What tweaks do slicers use to get rafts to stick? 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 &amp; 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.

Later reflection suggests that these introductory G-code snippets (I've now started using the PRUSA one of a line along the edge of the bed) risk problems  with flexible filament, so it might be best to err on the side of conservative for the extrude volume rather than assume that more is better here.

You can find other similar solutions in this question: Writing G-code : swiping at start of print