I realize this is an old post, but it came up on a google seach ...so potentially other people are finding it as well. I'm looking to add a heated bed to my Printrbot Simple XL. I've been using the wood bed with blue tape with good adhesion results, but with the larger surface I wanted to try something that might allow easier release with larger parts. Just some thoughts and questions....
The OP was having adhesion issues well into the print. My guess is that the glass surface temperature was to blame ...it actually makes a lot of sense. Glass is a pretty good thermal insualtor with a very low coefficient of thermal expansion ...much lower than PLA. So what happens? The bed heater comes to temperature, and the printer starts printing. At that point, the thermistor reads the correct temperature, but the top of the glass is much cooler. As the print continues, the glass slowly heats up (not expanding) while it also heats the first printed layer (which starts expanding). At some point, POP, the object comes off of the glass. Any takers? I'd think if the OP heated the bed for some time before starting the print it should solve the problem ...though I'm sure a solution has been found by now.
I was looking at the makerfarm heated bed ...and they recommend putting glass on it. I was considering using aluminum instead, with Kapton tape. Anyone tried this with good results? Aluminum has greater coefficient of expansion, not sure where it compares to PLA though, and how easy it would be to remove items. I'm satisfied with blu tape adhesion ...really just looking to make PLA easy to remove once stuck.
