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I've been thinking about alternatives to support materials for FDM printers, such as multi axis printers, dual head with soluble support material or printing in a gel/liquid which traps the material in place. Then it occurred to me that it might be possible to use the surface tension of a liquid to act as support material, same way small objects an insect can float on water even though they are heavier than water. This would also work great for cooling the extruded material. NOTE, we are still using a base plate like any other FDM printer, the only difference is that we have a water surface on the build plate which act as support material for large overhangs/steep angles. Everything is still attached to the build plate itself. We are not just printing stand alone parts on the surface of the water.

After some highly scientific testing (Dropping an extruded string of PLA and placing a thin printed PLA part on the top of water) I've concluded that it is very feasible to float PLA on the surface tension of water. See picture for refence: PLA floating on surface tension

As you can see the floating structure is not at all trivial, it has both smooth round shapes and many sharp corners and holes in it, it was still extremely simple to get floating (I could drop it in from a height of 5 cm and it would still not break the surface tension). I also tried my best to investigate the effects of having structures underneath which might mess up the surface tension by placing larger PLA parts under the floating ring and trying to sink the ring by touching the underside and corners without any success at sinking the floating part. The only way I could get this part to sink was to push it down under water. Even if the surface tension was only broken half way along the ring the ring would still float. If you are thinking that the ring acted as a boat, that is not the case. I made sure to fill the small indents with water so that there were no pockets of air keeping it afloat.

An even more encouraging result was another ring which was much thicker than it was wide. It still managed to float from the surface tension on the top side of the structure while the rest of it was submerged, accurately mimicking how the structure would actually be in the water during printing. See picture: Floating PLA ring thick

While this ring much easier to sink it was very resistant to being moved around. Again, basically the only way to sink it was to push it down under water, though as soon as the surface tension was broken on one side the whole ring quickly sank.

All this leads me to my question: Does anyone know of a 3D printer which uses the surface tension of water as support material?

I've searched around some on the web and I have not really found anything at all on the matter. I can foresee many potential problems (such as the extruder moving unsupported lines to the side when changing direction, layer adhesion, enclosed areas not filling with water etc) but the potential to print without support and only have to worry about keeping the newly extruded material in place could open up a lot of potential.

UPDATE: I also tried 60 degree Celsius water (In case you need hot water to help with layer adhesion) and the surface tension still was able to float the thicker ring piece, though it felt like the surface tension was weaker.

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    $\begingroup$ While I appreciate your enthusiasm, what's the gain by printing it on water? As you pointed to, your piece would most likely move during printing, which negates the entire thing. The purpose of the base is, well, to provide a base. It's a place where you have solidity and stability. Without it, you won't have precision. You won't have anything but a jumbled lump of plastic. Your idea is neat in and of itself, but what is the end gain for doing this? $\endgroup$
    – Pᴀᴜʟsᴛᴇʀ2
    Dec 22, 2019 at 15:05
  • $\begingroup$ @Pᴀᴜʟsᴛᴇʀ2 Oh, you misunderstand, I never said we are printing on the water alone. We are still using a base like a normal printer, only difference is that we use the water surface as support material so that we can print extremely steep angles (90 Deg angles). Your print is still attached to the base plate of the printer. I added a small description of this in the post in case it was too unclear. $\endgroup$
    – Beacon of Wierd
    Dec 22, 2019 at 20:49
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    $\begingroup$ Using surface tension sounds interesting, but I don't think it is even necessary. The buoyant forces and rapid cooling which are a result of printing on liquid might already be sufficient to help bridging and other types of overhangs. $\endgroup$
    – Ezra
    Aug 8, 2020 at 17:13

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It's not feasible as described with normal FDM technology.

FDM bases on depositing material in a single path. This needs the deposited material to stay at the same XY coordinates for subsequent paths. And exactly here is where a floating piece fails: a free-floating piece is by its very definition unrestricted in XY, and would move to follow the nozzle.

There are is also a whole plethora of factors that make this idea not feasible with the standard technology, meaning you'll have to develop the whole process, not just recombine two ideas. This means, you need to solve the following issues:

Heatsink Water

The extruded plastic needs to stay close to the melting point for some time, so it can fuse and bond with the lines next to them. However, water is known to be a very good method to get the heat away from items, as each liter of water can take about 4.1 MJ and only heat by one Kelvin. PLA on the other hand only stores about 1.8 MJ per Kilogram and Kelvin.

As a back of the envelope calculation, the temperature differential between room temperature and printing temperature PLA is about 180 Kelvin. Each gram of PLA is equivalent to 319.8 mm of filament (assuming a density of 1.3 g/cm³) or an extruded line of 9.6 kilometer length of 0.4 mm width and 0.2 mm height! That one gram contains about 324 Joules of energy that will be dispersed to the room temperature as it cools down. The water vat would not even get measurably warmer from sucking those few joules from a whole print!

While this could be, in the right setup, be used to rapidly cool the print and solidify it in shape, the result of the rapid healing will most likely also impact print quality negatively, as cross-layer bonding is reduced.

Separator water

It is a well-known trick in creating polymer fibers to extrude underwater, as the water not only cools (see above) but also acts as a separator between the fibers, for the very short timeframe they are still malleable. This would also strike when printing into the water - there'll be a water layer in between the deposited filament, which would need to either get pressed out or cooked out before any cross-extrusion bonding can occur. As a result, just extruding into the water should result in a print that has almost no sturdiness, and might fall apart on touch.

Floating

Water has a density of 1 g/cm³. PLA has a density of 1.3 g/cm³. So a solid chunk of PLA sinks. But we don't print solid, we include air. Not just a few percent but infill is usually below 20 %. I have just printed a cube. After smoothing the surface by sanding, the cube is 29,7x29.9x29.9 mm. It has 3 parameters, 20% infill, 5 top and bottom layers with 0.2 mm layer height and comes to 11 grams. 11 g/26.55 cm³=0.41 g/cm³. Or in other words: the cube would float, about 40 % under the water surface, 60 % above the surface. The print would be, as a result, quickly break the water surface and get no support from the water at all.

Submerging the bed?

The main issue of a free-floating object (position) might be mitigated if there was a bed that would be submerged, but one would open a new can of worms, that might be even worse: the volume of the print and the accuracy of pumps.

As the print goes on - quality be dammed - the print grows in volume. However, it doesn't grow entirely linear, depending on two factors:

  • Is the print happening on/below the water surface? Then we will include water in the print. The volumetric growth of the print is in this case just the deposited filament. You'll need an overflow to compensate for the print growing and keep the water level in position.
  • Is the print happening just above the surface of the water? then the displaced volume grows according to the depth of the print in the water. This would mitigate most of the problems from water preventing cross-bonding and causing floating, and even use the heatsink properties more beneficial, as any filament starting to sag will be stopped. It also would prevent water from being encased in the print. However, it does not use surface tension. Also, you'll need an overflow system to keep the water level steady.
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  • $\begingroup$ Did you even read the question before you posted this? I explicitly stated that the print will be attached to the bed. Poor bonding could be an issue, though you're calculating the wrong thing, we care about cooling times. Assuming a layer height of 0.2mm, half that for heat transfer, 210 C printing temperature and 60 C water it takes 77ms for the extruded PLA to reach 180 C. Floating isn't an issue since the print is stuck to the bed. I also asked if someone has done this, I know basic physics, I don't know what people have done, hence why I asked if someone had tried this. $\endgroup$
    – Beacon of Wierd
    Sep 9, 2020 at 23:32
  • $\begingroup$ @BeaconofWierd The surface tension simply is not supportive as stated. It's the rapid cooling that would make support unnecessary but result in really crap print. The photos you show are not surface tension, they are floating. That's normal floating. As in their density is below that of water. $\endgroup$
    – Trish
    Sep 10, 2020 at 8:59
  • $\begingroup$ @BeaconofWierd added other problems and adressed the submerging bed - but as described it is still not feasible with the normal technology - you need to solve so many issues $\endgroup$
    – Trish
    Sep 10, 2020 at 9:28
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I just had the same idea and googled it. My idea was to submerge the whole print platform in the water and lower it down so that the water level and the current layer line are always at the same height. Maybe a pump would be required to counteract the plastic displacing water. Completely floating parts of the print could have a tower with three contact points underneath to keep them in place without having to lift the whole structure.

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  • $\begingroup$ Though I appreciate your enthusiasm, this should probably have been a comment to the question if someone has done this rather than an answer. But yes, even with the bed going into the water you would still need some kind of pump or sufficiently large body of water so the displaced water from the extruded plastic does not change the water level significantly. $\endgroup$
    – Beacon of Wierd
    Aug 9, 2020 at 21:11
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This is exactly how the $100 Peachy Printer was supposed to work. Unfortunately, fraud sunk the Kickstarter campaign and no-one got their printer.

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  • $\begingroup$ Not really, it's a completely different technology. Peachy Printer used a laser to cure a resin, I'm talking about "regular" FDM printing on top of a liquid instead of using support material. $\endgroup$
    – Beacon of Wierd
    Aug 10, 2020 at 22:20
  • $\begingroup$ but it did use - allegedly - use water to support the resin structure $\endgroup$
    – scruss
    Aug 10, 2020 at 22:25
  • $\begingroup$ Oh, ok, so it floated the resin on top of the water? Then I guess it's similar, though it seems dubious since all the UV resin I can find has a density above water and would sink :S But I guess they could have used one which is lower density. $\endgroup$
    – Beacon of Wierd
    Aug 10, 2020 at 22:51
  • $\begingroup$ ISTR they used brine, which is considerably more dense $\endgroup$
    – scruss
    Aug 10, 2020 at 22:52

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