For last two weeks I have been struggling with flow rate adjustments. I'm using an UM2, colorFabb XT 1.75mm and Slic3r. Before printing a test cube I made flow rate adjustments using the "Perimeter test" (printed a hallow cube with 1 layer thin perimeter line). I then compared the desired value of the perimeter's width with the value from the g-code and adjusted the flow rate. I repeated this procedure until I got exactly the thickness I needed. After that, I printed a 15x15x15 mm cube and observed under extrusion on top layer. Also, the dimensions of the cube in the x-y plane were smaller than expected (0.4mm shorter than they should be). I have done this a few times now, and still get the same result. :/ Furthermore, I was unable to adjust width by dividing the desired value of the width of the perimeter by a real value, I got slightly smaller extrusion multiplier (One I got by division is 0.9 , but 9.5 relates to desired width). Do you have some suggestion guys?
I'll try to explain the calibration method I have used.
- I have made in Solidwork 15x15x15 mm cube.
- The cube was imported into Slic3r, where I have set the infill to 0% and number of sides shells equal to 1 and number of top and bottom shells equal to 0 (It gave me one layer thin wall in shape of cube).
- I printed it out and measured the width using caliper and compared with the perimeter's width from g-code.
- I have got something about 0.7, but the extrusion width in g-code is 0.65, so I changed extrusion multiplier by 0.65/0.7 = 0.93 (what is not actually true, because I got correct value on 0.95 only)
- I have repeated the procedure for many times, and always got wrong dimensions and under extrusion. There is the video about method I just have explained: https://www.youtube.com/watch?v=cnjE5udkNEA
It looks like you overtune it. I think it's because top layer of hollow object doesn't have any support (which is obvious) but because of that the filament is not oblate that's why it looks like underextruded.
If you really need hollow object then set number of top layers to 3..5 then check if it helps. I know it's not really what you expect but IMO this could help here. (I usually set 2 bottom layers and 3 top and it's quite enought.) You can also experiment with speed of top leyer. I would say the faster the better in this issue as the the first top layer should be as flat as possible then next top layer will have better support. And of course cooling should be set to max (as for the bridges).
Here is an explanation why you get underextrusion. enter image description here">
A is wall (perimeter) B is (ceiling) top layer.
Empty outlines are what you expect but filled shapes are what you really get. Differences are of course slightly smaller but it's to clearly explain the issue
Please also check Printer Settings > Advanced > Top solid infill.