How much weaker does PETG become when it absorbs as much moisture as it can hold?

Question

How much weaker does a PETG part become when it absorbs as much moisture as it can hold?

Answer 1

To my knowledge, there is no reason to expect a printed PETG object to become weaker by virtue of absorbing moisture. Unless you keep it in a controlled environment with desiccant or unless it's solid (100% infill) and so thick that it's hard for moisture to migrate deep into the interior, the printed object will rapidly reach an equilibrium with a certain moisture content. It's not something that happens slowly over a long time of exposure to excess moisture. Even an air conditioned indoor environment with 45-50% relative humidity has plenty for it to absorb.

Where moisture affects the part strength is when it's present in the filament you're trying to print. This is because:

1. Water has both an incredibly high specific heat and an even more extreme phase transition energy for the transition from liquid to vapor. This means a lot of your hotend's heat will be spent boiling off the water rather than getting the plastic sufficiently melted to flow well and bond to itself. This can largely be compensated for by increasing the temperature 15-30 °C. However...

2. When the water trapped in the filament boils, it produces bubbles. These either remain inside the extruded lines, making them less than solid, or they burst out, making holes in the extrusion. And while more temperature makes the melting issue above mostly go away, more temperature increases the violence of the bubbling and the corresponding damage to the print's integrity.

Neither of these is a concern with moisture absorbed into the part after printing.