Purge/Poop
Purge
When you are printing the pressure inside the nozzle is high and that helps force molten plastic out of the nozzle. If the pressure changes the amount of molten plastic coming out changes. If you have a nozzle full of molten plastic and you push solid filament in the filament does not go in until some of the molten plastic comes out. Pressure builds up.
So, you start with a nozzle full of molten plastic and the filament sitting behind it. The extruder moves and pushes the filament. The filament starts to push. It goes nowhere, cause there is no place for it to go. The molten plastic starts to have a pressure rise. Plastic starts to come out of the nozzle. The filament can not go into the nozzle. The pressure drops and the flow of plastic drops. The extruder moves a bit more and the cycle repeats.
At a microscopic level the flow is not steady, there are tiny pulses. The greatest time for a problem is right at the start. Some plastic leaked out, the pressure goes from 0 to printing. The whole thing moved off a state where all was at equilibrium and is not wildly moving to a new state of equilibrium. There might be a bit of a string hanging out and you might get a surge of plastic.
It’s a lot nicer if you can have this stuff not on your print. Hey, there’s the purge line. It gives the nozzle a bit of time to settle down, and if it looks horrible, who cares.
I don’t want to “waste” this tiny bit of plastic. Your printer does not do the line for jollies. It does it because the gcode in the file you are printing told it to do it. Edit the gcode to take it out. It probably came from the beginning gcode your slicer stuck on, edit that and always take it out. Remember that horrible bit of the purge line, that may not have always been there but usually was to some degree, well not it’s on your object.
Poop
The Bambu printers are famous for this. Every time you change colors they poop and “waste” a bunch of filament.
At the end of a color a black cuts off the current filament, leaving the molten filament and a but of hard filament in the hot end. Then the new filament comes along and pushes. The old filament comes out and the new filament comes in. There may be a little mixing where they meet so you want to get to the point were only the new filament is coming out.
Back when you sliced, in the prepare page, there was a button that said flushing volumes. If you click on that it brings up a table that lists the amount of filament to use when switching between colors. It starts off with numbers from Bambu. They are conservative so the numbers are probably higher than absolutely needed to do the flush. You can put lower values in here, if your printer can switch quicker or you don’t mind a bit of mixing. If the swap is hard, black to white is much harder than white to black, and you want excellent color swaps you might even increase the volume. If you don’t care about old color and new color mixing you can set the volume to 0.
Next is purge. When you are 24mm up you can not put the purge line on the plate. The slicer did a purge tower. It’s that rectangular thing on the plate that you did not put there. The wower will be as high as the last color change. For layers where there is no color change it will just build the layer with the same color. If you have a two color print, one color on the bottom and one on the top, you get a purge tower half way up made of the first color. half way it purges the second color, then stops the tower.
The purge tower does the same thing as the purge lines on the bed. If you do not want the purge tower you can have the print purge into the infill or not purge at all. Remember you might get horrible stuff in the infill or on your object.