Is printing directly on the bed a no-no?
No, you can print directly on the glass if you don’t have a pei sheet or some other kind of sheet. In fact my first 3D printer was setup like that to print directly on the glass. It worked very well, in fact I thought of doing this myself but the pei sheet is working good for me. I’m sure there are some materials you’ll not want to print directly on the glass, I don’t know what they are off the top of my head.
Thanks. I’ve been using pei, but why bother if printing on glass works?
When you pick a material profile in Cura, pay attention to the ‘notes’ field. It will usually tell you how to prepare the bed.
When you print with PLA onto glass you need something to help with bed adhesion. Typically that’s PVA glue stick (e.g. Elmer’s Glue Stick or equivalent). PVA dissolves in water … so a damp paper towel or rag can be used to clean it up.
Some stuff wants to print directly onto glass (with nothing on the glass) … TPU is like this. Other stuff needs something to either help with adhesion … or to help with release after the print is finished (because some stuff adheres a little too well.)