"For PLA, try a setting of 55-60C. If you go too hot, the PLA will stay liquid and can be pulled away from the bed by the cooling of subsequent layers. Too cold, and it won't stick."
From [
www.reprappro.com]
This is our troubleshooting page for the Tricolour Mendel, and has lots of useful tips; we are working on something similar for the Ormerod.
Ian
RepRapPro tech support