Inkjet duplex printing - How does it work?

February 3, 2013 2:02 PM

I just got a cheap-o HP printer that features two sided printing. It seems to print on one side, suck the paper back in, and print on the other side. Does anyone know how this works? Does it flip the paper over? My guess is that it loops it back in (back facing down), probably raises the heads, and "prints upside down" the other side. But I'd love to see a diagram or video. Can't find one, though.
posted by thirdgrader to technology (3 answers total)

It works just like you said, minus the "raises the heads" part, at least in this example.
posted by perspicio at 2:09 PM on February 3

Just want to say that this method isn't unique to inkjets. Our office laser printers also suck the paper back in and do something similar. Nothing changes with the orientation of the inkjet heads/imaging drums, it's just that the paper takes a slight detour on its second run through to get it in the right orientation.
posted by sbutler at 3:01 PM on February 3

The linked video is correct, for that kind of printer. In the simpler printers, however, there isn't another page working through the machine while the first one is getting printed. It just does one sheet at a time.

The trick is that it does a second direction reversal inside the duplexer where you can't see it. When it backs the paper up the first time, there is a gate-like thing that diverts it to a temporary holding area. Then it reverses the second time and feeds the sheet back into the printer more or less the same way it normally does.

For ease of understanding, imagine it like this: in a classic HP deskjet where the paper supply is right below the paper output. Remove the shelf that separates the two. It prints a page and then drops it right back into the paper tray, where the printer can pick it up again. But because the printer flipped it over while it was being printed, now the sheet is back side up and will get printed on its back when it goes through the second time. This is what the printer is doing when it duplexes, just behind the scenes and more complicated.

(That's for a printer that has a C shaped paper path. For a printer that has a S shaped paper path like most lasers, it does mostly the same thing but prints the pages back side first so they end up pointing in the right direction.)
posted by gjc at 4:52 PM on February 3