Printhead - Intermec EasyCoder PX4i Service Manual

Chapter 8 — Print Unit

8.6 Printhead

Description
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The printing is produced by the thermal printhead, which consists of a
line of very small, closely spaced resistors on a ceramic tile fi tted across the
media path. When a current is led through the resistors, commonly called
"dots," these will be heated very quickly. When the current is shut off, the
dots cool down just as fast.
As the media is fed past the dots, the hot dots will produce a number of
black spots on heat sensitive (direct thermal) media, or on other face mate-
rials via a thermal transfer ribbon. The spots can be combined into bitmap
patterns, which make up characters, bar codes, images, lines, and boxes.
The direct thermal printing method requires special media coated with a
thin layer of heat-sensitive chemicals. As the media is fed past the dots, the
heat from the dots will make the chemicals react, producing a dark salt,
which makes up the imprint under each dot.
In the thermal transfer printing method, a special "ink"-coated transfer
ribbon is used. When the ribbon is heated by a dot on the printhead, the
"ink" melts and sticks to the receiving face material, where the ink imme-
diately becomes solid again, producing a black spot. Transfer ribbons nor-
mally do not smear at room temperature, neither before nor after printing.
Nor do the printed labels smear, even if the printout may be smudged by
extensive rubbing if an unfortunate combination of ribbon and face mate-
rial is used. The thermal transfer method makes it possible to use a wide
range of face materials for printing, for example papers, boards, plastics,
foils, etc. However, an original transfer ribbon from Intermec should
always be used.
Information on how to switch between direct thermal and thermal transfer
printing is provided in the User's Guide.
The basis for all measures and positioning in both the Fingerprint and IPL
programming languages is the size of printhead dots.
In an 8 dots/mm (203.2 dpi) printhead, each dot has a nominal size of
0.125 mm (4.92 mils). This means that a heated dot under standardized
conditions will produce a black spot which has a diameter of 0.125 mm
(4.92 mils).
In an 11.81 dots/mm (300 dpi) printhead (Fingerprint only), each dot
has a nominal size of 0.086 mm (3.33 mils). This means that a heated dot
under standardized conditions will produce a black spot which has a diam-
eter of 0.086 mm (3.33 mils).
Depending on dot temperature, exposure time, media and ribbon charac-
teristics, etc., the spot may actually be somewhat smaller (weak print) or
larger (black print), but that does not affect the calculation of distances,
sizes, and positions.
At startup, the printer auto-detects the printhead density as well as the
head resistance. The printhead can also be checked using the Finger-
print instructions HEAD, FUNCTEST, and FUNCTEST$. In IPL, the
number and size of the dots in the printhead is returned to the host by the
command <STX><ESC>H<ETX>.
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