Chapter 6. Imaging
When talking about substrates used in printing, paper is usually what comes to mind. Paper is made most commonly from wood fibre. Today, many papers also have some percentage of recycled fibre as well as fillers and other additives. These all contribute to the quality of the paper itself and to the quality of the printed output. It’s important to understand some basic attributes of paper as they all have a direct impact on imaging processes and results.
Formation refers to the distribution of fibres, fillers, and additives in paper and how evenly they come together. When you hold a sheet up to a strong light source and look through it, the mix of dark and light areas are the result of formation. The more uniform the formation, the less mottling is observed in the paper. Papers with uniform formation accept inks and toners more evenly, have reduced print mottling, and enhance clarity.
In strict terms, opacity is the degree to which light is prevented from travelling through a paper. In practical terms, it’s how well a paper prevents the image on the backside of a sheet showing through to the front. This is measured on a scale from 1 to 100, where 100 is completely opaque. Opacity can be increased with fillers, pigments, or even coatings. In general, a thicker paper, coloured paper, or coated paper is more opaque than its counterparts. Opacity values are very important when projects require thinner paper stocks and both sides of the sheet are being printed.
Basis Weight and Grammage
When looking at the label on a ream of paper used in North America, you usually see two weight designations: the basis weight, designated in pounds (#) and the equivalent grammage, in grams per square metre (g/m2 or gsm). In most of the world, grammage is primarily used. In North America, the basis weight is more common. Grammage is simply how many grams per square metre paper weighs. No other factors are represented by this designation. So we can deduce that the higher the grammage, the thicker or denser the sheet. Basis weight is the weight of 500 sheets of paper at a specific size, known as the ‘parent’ sheet size, which varies based on the historical use of the specific paper. To understand this better, let’s examine two different basis weights.
Cover basis weight is based on a 20″ x 26″ parent sheet. So 500 sheets of 80# cover (the # symbol is used to indicate pounds) at the parent sheet size weighs 80 pounds. Likewise, 500 sheets of 80# text at the text-weight parent sheet size of 25″ x 38″ also weighs 80 pounds. This can be very confusing as a cut sheet of letter (8.5″ x 11″), 80# text, is much thinner than the same size of 80# cover. Table 6.1 shows common basis weights, parent sheet sizes, and typical uses.
|Table 6.1 Paper weights, sizes, and uses|
|Basis Weight||Parent Sheet Size||Typical Use|
|Bond||17″ x 22″||Historically used as writing paper and typically uncoated. Standard office paper is 20# bond, while colour prints are more commonly done on 24# or 28# bond due to the need for higher opacity.|
|Cover||20″ x 26″||Used for paperback book covers, business cards, post cards. Business cards have typically been 100# cover, but have been trending toward higher weights of 110# and 120#.|
|Text||25″ x 38″||Used for magazines and posters. Relatively thin sheets with higher opacity. Magazines typically use a coated text weight paper for both the cover and the body. Typical weights are 70# to 100#.|
|Index||25.5″ x 30.5″||Used for index cards and tab stock. Tab stocks are typically uncoated 90# index.|
Although basis weight is used as the primary weight on a paper label and description, a digital press will typically use grammage to define the weight property when assigning a paper to a tray. Paper weight is one of the key characteristics that affect many parameters on the digital press, including how much vacuum strength is used for feeding, how much charge is required to transfer toner to paper, and how much heat is required to maintain a consistent fusing temperature to bond toner to the paper, among others. Entering the wrong values for the paper weight can cause paper misfeeds, poor image quality, or toner not adhering to the paper. Using grammage simplifies data entry and avoids errors due to incorrect basis weight selection for the numeric weight value. It may, however, require one to do a conversion calculation if only basis weight is provided. The following conversion factors can be used to do these calculations.
Bond (lbs.) x 3.7606 = gsm
Cover (lbs.) x 2.7048 = gsm
Text (lbs.) x 1.4805 = gsm
Index (lbs.) x 1.8753 = gsm
In the paper manufacturing process, a slurry of fibre travels over a high-speed mesh conveyor belt that is oscillating side to side. This action and movement causes the fibres to interlace and develop a predominant alignment along the direction of movement. This predominant alignment of the fibres is called grain direction. Short grain refers to fibres running parallel to the short dimension of the sheet, and, conversely, long grain refers to fibres running parallel to the long dimension of the sheet.
It is important to keep grain direction in mind when choosing a paper for a project. You need to consider the print process and binding or finishing method you will use, as choosing the wrong grain direction can produce poor results or may be incompatible with the printing method you have chosen. Sheet fed offset lithography papers are often long grain and are most common. Digital presses require the grain to run perpendicular to the feed direction in order to feed properly and make the sharp turns typically found in a digital press. In this case, most sheets are fed into the press with the short edge first therefore requiring short grain paper. When folding is required, folds that run parallel to the grain will be smooth and sharp while folds that run across the grain will be ragged, and the fibres on the top of the sheet may pull apart. Toner used in digital printing bonds to the surface of the paper and does not penetrate. Folding across the grain will cause the toner to break apart where the fibres separate.
The second or underlined dimension of the sheet will indicate the direction of the grain. For example, 18″ x 12″ is a short grain sheet, and 12″ x 18″ is long grain. If the underline method is used, short grain would be 12″ x 18″ and long grain would be 12″ x 18“. If the dimensions are not noted or the sheet is not in its original packaging, grain direction can be determined by folding the sheet along both dimensions. As noted previously, a fold that runs parallel to the grain will be smooth and sharp while a fold that runs across the grain will be ragged. You can also gently bend the paper in either direction. The bend running in the direction offering the least resistance is the grain direction.
Caliper, unlike grammage and basis weight, is a measure of thickness. The most common measurement used in North America is thousandths of an inch, designated as points (common for paper) or mils (common for synthetic paper). This terminology can be confusing, however, as points can also refer to 1/72 of an inch when referring to font size, line thickness, and dimensions on a page. Mils can be confused with millimetres as well. A common misconception is that points and mils can be converted to grammage or basis weight. This is not true. The caliper can vary depending on the coatings or finish. In general, a rougher finished stock will have a higher caliper than the same weight of a smooth stock. Coatings can be heavier than paper fibre so coated paper can have a smaller caliper than the same weight of an uncoated counterpart. A process called calendaring, which irons the paper between two highly polished chrome rollers, improves smoothness and printability but also reduces the caliper without changing the weight of the paper.
Brightness and Whiteness
Brightness and whiteness define the optical properties of paper and differ mainly in how they are measured. Whiteness measures the reflective properties of the paper across the entire visible spectrum of light (defined by CIE). In other words, it defines how white the paper is. A perfect reflecting, non-fluorescent white material measures 100 whiteness. Brightness also measures the reflective properties of paper, on a scale of 1 to 100, but specifically in the blue area of the spectrum at a principal wavelength of 457 nanometres and 44 nanometres wide (defined by TAPPI and ISO standards). This wavelength coincides with lignin absorption. Lignin is what binds the cellulose fibres in wood and pulp and gives it its initial dark brown colour. The more bleaching done to the pulp, the more lignin is removed, and the higher the blue reflectance and therefore brightness. In most parts of the world, paper whiteness measurement is used; however, in North America, most papers use brightness measurement instead. Some papers have brightness values that exceed 100. This is due to the addition of fluorescent whitening agents (FWAs), which return additional blue light when exposed to UV light. The same is true for whiteness, as papers with higher blue reflectance levels tend to have higher whiteness levels.
Finish defines the look and feel of the paper’s surface and can be achieved during the paper-making process (on-machine) or after (off-machine). On-machine finishes are achieved by the application of a pattern onto the paper by a marking roller while it is still wet. Examples of on-machine finishes are smooth, vellum, laid and felt (see Table 6.2). Off-machine finishes are accomplished with rollers that press the pattern into the paper after it has been made. Off-machine finishes are also known as embossed finishes. Linen, stipple, and canvas are examples of these; Table 6.3 gives a description of each.
|Table 6.2 On-machine finishes|
|On-machine Finishes||Description||Typical Uses|
|Smooth||Paper is passed through various calendaring rollers, producing a finish that is uniform, flat, and smooth to the touch.||Ideal for general digital printing and copying as toner is applied to the surface and does not penetrate the fibres.|
|Vellum||A consistent eggshell appearance that is not quite as smooth as smooth finish but has a velvety feel. Not to be confused with the substrate called vellum, which is translucent.||Used most commonly for book paper.|
|Laid||Consists of a series of wide-spaced lines (chain lines) and more narrowly spaced lines (laid lines), which are at 90 degrees to the chain lines.||Used for letterhead, reports, presentations.|
|Felt||A felt-covered roller is used to produce this finish. The appearance resembles that of felt.||Used for letterhead, reports, presentations.|
|Table 6.3 Off-machine finishes|
|Off-machine Finishes||Description||Typical Uses|
|Linen||A cross-hatch pattern resembling linen fabric.||Used for personal stationery, letterhead, fine-dining menus, business cards.|
|Stipple||A fine bump texture that resembles the painted surface of a wall.||Used where a subtle uneven texture is desired.|
|Canvas||Simulates the surface of canvas.||Used for art prints or where a ‘painted’ appearance is desired.|
Coated papers have calcium carbonate or china clay applied to their surface. The coating fills in the spaces between the fibres on the paper’s surface, resulting in a smoother finish. The amount of coating and calendaring produces different finishes and gloss appearance. Examples of coated finishes are matte, dull, satin, silk, and gloss, described in Table 6.4.
|Table 6.4 Coated finishes|
|Coated Finish||Description||Gloss Level|
|Matte||Roughest surface of coated paper. Very flat, no lustre, no glare, no calendaring applied.||None|
|Dull||Smoother surface than matte. No luster, no glare, minimal calendaring.||Very low|
|Satin||Smooth and soft to the touch. Slight lustre, low glare, light calendaring.||Medium low|
|Silk||Smooth and silky to the touch. Low lustre, low glare, light calendaring.||Moderate|
|Gloss||Smooth and slick. Shiny, high calendaring.||High|
Cast coated paper has a very high gloss finish on the front side and is uncoated and rough on the back. The high gloss finish is created by applying a heated chrome roller to the coated surface to quickly dry it while moisture is released through the uncoated back of the sheet. Calendaring is not used, allowing the back surface to be rough and ideally suited for labels. Cast coated paper holds ink well, but the toner used in digital printing may not adhere to it.