Some time ago we were helping out the Biology Department at Walla Walla University get a printer dialed in for some work they wanted to do. In our work with them, they gave us these images they took with the help of Whitman College, using their scanning electron microscope and gave us permission to publish. It's been a while, but we ran onto those files again and thought the community might find them interesting.
All of the prints are done with the 4043D PLA we were selling at the time, and printed on a modified MakerBot Thing-O-Matic. The flat surfaces with the white lines, streaks, fuzz, or lightning-like patterns are where the plastic was parted with a razor blade to take the images of the infill.
This image was of particular interest, notice the twisting on the traces caused (presumably) by the changing directions of travel of the print-head and friction and bonding of the traces to prior layers. This was a grid / rectangular infill pattern at 45 degrees, cut on what we believe was the ZY plane.
Here is a closer view of the same piece. Here we get a much better view of the twisting in the traces, which is most pronounced near its interfaces with other traces. The lighting marks on the cut surface appear to be some combination of loose fibers of resin (seen better in a later photo) and embossed patterns we believe were created by the internal stresses of the traces formed during printing, cooling, and subsequent cutting.
In this next image we see the interface between the perimeter traces and infill traces. It is good to see that level of bonding between the traces toward the interior, but it looks like the outermost perimeter traces weren't very well bonded to the adjacent perimeter.
Here is another view of the infill that shows off the interface between opposing layers of the grid infill very well. Notice the twisting seen here as well as the distortion (smooshing) in the trace where it was laid down on the prior layer (oriented in on the perpendicular).
This last image was of the same design, printed with linear infill. For whatever reason we see less twisting in the traces, and less extreme deformation at the trace interfaces (which where there is less adherence when molten may actually mean a weaker part if there isn't as much of a mechanical bond from the deformation where the traces meet). The fibrous bits on the ends of the cut traces are just that - fibers of PLA resin smeared when the part was cut open. The fibers in this image are more numerous and larger than on other photos, possibly because of differences in the sharpness of the blade used, differences in printing conditions, or other uncontrolled factors.