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Duplex printers

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11th Aug 2009 | 21:32

I'm annoyed by how slow our office printer is at printing on both sides of the paper. It occurs to me that it ought to be possible to make it much faster (a similar speed to single-sided printing) without too much extra complexity.

At the moment it handles one sheet at a time, in the sequence feed / print / reverse / print / eject. The feed and eject phases can be overlapped but there's otherwise little opportunity for pipelining.

If the duplex mechanism includes a paper path that goes from the print mechanism's exit to its entrance while turning over the sheet, then the path through the printer can be one-way (except inside the duplexer) and the duplexer can operate at the same time as the printer. The whole thing can then handle two sheets at once. The sequence goes like this:

  • feed sheet 1
  • print page 2 on sheet 1
  • sheet 1 into duplexer / feed sheet 2
  • sheet 1 through duplexer / print page 4 on sheet 2
  • sheet 1 into printer / sheet 2 into duplexer
  • print page 1 on sheet 1 / sheet 2 through duplexer
  • sheet 1 exits / sheet 2 into printer
  • print page 3 on sheet 2
  • sheet 2 exits / feed sheet 3
  • ...

I have assumed a C-shaped or S-shaped paper path, where the printer prints on the top of the sheet which turns over before dropping into the hopper, so that you end up with the stack of paper face down in the correct order. Our printer's duplex mode presumably prints pages in even/odd order (i.e. page 2 then page 1 on sheet 1, page 4 then page 3 on sheet 2, etc.) so the output stack still ends up in the right order. My duplexer's 2/4/1/3 page ordering puts greater demands on the RIPper's memory but that shouldn't be a big deal these days. (On the other hand, why does our printer appear to wait while RIPping? Shouldn't that be instant nowadays?)

So I wonder if anyone makes a printer with a duplexer that works this way. It should be fairly obvious from the duplex / duplex / exit / exit rhythm.

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Comments {11}

Sheep with a guitar

from: sbp
date: 11th Aug 2009 21:16 (UTC)

Quick, patent it :-)

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Tony Finch

from: fanf
date: 11th Aug 2009 22:12 (UTC)

I'd prefer to have decent printers, not a patent :-)

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from: nonameyet
date: 11th Aug 2009 21:23 (UTC)

What sort of price point are you looking at ?
Our big photocopier/printers have about 6 sheets in flight at once (although some of those will be in the output stapler/sorter) as anyone trying to recover from a paper jam will find.

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Tony Finch

from: fanf
date: 11th Aug 2009 21:38 (UTC)

This would be for a workgroup printer. Big printers basically have two print engines so they don't have to turn sheets over or re-feed them.

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Jos Dingjan

from: happydisciple
date: 12th Aug 2009 07:18 (UTC)

There's a topological problem with printing double sided. Let's assume a single printing unit (more and we're in the realm of more expensive, higher volume printers.

To print on both sides, either 1) your paper goes through the printing unit in two directions, 2) your sheets of paper need to perform half a barrel roll, or 3) they need to reverse direction. 1) immediately goes against the idea of pipelining, and leads to complications in the ordering of various steps (illuminate drum, toner on drum, transfer to paper, fuse, in the case of laser printers). 2) is possible, but not practical in workgroup printers for reasons of space. So we're left with reversing direction, which is always going to have a time penalty compared with dimple transport.

HP has workgroup colour laser printers that do nearly what you describe: they feed page 1, print one side, duplex 1 while feeding 2, print one side of 2, duplex 2 while re-feeding 1, print the other side of 1, exit 1 while re-feeding 2, print the other side of 2, exit 2 & start afresh. You can tell by the fact that you see two half-printed sheets peeping out of the exit & being sucked in again, then two land in the output hopper. The limitation is in the reversing being done in the output path: if you had a separate path for that and fast paper-route switching, you could lengthen the above sequence. I've also seen printers accumulate a stack of half-printed sheets in a spare input/holding tray, then do all the other sides in one go.

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Tony Finch

from: fanf
date: 12th Aug 2009 10:32 (UTC)

Cool, that HP sounds just like what I have in mind :-)

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from: gareth_rees
date: 12th Aug 2009 11:34 (UTC)

half a barrel roll

I'm reminded of the ticket machines you get in some Japanese railway stations, where you can insert your ticket in any orientation but it always comes out in the correct orientation. (Of course a railway ticket is a lot smaller than A4.)

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from: kotturinn
date: 12th Aug 2009 09:45 (UTC)

I think the IPG's printer (and possibly the other one on the corridor) does something like this. It hiccups out two pieces of double-sided printout more-or-less simultaneously then works on the next two anyway. It's an HP. Come and see it/them in action - I haven't had enough sleep, or coffee, to be able to analyse the process accurately.

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from: kotturinn
date: 12th Aug 2009 09:46 (UTC)

That'll teach me to read more carefully. I see happydisciple's just described it. Coffee......

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from: anonymous
date: 7th Jun 2011 22:10 (UTC)

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Tony Finch

Re: Good

from: fanf
date: 8th Jun 2011 09:25 (UTC)

I think I'll leave this comment here because the sentiment is nice and it is expressed in such an unusual manner...

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