I have PLA and PETG filament.
I hear that 3D filament absorbs water and causes problems when printing but after printing they can be used with water and they are water proof.
So my question is why is it different after printing/what has changed to make it now waterproof?
Is PETG waterproof or does it absorb water?
Is there a limit on how much water PETG can absorb or will it keep going until it splits and turns to mush?
The properties of the maternal are not what changes after you print. It is how you use the material. PLA and ABS prints aren't usually subject to +100 °C after printing.
When PLA absorbs moisture it becomes brittle. PLA filament will break when trying to print. Your PLA print may be thick enough so that brittle doesn't cause a problem. However, this can be a problem if you intend your PLA print to be flexible.
The main issue with PETG absorbing moisture is the water changing to steam in the extruder. PETG becomes soft and will distort with force applied at +100 °C; so generally moisture isn't a issue with PETG prints, due to application.
PLA and PETG absorb moisture, but do not dissolve in water. Usually what a person means by waterproofing is water won't seep through the layers of the material and leak out of a container. Note: most glass will absorb small amounts of moisture, but are still waterproof.
If one wants a hermetic seal this is a similar issue. One person printing hermetic prints said thicker layers make it easier to be hermetic. Thicker layers also tend to make the printer stronger, but can also give less detail in the print.
Filament that absorbs water prior to printing is subject to boiling temperatures as it passes through the heater block. In extreme cases, steam will be visible and a spitting sound will be heard. The filament will expand as the water exits, causing multiple structural and printing problems.
Once printed, dry filament may absorb water from the atmosphere, but is unlikely to be subject to boiling temperatures.
Waterproofing as a general consideration usually means the ability to keep water out, which is possible if the model is sealed and some printing conditions will adhere each layer well enough to the previous one to provide floating-type waterproofing.
PETG is hydroscopic, which means it will absorb moisture from the air. When printed properly (layer adhesion), the model can be waterproof.
These terms are independent and should not be used interchangeably.
Let's distinguish first three things:
Now, hygroscopic behavior of filament has can create a huge hazzle when printing: Waterlogged filament creates lots of problems, like spazzing, steam, foaming and others. This can result in a watertight designed print being printed in a gappy fashion or result in lack of strength or many other results.
Watertightness of a shell - open or closed - is primarily a result of the design and well fused lines. In my experience, 2 shells can be watertight, 3 shells upwards is almost always watertight. The main problems are with tops, where I experienced that the minimum tops and bottom layers need to be 4, better even 5. But this is dependant on your print quality - and you might need more or less dependant on your settings.
Under stress, like in an RC boat hitting into waves, the inter-layer bonds are the ones that would break first, breaking the shell likewise. To strengthen both the shell and ensure water tightness, polymer lacquers like acrylic paint, guitar lacquer or even epoxy resin (2-component epoxy resin is a very common glue) have been used successfully to create much strengthened and polished surfaces in RC projects. But you don't need to go through these steps unless you really want to.
After printing, a hygroscopic filament still undergoes the same processes and might undergo strength shifts. Sintered-Nylon for example might become heavier and softer if waterlogged. Such material also can't be frozen, the contained water can destroy the print.
For a bath-plug, I just would print the item as a "solid" body from TPU - TPU is after all the opposite of Hygroscopic: it's Hydrophobic. The next best "easy" material is ABS (as you can seal the surface using a little acetone vapor and can withstand 80 °C water for some time without deforming), then PLA (it's barely hygroscopic, but does not like hot water), and likewise PETG will do for some time, possibly better with the heat. Unless you put it in a freezer, it will keep the water in the tub long enough to take a bath.
