I’ve been printing for a few years with a Reprap I’ve built and one of the ongoing struggles has been keeping parts stuck to the bed long enough to print. I’ve tried many, many things, but along with proper tram adjustment, nothing has worked as well as a thick piece of glass coated with Elmer’s Craft Bond (the specific flavor in the photo below):
I have three pieces of glass cut for printing (not sure the exact thickness but it’s pretty heavy glass) which I spray with a light coating of this adhesive and it is the perfect balance between enough stick to keep parts in place but not so much it’s impossible to remove them.
One application lasts a very long time, in fact I can’t say for sure that I’ve ever needed to reapply it (I have in some cases just to be cautious). Over time the surface becomes less sticky to the touch, but keeps working just as well (perhaps a function of the heat coming from the plastic during printing).
It also creates a little roughness to the surface of the glass that might play a role as well.
Using this setup I’ve been printing PLA on unheated glass for a number of months with only one or two failed prints due to adhesion issues, and in all cases I was pushing my luck (tall parts or circular overhangs without brim, etc.). I’ve also been able to use the entire printable area of my bed for the first time without curling, etc.
I’m not sure how well this works with other plastics because I haven’t tried many of them yet. I did do a little printing with Talman t-glase and it stayed put as well.
The best part is that both of these things can be had at most local art/craft supply stores. The adhesive is relatively inexpensive and goes a long way (be sure to follow the directions on clearing the nozzle after use!) and custom cut glass can be had from anywhere that makes picture frames (ask them to grind the edges if possible, much safer when handling the glass).