The main difference between these two excellent plastic card printers is that the Zebra 310i ID card printer prints on one side of ID cards, while the Zebra 420i prints both sides of identification cards.

Other differences between the two ID card machines follow from this main difference. This is seen, for example, in ID badge printer ribbon selection.

If a person printing security badges or employee ID cards on a duplex ID card printer wants a color photo on the front of the ID badge, but needs just black text and a bar code on its back side, he or she does not need to cycle through two complete sequences of YMCKO ribbon to again get to black. Instead, this ID printer can use a YMCKOK ribbon. These simply tack an extra black panel on to the end of the first sequence.

Thus single-sided P310i ID card printers take YMCKO color ribbons (which print 200 or 350 images), while the two-sided Zebra P420i can take either these (if color is desired on ID badge backsides) or 170 or 250 image YMCKOK color ribbons.