The extrusion multiplier

There has been some discussion on the Folger thread at http://forums.reprap.org about setting the extrusion multiplier. I've noticed in the past that my printer tends to over-extrude. You see it especially on holes which often print smaller than they should be, and on mechanical parts such as gears. Recently I've been trying to print this pieces for http://www.thingiverse.com/thing:1249221 and it's very apparent.

I did an experiment to get some data on this. When I was still fairly new to 3D printing, I was advised to set the extrusion multiplier to less than 1.0 in cases like this, but I've not previously collected any data on it.

I have already calibrated the E-steps for my extruder, so that if I manually extrude 100mm of filament, I really get that much to within 1 mm. To conduct the test, I created a 10mm cube and then extruded it in vase mode. Vase mode guarantees that there is a single strand of filament, so by measuring the thickness of the (vertical) walls you can see how much filament is being extruded. I did this for four filaments, at extrusion multipliers of 1.0, 0.9 and 0.8 using a 0.4mm nozzle. I also measured the diameter of the filament, by picking three places and at each measuring the diameter twice, with the measurements at right angles to each other. The diameter measurement are a little imprecise, while the wall measurements are probably more accurate. I use a E3D hot end, printing onto blue tape.

The results look like this:

Filament               Wall thickness (mm)          Ave filament diameter
                       x1.0    x0.9    x0.8         and range (mm)
MakerGeeks PETG        0.49    0.44    0.38         1.74 (1.71 to 1.78)
JustPLA PLA            0.50    0.46    0.38         1.78 (1.76 to 1.81)
Colorfabb PLA/PHA      0.48    0.43    0.39         1.75 (1.72 to 1.78)
Meltink3d PLA          0.44    0.42    0.36         1.69 (1.63 to 1.75)

It isn't clear to me why the value are so much larger than 0.40. The numbers looked so close to 0.5 that I even wondered whether I have the right nozzle. However, I use a E3D and the markings on it (dimples in the sides) indicate it is 0.4mm. Another hypothesis is that the weight of the next layer causes each one to squash out a bit; I can't really test this well, but trying to measure just the top layer suggests it isn't the case.

Whatever the reason, I'm going to try setting the multiplier to somewhere around 0.85 for a while and see if I like the results. It may be make PETG printing a little tricky. Reducing the multiplier makes the first layer stick less well, and with this particular brand of PETG (and maybe others, I don't know), that's already a problem. Making the first layer a bit thicker should help.