Bar Code
Printing Basics
Since the dependence on bar coded
information is continually growing, the ramifications for non-scannable
bar code symbols are severe. In addition to print quality,
there are other factors, which affect the scanning performance/
acceptance of bar code symbols. In fact, a recent retail
scanning performance study illustrated that one third of all bar
code rejections were caused by factors other than print quality.
Many of these problems resulted from errors in the design or
origination stages.
Design
Stage Considerations:
Quiet Zones
Bar codes require an area of free space on both the left and
right of a symbol. These spaces need to be of any stray
marks or graphics that the scanner may interpret as an errant
bar. 
Print Contrast
(Color)
A scanner reads a bar code by distinguishing the differences
between the bars and spaces. The spaces must be light
colored (highly reflective) and the bars must be dark (less
reflective). The best color combination is black bars on a
white background, but other color combinations will work as long
as the contrast between the bars and spaces is adequate.
If the contrast gets too low, the scanner can not differentiate
between the bars and spaces, therefore the bars code appears
invisible.
Height
Truncation (shortening of symbol height) should only be used as
a last alternative. The shorter the bar code height, the
more difficult it is to scan the code in most scanning
environments. Truncation has a dramatic effect on U.P.C.
symbols, and some retailers penalize suppliers for reducing the
height of a bar code on a package, which has the space for a
standard symbol. 
Encodation
This simply insures that the bars and spaces match the human
readable characters when a bar code is scanned. The human
readable characters should not be omitted or abbreviated.
Test
your knowledge... What's
Wrong with this Bar Code?
Printing Suggestions
Bar codes can be printed from a
multitude of printing methods ranging from wet ink processes to
on-demand digital printers. Symbol print quality
will vary depending on the printing process. Flexographic,
offset lithography, and silk screening qualify as “wet ink”
printing processes. Whereas laser and thermal transfer
printers are typical on-demand printers. The following
suggestions will help maximize bar code print quality:
Wet Ink
Processes
Precision bar
code artwork
All wet ink printing utilizes either printing plates or screens.
In order for a printing plate to accurately duplicate a bar code
image, the original bar code artwork must be precise and be
properly compensated for print growth. (For further
information go to Pre-Press Products.)
Ink Viscosity
If it is too thick, the bars of a symbol usually print thin,
because less ink transfers from the plate to the substrate.
Conversely, if it is too thin, the bars print wide because the
ink is slapped against the substrate, spreading the ink outward.
For flexible packaging, it is ink viscosity, which causes the
most problems with bar code print quality. When the law required
the removal of lead from ink, the opaque characteristics of the
ink has dropped. Bar code symbols are read by having
the bars absorb scanner beam of light, while the spaces between
the bars reflect the light. If white ink is printed on
flexible packaging under the symbol and the white ink is not
opaque, the scanner beam passes through the white instead of
reflecting off it. Thus, the contrast between the dark
bars and light spaces is lost.
Plate
Impression
When printing flexographically, ideally you want to use just
enough impression to achieve a clean, sharp detail with
appropriate ink coverage. This holds true for the bar code
symbol as well. Too much impression will cause the bars to
print wide and too little results in thin bars. Watch
other graphics that print on the same plate for indications of
inappropriate plate impression. When printing large solids
or reverses, bar code symbols will usually print wide because
the impression is increased to get suitable ink coverage.
Symbol
Orientation
The decision to run bar code symbols with or against the web
direction is press concern. We normally recommend running
symbols in the same directions as the press (picket fence
orientation). However, proper distortion can allow bar
codes to be printed in a ladder orientation.
Suggestions
for Digital Printing (Thermal Transfer)
Adjustment of
print-head temperature
Since thermal transfer printing involves imaging through tiny
heated pins contacting a ribbon onto a label, the print head
temperature has a dramatic effect on symbol quality. If
the heat setting is too high, the bars will print too heavy.
Conversely, if the setting is too cool, the bars within a symbol
will print too light. The heat setting can be adjusted
either as a printer function or through a software setting.
Adjustment control will vary between software and printers.
Print Speed
Although many thermal transfer printers boast a wide range of
print speeds (inches per second), the actual range for printing
bar codes is normally smaller. If the print speed is too
rapid, the heated pins can not fully transfer an image, and the
bars within a symbol will be too narrow.
Symbol
Orientation
It is highly recommended that bar code symbols be printed in the
direction of the printer. Bar
codes printed perpendicular to the print direction tends to
yield dimensional errors.
Printer
Maintenance
The quality of printed symbols tends to degrade as deposits
build up on the thermal print head. Regular cleaning of
the print head and guide surface in accordance with the
manufacturer’s recommendation is highly advised.
Thermal Print heads eventually
wear out to the point where one or more dot elements fail to
heat properly. When this occurs, the printed symbol may
longer be scannable. Obviously, if printed symbols are
routinely checked by a bar code print verifier, print head wears
will be easily detected. Another procedure we recommend is
to print a symbol with a horizontal bar going across the top of
the label. Any defective element module will be easy to
recognize (see example).
