Printed box leaks

[curious aardvark]

the thing with simplify 3d that is confusing is that 100% infill DOES NOT MAKE A SOLID PRINT.
Logically it should - but then logically, when creating a bridged layer on a cylinder, it should not start in the centre in empty space, but at the edges and build out - and it always starts in the centre - so logic and simplify3d's slicing algorithms rarely go together..

To get a completely solid and water tight print from simplify3d, you have to select the option that creates a solid layer of infill for every layer.
I'll post a pic when I get back on the workshop computer.
But it's at the bottom of the 'infill' option page.

And you can select to print a 'solid' layer every 5 or ten or 20 etc leyers or for every single layer.
I've made screw on 'bases' for bottles to provide drinking water for birds that can only be watertight with the solid layer option on for every layer.

I also use it to mnake standard prints stronger by printing a solid layer every 10 or 20 layers.

It does use alot more material - but it does make things properly watertight :-)

Also to help make a square box watertight you want to remove a rounded cube from the interior (minkowski) rather than a standard sharp angled cube. This will give you more material at the areas where leaks are more likely to occue: at the junctions of horizontal and vertical areas. By rounded all the sharp edges with a minkowski operation - you increase the material at these junctions and that will help enormously with the watertightness.

Umm - just realised I'm assuming you are using openscad - if you are not, then the above 'minkowski' blather won't make any sense at all :-)

But the rounded edges is still the way to go - no matter how your cad program generates them :-)