What does it take for melted plastic to stick to another plastic?

Hello,

When a new layer is put on top of the lower layer, what makes it to stick?

1) Does the lower layer need to be still in melt state, or at least semi-melt (soft?) state?
2) Or, does it only need to be warm.. warm enough?
3) Or, it doesn't even matter that as long as the new layer is placed with a melted plastic, it will bond to the lower layer, like its own heat will melt the surface of the lower layer just enough to make a strong bond?

I think #3 is very unlikely... well, it will be cool if it was, since if it was true I can stop printing in the middle, and continue from where I left off a day after.Also, it will make printing complex shapes easier. We may even be able to increase the structural strength, by having different layer direction in one object...

How about #2? If that's the case... as long as I can warm up the already printed object enough (using a UV ray?), then I may be able to do the things I said in my wish list above, right?

If it's case #1... well, that's not only disappointing, but it also means the size of the printed object will not only constrained by the size of the platform and length of the rods, but also speed of the printing - when printing a large layer, the area which was printed first may have cooled off/hardened by the time the extruder finally made its way back to print a new layer on top of it after finishing the rest of the lower layer...

IMHO, I think the less difference in temperature, the stronger the bond is.

If you compare it to welding metal, we don't need to heat the parts to get a strong link. But well, we aren't actually welding here. You've got a pretty good question.

You may be able to experiment it by yourself. Make a thin wall and modify the gcode to add a cooling time between each layer. I'm sure you will see difference between cooling times.

It depends on the plastic. PLA doesn't seem to care if the previous layer is still hot, ABS does. That's one reason why people generally use a fan with PLA to cool it quickly whereas they try to eliminate airflow with ABS (even so far as to build a chamber around their machine).