the part and the brass to increaes the pressure throughout the part

I expect the next attempt to produce a usable part.

There was some shrinkage, which we'll need to account for in slicing:

Outside diameter (x/y): 38.5mm (-1.5%) Inside diameter (x/y): 25mm (-2%) Height (z): 21.5mm (-4.5%) (but this was hard to measure and might not be accurate)

The surface detail is not quite as good as it was in the best places from the sintering attempts. It would be interesting to try doing this using the bronze-filled filament instead of plain PLA. The brass might mix with the bronze and we might get to keep the surface detail and get the part filled solid. Something to think about.

[jes]

This is what the temperature cool-down looked like at the end with the power off, but note I opened the lid after about 28 minutes.

Also, when I first got to the furnace and found the power off, after restoring power it was reading 681 degrees, which probably means the power had been off for about half an hour, and this was about an hour after it first reached 1050, so it looks like it tripped after half an hour of holding 1050 degrees.

[Waynewaynehello]

I've also wondered about the promise of sintering vs. casting if the "sintering" process actually ends up melting some of the metal. Certainly cast metal has the potential to be, you know, not full of holes. It's an interesting method to try and put the PLA part just underneath the metal but generally I've seen people burn out the PLA, then add the metal, maybe because of the problems you've described.