Web/offset printing is how most high volume printing is done. The paper (or substrate) comes in roll form, as compared to sheets, and runs continuously throug the press. If you've ever seen a newspaper being printed, you know what web printing is.
Printing on the plastic bags is actually flexographic printing, which is basically the same process as web printing. The only difference that I'm aware of is the press is confugured for, and prints on plastic versus paper.
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