Part of Metal Type’s Printing advice section, here Phil discusses printing paper bags.
There has been some preliminary discussion about printing paper bags of late. Questions about soft or hard packing and what kind of plates to use, and so forth.
Here is what I ended up doing after (in some cases) years of trial and error.
I used soft polymer plates; but with this kind of plate, I had to use a very minimal roller pressure or roller contact with the form. I found that too heavy a roller pressure caused very rapid wear on the plate. And I had to ask the plate maker to etch out the plate as deeply as he could.
For the most part, no makeready was required on the work that I was doing. Just put the job on the press and in this case I used Heidelberg platens, and set the rollers to give barely a sixteenth inch touch and run the job.
Excess Glue
The biggest problem that I had running paper bags was bags stuck together from sloppy or excess glue in the bag making process and or very curly bags at the feed edge. Most of these bags that I printed would have been much easier to run on a hand feed press but at this time in my career, I no longer had a hand fed press.
I also printed some grain bags that were made of very heavy kraft paper and double thickness with tar or bitumen between the layers of kraft to discourage mice and or rats from chewing through the bag to eat the grain contents. There was also a very clumsy fold over feature that allowed the user to seal the bag for shipping or mailing.
The overall difference in thickness of these grain bags was about 60 thousands and still no makeready was required. And in fact I ran these grain bags without packing except for a single tympan sheet to protect the steel platen from getting dirty with tar leaking from between the sheets of kraft.
I do not know if the grain and farm people still use such a bag. I have not seen any now for years; but we sure ran lots of these on our Heidelberg presses.
The bags were so thick that I had to set the elevating mechanism on our presses to maximum to be able to run the job. I could only put about 50 such bags in the feeder at a time.