I've always had trouble getting prints to stick directly on my bed, but had great reliability when use rafts. This didn't present a problem when I was using PLA, but I recently got a roll of PETG and it is very hard to remove rafts, so I am once again trying to figure out whats wrong.
I am printing the same file (currently a simple square for testing) with the same settings, except I am turning rafts on/off. When rafts are on, my print sticks perfectly. When rafts are off, my print doesn't stick at all - not even the skirt (which sticks fine on the raft print). With rafts on, I can watch my printer lay down a nice line for the skirt / first layer. With rafts off, I watched plastic ooze out of the nozzle and ride along with the print head, not sticking to the plate at all.
I have a stock Monoprice Maker Select v2 with the PEI surface it shipped with. I haven't tried any bed additions/changes to get better adhesion, since I know rafts stick fine with what I have. I figure there must be some settings I can change to make my first layer stick too. I am using Simplify3D & have tried a few basic tweaks, but I am largely stabbing in the dark.
What are the specific characteristics of a first layer raft, in comparison to a first layer of my print, that might make it stick better?
You could experiment with the 'cleaning' part of the startup gcode shown here
G1 X100 Y0 F4000 ; move half way along the front edge G1 Z1 ; move nozzle close to bed M109 S200 ; heat nozzle to 200 degC and wait until reached G4 P10000 ; wait 10 seconds for nozzle length to stabilize G1 E10 ; extrude 10 mm of filament G1 z15 F12000 E5 ; move 15 mm up, fast, while extruding 5mm G92 E0 ; reset extruder
I'm not completely persuaded that it helps a lot, but the idea is to fill the extruder (with some pressure from the bed at a 1mm gap) right before starting the skirt. Typically, I still find the first line of skirt may be quite blobby, but a 2nd line of skirt seems much better. Obviously you need the bed alignment right too. A raft seems to have enough 'extra' extrusion to mask these issues a little bit.