Printing dual sided in register on the HP Z3100
Paper sensors
The HP Z3100 and Z3200 like many other wide format inkjet printers measures
the sheet and roll paper position with an optical sensor when the media is
loaded. The sensor is located on the head carriage and most likely uses infra-red
light so is able to detect media types that are not white or have been printed
before. The sensor checks the skew of sheets and rolls at the right side edge of
the media and the loading (front) edge. For sheet loading there are two options:
“skew check on” and “skew check off”. The last was introduced with a firmware
upgrade of the Z3100 to enable the use of papers with a deckled edge etc. The
choice for “skew check off” is hidden deeper into the menu selection. The menu
Set Up > Printer Preferences > Skew Check On/Off. Which then becomes the
default for sheet loading. On the Z3200 the choice between the two is available
right after inserting media and will not become the default. Unlike the “Skew
Check On” method it will always ask the user to align the sheet to the blue lines
at the front of the printer and gives a wider tolerance on sensor measured skew
deviations. In that sense it should be less precise than the other loading method.
But when normal sheets with straight edges are loaded with this method and the
user makes custom register tabs at the front of the printer to align the sheet to,
the end result is an easier and better repeatable sheet position with more options
to align sheets for dual sided printing.
Register tabs
The front side of the printer beyond the blue line doesn’t provide a nice surface
for tabs, the surface has a slope and there are ribs. I use transparent
polycarbonate 1.3 mm thick to create a surface that allows smaller plastic tabs to
go beyond the blue line. Two sided adhesive tape is used for assembling the
parts and that allows changing the of the tab positions too. It is the classic
register tab system as used with conventional print methods like silkscreen.
There is however an important difference to that conventional, strictly manual
method: the Z3100 will in the end still measure the paper’s position with the
sensor on the head carriage. But there is an advantage using the tabs, adjusting
them makes it possible to get a precise parallel position of printed register marks
to the front sheet edge. Parallel also to the head carriage movement and the
Z3100 rotary cutter, which is important for another method discussed later. If
the application you print from is set for optimal (Qimage), economic use of the
sheet, the image printed starts at slightly less than 5mm from the front edge. If
the sheet sizes are both more than 28 cm and none is exceeding your printer’s
maximum width then use the widest dimension to align to the front. It gives the
best base for alignment and the head carriage movement is more precise than the
paper transport movement so one uses the shortest dimension for the last. Adjust
the image to be printed in portrait or landscape accordingly. Load the sheet with
“Skew Check Off”, place the tabs as much as possible near the corners but not at
the corners, symmetrical to the sheet is better too. At the right side this may not
always work as nice so the picture above is a bit in contradiction to this advice.
Make the print page in the application you print from as large as possible for the
sheet used, at least on the width of the sheet. Use centred to place the image in
the print page but shift the image to the front edge in the print page edit window
(Qimage), that is the fixed 5 mm printer margin, usually the upper one in the
preview window. Print the sheet with little image information (little ink) but
with register marks (Qimage’s Mark Corners has my preference as less paper is
wasted) and measure the two register marks to sheet edge. Replace one tab to
get the two right. Flip the sheet over but keep the same front edge to the tabs so
flip the sheet from right to left. !!!Never flip over a sheet that the trailing edge
from the first print becomes the front edge for the backside print, it doesn’t work
for various reasons!!! After printing lay the sheet on a light table or against a
window when there is daylight and check the position of the register marks. If
there’s no light available use a needle to mark the corners to the other side of the
sheet. They will be shifted on the width but should be at the same distance from
the front edge so be parallel. If not, more adjustment is needed but it is a good
idea to do that a bit later. First measure the horizontal shift between the register
marks. Take half of that number and use that to shift the image in the print page
edit window of Qimage so the horizontal register will be good between both
sides. If that print is done one should check any register errors on the 4 marks of
both sides, the error at the trailing edge side is most of the time more than on the
front side, the little skew still possible in sheet positioning will show there more.
One has to change one of the front tabs again to get that error minimised and
check then whether there is more adjustment needed in the Qimage positioning.
With some practice this goes fast and the same sheet can be used more times.
Don’t expect that you can adjust the front edge to register mark distance by
setting both front tabs more or less deep, the sensor measures the front edge in
the end so the only thing you can do is reduce the skew a little with the tabs.
On the picture above there is a tab used for the right side of the sheet, it is used
with the above method to align the sheet in both dual sided printing positions
but both at the right side. That is against all rules in conventional printing where
the tab should be used at the left side when the sheet is flipped over. But as the
sensor measures the actual right edge and the width of the paper, the tab used
left or right is in fact not needed if the image is more or less horizontally centred
on the sheet. It is however easier but not more precise to have one at the right
when loading than have none there. The method above requires sheets that have
exactly the same width and edge angles that are exactly 90 degrees, the height
(depth in the printer) plays no role here.
If humidity in the print room is consistent and the paper set to that humidity
(55% humidity is recommended but not always possible) then the print register
error will be well within 0.5 mm on sheets between 70 and 100 cm wide. Heavy
ink loads on the first printed side can influence that related to the paper weight.
It is better to let a sheet like that dry in the same room and do the other side after
the total first print run. Adjust with the tools described above for the backside
print run.
Mixed roll and sheet feeding for dual sided printing
In some cases I use roll printing for the front side printing, let the printer cut the
pages, which it does nicely leaving approximately 5 mm print margin at the
front edge of the next page. So I also let it cut a stroke at the front when the roll
is loaded to give the first page the same parallel edge. The printer measures the
roll front edge position after each print page and when the roll width is
consistent the precision of the backup print made on the cut sheets can be quite
high. Again depending on humidity and how heavy the first printed image was.
Not all dual sided paper comes on rolls though. Don´t forget to add at least 13
mm more paper at the trailing edge for the print pages that become the sheets as
the sheets need that extra margin for feeding later on. The height of the print
page should be at least 28 cm to use it as a sheet later on. Try it first before you
make a mistake on that as you can waste a lot of paper that way.
To fool the head carriage paper sensor
An even more manual register can be achieved by adhering a small square white
sticker at a specific position on the printer bed. At the right side of the printer
bed and with the dimensions and position as shown in the image above. The
sensor on the head carriage is fooled by the right edge of the sticker and
interpretes it as being the sheets right edge, that way the head position always
starts at 5 mm from the right side of the sticker. If that is used together with the
front tabs as described plus a right tab for the frontside print and a left tab for
the backside print the horizontal position can be adjusted manually by the tabs
and measured from just one edge of the paper. Depending on the image size
within the sheet size there should be at least 4 mm shift space available when
the image fills the sheet width and about 17 mm with smaller images.
Some additional comments
The “Skew Check Off” sheet loading method doesn’t measure the height of the
sheet (depth in the printer) and reports an arbitrary number for that dimension in
the printer driver. That should be totally ignored and instead one should be
aware of the true physical length of the paper. The “Skew Check On” method
measures the length of the paper but that number should be taken with a grain of
salt, the paper sensor at the insert side of media loading is a simple mechanical
switch that has three functions: give a signal that paper is inserted so the loading
can start (flips forward), report the end of a sheet or roll (flips upwards) and
measure the length of the paper in SCOn loading as it is released (flip upwards)
as soon as the trailing edge of the paper passes. It will flip backwards when the
sheet moves back after the length measurement. The switch is not as precise as
an optical sensor so one shouldn’t consider that paper height measurement as a
base to build the register on.
Some attention should be paid to the construction of that switch, if one tapes
waste sheets together to make proof print sheets etc large enough for loading the
switch may get stuck on overlapping edges. It is better to keep the assembled
sheet as plane as possible and cross the straight seem with a thin tape of 5 cm
wide. In general use the switch may catch paper lint etc that makes its
functioning less reliable. It can be cleaned quite easily.
For printing tasks Qimage is the application I use most of the time. It has a lot of
features including nesting of images on larger print pages, border creation etc.
too many features to mention here. For register control there’s more than enough
precision in the preview feedback numbers, feedback building on an even higher
internal precision. At least if you select imperial/inch units that show the
feedback in 0.01 inch in every menu. For people that are used to metrics the
feedback in some menu windows is rounded off to 1 mm, which is too rough for
jobs like this. Let us hope that one day this will be improved by the writer of
that program as there must be quite a number of users that are used to the metric
system only. Right now one has to check metric shifts made in the program by
trial and error or use workarounds, both a waste of time.
