Photo purchased from … www.depositphotos.com
What you see in the photo above is not a roll of commercial printing paper (a web) feeding into a heatset web press but a huge take-up roll of paper, winding up the custom printing stock as it is converted from a liquid pulp mixture into the dry, flat printing paper onto which presses will deposit ink. The Fourdrinier machine, as it is called, is almost as long as a football field. The paper pulp mixture travels through the machine on a wire mesh, and the water is allowed to drain off, yielding a flat, stable mesh of paper fibers suitable for commercial printing.
For the most part your reader will not notice the paper on which your job is printed, at least not consciously. Actually, this is a good thing. The tactile qualities of the sheet (coated or uncoated) as well as its thickness, color, and overall quality, will affect the reader’s subconscious and transmit a sense of quality, and perhaps an environmentally conscious feel. Therefore, it’s prudent to make sure the paper choice supports the message you are trying to convey.
That said, here are some more concepts relating to paper, to help you make more informed decisions and communicate more effectively with your printers.
Paper Reflects Light
Paper reflects light. Light travels through the layers of transparent process ink, bounces off the substrate, and travels to the reader’s eyes. The color or tone of the paper influences the reader’s perception of the ink colors. That is, a yellow-white (or cream, or natural) sheet will add a yellowish tone to the inks, which may be incompatible with the subject matter (such as people’s faces).
Interestingly enough, if you print 4-color process inks on a transparent acetate sheet (instead of paper), there is nothing behind the process ink to reflect light back to the viewer. So the colors will be very dull, if not difficult to see at all. To remedy this problem, printers lay down a background of white ink behind all 4-color process inks printed on clear plastic using an inkjet printer. This makes the process inks stand out.
So don’t underestimate the powerful influence of the substrate (paper or clear film) on the printed images.
Paper Density, Bulk, Caliper
Paper is composed of wood fibers, chemicals, and lots of air, among other things. (Paper is similar to a sponge. You can squeeze out the air or let it puff up.) A high-bulk paper and a more dense paper may both weigh the same (measured at, for instance, 500 sheets of 25” x 35” stock), but the high-bulk paper will feel thicker. (You can measure the thickness or caliper with a micrometer.) There are several benefits to using a high-bulk paper, which has not been flattened as much as a lower-bulk paper by the polished metal calender rollers on the paper-making machine.
These calender rollers flatten the paper and give it a harder surface, which allows ink to sit up on the surface of the sheet and makes for crisper photos (you can use more ink and a finer halftone screen than with an absorbent paper).
You can also give the reader more of a perception of quality with a higher bulk sheet (thicker paper feels more substantial). Or you could save money shipping a large job (let’s say 60,000 perfect bound print books) by using a lighter press sheet (perhaps 60# rather than 70#) that is of a higher bulk.
If your job is just text on paper (let’s say a novel), you may even want to use a rough 55# uncoated sheet that feels as thick as a 70# harder-surfaced, thinner sheet. For printing only text, the absorbency of the paper would not be as important as for printing photos.
Paper Formation
Paper formation pertains to the evenness of the custom printing sheet. Remember (from the earlier discussion of how paper is made on a Fourdrinier machine) that paper starts as a liquid and travels on a wire mesh through the papermaking machine, losing water as it proceeds. A high quality sheet will have good paper formation. This means there will be a smooth and even distribution of paper fibers, and the paper will be of consistent thickness. When you hold a good sheet up to the light, the paper will not appear blotchy, or denser in some areas and more transparent in others. Because of this, on a high quality press sheet printed areas (line work, screens, or heavy solids) will be consistent and not mottled.
Good formation also ensures adequate opacity (or the blocking of light traveling through the paper). This means you won’t be distracted by what is printed on the back of a printed page when you’re looking at the front of the page.
Paper Grain
In prior blog articles on commercial printing paper, I have mentioned that most paper fibers line up in the same direction when the paper is made. (This direction is parallel to the direction the paper travels through the Fourdrinier machine.) Whether paper is grain long or grain short then depends on how the web (roll) of paper is then chopped up into flat, cut press sheets.
Paper folds easier/better parallel to the grain and is stronger against the grain (perpendicular to the grain).
But how can you determine the direction of the grain (other than by reading the label on the carton in which it arrives)?
- You can put a sheet of paper on a table and extend it off the edge until it droops a bit. In the direction parallel to the grain (with the grain), the sheet will hang lower. (That is, it will not have the strength of the paper fibers holding up the sheet and making it more rigid in the opposite direction.)
- You can tear the paper. When you tear a sheet with the grain, it will tear smoothly, in a straight line (basically, you’re tearing the sheet between rows of paper fibers). In contrast, if you tear a sheet against the grain, the tear will be harder to achieve (because you will be breaking the paper fibers). The tear will be jagged and uneven.
Why does this matter? For one thing, paper in a print book must have the paper grain running parallel to the spine for the pages to open and turn easily and for the book (such as a saddle-stitched product) to lie flat when folded shut.
Aside from this, your printer may choose a grain direction for durability, since paper folded against the grain is stronger at the fold than is paper folded with the grain. But if he does this, to avoid having the paper fibers break, he will usually score the stock with a metal rule first, before he prints ink on the paper. This allows for smoother, more even folds with less chance for the paper or ink coating to crack.
Dimensional Stability
Because of the grain alignment in a sheet of paper, paper dimensions are important. You need a trimmed sheet to have a consistent width vs. height, and you need the sheet to be trimmed square, with four 90 degree angles at the corners. Anything else will cause major headaches on press.
What makes this a challenge is that paper absorbs moisture from the surrounding air and expands. Moreover, this growth is not consistent when you compare the side parallel to the paper grain to the side perpendicular to the grain.
To say this in a different way, paper expands up to 300 percent more against the grain than with the grain. (This will be clearer if you picture the rows of paper fibers spreading apart from one another but the fibers themselves not lengthening.)
So paper is really a bit like a sponge, and if you don’t adequately control the temperature and humidity at which it is stored before being used on press, you will have serious problems with dimensional stability. And this is why your printer takes delivery of cartons (or rolls) of paper and then lets them gradually become acclimated to the proper ambient conditions before being brought into the pressroom and used to produce your custom printing job.
Problems can become even more challenging in the high humidity of summer or the cold weather of winter, when you consider the ambient conditions of the paper as it transitions from the delivery trailer truck to the printer’s loading dock and then into the pressroom.
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on Monday, March 14th, 2022 at 12:22 am and is filed under Paper and finishing.
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