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Archive for the ‘Paper and Plastic Bags’ Category

Plastic and Paper Bags: How Are They Printed?

Friday, September 28th, 2012

I was curious recently as to how plastic and paper bags are printed, since they are everywhere you look. I assumed that the options would be flexography and screen printing, but since I wasn’t sure, I did some research. I found the answer interesting, so I wanted to share my findings with you.

Long Print Runs of Plastic Bags

For the most part, I found that plastic bags were printed in different ways depending on the size of the press run. One vendor put the cut-off at 3,000 copies. For the longer runs, the more economical technique involved a large web-press. A roll of plastic (composed of two sheets, one atop the other) is held in tension by the press as the plastic sheet travels through a number of inking units. Further research revealed that flexography, not offset lithography, was the appropriate commercial printing method.

Flexography is a process in which the raised image area of rubber printing plates deposits ink on the substrate. (This is unlike offset lithography, which uses flat, or planographic, plates.) The flexo plates are affixed to the press rollers with adhesive, and as the rollers turn and the plates strike the ribbon of sheet plastic traveling through the web press, the plates print the graphic designs on the plastic. This process allows for printing of a number of colors, but it does not allow for as tight register of the colors as does offset lithography.

Once the ribbon of sheet plastic has traveled through the press and has been rewound at the delivery end of the equipment, a post-printing step can be done in which the bags are cut off the single sheet of plastic (i.e., cut at the top and bottom) and fused with heat (at the top, bottom, and sides), creating a plastic bag.

Short Print Runs of Plastic Bags

I also found a video from Asia showing thicker plastic bags being placed one at a time in a platen press, a letterpress in which a round “platen” (pressure plate) comes down onto the bed of the equipment to press the inked plate (with raised image areas) against the plastic bags. In the video it was clear that the process was slow and manual, in direct contrast to the web flexo press. The video also showed that one color was being printed at a time.

Screen Printing on Plastic Bags

Unlike the web of uncut plastic used on the flexo presses, videos of the screen printing option showed the custom screen printing operator placing an individual bag around a wide, flat support structure not unlike an ironing board (also called a “platen”). This kept the bags stable as the operator lowered the metal screen onto the bag and dragged the squeegie and ink across the screen. Like the platen letterpress from Asia, this seemed to be a labor intensive process (i.e., only good for short runs of the bags, printed with only a few colors).

Inkjet Imprinting on Plastic Bags

I also learned about the use of inkjet printers with plastic bags, although this technology seemed to be more appropriate for imprinting tracking numbers than for printing graphic images.

Ink Colors for Printing Plastic Bags

In my research I found that most printers would produce bags within a limited palette of “standard” colors (for instance, one or two distinct blues instead of all possible PMS blues). For an additional fee, however, they would mix a PMS color.

Printing on Paper Grocery Bags

I found videos showing brown paper grocery bags composed of a single, thick sheet of Kraft stock being printed on custom screen printing equipment. Operators were printing the bags by hand one at a time, laying the open bags over a “platen” (the flat, ironing board-like structure), and then lowering the screen and applying ink through the screen with a squeegie.

Printing on Luxury Department Store Paper Bags

I found that high-end department store bags (like Tiffany bags) were printed in two different ways. One video showed an automated screen press turning a pre-made high-end department store bag over to allow the ink screen and squeegie access to either side of the paper bag. This was for a simple design screen printed in one color.

For more complex design work, I learned through my research that the bags are initially offset printed on flat press sheets and then die cut, folded, and converted into laminated bags with the exterior paper stock folded over the edges of the bags and pasted into their interior. These often have specialized rope or fabric handles affixed to the bags through die cut holes. As with other offset press work, various coatings, hot-stamp foils, and embossing effects can be used. In some cases an additional layer of paper covers the interior of the bags. These are high-end, specialized works of art in themselves produced by highly skilled commercial printing establishments.

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