Light reflection is the number-one way to kill a UV printer. When printing on glass, you will always need to use jigs to eliminate the UV light from reflection back up into the print heads. Your jigs need to be non-reflective and 1-2 mm lower than the printed product. On glass and crystal that has the beveled edges, you need to tape the edges to keep UV light from being directed back into the print heads.
-Steve Weist, PDS Equipment