BY DR. BRADLEY A. PAULSON, FARGO ELECTRONICS CORPORATION

Introduction

Critical issues in fabricating photographic identification cards are the following:

1. Print quality
2. Security
3. Reliability
4. Cost

The quality of the printed image on the identification card is dependent on the materials that are used, the configuration of the printer, and the method of printing. The cost of the finished, printed cards depends almost entirely on the cost of the supplies; this frequently has a direct relationship with the print quality of the finished card, as more expensive supplies are generally required for a higher quality product. The security and reliability (or durability) of the printed cards is a function of the card construction; again, higher durability is usually accomplished with more expensive supplies and printers. High Definition Printing™ - or HDP™ - technology is designed to produce identification cards with higher quality images, more security, and increased reliability and durability - all at a competitive cost.

Dye diffusion thermal transfer

A common method of fabricating printed identification cards uses dye diffusion thermal transfer - or D2T2, often referred to as dye-sublimation - to transfer colored dyes from a print ribbon to the blank identification card. In this process, a ribbon carrying dye in a polymeric binder is heated from behind with a thermal printhead (TPH), while in contact with a vinyl (or PVC) surface. As each pixel heats the ribbon, the dye melts and diffuses from the ribbon into the vinyl surface. Printing with successive yellow, magenta, and cyan panels across the substrate creates a three-color image in the surface. True color representation is possible in the printed image because the amount of transferred dye is directly proportional to the amount of heat from the corresponding pixel.

When using D2T2 methods to print to hard, plastic identification cards, the apparent quality of the printed image frequently depends on the ability of the mechanical printer systems to accurately register each of the printing sequences (yellow, magenta, cyan, and black) and to smoothly move the substrate and ribbon beneath the TPH during the print sequences. Since the efficiency of transferring the dye from the heated pixel to the surface of the card depends on close, intimate contact, the presence of dirt, debris, or surface imperfections will preclude contact of the ribbon with the surface; this leaves corresponding voids and vacancies in the printed image.

Frequently, the D2T2 print ribbon also contains a resin panel (to print black resin for barcode applications) and a clear protective overlaminate panel (for application over the printed image). Mass transfer of both the resin and overlaminate panels is accomplished as the TPH heats the ribbon. The material releases from the ribbon and adheres to the surface of the card. Again, the apparent quality of the printed resin image, and the coherence of the overlaminate panel, depend on the ability of the mechanical printer system to accurately register each of the ribbon panels and smoothly move the card and ribbon beneath the TPH during the print sequences. However, while imperfections in surface smoothness interfere with dye transfer, it is surface contamination - such as oils - that interfere with the adhesion of the resin and overlaminate to the surface of the card.

High definition printing

The principle of the High Definition Printing process is that a mirror image is printed onto the HDP film with the D2T2 process. The two-component HDP film - the printed receptive layer with a protective coating underneath - is then transferred onto the desired card substrate. The film adheres to the card surface, with the printed image against the card surface and the protective coating exposed. Once transferred to the substrate, the original, "unmirrored" image (in the receptive portion of the film) is visible through the transparent, protective portion of the film.

With this method, printed images can be placed on cards that are not normally receptive to printing with current, direct-to-card D2T2 printing methods, including radio frequency proximity cards, IC smart cards, and other cards with rough or matte surfaces. Furthermore, the image transfer film adheres to polyvinyl chloride (PVC), and alternative card materials, such as acrylonitrile butadiene styrene (ABS), polyethylene terephthalate (PET), and polycarbonate. It also adheres to mazin, teslin, and natural paper products. This makes HDP technology attractive in applications where PVC is not acceptable.

Print quality

The pixel density of the TPH primarily determines the inherent quality of the printed image. However, the mechanical and electronic systems of the printer can either enhance or detract from this baseline. Mechanically, the HDP printers print to the image transfer film, rather than a hard identification card; this allows smoother, more controlled movement of the receptive surface under the TPH. By controlling movement at 600 dpi, a smoother, more defined image is achieved, while still using a 300 dpi thermal printhead. The effect of these mechanical, electronic, and software improvements is a high-quality image that can reach 256 shades of gray. In addition, dirt, debris, oils, and other surface imperfections are greatly reduced by unwinding the HDP film as it is printed.

Security

The security of the identification cards can be defined as the ability of the fabricated cards to resist tampering or reconstruction. During the standard direct-to-card D2T2 printing process, the image migrates into the surface of the identification card. However, unless some sort of protective material is placed over the image to secure it, the image remains readily accessible for fraudulent alteration. Unfortunately, the protective material itself can contribute to the degradation of the identification card.

With HDP technology, however, the image is effectively sealed within the surface of the fabricated identification card by transferring the printed HDP film to the blank substrate. Therefore, an effective protective film is already in place, above the printed image. Furthermore, in the same way that dye migrates into the surface of a PVC card during the standard direct-to-card D2T2 process, dye migrates into the surface of the card during the thermal lamination required for transfer of the printed HDP film. Thus a latent image is produced in the substrate, making it difficult to reconstruct the identification card with counterfeit information if the protective surface of the HDP film is compromised.

Reliability

Reliability of identification cards is related to the durability of the fabricated card under expected wear. Since the completed identification card is composed of the image receptive layer sealed between the protective layer and the card substrate, the reliability of the identification card is determined, primarily, by the robustness of the composite structure and the strength of the protective film. To evaluate the durability of identification cards fabricated with the HDP printer, samples were submitted for testing with three conventional constructions as benchmarks. The three conventional constructions were obtained by direct printing a test image on blank, graphics-quality identification cards with the standard direct-to-card D2T2 process. Then, one of three standard protective overlaminates was used: print ribbon clear protective overlay (OP), clear thermal thin film (TTF), and 0.001" adhesive backed, clear polyester film (patch).

Dye Migration Resistance

A piece of plasticized vinyl was placed on top of each of the cards representing each card type, under a nine pound load, at 82oC (180oF) for 32 hours. The cards were examined for signs of discoloration or any other deterioration. After exposure, the print ribbon clear protective overlaminate allowed severe transfer of the image to the plasticized vinyl. The clear thermal thin film allowed significant transfer of the image to the plasticized vinyl. The HDP sample card - composed of the two-component image transfer film - exhibited protective characteristics comparable to the sample card protected with the 0.001" adhesive backed, clear polyester film, as illustrated in the table below.

Dye Migration Resistance Comparison
OP	TTF	HDP	Patch
severe image transfer	significant image transfer	no effect	no effect

Abrasion Resistance

The printed, protected face of sample cards of each type were subjected to Taber Abrasion cycles using dual CS10F wheels weighted with 250 grams on each wheel. At regular intervals, the cards were examined for loss of printed information, until a maximum of 4,000 cycles without loss of print image was reached. As illustrated in the table below, only the 0.001" adhesive backed, clear polyester film offered sufficient protection to protect the printed image through 4000 cycles. However the image transfer film offers abrasion resistance comparable to that offered by the thermal thin film and significantly superior to that offered by the print ribbon clear protective overlay.

Abrasion Resistance Comparison
Cycles	OP	TTF	HDP	patch
500	image loss	no effect	no effect	no effect
1000	--	no effect	no effect	no effect
2000	--	image loss	image loss	no effect
4000	--	--	--	no effect

Chemical Resistance

Cards of each type were submerged in water for 24 hours. Additional cards were also placed in solutions of 2% dish washing liquid, 25% salt water, 70% isopropyl alcohol, beer, Pepsi®-cola, bleach, and brewed coffee for a period of four hours. The cards were examined for structural deterioration, discoloration, or any other changes. As illustrated in the table below, of the combinations tested, the thermal thin film was susceptible to attack by the 70% isopropyl alcohol. All the other identification card constructions exhibited no apparent effects to exposure to the selected chemicals.

Chemical Resistance Comparison
Chemical	TTF	OP	patch	HDP
Water	no effect	no effect	no effect	no effect
2% Dish Washing Liquid	no effect	no effect	no effect	no effect
25% Salt Water	no effect	no effect	no effect	no effect
70% Isopropyl Alcohol	film fogs	no effect	no effect	no effect
Beer	no effect	no effect	no effect	no effect
Pepsi Cola	no effect	no effect	no effect	no effect
Bleach	no effect	no effect	no effect	no effect
Brewed Coffee	no effect	no effect	no effect	no effect

Cost

As previously indicated, the standard direct-to-card D2T2 process requires a highly polished, defect free surface to print directly to the identification cards. The equipment and facilities required to produce such prime identification cards are relatively costly, which translates to higher card costs.

HDP technology was designed to provide a method for printing to the difficult surfaces presented by IC smart cards and radio frequency proximity cards. These surfaces are not acceptable for direct printing with the standard direct-to-card D2T2 process. Non-graphic quality solid plastic blank cards, which are not suited for the standard direct-to-card D2T2 process, may also be used for the HDP process. These cards are easily fabricated, and are less costly than the graphic-quality cards required for the direct-to-card D2T2 process.

The net result is that HDP technology fabricates a more durable card at a cost that is competitive with the less durable, conventional card constructions - identification cards with print ribbon clear protective overlay (OP) and clear thermal thin film overlaminate (TTF). HDP technology also offers a significantly less expensive option when compared to the price of printing the more durable - and considerably more expensive - identification card protected with the 0.001" clear polyester film overlaminate (patch).

Conclusions

HDP technology produces superior printed images compared to the images produced by direct printing to blank identification cards. Further, the construction of the fabricated identification cards provides for the printed data to be contained within the surface of the card, under a durable, protective coating, yielding a secure and reliable identification card. Finally, the ability to fabricate identification cards from non-premium card stock - including IC smart cards and radio frequency proximity cards - produces a superior identification card at a reduced cost.

References

Oshima, Katsuyuki. 1998. New Thermal Dye Transfer Recording Method by Using an Intermediate Transfer Recording Medium. 1998 International Conference on Digital Printing Technologies.

© 1999 FARGO Electronics, Inc. All rights reserved. FARGO, High Definition Printing, and HDP are marks of FARGO Electronics, Inc. Pepsi is a registered trademark of PepsiCo, Inc. All other marks are the property of their respective owners.