Having problems printing a smooth bottom layer on my Makergear M2. This afternoon I ran a series of trials to see if I could get a better handle on first layer issues. Creator seems to treat Solid Layers (SL) differently that Fill Layers (FL). I was printing with 1.75 mm ABS, 0.25 mm layer height, a simple rectangle plate 3D part 45X45 mm. Extrusion multiplier was set to 1.0 with Auto Extrusion Width selected.
I ran the part with a single layer skirt, 2 layer outside wall and 2 bottom solid layers. I would stop the print when the first layer completed so I could use my calipers to measure skirt height, outer wall height and SL fill height. I observed the printing activity through a 1.75 power magnification lamp. The skirts and outside walls print nice but the first SL was another story. The nozzle's proximity to the print surface (kapton tape layer) seems to force material outward excessively which caused the formation of little ridges between the beads. The skirt and outside walls were measuring around 0.21-0.23 mm height which seems right for a print layer height setting of 0.25mm. The first SL, however, measured a height anywhere from 0.45-0.52 mm. The height is caused by the ridges.
This phenomenon is the reason I was hearing/feeling the nozzle rumble over the first layer when printing the second layer and then even bumping on infill wall perimeters at higher layer counts. In short, the 0.5 mm solid first layer height (height measured at the peaks of the ridges) introduces and error that effects all subsequent layer. The nozzle is 0.25 mm closer to the print surface for each successive layer than the controller believes it to be.
I made additional trials to explore this problem further. I set Bottom Solid Layers to zero and the Infill Percentage to 60% (I actually started at 90% and stepped it down in 5% increments). With these print settings I get a 0.23-0.25 mm skirt, outer wall and first layer infill height. The first layer bead edges are fused together completely, no separation which I would consider solid. Its actually pretty impressive that the machine can print something this thin, this consistently. I then ran another trial and let the G-code continue executing until half-way through the second layer. Skirt and first infill layer measured 0.23 - 0.25 mm. Second layer infill and outer wall measured 0.49-0.51 mm; exactly what one would expect with a 0.25 mm print height setting. No ridges between the beads on the first layer; beads completely fused. However, the second layer beads are separated by about 0.5 mm with a bead width of about 0.26 mm. This makes sense as the Infill Percentage was set to 60%. So the height control is spot on with these settings. The first layer ends up being solid because the nozzles proximity to the print surface force a wider than normal aspect ratio so the bead edges close with one another.
I could leave the setting like this and probably have good luck with most parts as long as I did not want to change the infill percentage and would never need more than one solid first layer. But I will usually want at least 2 solid bottom and top layers and the infill percentage will be driven by part loading conditions. Long and short of it, I understand more about whats going on but still have an issue.
Can anyone explain whats happening in Creator when its printing Bottom Solid Layers? It seems to me that perhaps the software is flowing too much material during the formation of the Bottom Solid Layer. Biggest reason for thinking this is that the outer wall height is nearly perfect and no ridge forms between the the two beads that form the outer wall. This makes me think that Creator is extruding the Bottom Layer infill at a higher flow rate than it does when printing outer wall and normal infill. Is there a way to control the extrusion rate for solid layers?
Thanks