I've been thinking about this a bit, and I think you're likely to see some behavior differences between different polymers. Because FFF 3D printing deposition doesn't remelt the previous layer for a true fusion weld, to achieve full bulk property performance at the interface I think we have to rely on either
crystal growth effects or
diffusion bonding for adhesion between adjacent strands/layers. Simple van der waals type adhesion such as you see on a glass build plate isn't as strong as interlayer bonding, so I think there has to be something mechanically connecting the layers together.
- With semi-crystalline plastics, adjacent polymer chains have significant tendency to self-order into micro-crystal domains as they cool, and it stands to reason that the previous layer's exposed surface has crystalline domains which nucleate growth of those zones into the cooling fresh extrusion. If the crystalline zones grow across the interface, you should get very high mechanical performance at the interface equivalent to bulk properties. PLA and nylon both have this kind of semi-crystalline behavior. (As do most polyethylene and polypropylene variants).
- For amorphous plastics like PETG and ABS, there's no crystalline domains to speak of. Thus, diffusion bonding should dominate, where you get bonding because free ends of the molten polymer molecules are permeating into / tangling with the warm polymer molecules on the other side of the interface. Diffusion bonding is a "time at temp" process where the speed depends on temp (and the strength of the bond can be theoretically limited by rapid cooling).
Then, we also know that warping stress puts shear force on the interlayer bond and weakens it by "using up" some of the bond strength. So it makes a lot of sense that a low-warp, semi-crystalline plastic like PLA will have full bulk properties at the interlayer interface, subject only to interstitial void strength loss.
My question is, does the diffusion bonding of ABS and PETG happen fast enough to reliably reach full bulk strength? PETG tends to have high interlayer bond strength, while ABS is well-known for cracking at layer lines, but does ABS actually develop full bond strength and lose capacity due to interlayer warping shear stresses, or does ABS not develop full bond strength if it doesn't have enough hot contact time to get a good diffusion bond? The same environmental variables are going to affect both of these effects, e.g. hotter chambers or hotter nozzle temp have more diffusion time and also put more heat into lower layers so less warping stress. Can we separate these effects?