A wide range of papers and other substrates are available for digitally printed applications. Continuous improvements in digital printing technology allow applications to be printed on nearly every type of substrate that can be used for projects printed with conventional printing processes. Not only can digital print projects be printed on hundreds of paper stocks of varying weights and finishes, but other materials such as polyester surfaces, vinyl substances, transparencies, self-adhesive label stocks, textiles, and foils are among the many substrates that can also be used.

It is unwise to assume that any paper that can be used for conventional offset printing can be used for digital printers and presses as well. Paper used for applications printed with digital equipment must be approved for use with digital printing technology. Digital printing offers a unique set of criteria that paper must conform to in order to be used with the equipment. Paper specifically produced for digital printing equipment is known as digitally optimized paper.

The following paper primer describes the basic properties of paper used for the printing industry and the unique characteristics of paper used for digital printing.

Paper types can be classified using a number of differing criteria, but most papers used in the printing industry fall into two classifications: groundwood and freesheet.

• Groundwood
  
  Paper stocks are classified as groundwood when more than 10% of the pulp content is derived from a mechanical process that grinds wood into pulp. The cellulose in the wood is the major component of the pulp, but since other undesirable components, such as lignin and resins, are not removed, they also become part of the pulp. The lignin is what causes paper to yellow quickly and become brittle, so groundwood stocks are good choices for non-permanent printed items such as newspapers. In addition to this, the wood grinding process produces a pulp with very short fibers so the resulting paper has low tear strength. Groundwood stocks are less bright and less permanent than paper produced from chemical pulp. Chemical fibers are added to many groundwood stocks to give them added strength.
  
  
• Freesheet
  
  Freesheet refers to paper that is free of pulp produced from a mechanical grinding process. Chemicals, rather than grinding, are used to reduce the pulp to fibers. Wood chips are cooked in chemical baths that remove the undesirable ingredients, such as lignin and resins. Most of the cellulose remains, resulting in fibers that are longer, stronger, and cleaner, which produces paper that is brighter and much more permanent than groundwood stocks. Most digital paper stocks are produced from freesheet pulp.

Both groundwood and freesheet paper types can be separated into uncoated and coated stock subcategories.

• Uncoated Stocks
  
  Uncoated stock is paper that has no coated pigment applied to reduce the absorbency or increase the smoothness. The uncoated finishes can be described as vellum, antique, wove, or smooth.
  
  
• Coated Stocks
  
  A coated stock has a surface coating that has been applied to make the surface more receptive for the reproduction of text and images in order to achieve sharper detail and improved color density. A coated clay pigment, for example, improves the smoothness of the paper and reduces the absorbency. Coatings added to groundwood papers provide them with a greater degree of permanency. The natural tendency of groundwood papers to yellow is also reduced.
  
  Coated paper finishes can be categorized as matte, dull, cast, gloss, and high gloss. The coating can be on both sides of the stock (coated two sides, "C2S") or on one side only (coated one side, "C1S"). Coatings can be applied online during the paper manufacturing process or afterward by an offline coater.

From the classifications and subcategories, paper stocks are then separated into types, such as offset, bond, cover, index, vellum bristol, and so on.

The grade of a paper refers to the quality category of a paper stock determined by the method of manufacturing and by the contents of the paper, which provide the various surface characteristics. Grade classifications range from Premium, which is the highest level, to #5, which is the lowest level. Some text and cover stocks are listed simply as A or B grades since fewer grades of text and cover stock are produced.

Paper grade also refers to the specific type of paper, such as coated, uncoated, bond, or index. Digital paper grades may include laser, ink-jet, or multipurpose papers.

Basic Size and Basis Weight

When industry professionals refer to the basis weight of a particular paper, they are referring to the weight in pounds of 500 sheets of paper when it has been cut to that paper's standard basic size. The basic size of Bond paper, for example, is 17 x 22 inches. If 500 sheets (a ream) of Bond paper is cut to its basic size of 17 x 22 inches and weighs 20 pounds, it is classified as 20 lb. bond. If a 17 x 22" ream of Bond paper weighs 24 pounds, it is called 24 lb. Bond, and so on.

The chart below lists some common paper types and their basic size:

ISO Paper Size Standards

The International Organization for Standardization (ISO) has established standards for paper sizes based on the metric system with length and width measurements designated in millimeters. The standards have been arranged into three different groups of requirements:

• A-series: for general printing
  
  
• B-series: for posters
  
  
• C-series: for envelopes, postcards, and folders

The "A" series is the most commonly used with sizes ranging from A0, which is the largest, down to A8. The A-series sizes are all represented as a part of the area of one square meter with a length to width ratio of 1.414. The size A0 is equivalent to the area of a square meter with each smaller size being 50% of the size of the preceding one. A1 is 50% of the area of A0, A2 is 50% of A1, and so on. Another way to look at it is that when an A0 sheet is cut in half, two A1 sheets are produced, and when an A1 sheet is cut in half, two A2 sheets are produced. Some of the sizes for the A-series are illustrated below.

North American Paper Size Standards

Although the ISO size standards are common in many parts of the world where the metric system is the established standard for measurement, North American sheet sizes are based on inches and are illustrated below.

Universal Paper Web Sizes

The paper rolls shown below are the standard widths used for web-fed printing presses. The illustration also shows the standard ISO sheet sizes and the standard North American sheet sizes that can be obtained from each of the various roll widths.

Caliper Readings: The chart below shows the actual thickness of various weights and grades of paper. The readings are determined with the use of a caliper or micrometer gauge, which measure the thickness of the paper in thousandths of an inch.

The finish refers to the surface characteristics of the paper. When determining the proper digital paper to use for a project, there are many questions to consider before selecting the correct stock. For example, does the paper need to feel smooth or rough? Does the application require a paper that has a glossy appearance or does the paper need to appear dull? Will the paper play an integral role in enhancing the look of the printed piece or is the paper's use purely functional? Does the print project require a paper with a high ink absorption rate? Your print supplier can provide assistance in selecting the proper paper type and finish for any application.

Finishes can be applied to paper during the paper
manufacturing process or they can be applied as an off-line process.

• Online: A finish such as Laid can be created while the paper is being manufactured. A marking roller is used that forms the pattern in the paper while the paper is still wet.
  
  
• Off-line: Paper finishes produced off-line are usually accomplished with steel rollers that press the pattern into the paper. The off-line finishes are known as embossed finishes.

Some common paper finishes are described below.

• Cockle: A cockle finish simulates characteristics of hand made paper with a wavy, rippled, puckered finish. The effect is obtained by air drying the paper under minimum tension.
  
  
  
  
  
  
  
• Felt: Felt is a soft texture on uncoated paper that is created during the papermaking process with either a felt covered roller or a rubber roller with a felt pattern that creates the finish. It can also be accomplished as an off-line process. The felt finish does not affect the strength of the paper.
  
  
• Gloss: A gloss finish produces a shiny and reflective surface on one or both sides of certain coated papers. A higher gloss is usually seen on higher quality coated papers. The gloss finish is produced from compounds added during the paper making process.
  
  
  
  
  
• Laid: A laid finish has the appearance of translucent lines running horizontally and vertically in the paper. It is produced during the papermaking process with a special roller that creates the pattern in the wet paper.
  
  
  
  
  
  
  
  
• Linen: Linen finished paper resembles linen cloth and is usually produced after the papermaking process as an off-line embossing process.
  
  
  
  
  
  
  
  
  
• Matte: A finish on certain coated papers that is smooth but gives a dull appearance. A matte finish, as well as other types of coated paper, are good choices for print jobs in which high quality is required.
  
  
  
  
• Parchment: A paper finish that has an old or antique appearance and is the result of washing sulfuric acid over the paper and then quickly neutralizing the acid wash. This process melts the outer paper fibers which fill the voids in the rest of the paper. Parchment is very durable and grease resistant.
  
  
  
• Smooth: A smooth finish is the result of the paper passing through sets of rollers during the papermaking process. This process is known as calendering.
  
  
  
  
  
  
• Vellum: A vellum finish has an eggshell appearance. The finish is consistent and even, but not to the extent of a smooth finish. Vellum is one of the most popular uncoated finishes and paper with this finish has a high ink absorbency rate.
  
  
  
  
  
  
  
• Wove: A type of even finish in uncoated paper. It has a slight texture that is created by a felt roller covered in woven wire.

Paper grain refers to the fibers of the paper. The grain can be either long grain or short grain. Long grain refers to paper in which the fibers line up in the same direction as the longest measurement of the paper. For digital web-fed presses, the grain of the paper is parallel with the length of the paper web. When paper is torn parallel with the grain, it tears easily. The tear is also fairly straight. Short grain refers to paper in which the fibers line up in the same direction as the shortest measurement of the paper, or perpendicular to the long dimension.

Paper grain plays a key role in the success or failure of a digital print application in terms of production and proper use. Paper folded parallel with the grain, produces a cleaner fold than if folded across the grain. This is an important consideration for applications printed with toner since folding against the grain may cause toner to flake off in the folded area, although because of improvements in technology, this is not as common as it once was. Laser printers require the used of long grain paper for the best results. Short grain paper may not feed properly into a laser printer and the heat produced by a laser printer may result in excessive curling of the paper as the sheets exit the printer.

Brightness

Brightness refers to the percentage of light reflected by a sheet of paper. The paper brightness is measured by a light meter. As the paper brightness increases, the degree of contrast between the paper and the print also increases. This is often an important element when printing photographs or colorful graphics. Paper with lower brightness is often best for books that do not include images because extended reading of text on high brightness paper can strain the eyes.

Brightness levels are usually indicated on the paper packaging. The levels usually range from 84 to 98, which are adequate for most digitally printed applications. Brightness levels of 94 to 98 are most often used on color digital printers and presses.

Whiteness

The whiteness of paper is the measure of its ability to reflect the colors of light equally. The more evenly a paper reflects all colors of the spectrum, the better suited it is for presenting an accurate color representation of the printed images. Some papers may reflect cool colors back to our eyes and give the illusion that the sheet is actually brighter than white paper. If white paper has a slight warm appearance it will not appear as bright as a sheet that reflects a cool color, however warm colors printed on a warm sheet will appear stronger than they would if printed on a cool sheet. Cool colors printed on a cool white sheet are enhanced in the same way.

There really is no way to claim that a particular paper appears pure white because the white that we see is influenced by the lighting conditions of the environment and the colored reflections from surrounding objects. The whiteness of digital papers is usually classified as true white, cream white, or blue white. Although true white varieties of paper reflect all colors of the spectrum equally, blue white varieties actually appear whiter to the human eye. True white paper varieties are often used for applications containing a high proportion of graphics and images. Blue white varieties are used for this purpose also, but the overall appearance of the printed document often appears cooler. Cream white papers, which provide a slightly yellow tone, are popular for digital book publishing. Paper whiteness should always be a consideration and should be appropriate for the colors of the graphics and images of the particular print project.

Opacity

Opacity is the measure (percent) of the amount of light passing through a sheet of paper. Some papers have more fibers and/or fillers and as a result are more opaque than others. Papers containing more fibers and fillers have the ability to hold a printed image without it showing through to the backside. A paper that is thick does not necessarily guarantee that it is more opaque than a thinner paper. Some thinner papers may be more opaque because they contain a greater number of fibers and/or fillers in their composition.

Smoothness

The smoothness level is a measure of the surface characteristics of paper. The flatter or more even the surface, the higher the level of smoothness. With a smoother surface, the stock can provide a fully shaped ink dot resulting in a sharper and higher quality image.

Holdout

Acidity/Alkalinity

The pH (potential for Hydrogen) measurement of paper determines the degree of acidity and alkalinity in the stock. The pH scale has readings of 0 to 14 with 7 being neutral. Readings below a pH of 7.0 are acidic and readings above 7.0 are alkaline. Each single digit actually equals a measure of 10, so a paper stock with a pH of 4.0 is 10 times more acidic than one with a pH of 5.0. Paper can have an acid base, an alkaline base, or it can be neutral. Most paper manufactured in the 20th century was of an acid base. Acidic papers deteriorate in a relatively short period of time, and should never be used for printed items that are intended to last for many years. Since the 1970's, most of the paper used for book publishing and other printed materials in which permanence is of importance, has been produced with an alkaline base. Alkaline paper is manufactured with fillers such as calcium carbonate, which bring the pH above 7. An acidic paper, such as newsprint, has a pH of about 4.5, which becomes lower once it has been used for printing. The acid level tends to break down the paper and it can deteriorate rapidly, which is why newspapers tend to yellow and fall apart. Alkaline paper (a pH above 7) is said to be permanent, but papers that have a neutral pH are best for preserving items such as photographic materials and matte boards for artwork.

Paper is very sensitive to changes in temperature and humidity. Excessive moisture will cause flat sheet paper to curl. The edges of roll paper for web-fed presses will become slack if excess moisture is present. If paper is unloaded from a truck that has been in freezing temperatures and the paper is immediately used for printing, problems will occur. When the paper is unwrapped in the warmer environment, it will immediately absorb moisture. Some of this moisture will disappear as the paper becomes warmer, but the edges of the paper will curl and stretch. It is almost impossible to use paper that has curled or stretched.

Print producers should always order paper stock with enough lead time so that the paper has an opportunity to become acclimated to the temperature and humidity conditions of the printing facility. Paper should be left in its original wrapping and it should remain unopened until it becomes conditioned to the humidity and temperature of the pressroom. Many printing companies have temperature and humidity controlled environments to lessen the effects of changing environmental conditions.

The time necessary for the paper to be conditioned properly will range from a few hours up to several days depending upon the quantity of paper to be used and the differences in the temperature and humidity levels between the pressroom and the environment where the paper had been stored previously. The chart below shows the number of hours necessary for proper conditioning.

Compared to papers used for conventional offset and other printing processes, digital papers have some unique characteristics:

• Lack of Naturally Occurring Static Charge: Compared to conventional paper, digital paper builds up less static charge as the paper travels through the press. This is an important property because many digital papers are used on equipment that prints with electrophotographic imaging. During the electrophotographic process, the paper is given an electrical charge that attracts dry toners, which are then heated and fused to the paper. Electrophotography would not function properly if the paper built up too much of an electrical (static) charge as it traveled through the press, which paper traveling through a conventional press has a tendency to do. The digital paper must be charged at the appropriate time to attract the toner properly. Digital papers are also smoother than offset papers to allow for better adhesion of the toner.
  
  
• Lack of Acidity: Another important property to consider is the pH of the paper (acid or alkaline base). Many papers manufactured today for digital and conventional printing are alkaline based. Alkaline paper is much better than acid papers in preserving a printed piece for long periods. Anyone who has seen an old newspaper that has become yellow and brittle has seen the effects that acid based paper has on preservation.
  
  
• Uniformity: Sheets that vary in their uniformity may produce inconsistent results in the print quality. Digital papers are produced to the highest standards to ensure that the best possible print quality is achieved.
  
  
• Thickness: Many digital presses have limitations on the thickness of the substrates that can be used for printing (as do conventional presses). While limitations on thickness are becoming less of an issue as technology improves, it is best to consult with your print supplier for their capabilities before deciding on a specific paper stock for your application.