In-Line Printing on Tyvek

Minimize registration problems, feathering, and other complications by choosing the right printer and supplies.

by Earl T. Hackett, Jr., Technical Marketing Associate, DuPont Medical Packaging

The Sirius line of thermal-transfer ribbons from Dynic USA Corp. (Hillsboro, OR) are available in wax/resin blends and color
formulations for printing on Tyvek.
The need to print variable information on medical packages has resulted in the increased use of electronically controlled printing processes. These devices can output variable information, such as lot numbers, production dates, sequential numbering, product codes, and bar codes. Often referred to as print on demand, printing variable information is a growing trend in medical packaging, especially for packaging created on form-fill-seal machines. The ability to print different labels without changing the top web is the most obvious advantage of such a system.

When considering the purchase of print-on-demand systems, careful attention should be given to the method by which the top web is conveyed through the printing mechanism. If the top web is pulled through the printing mechanism by the movement of a chain holding the bottom web, all the force required to pull the top web is transmitted through the heat seal. This puts a premium on the adhesive’s hot-tack performance. And, if the path through the printer consists of more than one or two rollers, there is a good probability that the seal will lift slightly on the trailing edge of the package.

Although your present packaging system might perform well, the high web tension characteristic of a chain-pulled system may hinder the adoption of newer packaging systems in the future. A powered drive in the printer will reduce the need for hot tack in the adhesive. It will also reduce tension in the top web as it feeds into the sealing die, minimizing problems with loss of registration and package bending.

The most common process for printing variable information is thermal transfer. This process uses heated pins to activate a pigmented wax, resin, or wax/resin blend that is carried on a ribbon. The image is created when the molten ink is transferred to the substrate. For medical-grade styles of Tyvek, which are not corona treated, wax ribbons offer the best results. However, image durability is marginal. If more durability is required, a wax/resin-blend ribbon should be used. A 90%/10% wax/resin-blend ribbon yields good results. It is important to note, however, that this blend may need to be custom manufactured because many ribbon manufacturers only stock 50%/50%-blend ribbons.

Excellent results in printing alpha-numeric information have been achieved using 300- to 600-dpi printers. Low- or medium-density bar codes can be printed to a D-C ANSI bar code quality using the thermal-transfer process. On the other hand, bar codes with a consistent C rating can be printed on Tyvek 2FS because it has a higher contrast than either Tyvek 1073B or Tyvek 1059B because of the addition of an opacifier.

Ink-jet printing is another method used for printing variable information on medical packaging. Medical-grade styles of Tyvek have been printed on successfully using continuous and drop-on-demand ink-jet printers. However, because Tyvek is made of high-density polyethylene and does not absorb water, solvent-based inks must be used. Most water-based inks are slower drying and tend to feather on Tyvek, resulting in a blurry image. Acceptable results have also been achieved with ultraviolet and change-of-phase inks, which cure almost instantly. Typically, 200- to 300-dpi print heads are used.

Although the graphics industry has seen many improvements in printing materials and techniques over the past few years, most of these advances are not available to the medical packaging industry because of the stringent requirements associated with sterile packaging. However, under the proper conditions, excellent printing results can be obtained on medical packages using the equipment and materials currently available.

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