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A variety of tests are used to determine ink and paper and paperboard quality, and to measure their interactions. They are necessary to balance print quality, cost, and wear on the press. Some of the important paper and ink tests are listed here:
Paper and ink testing Tests for paper and ink interaction
A variety of tests are used to determine ink and paper and paperboard quality, and to measure their interactions. They are necessary to balance print quality, cost, and wear on the press. Some of the important paper and ink tests are listed here:
Paper test A wide variety of paper tests are available, depending on the specific needs and on the governing contract or specification:.
Major paper properties are widely classified into four categories.
The mass per area of the paper is measured as GSM (grams per square meter). (Mass is usually measured as weight at a location with sea-level gravity.) Paper sheets are cut to a size of 10 cm × 10 cm, then the sheets are weighed individually. The mean of the weights is used to calculate the mass of the paper per square meter, and the standard deviation is determined from all samples.
Defines the thickness of the paper. Consistency of caliper throughout the paper web is an important consideration. An abrupt increase or decrease in caliper can affect the extent to which the printing plate or blanket contacts the paper and transfers a complete printed image, as well as other printability and runability issues.
Thickness of the paper in relation with GSM. Not only is consistent thickness required in web offset lithography, where varying bulk within a single roll can cause the roll to unwind with uneven tension, which is responsible for various printing defects as well as web breaks. Higher bulk of the same GSM will reduce the show through defects.
A degree of peaks and valleys on the surface of the paper. High roughness of the paper results in more consumption and many other printability defects, too.
A measure of the extent to which a paper will resist a change in size as the result of a change in moisture content. Dimensional instability will result in mis-registration in the press.
Opacity is a property of paper that describes the amount of light which is transmitted through it, with paper having a low degree of opacity being more translucent and allowing more light to pass through it. A paper's opacity determines the extent to which printing on a particular side of paper will be visible from the reverse side (called show-through)
Paper shade plays a significant role in the quality of colour reproduction. The shade of the paper affects the colour values of the primary and secondary colours and hence the colour gamut of a press. The standard for paper shade of newsprint as per ISO 12647-3 is mentioned in the table.
Backing | L* | a* | b* |
---|---|---|---|
Black backing | 82 | 0 | 3 |
White Backing | 85.2 | 0.9 | 5.2 |
The maximum tolerance specified is ∆L 4, ∆a 2 and ∆b 2. When selecting a paper for production, it is very important to measure the paper shade. A spectrophotometer or a spectrodensitometer can be used. Publishers should also check for batch to batch variations. The ultimate aim is to use the paper which has a shade closer to ISO specification.
Bursting strength the maximum pressure that a paper can tolerate before rupture. It is an important property for packaging materials, not so important for newsprint / commercial papers.
It is measured directly by the Mullen Tester which provides a value in units of pressure. Other tests used for packaging papers are the Ring Crush Test and the Short-Span Compression Test which provide values in units of force.
The maximum length a paper can be stretched without breaking, expressed as a percentage of the material's original length. An important parameter to relate in terms of breaks.
The degree of reduction in thickness under compressive forces or pressure is known as compressibility of the paper. It influences the ability of paper to change its surface contour and conform to make contact with the printing plate or blanket during print production.
Not to be confused with Compression Strength, a mechanical property of all materials, with units of pressure.
Tearing strength is the ability of the paper to withstand any tearing force without break. It is useful to evaluate web runability, controlling the quality of newsprint and characterizing the toughness of packaging paper.
Tear strength for paper is measured in units of force, frequently measured with the Elmendorf Tester. Tear Factor is tear strength per unit basis weight, Tear_Strength/Grammage, or N/g/m2.
This is the ability of the paper to withstand a stretching force without break. The higher the tensile strength of the paper is, the less the chances of web breaks due to high tension at press.
% of moisture content in the paper is known as relative moisture content. High moisture content will increase the weight of the paper and increase the cost of paper. At the same time, low moisture content will make the paper brittle and increases the chances of break in production.
Roughness indicates varying degree of porosity of the paper. Porosity is a direct indication of paper to accept ink or water.
The residue left after complete combustion of paper at high temperature is known as ash content of the paper. It is generally expressed as percent of the original test sample and represents filler content in the paper. Ash content is not an important property of paper but it helps to know the level of fillers in the paper and grade of papers.
The pH value of paper indicates the residual acidic/alkaline chemicals in pulp, or atmospheric pollutants. It is an environmental KPI parameter in paper making. Acidic paper made from wood-based pulp that still contains lignin will deteriorate over time by becoming brittle and turning yellow. [1] For document preservation, acid-free papers are therefore gaining popularity and have pH of 7 or slightly higher.
A pick test gives an indication of fluff accumulation in the newsprint. Decides the strength of the fiber bonding on the paper surface. Often studied to relate fluff accumulation on the Press.
The water absorbent capacity of the paper in terms of kilogram per square metre is known as water absorbency.
The fineness of grind and size of particles in the ink. Optimum pigment size improves the dispersion of pigments into the vehicle (solvent). Pigment size could be related to image wear on the press, larger pigment size may lead to more abrasion on the image in the plate and may result in image wear.
Total amount of surface area a given quantity of ink to produce the standard density. Different ink brands and different batches can give different mileage. As ink is one of the major materials, an ideal study of ink mileage will help to save the cost.
The colour produced by ink depends on the ink composition, pigments and also the paper shade. This test can be done in two ways. 1. Same ink printed over different paper substrates and 2. Different ink samples printed over a standard newsprint that matches ISO specification.
The most important thing that should be noted is the colour of the inks at the ISO specified density i.e. C 0.9, M 0.9, Y 0.9 and K 1.1. The ∆E value between ISO specified colours and the colours produced by the sample ink gives the ink's deviation from standard.
The colour values of the secondary colours (R, G and B) are also measured. From the colour values of C, M, Y, R, G and B, a 2-D curve of the colour gamut can be plotted and compared with the ISO colour values.
Ink requirement is defined as the amount of ink needed to print a unit area with a standard Solid Ink Density. For newsprint, the ISO 12647-3 specification is C 0.90, M 0.90, Y 0.90 and K 1.10, Status E, D50, 2o, density minus paper with polarization filter. Ink requirement is measured as g/m2 (grams of ink required to print 1 m2 of paper)
Different paper substrates require different amounts of ink to achieve the standard SID. A highly porous substrate generally requires a higher amount of ink compared to a less porous substrate. Therefore, to study the ink mileage of different paper substrates, this test can be done on all the newsprint samples.
Ink requirement also depends upon the ink formulation and the colour strength of the pigments. From this test, the ink mileage of all the four process colours, when printed in the same paper, can be studied.
Trap is a measure of the ability of a wet ink film printed on the paper to accept the next ink printed on top of it. Preucil’s formula is used to measure trap. % Trap = {(Density of overprint – Density of 1st ink)/Density of 2nd ink} x 100
Trap affects the colour values of the secondary colours, and plays a major role in defining the color gamut of a press. Poor trap leads to colour shift in the secondary colours. Trap is also affected by the ink tack, viscosity, ink film thickness and the impression pressure.
Set-off is defined as the tendency of ink to transfer from a freshly printed surface to another paper when pressed without any friction. Set-off is an unwanted behavior of paper and ink. It is influenced by the characteristics of the paper and ink: a porous paper surface absorbs the ink fast results in lesser set-off, as do inks that dry (or set) quickly. The higher the speed of the press, the higher the set-off will be.
Set-off is measured at two different time intervals, 0.35s and 3s. A freshly printed surface is pressed against a paper after 0.35s and after 3s. Then the set-off density is measured. The idea behind selecting these two timings is to know what will be the set-off when the materials are used in a high-speed press and a low-speed press. The table below gives an example of set-off measurement.
Ink | 0.35 s | 3 s |
---|---|---|
Cyan | 0.09 | 0.07 |
Magenta | 0.11 | 0.09 |
Yellow | 0.09 | 0.07 |
Black | 0.10 | 0.08 |
Print through is measured as the print density of the reverse side of the printed-paper which is printed at a standard print density. The lower the print through, the better the paper.
Fineness of ink grind is an important parameter that describes the quality of dispersion of the pigments in the ink. A grindometer is used to test the fineness of the pigment particles. The lower the particle size the better is the dispersion of the ink.
The following tests are considered as printability tests as these properties are influenced by characteristics of both paper and Ink and also the type of print process.
A pigment is a powder used to add color or change visual appearance. Pigments are completely or nearly insoluble and chemically unreactive in water or another medium; in contrast, dyes are colored substances which are soluble or go into solution at some stage in their use. Dyes are often organic compounds whereas pigments are often inorganic. Pigments of prehistoric and historic value include ochre, charcoal, and lapis lazuli.
A printed circuit board (PCB), also called printed wiring board (PWB), is a medium used to connect or "wire" components to one another in a circuit. It takes the form of a laminated sandwich structure of conductive and insulating layers: each of the conductive layers is designed with a pattern of traces, planes and other features etched from one or more sheet layers of copper laminated onto and/or between sheet layers of a non-conductive substrate. Electrical components may be fixed to conductive pads on the outer layers in the shape designed to accept the component's terminals, generally by means of soldering, to both electrically connect and mechanically fasten them to it. Another manufacturing process adds vias, plated-through holes that allow interconnections between layers.
Paperboard is a thick paper-based material. While there is no rigid differentiation between paper and paperboard, paperboard is generally thicker than paper and has certain superior attributes such as foldability and rigidity. According to ISO standards, paperboard is a paper with a grammage above 250 g/m2, but there are exceptions. Paperboard can be single- or multi-ply.
Security printing is the field of the printing industry that deals with the printing of items such as banknotes, cheques, passports, tamper-evident labels, security tapes, product authentication, stock certificates, postage stamps and identity cards. The main goal of security printing is to prevent forgery, tampering, or counterfeiting. More recently many of the techniques used to protect these high-value documents have become more available to commercial printers, whether they are using the more traditional offset and flexographic presses or the newer digital platforms. Businesses are protecting their lesser-value documents such as transcripts, coupons and prescription pads by incorporating some of the features listed below to ensure that they cannot be forged or that alteration of the data cannot occur undetected.
Wood stain is a type of paint used to colour wood comprising colourants dissolved and/or suspended in a vehicle or solvent. Pigments and/or dyes are largely used as colourants in most stains.
Coal analysis techniques are specific analytical methods designed to measure the particular physical and chemical properties of coals. These methods are used primarily to determine the suitability of coal for coking, power generation or for iron ore smelting in the manufacture of steel.
Tracing paper is paper made to have low opacity, allowing light to pass through. Its origins date back to at least the 1300s where it was used by artists of the Italian Renaissance. In the 1880s, tracing paper was produced en masse, used by architects, design engineers, and artists. Tracing paper was key in creating drawings that could be copied precisely using the diazo copy process. It then found many other uses. The original use for drawing and tracing was largely superseded by technologies that do not require diazo copying or manual copying of drawings.
Inkjet paper is a special fine paper designed for inkjet printers, typically classified by its weight, brightness and smoothness, and sometimes by its opacity.
In color management, an ICC profile is a set of data that characterizes a color input or output device, or a color space, according to standards promulgated by the International Color Consortium (ICC). Profiles describe the color attributes of a particular device or viewing requirement by defining a mapping between the device source or target color space and a profile connection space (PCS). This PCS is either CIELAB (L*a*b*) or CIEXYZ. Mappings may be specified using tables, to which interpolation is applied, or through a series of parameters for transformations.
Grammage and basis weight, in the pulp and paper industry, are the area density of a paper product, that is, its mass per unit of area. Two ways of expressing grammage are commonly used:
There are three types of color mixing models, depending on the relative brightness of the resultant mixture: additive, subtractive, and average. In these models, mixing black and white will yield white, black and gray, respectively. Physical mixing processes, e.g. mixing light beams or oil paints, will follow one or a hybrid of these 3 models. Each mixing model is associated with several color models, depending on the approximate primary colors used. The most common color models are optimized to human trichromatic color vision, therefore comprising three primary colors.
Special fine paper is a classification of paper used for copying and digital printing.
Lightfastness is a property of a colourant such as dye or pigment that describes its resistance to fading when exposed to light. Dyes and pigments are used for example for dyeing of fabrics, plastics or other materials and manufacturing paints or printing inks.
Decorative laminates are laminated products primarily used as furniture surface materials or wall paneling. It can be manufactured as either high- or low-pressure laminate, with the two processes not much different from each other except for the pressure applied in the pressing process. Also, laminate can be produced either in batches or in a continuous process; the latter is called continuous pressure laminate (CPL).
There are two different types of haze that can occur in materials:
Media Standard Print is a publication of the Bundesverband Druck und Medien (BVDM), available on its website. The publication contains instructions on how to produce data and proofs that are to be sent to a printer. It is based on ProcessStandard Offset and therefore on the ISO standards 12647 and 15930. As such, it serves as the foundation for smooth cooperation between customer, prepress service provider and printer during media production, covering data formats, colour spaces, printing conditions, workflows, means of proofing, standards, black composition and much more.
Textile testing is the process of measuring the properties and performance of textile materials—textile testing includes physical and chemical testing of raw materials to finished products.
The hiding power is an ability of a paint to hide the surface that the paint was applied to. Numerically, it is defined as an area of surface coated by a volume of paint at which the "complete hiding" of the underlying surface occurs.