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I am trying to print a shape (e.g. square container) using clear resin. However, I am facing the problem of the design sinking when I put it water. I am right now trying to make it hollow from the inside in some spots to reduce its density, but not sure how far can that take me. It is necessary for me that the print floats! enter image description here

Do you guys have any tips? like a different print mater? or a change in shape that will help the design float?

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  • $\begingroup$ My understanding of resin is that it's less suitable for this than FDM because it doesn't admit leaving air gaps in the interior. You probably need to make it hollow with a hole for a plug. $\endgroup$ Apr 4 at 15:55
  • $\begingroup$ @github Resin printing is generally better for water and gas-tight fittings than fdm, which must be painstakingly calibrated and sliced to not have little pinhole gaps that leak. Suggestion of leaving a drain and making/finding a plug is good. $\endgroup$ Apr 5 at 1:40
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    $\begingroup$ It appears your design has holes in the side near the bottom. It might be that these holes are letting water in and that's causing it to float. Simply tape over the holes to see if it will float, and if so, reprint without the holes. After all, boats float. At least, until they get holes in the hull... $\endgroup$
    – FreeMan
    Apr 5 at 17:38
  • $\begingroup$ @ChinchillaWafers: That was traditionally an issue but is becoming less and less so because of better hardware and software. Particularly, the advent of geared extruders with serious grip on the filament, rather than the ungeared flat hob stuff like a stock Ender 3, has made it practical to actually push out very close to the exact amount of material requested. And firmware advances (linear advance/pressure advance) as well as slicer advances (not leaving stupid gaps) have made it possible to actually get that material put in the right places. $\endgroup$ Apr 5 at 22:32

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The material doesn't matter*

Floating designs are exactly that: designs. If the design is made to float, and your printer can create a watertight shell (which is a different problem than the material), then you can use any material. Well, almost any, because:

Only one caveat remains

The only type of material that isn't suitable is any material that will dissolve in the fluid it will swim in. Like PVA in water.

Otherwise, the material only matters for other properties, but not for "it can float".

How to design a floating item?

When does a design float? Well, the rule described by Archimedes is often described as "An item floats if when the volume of water it displaces is lighter than the item." In other words: if its density is lower than water.

However, it's not that simple after the first look: What is the volume and weight of the boat? After all, we can make steel, which weighs upwards of 3 tons per cubic meter float while water is only 1 ton a cubic meter. What is the mystery part? It is simple: the underwater ship does not allow ingress.

Draft depth

So, we want a solid wall from the lowest part to the line the item will sink into when it reaches equilibrium. That's the draft depth. For ships, there is a formula to quickly calculate the draft depth based on knowing how much water will be displaced:

  • Seal all the holes and gaps in the boat. Now measure the volume the item displaces when it is put onto the water. Call it $V$.
  • Take the surface area of the boat. Call it $A$.
  • The Draft $d$ is now $d=\frac V A$

In the alternative:

  • Make a volume item by removing the holes into the contained volume and filling those voids to the upper rim. Have your CAD tell you the contained Volume. This is $V_w$.
  • Weigh your printed item. This is $M_p$. We turn that into Displaced Volume $V_d$ by using water density: $V_d=M_p*1000\frac {\text {mm}^3}{\text g}$
  • Make a plane cut of our volume item, so that the lower part of the item has the same volume as the displaced volume. That is how much of the boat will be underwater.

The second method also works for tilted or very asymmetric shapes (like a bot listing), while the first is roughly over the thumb.

Now, how does that help with the design? Remember, we had sealed the slots earlier. If the slots are in any way below our draft depth, then water ingress happens through that slot - and the slot will have to be moved above the draft depth.

Dasign adjustments

There are several adjustments to get the item to have a lower draft, but all of them boil down to reducing the density.

In the case of your design, the hanging floor of the basin can have a lot of it cut away and turned into a grid of sorts, which would reduce the impact this part of the print has to the total density.

You could also move the slots up enough so that it will be above the waterline, even if filled to an expected degree.

If you really need to include voids, you will need to alter your printing code in between layers, because resin printers don't typically raise the print fuly out of the resin vat and have such hollows filled with resin as a result. And please try to avoid Cupping

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