Terms and concepts
Absorption and drying
The absorption and drying properties of paperboard are crucial for achieving a perfect result when printing and varnishing. The measurable properties normally related to absorption and drying are ink absorption and surface pH. Both are strongly influenced by the constituents of the coating and the uniformity of the coating characteristics.
- Ink absorption is the ability of the paperboard surface to absorb ink optimally. If too much pigment is absorbed this leads to less bright colour reproduction. If the absorption is too slow, there is a risk of set-off.
- The correct level of pH is important for the ink-drying process. Normally a pH level between 6 and 8 is required. Below 5 would be too low.
There is an important connection between the absorption and drying processes. Depending on the printing method, the fluent ink vehicle can be dried either by evaporation or by chemical change as follows:
- When the ink vehicle consists of water or organic solvents (e.g. alcohols, esters, or ketones) the vehicle is dried by evaporation. Hot air or infrared dryers (IR dryers) can be used.
- When the vehicle consists of organic oil (e.g. linseed oil) the first step in the drying process is absorption (ink setting). After that the drying process continues by chemical change, i.e. oxidation polymerisation of the oil.
- There are also inks and varnishes that dry virtually instantaneously on the press or varnishing machine by ultraviolet radiation (UV drying). In this case the drying is a chemical cross-linking polymerisation process.
Good absorption and drying properties come from the use of virgin fibres, the formation and treatment of the baseboard, and the coating properties (quality, formulation, application method, smoothness, and finishing). The printing and varnishing results depend greatly on both the baseboard and the coating having very uniform absorption and drying characteristics.
Bending stiffness, bending moment, and bending resistance
Bending stiffness is the force required to bend a paperboard sample to 5º. The bending resistance is the force required to bend it to 15º, while the bending moment is the movement required to bend it to 15º.
Bleached chemical pulp
Chemically processed virgin fibres are flexible and soft, and can therefore retain their original length and strength. The paperboard becomes very strong, with good creasing, embossing, and cutting properties, as well as low dust generation. When bleached, the chemical pulp provides paperboard with high whiteness, brightness, and good light stability. The pulp is also a clean product of known origin.
To make a truly white paperboard, all its components have to be white. Bleached paperboard is made from bleached chemical pulp.
Brightness, see Whiteness
Bulk is a paperboard property closely related to stiffness. It is calculated as the paperboard thickness in relation to its grammage. Graphic paperboard offers higher stiffness at lower grammage than paper—you achieve the same stiffness at about 25% lower grammage.
Cellulose, see Virgin fibres
Chemical pulp, see Bleached chemical pulp
Clean edges and surfaces
Clean edges and surfaces promote good print quality, printability and runnability. A paperboard with edges and surfaces free from dust and fibrous debris is a prime consideration when assessing a material for optimal process efficiency, with low down time and no unnecessary material waste. Once debris has built up on the printing press, there is no alternative but to shut it down to clean plates and blankets.
White-pigmented coating is applied on the paperboard surface (on one or both sides) in liquid form, then blade smoothed and dried. This coating provides a surface suitable for printing and varnishing, and facilitates whiteness, smoothness, and uniform absorption and drying properties.
The colour gamut is the number of colours that can be reproduced with a certain colour system relative to the substrate, printing method, and printing ink used. High colour gamut means excellent reproduction in true colours of e.g. photographic images, and is enhanced by a very smooth and uniform paperboard surface.
Compression strength is the paperboard’s ability to resist compression forces. Multi-ply paperboard has a unique relationship between compression strength and tensile strength: the compression strength is always two to three times lower than the tensile strength. This relationship is a key factor for permitting creasing, folding, and embossing/debossing.
Consistency, see Uniform quality
Paperboard should always be creased before it is folded. Creasing facilitates distinct, durable and narrow fold lines, with no cracks on the folds.
Creasability is the paperboard’s ability to permit deep and narrow creases along well defined fold lines. Since the creasing operation involves an intentional and well controlled delamination of the inner plies of the paperboard without cracking or damaging the surface plies and coatings, creasability depends on a combination of many paperboard features. Key properties are elasticity, elongation, bending moment, bending resistance, tensile strength, compression strength, delamination strength, surface strength, flatness, and dimensional stability.
When the pulp is processed on the paperboard machine, most of the fibres become oriented lengthwise, i.e. along the machine direction (MD). The paperboard is stronger in the machine direction than in the cross-fibre direction (CD).
Curl is a symmetrical (circular) deviation from flatness. Curl is not likely to occur in a high quality paperboard as long as the board is well protected from changes to its original moisture content.
Debris is particles either of paperboard origin or coming from the ink or press room. Debris may cause print blemishes or spots. A paperboard with clean edges and surfaces helps to avoid this problem.
Delamination is the separation of the different plies of multi-ply paperboard. In some processes, such as creasing, it is desirable to delaminate the inner plies without damaging the outer plies.
Delamination strength is the force required to separate (or delaminate) the bonding between different plies of multi-ply paperboard. The delamination strength should be high enough to prevent edges, corners, and flaps from being damaged, but low enough to allow for good delamination during creasing and folding. Chemically processed virgin fibres from pine or spruce give the optimal delamination strength.
Density is the compactness of the paperboard (i.e. the inverse of its bulk). Density is calculated as the paperboard grammage in relation to its thickness.
Dimensional stability and flatness
Dimensional stability is the paperboard’s ability to resist dimensional changes due to moisture changes. Flatness is the paperboard’s ability to remain flat after the printing and finishing operations. Both are crucial properties, since any change to the sheet’s shape and dimensions affects the runnability of the printing and finishing processes and the quality of the finished graphic product.
Since paperboard is a hygroscopic material, it will change its moisture content when exposed to changing humidity. When the two surfaces of the sheet are equal in terms of their capacity for moisture expansion, the paperboard is as close to the ideal symmetric sheet as possible. However, in practice almost all paperboard products are more or less asymmetric. Therefore the paperboard is manufactured to meet an ideal shape at a predetermined level of moisture content (50% relative humidity).
The choice and composition of raw materials and firmly controlled manufacturing processes are crucial for producing a paperboard that retains its dimensional stability and flatness throughout all operations. It is therefore important to keep the relative humidity of the surroundings very close to the 50% level before, after, and during the printing and finishing operations.
Dot gain (enlargement of the dots) is to some extent inevitable when printing. To achieve a high quality graphic presentation and to avoid unintended colour variation, the screen ruling, ink, and paperboard surface should be carefully selected for the specific print job.
Drying, see Absorption and drying
Elasticity is the paperboard’s ability to regain its original shape after released stress. This property is closely related to tensile strength.
Flat-bed equipment for e.g. cutting, creasing, and embossing means that these operations are carried out with the tool acting in a vertical direction towards a horizontal make-ready (or counter-die).
see Dimensional stability and flatness
After the paperboard has been creased, the folding operation should always be carried out towards the bead.
Foldability is the ability of creases to be folded so that they show permanency, good definition, and low spring-back forces. Ideally, the fold should act like a hinge. This ability depends on a complex combination of factors such as tensile strength, compression strength, delamination strength, bending resistance, flatness, and dimensional stability. These are crucial properties for the paperboard’s ability to “forgive” the permanent deformation of deep and narrow creases and to retain the intended shape of the folds.
The folding factor is the ratio between bending resistance for creased and uncreased paperboard respectively. Folding factor 0 corresponds to uncreased paperboard, folding factor 100% corresponds to a perfect hinge.
High folding resistance requires high bending force. Deep and narrow creases along well-defined fold lines provide low folding resistance.
Paperboard is often used for complex two- or three-dimensional shapes. Formability is related to features such as elasticity, creasability, foldability, and different strength characteristics.
Form stability is the ability to retain a certain form during handling and use. Important features are compression strength, delamination strength, and dimensional stability.
Gloss, see Smoothness and Surface finishing
The established criterion for gluability is the tear behaviour of a glue seam between the pigment-coated surface and (usually) the reverse side of the paperboard. Since graphic paperboard can have a different finish on its reverse side, a paperboard with suitable gluing properties can always be chosen. Of course the gluability is different for different gluing systems.
The grammage specifies the weight of the paperboard per unit area (g/m²).
Graphical paperboard is paperboard in which both the baseboard and the coatings are constructed to provide excellent print quality and high runnability in all printing and finishing operations. Sometimes we use the terms paperboard and multi-ply paperboard interchangeably. The reason is that all our paperboard products are of multi-ply construction.
By using different types of pulp in different plies (layers) and combining this with various sheet constructions and coatings, the multi-ply technique allows us to tailor the paperboard features to suit a wide variety of high quality demands in the graphics industry. To achieve a smooth surface, short fibres are used in the surface layers. For elasticity, strength and the sense of quality from a dense paperboard, a greater proportion of long fibres are used in the middle layers. It is also possible to use the same fibres for every layer and apply different preparation treatments to them.
Paperboard is a hygroscopic material, which means that it adapts to the environmental moisture content and reacts to changes in humidity by swelling during moisture uptake and shrinking when losing moisture.
Light fastness is the ability of the paperboard to remain white despite exposure to light. Bleached chemical pulp fibres make the paperboard light stable for a long time. The patented coating formula provides outstanding lightfastness further extending the end products shelf lives.
Light stability is the ability of the paperboard to remain white despite exposure to light. Bleached chemically pulped virgin fibres and suitable coating constituents make the paperboard light stable for a long time.
Lignin is the substance that is removed from the wood in the pulping processes in order to extract the virgin fibres.
Linting, see Picking
Machine direction (MD)
When the pulp is processed in the paperboard mill, most of the fibres become oriented lengthwise, in the machine direction (MD). The paperboard is stronger in the machine direction than in the cross direction (CD).
Mechanically processed fibres are harder and more rigid than chemically processed fibres. Paperboard made solely from mechanical pulp is relatively weak but is stiff and bulky with high dimensional stability.
Misregister, see Register
Mottle, i.e. variable print impression or lack of uniformity in print appearance, can arise from one of the following possible causes.
- Non-uniformities in the base sheet of the paperboard
- Local variation in the surface finishing
- Variable absorbency of the ink from the first printing unit onto the sheet causing variations at the second blanket, which then back traps in a variable way onto the succeeding sheet (back trap mottle)
- Due to the variable absorbency of water into the sheet, ink at subsequent sheets may be repelled (water induced mottle)
- Excessive ink emulsification
To prevent mottle, a paperboard with high and uniform quality of both the baseboard and the coating is required.
The multi-ply technique involves bringing together several layers of pulp in the wet state. The big advantage is that this construction allows us to optimise the paperboard features by using a combination of different types of pulp and fibres from different wood species. As both stiffness and strength are important, a compromise is needed to achieve the best result. The strength, flexibility, and consolidation behaviour of chemically processed fibres result in well formed, dense and strong paperboard products. Mechanically processed fibres have the opposite characteristics, resulting in open, bulky, and weak, but very stiff products.
By using different types of pulp in different layers and combining this with various sheet constructions and coatings, the multi-ply technique allows us to tailor the paperboard features to suit many different market requirements. To achieve a smooth surface, short fibres are used in the surface layers. To achieve elasticity, strength, and the sense of quality of a dense paperboard, a greater proportion of long fibres is used in the middle layers. It is also possible to use the same fibres for every layer and apply different preparation treatments to them.
When a sheet of paperboard is die cut, the cut blanks remain linked to each other by uncut parts (nicks) to facilitate easy handling in the subsequent conversion operations.
Good ply bond depends on the forming of a strong and elastic network inside the baseboard. This is influenced by the fibre characteristics and consistent sheet formation on the paperboard machine.
Picking, shown as small white areas in solid print, occurs when small parts of print and coating are removed from the sheet during the printing process. Picking is usually due to too high ink tack, but might also be caused by running the press too fast or with too high pressure.
Printability and print quality
Printability is the paperboard’s ability to reproduce images and texts. The measurable paperboard properties that are most related to printability are surface strength and surface tension. In modern multicolour printing, high print speeds in combination with tacky inks place very high demands on a high and uniform surface strength. Surface tension involves the adhesion ability of a specific liquid (e.g. ink). The better and more uniform the ink adheres to the surface, the better results you get when printing.
Important paperboard features for printability and high print quality are whiteness, smoothness, gloss, ink absorption and drying, flatness and dimensional stability, moisture content, rub resistance, ink and varnish gloss enhancement, and clean and dust-free edges and surfaces.
Purity, see Virgin fibres
Unlike virgin fibres, recovered or recycled fibres are derived from recycled paper waste. Since recovered fibres are not of the same known origin as virgin fibres, they may contain impurities.
A perfect register is the paperboard’s ability to retain its shape and dimensions throughout all printing and finishing operations. This ability is crucial for achieving excellent precision in multi-step processing. Of course, even in a single operation such as multi-colour printing, the flatness and dimensions of the paperboard must remain exactly the same after passing each different printing unit.
Rub and scratch resistance
It is essential for the printed ink or varnish film to be durable during normal handling. Rub and scratch resistance depend greatly on having a smooth paperboard that has good absorption and drying properties. The interrelationships between the paperboard surface, the printing and varnishing processes, and the ink and varnish formulation are crucial for achieving good rub and scratch resistance.
Runnability can be defined as the paperboard’s ability to be printed and finished with the minimum of disruption at a satisfactory level of efficiency. Speaking generally, the multi-ply construction, the surface treatments, and the uniform characteristics of graphic paperboard contribute to high runnability with low downtime and low material waste.
Scratch resistance, see Rub and scratch resistance
Screen ruling is a measure of the information density of a picture. It is measured in dpi or ppi (dots per inch or pixels per inch). High screen ruling means printing with high resolution, which requires a very smooth paperboard with very uniform surface characteristics.
Set-off, see Absorption and drying
A smooth paperboard surface is vital for achieving high-definition print results and faithful colour reproduction. If lamination or hot foil stamping is intended, this will require even higher smoothness. Smoothness is expressed in terms of the average surface profile. A property closely related to smoothness is gloss, i.e. the “mirror-like” impression created by the surface. A white, smooth, and glossy surface reflects light, giving the best possible brightness to printed colours.
Without its stiffness, paperboard would not be able to perform its function. Paperboard offers higher stiffness at lower grammage than paper (the same stiffness at about 25% lower grammage). This is one of the factors that specifiers usually consider most when choosing paperboard. The measurable properties normally related to stiffness are bending stiffness, bending resistance, and bending moment. Another property closely related to stiffness is bulk (i.e. grammage in relation to paperboard thickness).
Strength and toughness
Strength and toughness are composite terms used to describe a complex combination of the mechanical characteristics of paperboard. Measurable properties normally related to strength and toughness are tensile strength, tearing resistance, delamination strength, and compression strength. These factors are crucial for achieving sophisticated designs, such as embossing or complex structural shapes.
In order to provide a very smooth surface with uniform characteristics for printing and varnishing, the paperboard coating is treated with brush burnishing or gloss calendering. The surface can also be finished with embossing in order to create a special surface texture (e.g. linen structure).
Surface sizing is done on a section of the paperboard machine and involves the application of a pigmented starch solution to coat the paperboard. Surface sizing binds the fibres to the surface, making the paperboard more uniform and dense.
Surface strength is the paperboard’s ability to withstand forces applied to its surface. During the printing process the paperboard must be able to resist forces caused by print speed or ink tack. In finishing operations such as creasing or embossing the surface must endure high forces without cracking.
Surface structure, see Smoothness and Surface finishing
Surface tension is a measure of the wettability of plastic-coated paperboard surfaces. This property influences printability and gluability.
Tearing resistance is the force required to tear the paperboard from an initial cut. High tearing resistance depends on the general strength level and the amount of long, well bonded fibres.
A paperboard with high tensile strength is generally produced by using chemically processed virgin fibres, preferably long fibres from spruce and pine. The sheet forming process and the multi-ply construction further enhance the tensile strength. This property is measured in terms of the force required to break a 15 mm wide paperboard strip.
The thickness of paperboard is of course directly related to its stiffness. But thickness is not the only factor to consider. With a dense paperboard you need higher thickness to achieve the same stiffness compared to a bulky paperboard.
Tone scale is all possible tones between 0 and 100% of a certain colour.
Toughness, see Strength and toughness
All good characteristics and features of graphic paperboard would not be of much value if they were not also uniform. What counts is uniform quality in the finest details, both consistent within a single delivery as well as between different deliveries.
UV drying/curing, see Absorption and drying
Using virgin fibres (wood fibres) to produce graphic paperboard means using a natural raw material. Virgin fibres are by nature strong and elastic. They are also pure and of known origin. All virgin fibres share some natural properties to a greater or lesser degree, and there are also other specific properties associated with fibres of particular wood species.
Basically, the choice of virgin fibres is between long fibres (spruce and pine) and short fibres (birch). Long fibres are stronger and more elastic, while the short ones are stiffer. The fibre characteristics are also influenced by the method of pulping (chemical or mechanical pulp).
The natural properties shared by all virgin fibres with regard to pulping and board making are:
- The capacity to be modified by processing
- The flexibility, shape and dimensional features that enable the fibres to form a uniform interlaced network
- The ability to bond together at points of contact as the web dries
This is known as consolidation of the web, and it provides the strength properties of the paperboard sheet.
Viscosity is a measure of the inner friction of e.g. inks and varnishes. A highly viscous ink is more adhesive and tacky than an ink of low viscosity.
The whiteness of the paperboard is crucial for achieving perfect reproduction of illustrations and photographic images in true colours. In addition to the degree of whiteness itself, the measurable properties normally related to whiteness, are brightness and opacity. Whiteness depends on both the whiteness of the baseboard (made from bleached chemically pulped virgin fibres) and the coating (white-pigmented, smooth, and well finished).