Hi,
Mased on the following link, 236gsm (145lb) is the max number:
http://h10025.www1.hp.com/ewfrf/wc/document?cc=us&lc=en&dlc=en&docname=c01490922#N721
You may want to try feed paper from the back but I believe it will stop becuase the back is not close.
Regards.