Section 2: Selecting A Print Technology
All of the major print technologies have some advantages and disadvantages when used in a demand label application. It is possible to make a particular technology fit the application, but this generally involves compromises that may not make sense. It is more
practical to select a printer based on a set of reasonable tradeoffs. For instance, a label printer is not often used for printing text documents, so this capability should not be included in the primary selection criteria.
Before requirements can be matched to a particular type of printer, something must be known about the different characteristics of the technology and what they mean to bar code printing. Most of the familiar print technologies were developed for producing human
readable text information. They can be modified to print bar codes, but what makes a good text printer does not necessarily make a good bar code printer. The more successful bar code printers have been optimized for this purpose. An example of optimization for bar code printing is the shape of the
print dot used by matrix printers. A square or rectangular dot makes a bar with a very defined edge, something scanners like to see. A round dot produces a bar with a scalloped edge, harder for the bar code scanner to read, but it makes a human readable character that is more pleasing to the eye
than the harsh corners of the square dot. Square dots make better bar codes while round dots are better suited for text documents. Another differentiation is the size of the print field. Text printers are designed to print document size pages, while label printers limit themselves to practical
label sizes.
Direct Thermal
Direct thermal printing has more of a public image problem than a performance problem. While it is sensitive to heat and ultraviolet light, the degree is much less than most people suppose. It does offer one unique advantage not available in any other of the
technologies presented here. It does not depend upon a secondary substance transfer to generate a mark on the paper. Direct thermal printing chemically alters a coating to produce the desired image. There is no secondary ink substance that must be disassociated from a carrier and made to adhere to
the label surface. If the thermally active layer is covered by a protective coating, as all thermal printer manufacturers recommend, the image is shielded from surface abuse and contamination. The only thing that can get to it without first having to break down the protective layer is radiated
energy in the form of either heat or ultraviolet light. Foreign contaminates or surface abuse must first destroy the protective coating before affecting the image.
Thermal Printing
� ADVANTAGES
- Quality - Because of the square image elements and the non-reliance on a secondary substance transfer, direct thermal labels produce high quality bar codes with excellent bar edge definition. � Simplicity - The absence of any ribbon/toner mechanisms makes the direct
thermal mechanism inherently simple. This results in a less complex media loading path for more user friendly consumables handling.
- Resolution - The thermal print elements can produce a consistent dot pattern down to 5.0 mils (200 dpi) when printing in a horizontal mode (parallel to the paper movement). This allows ultra high density bar code printing. When printing in a vertical mode however, the
speed must be reduced significantly to allow the elements to cool down before stepping to the next print position. While it is possible to print bars down to 5.0 mils when the print speed, paper sensitivity and power applied to the elements are carefully matched, it is best to avoid printing
narrow dimensions of less than 10 mils in the horizontal mode and 15 mils in the vertical mode.
- Cost - The elimination of any ribbon/toner mechanism results in a lower initial printer cost. It also results in a more favorable consumable cost when compared to either thermal transfer or laser printers.
� LIMITATIONS
- Environment - The two most severe environmental limitations are exposure to high temperatures or prolonged exposure to direct sunlight. The development of new thermal papers has pushed the upper temperature range upward to about 2120F and special ultraviolet filter
coatings can be applied that will extend the exposure time to sunlight from weeks to months and will retain an acceptable quality image for most purposes.
- Spectral Response - The bars created by the standard thermal dyes used in the label coatings are relatively transparent to infrared light, limiting the usefulness of standard label media to visible light scanners. Special label coatings are available that work with both
visible and infrared light sources, but also increase the cost of the label. If it is to be read by both types of scanners, then the infrared stock should be specified.
- Media - The thermal paper used must match the characteristics for which the printer was designed. Because of the various "speeds" (sensitivity to heat) available in thermal papers, not all printer and paper combinations are compatible. When properly matched
however, they will yield excellent quality bar code symbols.
SATO M-5900 Thermal Printer
Thermal Transfer
Thermal transfer printing is basically a direct thermal process that has a ribbon interposed between the head and the label. The heat from the print head is used to release the ink from a mylar ribbon and make it adhere to the label surface. Since this type of
thermal process now relies upon a secondary substance transfer, some of the advantages of direct thermal are lost, but heat restrictions have been improved and sensitivity to ultraviolet light has been eliminated.
� ADVANTAGES
- Quality - The ink transferred to the label surface produces excellent bars with quite high contrast ratios which are very stable and resist deterioration. The use of square print dot elements also gives excellent bar edge quality.
- Resolution - The thermal print elements can produce a consistent dot pattern down to 3.3 mils (300 dpi) when printing in a horizontal mode (parallel to the paper movement), allowing ultra high density bar code printing. When printing in a vertical mode however, the speed
must be reduced significantly to allow the elements to cool down before stepping to the next print position. While it is possible to print bars down to 5.0 mils when the print speed, paper, ribbon and power applied to the elements are carefully matched, it is best to avoid printing bar codes with
a narrow dimension of less than 10 mils in the horizontal mode and 15 mils in the vertical mode.
- Speed - Because it takes less energy to release the ink from the ribbon than it does to develop a dot using thermally sensitive paper, the print speed of thermal transfer printers is faster than their direct thermal cousins. With the same print head and mechanism, it is
typically twice as fast when printing comparable images in the direct thermal mode.
- Dual Mode Printing - Since the thermal transfer printer is essentially a thermal mechanism with a ribbon positioned between the head and the paper, it can also be used to print in a direct thermal mode if the ribbon is not used. However, the head life will be considerably
reduced because it is no longer protected by the ribbon.
- Media Selection - Bar code symbols can be generated using a wider range of paper and vinyl substrates that are more resistive to heat, water and light than in the direct thermal process. By proper selection of the label material and the ribbon, a very strong bond may be
obtained between the ink and the label surface, giving performance comparable to direct thermal labels with protective surface laminations.
� LIMITATIONS
- Ribbons - The ribbons are single pass and the printing of a single dot in a row wastes the remaining ink on that row. This results in a very high ribbon usage and associated cost. The ribbons are a thin mylar or similar material coated with ink on one side and can be
difficult to handle, especially if they are very wide.
- Label Cost - Because of the high ribbon usage, usually on the order of one ribbon roll for each two rolls of labels, the cost of labels is higher than with most of the other common technologies.
- Ribbon/Media Compatibility - The adherence of the ink to the surface is the source of most of the durability problems with thermal transfer images. If improper ribbon formulations are used, the transferred ink may flake at the edges when contact scanners are used, or
become smudged from oily finger prints.
Thermal Transfer Printing
Press Printing
This category encompasses all of the press technology available for off-site printing. This includes film masters, flexography, offset lithography, gravure, letterset, hot stamping and many others. It offers the widest range of quality labels available for bar code
printing. The most important step for procuring off-site printed labels is the generation of a good specification for the label, detailing the precise dimensions of the symbols and media upon which they are to be printed. Selection of a label vendor should be based primarily upon their experience
and reputation. This probably won't be the lowest cost bidder, but will most likely represent the lowest overall cost if headaches and mistakes are taken into consideration.
� ADVANTAGES
- Label Material Selection - Because of the many print processes available for use, the range of materials available on which bar codes may be printed is almost limitless. Special materials are available for high temperatures, and even metal plates can be imprinted with bar
coded information.
- Cost - If a large quantity of identical labels are required and they do not have any special requirements to drive up the price, cost per label will be relatively inexpensive.
- Quality - High quality should be expected for press printed labels. While it is possible to get low quality labels using off-site vendors, the cause can usually be traced to poor workmanship or a lack of understanding of the bar code symbol requirements. Excellent quality
labels that survive under harsh conditions can be obtained by the selection of the correct print process and base label material.
- Label Size - The printable label size is limited only by the dimensions of the press web, which can be very large.
� LIMITATIONS
- Variable Data - It is difficult to print labels where each one must contain different data. Even sequential numbering using bar code symbols is a task. It involves not just the changing of a single character, but the rearrangement of a series of bars and spaces.
- Advance Data - The data for the label(s) to be printed must be known far enough in advance to allow for the preparation of the print masters and scheduling of press time. If special labels are needed, extra time must be allowed to obtain any non-standard materials.
- Cost - If only a small number of labels are needed, the cost per label can be high, reflecting set-up charges that are now prorated over fewer labels.
Solid Font Impact
Solid font impact printers come the closest to typewriters in technology and limitations. They print the bar codes by constructing the symbols with preformed bars or by printing the complete character with a single hammer blow. The constructed code method must be
used if the symbol to be printed represents a large area per hammer blow, or if a continuous code is chosen.
� ADVANTAGES
- Simple Interfacing - Send the printer a character code and that is what it prints. No worrying about aspect ratios, different code symbologies, etc.
- Quality - Since each bar is precisely formed, the edge definition is excellent and good quality bar code symbols are produced.
� LIMITATIONS
- Fixed Format - Since each bar or symbol is preformed, the label format, symbology codes or character densities cannot be changed without changing the code wheel. Alpha characters may be printed, but are restricted to a particular location determined by their position on
the code wheel. All symbols must be printed in the same orientation, and orthogonally (i.e. at right angles) printed symbols cannot be produced.
- Speed - Each bar or symbol is located sequentially on the code wheel and the print speed is limited by the rotational speed of the wheel.
- Mechanical - The operation of the mechanism depends upon numerous moving parts with large masses. The hammer blows and code wheel rotation must be properly synchronized or smearing will result.
- Symbol Area - If small area symbols are printed, there is a tendency for "ticking" to occur. This is ink transfer in areas between characters or on edge of adjacent characters.
Impact Dot Matrix
Dot matrix printers are very popular in the computer industry for document printing. Many have been pressed into bar code service because they are cheap and everyone with a computer probably already has one. However, the ones best suited for bar code applications are
the line printer types, the ones that are not cheap and only used in limited computer applications. The most effective dot matrix printers generally have additional intelligence in the form of a graphics controller card to reduce transmission time for complex labels.
� ADVANTAGES
- Versatility - Dot matrix printers are one of the most versatile types available. They can print bar code symbols or text documents in any orientation and with various height and width symbols. They are especially adept at printing multi-copy forms that include bar code
symbols for document tracking and control. When printing bar codes on multi-copy forms however, the top copy will be the only one readable by a scanner.
- Equipment Cost - Dot matrix printers are used in large quantities by the computer industry and the initial cost of the equipment is low. However, care should be exercised in selecting a dot matrix printer for bar code applications on the basis of price or availability.
Also make sure it can print suitable quality symbols.
� LIMITATIONS
- Carriage Width - Most of the common dot matrix printers were designed primarily for printing documents. The width of the carriage is determined by the smallest sheet size (usually 8.5 x 11'). Bar code labels tend to be much smaller and have to be printed "multiple
up" to take advantage of the print speed. Also, if the printer uses the standard puller tractor arrangement, the label must clear the tractor assembly before it is easily accessible. Fitting a label stripper or cutter to such a printer presents some mechanical challenges.
- Quality - When used to print bar code symbols, dot matrix printers have several factors going against them. First, the edge definition of the bar is poor and overlapping dots must be used to meet the specifications. When overprinting to increase the bar density and fill
in the ragged edges, new ribbons will bleed excessively at the bar edge causing bars to be too wide and the adjacent spaces too narrow, while worn ribbons not containing enough ink will tend to print the bars will too narrow with adjacent spaces being too wide. Second, irregular paper spacing will
cause problems with vertically printed symbols. If the printer uses a serially driven head, then irregular head motion will result in problems with horizontally printed symbols.
- Spectral Response - The standard ink in a dot matrix printer ribbon is not readable to scanners using infrared light. Carbon must be added to make it infrared scannable, but carbon will cause the print head to wear out prematurely. Special mylar ribbons using carbon have
been developed to overcome this problem, but most are single pass and have a very limited lifetime. The dry carbon transferred from the mylar ribbons must adhere to the surface since it is not absorbed by the label material.
- Resolution - The best resolution offered by dot matrix printers comes from the 24 pin print heads that use 8 mil diameter print wires. When ink bleed is taken into consideration, the dot size will end up between 9 and 10 mils, giving a medium density bar code symbol at
best. When printing low density symbols such as those called for by the AIAG B-3 Shipping Label, the 8 mil wire requires several overlapping dot rows to print the proper bar width. Conversely, printers using nine pin heads with 12 mil print wires can create the symbol more easily, but are limited
to printing only low density bar codes.
Laser
The term laser is used here to refer to any printer using a xerographic or similar type of printing process. Liquid Crystal Shutter (LCS) arrays, Light Emitting Diode (LED) arrays or lasers are used to expose the surface of the image drum, with the laser being the
most popular. These printers are invariably page printers and are not well suited for demand label applications. They are most commonly sheet fed printers, but some of the newer laser printers designed for bar code applications employ a tractor feed system whereby continuous forms may be used.
� ADVANTAGES
- Aesthetics - Laser printers commonly have a resolution of at least 300 dots per inch, with some of the newer models being capable of printing at 600 or 1200 dpi. This allows them to print almost typeset quality characters and high resolution graphic images. These can be
combined with bar code symbols on a label to produce very nice looking labels with complicated designs.
- Resolution - At 300 dots per inch resolution (a 3.3 mil dot size), laser printers can create ultra high density bar code symbols. However, it is not generally recommended that bars be printed less than 6.6 mils (two dots wide on a 300 dpi printer) unless the symbol is to
be scanned in closed loop applications where the scanning equipment used can be controlled.
- Multi-Application - Lasers are the printers of choice for documents where bar code symbols and quality document printing must be intermixed.
� LIMITATIONS
- Page Printer - Since laser printers are basically page printers, it takes as much time and media to print a single small label as it does a large label. If it prints at six pages per minute, it takes 10 seconds to print a whole page or a single bar code symbol. Some of
the newer laser printers designed for bar code applications have a variable page length feature, but the physical distance between the imaging drum and the fusing roller prevents an image from being placed immediately following the previous image. They are therefore poor choices for typical demand
label applications.
- Heat - Laser printers use heat to fuse the toner to the surface of the paper, a lot of heat. The adhesive coating on the back of a label tends to seep out between the die cut edges when heat is applied. In ion deposition printers, the heat is replaced by pressure, but the
result can be the same. Special adhesives must be specified for these printers.
- Label Imaging Time - Because of the large number of dots per inch, the time required to image a label can be substantial. Compounding the problem is the requirement for a complete page to be imaged even if it consists of a number of identical smaller labels.
Ink Jet
Ink jet printing has several drawbacks when used for printing bar code symbols, but does find some use in specialized applications. The main problems with bar code printing are involved with the formation and control of the dot. Since the ink is absorbed, the paper
porosity, ink viscosity and drying time must be carefully controlled. The most pronounced symbol quality problems are associated with the edge definition, contrast ratio and consistent bar widths. Some of the new ink jet techniques use a solid ink that is liquefied with heat and placed on the
paper where it reverts to its solid state. This gives better edge definition and contrast ratios but leaves a raised print image which can cause problems with contact scanners. Special ink jet systems have been developed to print low resolution bar codes directly on corrugated surfaces. Using
independently mounted nozzles, the images are formed as the container is moved by on a conveyor system.
� ADVANTAGES
- Non-contact - The primary advantages of ink-jet printing is that it is a non-contact printing technology. This removes any wear on the head due to contact abrasion. This also allows surfaces with irregular finishes to be printed if they are compatible with the ink.
� LIMITATIONS
- Media Surface - The ink must be absorbed into the surface of the media, requiring a controlled surface porosity and finish. Because bar codes involve much higher printing densities than text, the media requires a longer period of time to dry sufficiently before it can be
handled without smearing.
- Ink Formulation - The ink must dry quickly, but not in the nozzle. The compromise between these two extremes requires the head to be purged or cleaned between print jobs. Water based inks are also very susceptible to exposure to moisture. A single drop of water can render
a bar code unreadable.
- Infrared Response - If infrared scanners are used, carbon or some other infrared absorbing material must be added to the ink. This can cause excessive wear on the ink nozzles.
Matching Technology and Requirements
The selection of the proper technology for generating bar coded labels is a very complex process. It involves not only an understanding of the print technology to be used, but the label usage requirements as well. These two bodies of knowledge must be combined in the
selection process to ensure that the bar code labels produced will give satisfactory performance for their required lifetime.
Some of the factors to be considered are:
- What are the environmental requirements?
- What is the total cost target per label?
- Does the data content change?
- How will the label be scanned?
- How will the label be applied?
- How often will a label be required i.e., (10 labels per second, etc)?
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