Terminology

 

Basis Weight

The weight in pounds per ream of paper cut to its basic size. Uniform basis weight is necessary for other paper properties to be uniform.

Brightness

Brightness is the percentage of light at a certain wavelength (457 nanometers) that is reflected from the surface of the paper. High brightness papers give greater contrast with black inks and a more lively appearance of ink colors.

Caliper

The thickness of a sheet of paper, expressed as thousands of an inch, or points. Caliper is important in offset papers where the page count per inch is specified. Uniform caliper is needed for uniform printing and for runnability in converting processes.

Curl

Curl is built into copier papers so that after they run through the complicated path inside a copier, the paper will be flat. Printing papers should not have curl, or they will not feed well on presses, finishing or binding equipment. Moisture curl is the result of the paper’s tendency to come into equilibrium with the surrounding atmosphere. Thin papers curl more readily than thick papers. To avoid curl, keep tight controls on the temperature and humidity of the pressroom. Ideal conditions are 75 °F (24 °C) and 45% relative humidity. Temperature acclimatization is a good idea.  Bring the paper into the room where the copier or press is located 24 hours before opening cartons or removing ream wrappers. This is especially important during cold weather.

Felt and Wire Sides

Refers to the top and bottom surfaces, respectively, of paper made on conventional fourdrinier paper machines. In general, the felt side contains a higher percentage of fillers and short fibers than the wire side. However, modern paper machines, such as twin wire machines, largely eliminate these differences

Formation

Formation is judged by transmitting light through the sheet and looking at its structure and degree of uniformity. Paper with good formation prints with less mottling and has more uniform opacity.

Grain

Refers to the alignment of fibers in the direction of their flow on the paper machine. Folding and scoring work best when done in the paper’s grain direction.  Grain also affects tear strength, stiffness and dimensional stability.

Ink Receptivity

It is vital for paper to accept ink uniformly for even coverage, and thoroughly to prevent offsetting in printing.  Smoother, less porous papers require and absorb less ink.

Moisture Content

This is the amount of moisture contained by paper, expressed as a percentage of its total weight. Uniform moisture is a necessity in all grades of paper.

Opacity

The ability of paper to obstruct light transmission and the show-through of printing. It is particularly important in two-sided printing. It also affects readability and overall appearance.  Opacity is improved by scattering, absorbing or reflecting light. Fillers such as titanium dioxide and calcium carbonate scatter light, while blue and violet dyes absorb it. Therefore, more opaque papers are generally a more blue-white shade.

Pen & Ink

Bond papers traditionally carry sufficient internal and surface sizing to withstand feathering when written on with fountain pen ink. Ledger papers withstand fountain pen ink and also have good receptivity for ruling ink.

Porosity

Refers to the resistance paper has to air permeation. A higher number means less porosity. High porosity is needed in duplicator papers for brighter copies, for Offset papers to decrease ink consumption, and for papers that will be converted to avoid misfeeds or double feeds in vacuum pick-up and transfer systems. A low porosity is desired in mimeo to allow oil-based inks to absorb and dry.

Sizing

Internal sizing in the paper affects absorbency, strength and permanence. External sizing improves resistance to water, ink and other fluids, seals down surface fibers and improves surface strength. Typical sizing agents are rosin, glue, gelatin starch resins, waxes, etc.

Smoothness

Refers to the evenness of the paper’s surface. A smoother sheet generally results in cleaner printing results for offset, letterpress, gravure and spirit duplicating. Mimeo, however, requires a vellum (rougher) finish to help prevent set-off.

Stiffness

Stiffness is basically determined by the weight/thickness ratio of the paper.  It adds to the snap and crackle of bond papers, helps in feeding for lighter weight papers, and is required for ledger to stand up properly in files.

Strength

Measured by tests for burst, tear, tensile and folding strength. All printing papers need strength to run through presses at normal production speeds. (Web papers require more tensile strength than sheet-fed papers.)

Surface Bonding

Refers to resistance of surface fibers to separating or “picking.” Picking can become a problem if the ink is too tacky or if there is too much pressure on the impression cylinder.

 

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