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. Then the selection should be based primarily upon the experience and reputation of the supplier. 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 low compared to other choices.
• Quality 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 size of the label that can be printed 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 be 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 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 it 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 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 synced 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.

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 everyone most likely doesn't have. 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 though, the top copy will be the only one usable.
• 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 is selecting a dot matrix printer for bar code applications on the basis of price or availability. Make sure it can print suitable quality symbols.

Limitations

• Carriage Design 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 problems.
• 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 will not contain enough ink to print an acceptable contrast ratio and the bars will tend to be too narrow with adjacent spaces 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 wire. 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. Also, the 8 mil wire requires several overlapping dot rows to properly print an AIAG B 3 shipping label. Conversely, printers using a nine pin heads with 12 mil print wires can create the AIAG symbol easier, but are limited to low density code applications.

The term laser is used here to refer to any printer using a xerographic or similar type of printing process. Liquid Crystal Shutters (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. 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 appearing labels with complicated designs.
• Resolution The 300 dots per inch resolution (a 3.3 mil dot size) allows for ultra high density bar code symbols to be printed. However, it is not generally recommended that bars be printed less than two dots wide (6.6 mils) unless the symbol is to be scanned in closed loop applications where the scanning equipment used can be controlled.
• Multi Application They are the printers of choice for documents where bar code symbols and quality document printing must be intermixed.

Limitations

• Page Printer Since they 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 most demand label applications.
• Heat Most xerographic 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.

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 that 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 substances 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. Degradation by foreign contaminates or surface abuse must first destroy the protective coating before affecting the image.

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 simpler. This results in a less complex mechanism with more user friendly consumables loading.
• Resolution The thermal print elements can produce a very consistent dot pattern down to about 5 mils. This allows ultra high density bar code printing.
• Symbol Orientation Since a square dot is used, it is as easy to produce bars horizontally as it is vertically, or even intermix them on the same label. Some thermal printers may have to slow down when printing vertically, depending upon the imaging techniques employed.

Limitations

• Environment The two most severe environmental limitations are exposure to temperatures above 140° F or prolonged exposure (around 30 days for typical labels) to direct sunlight. Below these limits, the labels will retain an acceptable quality image most purposes.
• Spectral Response The bars created by the standard thermal dyes used in the label coatings are relatively transparent to infrared light, limiting its usefulness to visible light scanners. Special label coatings are available that work with both visible and infrared light sources, but 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 though, they will yield excellent quality bar code symbols.

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 very high contrast ratios which are very stable and resist deterioration. The use of square print dot elements also gives excellent bar edge quality.
• Media Selection The 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 with 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 labels with protective surface laminations.
• Label Orientation The square dot image forming elements allow bars to be produced horizontally and vertically and intermixed on the label with other HRI and graphics information.

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 difficult to handle, especially if they are very wide.
• Label Cost Because of the high ribbon usage, usually on the order of two rolls of labels per ribbon, the cost of comparable 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.

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 liquified with heat and placed on the paper where it reverts to its solid state. This gives much 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.

Disadvantages

• Media Surface The ink must be absorbed into the surface of the media, requiring a controlled surface porosity and finish. Because bar code involves 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 during intervals in the printing.
• 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

By knowing the conditions under which the label must exist and the limitations of the technology choices, the process of selecting a satisfactory print method is simplified. The best implementation is a straight matrix approach, listing the requirements down one side and the potential technologies across the top. A generalized matrix is given in Table 1 indexing label requirements versus the technologies discussed previously. Remember that this is only a recommendation list and exceptions can always be found. An illustration is the dot matrix category which is not recommended for single label demand use. Some dot matrix printers use pin feed mechanisms that do not trap the last form printed. Therefore, if the label does not have to be presented in a self stripped form, this type of printer could be used.

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.

Plain Paper, a Myth

There are many definitions of "plain paper". They range from "untreated" (whatever that means) to what can be purchased at the corner drugstore at 1:00 AM Sunday morning. The later definition is probably closer, but still is not the way to purchase bar code label stock. For bar code labeling applications, nothing is "plain paper". Most labels are made of paper coated on one side with an adhesive and placed on a carrier strip. They are then die cut to a specific size so that individual labels can be easily peeled off and affixed to the desired article. Once on the article, they must adhere without coming off, and there may even be a requirement for the label to destruct if someone tries to remove it. The labels must be made of a material that is compatible with the print process being used. This can range from coating with a thermal sensitive layer for direct thermal labels to controlling the surface smoothness for transferred inks. In print processes that use ink absorption, like ink jet, the porosity of the paper must also be controlled.

By the time the correct label material, adhesive, carrier strip, die cut size, etc. are specified, it is easy to see that it has passed far beyond the "plain paper" definition. Indeed, most of the cost of the label is contained in these secondary processes, completely over shadowing the cost of the base label material.

More Information

This GUIDE was prepared to give a quick review of the many and complex factors to be considered when printing bar code symbols. It is not intended as a complete reference for such applications. For additional information, please contact CMS directly, or it is recommended that you obtain a copy of the book Bar Code Printing. This book is a part of a five volume series covering all the aspects of bar code technology and implementation. Other volumes in the series are entitled:

• Bar Code Systems and Symbologies
• Bar Code Scanning and Reading
• Data Collection in Bar Code Systems
• Bar Code Applications and Software