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I created a re-usable 3D printed mould to cast little plant pots from concrete. Here is what went well and what went wrong [Thread]
![I created a re-usable 3D printed mould to cast little plant pots from concrete. Here is what went well and what went wrong [Thread]](https://pbs.twimg.com/media/EfeUC4GWsAA0hh8.jpg "I created a re-usable 3D printed mould to cast little plant pots from concrete. Here is what went well and what went wrong [Thread]")
At first I designed it as a 2 part mould in @adskFusion360
You can see more about the initial design here: https://twitter.com/timonsku/status/1287443895857577984
You can see more about the initial design here: https://twitter.com/timonsku/status/1287443895857577984
The idea was simple, have a big hole on the bottom to pour the concrete in and a few smaller ones to let the air out. Uneven spots on the bottom wouldn't visible and its easy to sand. That was at least the idea.
Reality looked different though. It has been years or even decades since the last time I worked with concrete. I forgot how thick this stuff is and pouring it into my small hole was not gonna work.
I came up with a quick plan B before the concrete hardened further and just covered the holes and poured everything into the bottom mould and squeezed the top half into the soup until it squished over at the top. This worked surprisingly ok.
When I opened it up the next day it looked great! The top mould came out quite well (though the threaded inserts chose to get stuck in the top part and I pulled those out of the bottom mould in the process. They were a bad choice for the forces at play in this mould process.
But I was not able to remove the pot from the bottom mould in any way. I ended up tapping it from all side and tried to push it out through the holes on the bottom but no luck, I ended up blasting a hole through it. I only managed to get it out after completely destroying it.
I used a bit too much water and the broken concrete still looked kinda wet so I thought the issue was just that it didn't harden enough and had too much water content and otherwise I could have pushed it out without damage so I tried again with a better mixture.
Again it was super hard to get it out. This time I tried putting it into the fridge in the hope that it shrinks the concrete and removes tension. (looks like sourdough with those bubbles)
But well, it was no use and it ended up getting destroyed again. It was really quite fascinating just how strong the tension was. I hammered everything to pieces but it was still completely stuck until I forcibly removed the last bit that was forming a ring in the mould.
So there were was clearly an insane amount of tension going on that I could not get rid of so I decided to redo the bottom and split it in two halfs.
The top part had I to fix after the first pour because the layer adhesion was not good enough for the huge forces also completely broke the second time around and I had to reprint it with a stronger handle (pictured the old epoxy bodged part and then the new parts)
Yay success I could remove the outer shell easily!
but whomp whomp, I could not get the top part out any more which was fairly easy previously. I felt betrayed at that point. I decided to heat up the part and cut a relief cut into the side to remove the tensions from the top mould. That process caused a crack but I got it out.
So after a little fix-up with resin this was a usable part as it was just one large clean break but it was far from great process wise. I want a re-usable mould after all.
It did look good though.
It did look good though.
This time I added a bunch of copper wires harvested from old audio cables to re-inforce the concrete.
I was able to get the mould out this time but still made a hole in the part but at least stayed in tact and the hole was held together by the copper wires, it was a again a resin fix up though.
Part looked great again, the best one so far.
I then cut the small relief cut I made way deeper and also added it to the other side. I taped the cuts to avoid the concrete oozing inwards which worked out just fine.
But once again it broke when I pried on the relief cuts to get the mould out :|
Though this time I also bodged the mixture once again and it looks a lot more brittle than the previous two casts.
Though this time I also bodged the mixture once again and it looks a lot more brittle than the previous two casts.
But again, this shouldn't be so hard and a not so perfect mixture shouldn't be destroyed just because the mould is too stuck but I feel this is on the right path and that I will also have to redesign the top part as well with some form of settable relief mechanism.
I'm currently think of splitting the top part as well, if at all possible the same way as the top/outer part but I think there is not enough space to tighten a screw so I might have to think of a different solution there.
I kinda glossed over the finishing of the 3D prints. I wrote in more detail bit about that here but tldr; spray with filler, sand, spray with car clear coat, results in ultra smooth surface https://twitter.com/timonsku/status/1288593476477648901
Oh and I also forgot, I oiled the moulds up really well each time. Definitely helps but not enough.
