Prepress 5 substrates and inks

Graphic Prepress

Substrates and inks

Substrates Printing substrates include coated and non-coated papers, coated and non-coated board, release papers for the food industry, foils, and metallised papers. Less common substrates are cellophane, polyurethane, tissue and tinplate. Coated papers and board probably make up the bulk of the more common printing substrates. One of the more popular coatings used is clay. This coating is generally applied when the paper or board is manufactured. There are single, double, one-sided, and twosided coated papers.

Substrates have an impact on several parts of the printing process. The substrate can affect how the ink is transferred to the surface, how the ink lies on the surface, how well the ink dries and is absorbed by the surface, how well the press operator can control the register of the printed product and how the substrate can be converted and finished. Each printing process will produce different qualities of print on different types of substrates. The quality of the paper has a lot to do with the results of a colour print job. A poorly made sheet, with a lot of “dust� will cause sever hickey problems. A substrate that is not well manufactured will cause the press operator difficulties, including misfeeds and paper jams.

Substrates for sheetfed offset lithography Paper used for sheetfed offset lithography must have higher surface and internal bonding strength than that used for other printing processes, so that it can withstand the tackier ink films. Water resistance is needed for two reasons. One reason is the prevention of a softening and weakening of the paper surface, which can cause picking and a transfer of fibres or coating to the blanket. A second reason is the avoidance of excessive moisture pickup from the press dampening system, which could cause curl and intolerable changes in paper dimension.

To maintain register, paper must remain flat and not change its dimensions during printing. Its moisture content at the time of printing should be at least in reasonably close balance with the relative humidity of the pressroom. For pressrooms that are not conditioned, and whose relative humidity varies widely depending upon changing weather conditions, it is impossible to predict what the relative humidity will be at the time the paper is printed.

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Substrates and inks

Substrates for web offset lithography The requirements for web offset lithography are basically the same as those for sheetfed offset lithography, except for the different aspects of sheet and web printing. For web printing, the paper’s grain direction is always the same direction as the web travel.

The pick and moisture resistance for web offset papers need not be as high as that for sheetfed offset, as web offset inks generally have lower tack. For satisfactory runability, paper webs must be flat enough to pass through the printing nip of the two blanket cylinders without wrinkling or becoming distorted.

The moisture content requirements of paper used for web offset lithography may differ from those for sheetfed offset lithography because heatset drying in web offset lowers paper’s moisture content considerably.

Substrates for the letterpress process The major paper requirements for letterpress printing relate to printability. Pinpoint smoothness is the single most important requirement for complete image transfer. Inadequate smoothness, including the presence of pits or craters in the paper’s surface, results in missing or broken halftone dots and incomplete image transfer.

Gravure, flexography and offset lithography continue to replace letterpress printing. Letterpress is still used for some newspapers, labels and packaging printing, business forms and job printing.

Substrates for the flexography process There are very few special requirements for flexographic printing. It is adaptable to a wide variety of papers and paperboards. Flexographic inks have a very low tack and dry by evaporation, often aided by heat.

Flexography has replaced letterpress as the major relief printing process.

It is widely used for printing narrow-web, corrugated board, flexible packaging, milk cartons and paperback books.

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Graphic Prepress

Substrates and inks

Substrates for the gravure process Sheetfed gravure and rotogravure printing each have specific paper requirements. Pinpoint smoothness, or micro-smoothness, of the paper’s surface and the absence of pits or voids are critical requirements.

Inadequate smoothness and compressibility as well as minute surface depressions or voids prevent ink transfer, because small paper areas fail to contact inked cells of the gravure cylinder.

Gravure prints well on lightweight papers, boards, plastics and foils and is most suitable for long-run, high-volume printing of catalogues, magazines, and a wide variety of packaging applications.

Substrates for the screen printing process An almost unlimited range of substrates are suitable for screen printing.

These include textiles, ceramics, glass, plastics, metals and most paper and board surfaces.

Although screen printing can be applied to almost any substrate and any surface, flat and relatively bulky paper is an advantage. This prevents warping due to the heavy layer of ink normally applied in this process.

Uncoated and rougher surfaces are less susceptible to smearing and drying difficulties. However, when printing with U.V. inks - uncoated stocks act like sponges and absorb the ink. Cast coated paper and boards having 2-3 times more clay will also act as an absorber.

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Substrates and inks

Substrates for the digital printing process Digital printing is capable of imaging on a wide range of coated and uncoated paper, high gloss films, vinyl, canvas and textiles.

Uncoated substrates Electrophotographic printers have the ability to print on most traditional uncoated paper surfaces. Uncoated paper is not suitable for quality liquid ink jet printing as the ink bleeds or wicks along the cellulose fibre structures. As a result, the image loses sharpness, colour density and brilliance. Paper used for digital printing should ideally, have a slightly lower moisture content of between 4.7% and 5.5%.

Surface sizing can be added to seal the fibres, and using pigments such as calcium carbonate or titanium dioxide, will reduce ink bleeding and improve paper whiteness.

Coated substrates Whereas dry toners can be fused to the media within the digital laser printer hardware, most ink jet printers have no built in drying capabilities. Specific ink jet applications require careful matching of the ink and the substrate. This will affect the ink binding ability, drying time, optical density, water fastness and light fastness.

Ink jet printing requires coated media to achieve photo quality images.

These coating technologies include: • polymer coatings • resin coatings • polyethylene coatings • baryta coatings • inorganic oxides.

With inorganic oxides, special receiver layers of nanoporous media, absorb the ink dyes into capillaries. This will result in fast drying, excellent optical density and a large colour gamut.

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Graphic Prepress

Substrates and inks

International paper sizes A, B, C system of standard paper sizes International system originally established by the German standards institution (DIN) and subsequently adopted by the International Standards Organisation (ISO) in Switzerland. The 'A' series covers stationary and books; the 'B' series is an alternative intermediate range normally used for posters and wall-charts,etc.; the 'C' series is exclusively intended for envelopes. the 'A' sizes are based on a rectangle one square metre in area. The sides of all sheets are in the proportion of 1:1.4142, with these proportions remaining constant however many times the sheet is folded.

‘A’ series of paper sizes are as follows: • The height divided by the width of all formats is the square root of two (1.4142).

• Format A0 has an area of one square meter.

• Format A1 is A0 cut into two equal pieces, i.e. A1 is as high as A0 is wide and A1 is half as wide as A0 is high.

• All smaller A series formats are defined in the same way by cutting the next larger format in the series parallel to its shorter side into two equal pieces.

• The standardised height and width of the paper formats is a rounded number of millimetres.

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A series paper sizes

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Substrates and inks

For applications where the ISO A series does not provide an adequate format, the B series has been introduced to cover a wider range of paper sizes. The C series of formats has been defined for envelopes. • The width and height of a B series format is the geometric mean between the corresponding A format and the next larger A format.

For instance, B1 is the geometric mean between A1 and A0, that means the magnification factor that scales A1 to B1 also scales B1 to A0. • Similarly, the formats of the C series are the geometric mean between the A and B series formats with the same number. For example, an A4 letter fits nicely into a C4 envelope, which in turn fits as nicely into a B4 envelope. If you fold this letter once to A5 format, then it will fit nicely into a C5 envelope.

Format sizes

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Graphic Prepress

Substrates and inks

Application examples The ISO standard paper size system covers a wide range of formats, but not all of them are widely used in practice. Among all formats, A4 is clearly the most important one for daily office use. Some main applications of the most popular formats can be summarised as: A0, A1: technical drawings, posters A2, A3: drawings, diagrams A4: letters, magazines, forms, catalogues, laser printer and copying machine output A5: note pads A6: postcards B5, A5, B6, A6: books C4, C5, C6: envelopes for A4 letters: unfolded (C4), folded once (C5), folded twice (C6) B4, A3: newspapers, supported by most copying machines in addition to A4 Envelope formats For postal purposes, the following are the envelope formats:

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Paper properties Weight Paper weights are measured in grams per square metre (gsm), in other words a one metre square piece of paper weighs so many grams. The accepted tolerance for weight is ±5%. Paper is usually sold by weight and nominal weight is measured in kilograms per 1000 sheets (kg/1000).

Bulk or caliper Caliper is usually measured in microns (µm). The caliper of paper is important for imposition. Depending on the grade of paper the caliper for any given weight can vary considerably.

Papers with higher caliper at a given weight will have rougher finish and greater porosity and ink absorbency than those of lower caliper.

Acceptable tolerance for caliper is ±7.5%.

Opacity This is how much light is transmitted through the paper and therefore how much show through there is. It is measured on a scale from 0% – transparent to 100% – completely opaque.

Gloss The shininess of paper, measured as a ratio of reflected and incident light.

Gloss varies when measured in the machine direction or across the machine direction so both are measured and the average used.

The measured gloss is referenced to a gloss standard in which a polished black glass has 100 gloss units. Matt papers have from a few to 20 gloss units, glossy enamels are above 40 units.

Brightness Often confused with whiteness; this is in fact the percentage of reflectance of a standard (blue) wave length of light.

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Substrates and inks

Smoothness The evenness of the surface contour. A smooth finish is important for high quality colour work. Smoothness is measured on a Sheffield smoothness instrument. The higher the reading the rougher the paper.

Bond paper has a reading of about 120-150.

Porosity The ease with which air passes through the paper. Porosity greatly affects ink drying ability. Porous papers like newsprint are quick drying while coated papers are slow drying. The rate of drying can be increased by applying heat through Infra-red (IR) devices or hot air. UV inks dry on exposure to UV radiation.

Ink absorbency The rate at which ink penetrates the paper. This affects drying and the possibility of set off. Coated papers tend to be less absorbent than uncoated ones. High absorbency does however reduce ink gloss.

Whiteness This is measured by the even reflectivity of all light wave lengths. Most paper merchants have a variety of whites (possibly all called white). For four colour process work a neutral white is ideal, while for text only work slightly off whites can be more legible.

Colours Virtually no coloured paper is light fast, that is they will all fade. When matching colours you should use the standard viewing conditions of an illumination of 5,000K (or daylight). View samples at 45° and make sure that samples are the same size, and you are viewing the same side (felt or wire) and grain direction.

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Substrates and inks

Prepress and printing considerations Uncoated paper generally requires adjustment during prepress preparation. This is due to dot gain, which is an increase in the size of the dot due to increased paper absorbency. To counter this affect and therefore reduce problems with images "filling in" and becoming muddy, dots can be reduced across all tonal ranges. Ensure that all involved with the production process are aware of the stock selection and resulting film requirements.

Ideally for reproduction to give the best possible result the printing press needs to be "fingerprinted" and the reproduction system tailored to the print system.

Proofing If possible this should always be done on the same paper as the final job.

Proofing processes that allow for simulation of the printing process and dot gain are suitable alternatives.

Imposition It is important to know the caliper or bulk of the paper so that image creep and shingle can be avoided and pages will line up from the start to the finish.

Printability The printability of any image depends on the paper. Fine lines and detail do not print well on coarse uncoated papers.

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Graphic Prepress

Substrates and inks

Dot gain and screen rulings High screen rulings produce high fidelity reproduction but they only print well on smooth coated stocks. Typical screen rulings used in magazines and catalogues are: • heavy coated stock 115–150 gsm, (e.g. magazine covers) 175 lpi; • medium coated papers 80-100 gsm (quality magazines) 150 lpi; • light coated papers 52–60 gsm (high volume magazines) 133 lpi; • newsprint (low cost catalogues) 120 lpi or less.

Just to make things complicated the effect of dot gain is greater on finer screen rulings and can be improved by reducing the thickness of the ink coating, which may reduce brightness and colour saturation.

Compromises may need to be made for the appearance of the final printed product.

Colour and tone reproduction Colour and tone reproduction will depend on the whiteness and brightness of the paper. It is critical that proofs should be done on the same paper as the final job or at least be able to simulate print conditions.

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Paper grain The grain direction in paper is a result of water suspended fibres flowing onto the moving wire of the paper making machine.

The printer needs to understand grain direction as it can have an effect on the final print quality. If the fibres lay parallel to the longest side of the sheet, it is referred to as ‘long grain’. If the fibres run in the same direction as the shorter side of the sheet, it is called ‘short grain’. Long grain stock is preferred by litho printers when printing colour work, as they can compensate for circumferential fit problems by altering plate packing. Short grain stock has better blanket release, but can sometimes cause marking problems on board, as the sheet is stiffer and does not easily conform to impression and transfer cylinders.

Consideration of the grain direction for the binders is important as the grain direction will assist in folding and binding of a printed book or magazine, as the spine will be sharper and less distorted. However if the paper is to be used in a loose-leaf binder, having the grain perpendicular with the bound edge, gives the turned pages greater strength.

A showcard may not be satisfactory if the grain of the card used is incorrect. The showcard could develop a bend or bow which could make it unacceptable to the client.

To determine grain direction of paper or card refer to the packaging information on the outside of each ream or conduct the following tests.

1. Cut a sample about 5 cm square from the sheet marking its original position in the sheet. Wet it. As it dries it will curl. The grain is parallel to the hollow that is formed as the paper curls.

2. Moisten two right-angle edges of the sheet and press between the fingers. As the sheet dries, the edge across the grain will be wavy, with the grain straight.

3. Tear the sheet in two directions. It tears straighter and cleaner with the grain.

4. Fold the sheet in two directions. It folds easier and smoother with the grain

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Graphic Prepress

Substrates and inks

Inks Printing inks are not all the same. Ink must not only satisfy the needs of the process, but also end use requirements and appearance.

Inks in general contain: • Pigment or dyes - which give the ink its colour.

• Resins - which are dispersed and dissolved in solvents to make a varnish.

• Solvents - which disperse and dissolve the resin.

• Additives - which are used to give special properties to the finished ink.

• The varnish or vehicle - ‘carries’ the colour and binds it to the material being printed.

The solvents also control the viscosity (the flow properties) of the ink and the speed at which the ink dries.

Lithographic and letterpress inks are fairly stiff compared to inks used for other printing processes, such as flexography and gravure. These inks tend to be very fluid in comparison.

Lithographic inks have a higher concentration of pigment than letterpress inks. This is due to the thinner ink film deposited on the substrate by the lithographic process. They must have a high resistance to water to reduce emulsification. Most sheetfed inks are of the quickset variety with hard resins that give a higher gloss. Whereas, inks for the web offset printing process have a lower viscosity and tack. There are two classifications of web offset inks – nonheatset and heatset.

Gravure inks are classified as low-viscosity liquid inks. Most of the inks used in this process are solvent based that dry by evaporation.

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Flexographic inks are also low viscosity liquid inks. These inks can either be water based or solvent based. Solvent based inks are used mainly for printing on nonporous substrates such as plastic or cellophane.

Different types of inks available to printers include: • Fluorescent inks appear brighter to the eye because they have the ability to reflect more light in the various wavelengths than do other types of inks.

• Metallic inks appears to shimmer because of tiny metallic particles (either bronze or aluminium) in the ink. The metallic particles float to the surface and overlap, presenting the appearance of a continuous shining surface. There is a very good palette selection of metallic inks and coloured metallic blends have been introduced.

• Opaque inks use pigments that are naturally opaque or add titanium oxide to transparent pigments. Pigments used in opaque inks reflect light from their outer surface, and cover or hide the background surface on which they are printed.

• Transparent inks allow light to pass through the ink film. This allows the light to pass through the ink film and reflect off the surface of the substrate.

• Varnishes are inks without pigments. They are applied over selected areas of the print (spot varnish), or over the whole sheet or product. Varnish comes in both gloss and matt varieties. It is used to enhance and protect the product. Varnishes can be tinted to give a special effect.

Conventional varnishes have been largely replaced by infra-red and ultra-violet coatings. Their advantages include greater gloss, quicker drying and improved protection.

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Substrates and inks

Properties of ink Some properties of ink that are important to printers are colour, body, tack and drying characteristics.

• Colour: Pigment is the ingredient that gives ink its colour.

Pigments also have other important characteristics such as opacity, permanence, and bleed. Ink manufacturers use quality controls to guarantee consistent ink colour. It is important to appreciate that ink colour is influenced by the substrate colour and surface characteristics.

• Body: This refers to the consistency, softness or stiffness of the ink.

As mentioned previously, viscosity of the ink is a major consideration when choosing inks for different processes of printing. Viscosity is the resistance of a liquid to flow.

• Tack: Tack is the ‘stickiness’ of the ink, or the force required to split an ink film between two surfaces.

Applications Inks should have other characteristics that must be considered for the variety of uses and end uses of the printed product. Inks must dry so that they resist smudging and rubbing. Whether the product requires diecutting, embossing or hot foil stamping, it can be further enhanced by the application of a coating. Products that are constantly handled, such as catalogues and books can be coated or laminated for protection. The use of inline coating application methods is growing in the finishing process.

This type of coating is applied as part of a continuous operation directly after the printing section on a press, or as a single piece of equipment as part of the binding process. This is also referred to as off-line coating.

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Review questions for substrates and inks Q1 How many A4 sheets can be cut from an A0 sheet? ............................................................................................................................

Q2 Describe six (6) properties of paper. ............................................................................................................................ ............................................................................................................................ ............................................................................................................................ ............................................................................................................................ ............................................................................................................................ ............................................................................................................................

Q3 Describe three (3) methods of testing for paper grain direction. ............................................................................................................................ ............................................................................................................................ ............................................................................................................................ ............................................................................................................................ ............................................................................................................................ ............................................................................................................................

Q4 When planning for impositions, why is it important to know the caliper of the printing stock?. ............................................................................................................................ ............................................................................................................................ ............................................................................................................................

Q5 List three (3) different types of inks used by printers. ............................................................................................................................ ............................................................................................................................ ............................................................................................................................

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