On Fri, 06 Nov 2009 13:05:41 +1000, Bob Larter
wrote:

Well, something is clearly wrong. I serviced Canon-engine lasers for
many years, & all the larger models (ie; with more metal than plastic,
such as the CX, SX & MX engines)

Ummm... I think you mean CX, SX, and NX. There is no MX.
http://www.printerworks.com/Catalogs/LaserJets.html
All the HP (something)X LaserJet printers were made by Canon.

had really good paper-handling, & could
cope just fine with double-sided printing.

Let me try to explain again, for the third time (with some additions).

The printers expect the pages to be flat. Pages that have gone
through the printer tend to be slightly warped. You can demonstrate
this by finding a truly flat and smooth surface, and plopping a piece
of printed paper onto this surface. Flip it over and compare how it
lies on the flat surface to that of a clean unprinted page. It
invariable has a lump on one side or other. That's fatal to many feed
mechanism, especially those that use only a single foot or roller in
the middle of the page.

Remelting the toner on the back side of the page sometimes causes the
toner to stick to the rubber roller in the fuser, causing exit jams.
Take a page of recently printed paper and slide an unprinted page over
the printed page on the flat surface. If you're careful, you can feel
the increased and erratic drag. If your not a believer, hang an
alligator clip on the end of a fish scale. Plop the printed page into
a stack of new unprinted paper. Pull and measure the tension required
to pull the paper out of the stack. Now, do the same think with an
unprinted page. Depending on the height of the stack, the printed
page has increased drag over the unprinted page. Yes, I know that the
printer does not pull paper from the middle of the stack. The weight
of the paper pile is to simulate the pressure supplied by the rubber
feed roller.

That's an especially bad problem if you print a stack of paper, and
then feed the stack back through the printer again to print on the
back side. The pages will stick together because the black plastic
toner is fairly lumpy and slightly sticky. Printing on the back side
also tend to transfer some toner from the back side to the rubber
roller in the fuser assembly. The HPII and HPIII were especially bad
at leaving impressions on the roller. Later printers uses a mylar
sheet between the fuser and the rubber roller. Reheated toner on the
back of the page would not stick to the mylar.

The rubber "feet" above the feed tray don't like to push against
printed paper, where the black areas are much slicker than the
unprinted white paper. It's usually not a problem with a fairly new
printer, but after the rubber feet get worn and slick, things tend to
slide. I just fixed an HP 4000 printer with this problem.

In my never humble opinion, the Canon printer feed mechanism sucks.
It's major problems are the reliance on a single rubber feed roller
located in the middle of the page. When everything is proper (new
rubber, good flat 20 lb paper, no printing on the back, etc), it works
just fine. When something changes, it has so little friction surface,
that it screws up badly. If there's any crud in mechanism, the center
feed will cause the leading edge to misalign or corrugate, resulting
in a guaranteed paper jam.

The solution was provided in the 4000 thru 4300 series printers (and
some others). It has 4ea "D" shaped rubber feet to feed the paper off
the top of the stack. If one foot slips a little because it's sitting
on some printing or a bulge, the others will still feed the page in a
fairly straight line. The "D" feet are also very easy to replace.

These later printers also solved another problem. The HP LJ4 series
had two sets of coupled rollers contact the paper at the same time. If
there was even a slight difference in roller diameter, the paper would
either wrinkle, or the rollers would slip, and the paper would jam.
Bad design. This was fixed in later models by either having only one
set of rollers make contact, or adding a clutch to allow for changes
in roller diameter.

Incidentally, the HP4300 records the number of pages printed on a chip
attached to the toner cart. The printer records errors including
paper jams. I just connected to a customers network via VNC and
looked at their 4300 log file. About 10,000 pages on the toner cart
with 8 paper jams. I don't think I could ever come close to that with
a Canon SX series printer. This particular machine has a duplexer,
but it's normally not used for tax returns. I've seen this printer
grind through piles and piles of printed tax returns without ever
jamming. Their previous printer was an HP4si, which would jam at
least 2-4 times per ream of paper.


--
Jeff Liebermann
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558