Paper characteristics for digital printing
With new technological developments in electrophotographic printing, more stringent demands are being placed on paper performance. With higher run speeds and higher image quality expectations, paper manufacturers are challenged to produce papers with the appropriate characteristics at acceptable price points. Print buyers have higher expectations than ever before, requiring “on-demand” print solutions with near photo-quality colour output. Therefore, digital substrates must be able to handle higher levels of toner from four component colours while maintaining sharp line edge acuity and accurate dot placement. End users are demanding snappy colours, defect-free areas of solid colour, and sharp text with high edge sharpness.
Paper performance for printing may be broken down into three functional areas: runnability, printability, and fitness for use. Runnability is generally understood to encompass the performance of papers in press operation, such that sheets will run smoothly through the print engine without jamming. Printability relates to the image quality and overall appearance of the printed piece. Fitness for use or usability of the final printed piece is assessed in terms of grade-related properties such as colour, texture, and basis weight, the ability to be finished and distributed in the required manner, and the ability of the image to meet permanence requirements for the specific use.
In order to discuss the technical requirements of digital papers, it is useful to consider the paper related steps of the electrophotographic marking event. Two steps in this process are critically related to paper properties: toner transfer and fusing. Inside an electrophotographic printer, the image is written using a laser or other light-based system to a photosensitive drum or belt known as the photoreceptor. Charged toner is attracted to the image areas of photoreceptor, which are charged differently than the background (or non-image) areas. The dielectric force that drives the toner transfer arises from a charge placed on the paper before it reaches the transfer “gap.” The strength and uniformity of this force determines the efficiency of toner transfer.
Discontinuities and variation in this force result in mottle (uneven print density) and low image quality. Toner transfer efficiency is related to the distribution and density of fillers within the paper structure, and also is significantly affected by thickness variations. Toner penetrates very little into the paper surface, and so mottle or print density variations are primarily due to the factors that control toner transfer. Other factors include surface roughness, and moisture non-uniformity, which even on a very localized level can affect the dielectric force strength sufficiently to produce a visible optical density fluctuation. Where toner transfer is inefficient, residual toner remains on the photoreceptor and may be transferred to the next image, increasing background speckle or producing “ghosting.” Background speckle can reduce the apparent brightness of papers and can lead to lower relative contrast, reducing image quality.
Once on the paper, the toned image must be fused to become permanent, and this done through heat and pressure. A commonly-used method is hot roll fusing under applied pressure. In the fusing process, toner melts under heat and pressure exerted by the nip forming rollers. The degree of toner penetration into the paper voids and pores depends on process conditions, toner rheology, and paper properties. In plain papers, toner just penetrates the voids at the paper surface. In some systems toner penetration is minimal, and the bonding between the toner and paper may be inadequate for permanence and rub resistance. Paper permeability is a parameter that encompasses the shape, size, spacing and distribution of surface voids and pores, and this parameter is frequently related to fusing efficiency and toner penetration. Other paper properties that influence fusing include the thermal properties of the sheet, moisture content, surface energy, roughness, and. Generally, fusing quality decreases as the surface roughness of the paper.
Digital Substrate Range
In order to offer new and exciting applications for digital print and move the market perception away from a commodity service to a value-added manufacturing process, a wide range of substrates must be available to designers. To fully utilize the flexibility of digital printing, a full range of colours, textures, sizes, and basis weights is needed.
A printing operation using both offset and digital technologies may prefer to work with the “same” paper grades on each press; this means purchasing technology-specific grades designed with a similar look and feel, e.g., colour, finish, and basis weight . The development of matching text weights and covers within a product range for both offset and digital printing increases the flexibility of making real-time decisions in the pressroom about which technology to use. The digital/offset cost breakeven point may be less important in making the decision about choice of press than schedule availability and other logistical factors. Such a decision may be made just before printing, so the stock range must be available. Where several print technologies are functioning within one print operation, a universal paper for the different print technology has significant economic advantages. However, the best runnability and image quality for digital printing is obtained from papers designed specifically for electrophotographic applications.
Specialty grades: As customers move into new markets with personalized applications, the need for synthetic substrates, ID tags, and value-added niche substrates grows. With a full range of substrate materials, designers can plan the development of a complete marketing kit, encompassing decals, labels, mailers, brochures, etc. The field of security printing is developing in sophistication, requiring new materials and inks/toners. Carbonless papers are now available for some digital colour printers to further open up the business forms, transactional, and healthcare markets to digital production.
Runnability
The trend towards short-run, variable data electrophotographic printing for targeted marketing applications requires robust paper runnability. Downtime is as expensive in a digital printing environment as anywhere else, but is a particular issue in variable data printing, where the loss of a single sheet can disrupt the integrity of the print run. The challenge for paper manufacturers is to design papers with appropriate runnability characteristics that can operate across the full range of digital print engines currently in use.
As labour costs increase and operational workflows are reconstructed to output more work with fewer people, digital technologies can offer improved throughput speeds, unattended printer operation, and in-line finishing operations, all of which can lead to lower levels of human involvement in a press run. Such functionality involves more complex paper paths and feed mechanisms, and hence requires tighter tolerances on dimensional stability and sheet uniformity. Runnability issues are common across all printing processes, but some are specific to digital printing. A leading cause of paper jams is out-of-plane deformation (such as curl or cockle), a problem that is exacerbated at the higher toner levels and fuser temperatures used in full colour printing. Compared with many offset press requirements, sheet properties for digital printing must be more stringently controlled in terms of stiffness, moisture level, edge quality and dimensional integrity in order to meet the jam-free requirements of complex high-speed paper paths.
Strength Properties
Stiffness is the ability of a sheet to resist an applied bending force, and has a significant effect on runnability. It is closely related to formation, thickness, and moisture level. High stiffness may be an end-use requirement, but can also inhibit smooth transport around paper paths with tight curvature. Digital press manufacturers are now promoting a “straight paper path” as a runnability enabler in order to cope with the broadening range of digital papers. Moisture non-uniformities, large void, wrinkles and uneven formation can also result in areas of weakness. These requirements are similar for digital and traditional web presses.
Caliper (Thickness)
Automatic feed systems, high capacity stackers and inline finishing equipment function effectively only if paper caliper is sufficiently uniform. Some systems employ real-time inline thickness measurement to detect and compensate for variation. The stack thickness of a collated document or book can vary significantly with only a small variation in sheet thickness, which introduces complexity into inline finishing involving covers and binding. However, there is a more urgent reason to manage caliper in digital printing, because the magnitude of the electrostatic force which pulls toner towards the sheet surface in the toner transfer step depends on how much material is beneath the surface. Sheet thickness variation and non-uniformity, has been shown to be a significant factor in the variation of surface charge density. Additionally, the distribution of fillers both close to the surface and within the body of the paper affects this dielectric force Thus, image density non-uniformities (print mottle) can result from thickness variations and non-uniformities in filler distribution within a sheet. Formation and thickness must therefore be controlled more tightly than in papers designed for non-electrostatic printing methods.
Grain Direction
The grain direction, or the direction in which most fibres lie in a sheet, determines the relative level of a range of physical properties that can vary between the width and length of a sheet. This is particularly true of stiffness, a key runnability factor. All print technologies require specific alignment of web and sheet grain direction in order to optimize the strength, stiffness and other performance characteristics on press. In digital presses, feeding sheets with the grain in the wrong direction can cause paper jams if the stiffness is not in the functional range.
Formation
Formation is the arrangement of fibres and other components in the sheet, and expresses the orientation and distribution of fibres, fillers, pores and voids. The performance of paper in digital printing has been shown to be very closely related to formation. Void and pore structures play a key role in the flow and subsequent bonding of molten toner to the paper surface in the fusing step, and is a factor in managing toner adhesion. Sheet formation non-uniformity contributes to appear mottle, cloudy and have uneven appearance. Strength and dimensional stability are also affected by formation since the degree of fibre contact and bonding dictates the strength properties, particularly stiffness.
Surface Properties and Print Quality
Print quality is all about ink\toner and paper interactions, and so the surface characteristics of paper must be matched to the specific ink or toner as much as to the press technology. Surface characteristics important to toner printing are uniformity, adhesion, strength, and smoothness.
Fluctuations in paper surface composition can result in variations in surface resistivity, and hence toner density, degrading the print quality of graphic images. This use of colour graphics therefore puts new pressures on paper manufacturers for microstructural uniformity. This is not only important in the lateral dimension (in-plane with the surface) but also in the feed direction. The distribution of fillers within the body of the paper both laterally and perpendicularly to the surface will affect the charge density at the surface, and hence influence the toner transfer step.
Toner Adhesion
Toner adhesion is important not only for the long-term permanence of an image, but also for the general handling and processing involved in finishing and distribution. Adhesion is determined both by toner characteristics and the paper’s surface energy, resistivity and moisture levels. Poor adhesion leads to rub-off, scuffing and scratching, and is especially an issue with mailed pieces and booklet covers.
In offset printing and liquid ink digital technologies, ink holdout (ink remaining on the surface) is balanced with vehicle penetration (non-colorant components moving into the sheet). The objective is immobilization of the colorant at the point it is placed, preferably without too much lateral spread (dot gain). Dry toners used in digital printing generally penetrate much less into the surface, even though there is a molten phase in which some liquid polymer or resin is able to penetrate pores and voids. Thus there is a higher concentration of colorant on the surface than with similar offset inking levels. Coated papers retain more toner on the surfaces, but do still rely on some pore penetration for effective adhesion.
Surface Strength
In the toner fusing stage, paper surface strength must be adequate to prevent delamination of coatings, or fibre-picking with uncoated papers. Surface control agents on the toner particles themselves maybe used to enable release from fuser rolls.
Smoothness
The smoothness of the paper surface is often described in marketing terms as its “finish,” and a wide range of finishes are available, from cast-coated gloss with an almost mirror finish, to low-gloss matte surfaces, to rough-textured surfaces such as linen. Special embossed finishes with specific patterns can add interest, but these substrates are notoriously difficult to print on most dry toner systems.
Very smooth surfaces cause high levels of light reflection from the paper surface, or gloss. One disadvantage of powder toner systems is that the substrate finish is dominated by the toner gloss. In areas with differential toner coverage, or if fusing is non-uniform, differential gloss across solid tones can be distracting. New toner systems with release additives on toner particles allow gloss levels to be managed, and reduce the difference in gloss between toned areas and substrates. Gloss coatings are achieved with base papers of high smoothness and with highly uniform coatings. The resulting uniformity in surface, thickness and formation yields a uniform dielectric force and uniform toner transfer. Gloss stock can blister if the underlying moisture is heated in the fusing step and the steam has nowhere to go. Therefore environmental conditioning and low, uniform moisture levels are particularly important with high-gloss digital papers.
Generally, smoother surfaces produce better quality images with improved sharper line edge acuity, dot integrity and the ability to render fine detail. A rough surface will show less continuity in dielectric force across a sheet, and therefore uneven toner transfer can result. Where toner particles are unable to penetrate the valleys of a rough surface, density variations will result, leading to mottled image areas. There is increasing demand for textured papers for special applications. Newer technologies use various mechanisms to encourage toner particles to enter valleys in uneven surfaces. Runnability can also be affected by smoothness in friction feed systems—some friction is necessary for grippers to function however digital manufactures have adopted suction or vacuum fed paper systems that work in the same manner as traditional litho machines.
Dimensional Stability
Dimensional stability refers to the change in shape or dimension of a sheet or web, and also can refer to the change in planarity. In a digital press, papers are subjected to heat, pressure and variety of forces, most of which are imposed in the fusing cycle. High temperatures can cause expansion, contraction, curl, cockle (an uneven wavy surface), and in some cases accelerated creep. Curl occurs when extreme temperatures and pressures are exerted differentially on the paper, so that one surface heats and contracts more than another. Some presses have anti-curl systems to compensate for this out-of-plane deformation. Curl is related to fibre orientation, formation and previous drying and moisture history, and is a leading cause of poor runnability in digital presses. This is one reason why the moisture level of digital papers must be maintained at a low, specified level, and must be uniform across a sheet. After fusing, even a non-curled sheet can experience dimensional instability if the moisture level in the environment is high, resulting in fast and uneven adsorption of the toner into the sheet. Cockle is related to uneven moisture levels, and non-uniform formation and fillers. Dimensional stability differences between sheet surfaces can also result in cockle, which is mostly an issue in two-sided printing. A cockled sheet will not experience efficient toner transfer on the second side due to the variation in transfer distance. Fusing pressure can also result in compression in the lead direction followed by some level of elastic recovery.
Papers must be able to maintain adequate dimensional stability in fusing cycles up to 200 degrees Celsius to enable the accurate registration of images on both sides of the paper. In duplex printing, sheets pass through the fuser system twice, so the toned sheet must also survive the second heat and charge exposure without cockling and curling.
Moisture
Of all digital paper properties, the moisture level and moisture history are arguably the most critical, and are often the only rigid paper specifications. Moisture affects resistivity, which in turn affects the magnitude of the dielectric force in toner transfer, and hence the resulting image quality. Non-uniformities in moisture level will result in variations in this dielectric force, leading to print mottle. Manufacturing specifications for both level and uniformity of moisture across the sheet are tight, and this is one reason why digital papers may cost more to produce.
In the fusing cycle, the image side of a sheet may be exposed to high heat, driving off moisture unevenly. This can result in cockle if the paper’s initial moisture level was inappropriate. Paper that is too dry may result in static discharge within the print engine, resulting in paper jams. Too much moisture causes print defects, curl, and again, jamming. Thus the runnability of paper is strongly dependent on its humidity and temperature. However, the moisture history is also a factor: paper “remembers” moisture and temperature exposures, and may not fully recover from an inappropriate environmental exposure.
Paper Conditioning
Dimensional stability on press requires sufficient paper conditioning time, this means allowing paper to come to equilibrium with the relative humidity and temperature in the press room or storage area, but at a specified rate of change. The range of paper sizes needed to operate a true print-on-demand environment means providing temperature and humidity controlled warehousing for papers.
Wrapping and packaging can be important in managing the challenge of significant variations in environmental conditions across climates and seasons. A recyclable ream wrap with a moisture barrier is used in some digital papers. Recommendation for storage in wrappers on pallets or shelves until the press run commences, under conditions of 20–25 degrees Celsius and relative humidity of 35–55%. Conditioning should be a minimum of 24 hours, and with coated papers a minimum of 48 hours. Stacking too many cartons can result in excessive forces that will compress and deform paper.
Charging Characteristics
Digital papers must be able to take and hold a charge in order to affect a clean and efficient image transfer. The characteristics that relate to efficient toner transfer include the paper’s intrinsic conductivity and also the charge injection and charge lifetimes. Thus the charging characteristics are a result of a number of interrelated complex phenomena. Due to the effects of temperature and humidity on this mechanism, in some high-end presses the print engine is enclosed in an environmentally-controlled environment. The charging characteristics of paper in electrophotographic print processes are related to moisture level. In general, if the surface charges on the paper is too low, low toner adhesion results. Too high a charge may lead to static discharge and paper jamming. Highly charged sheets will adhere together and will not feed appropriately.
Static properties of papers are generally expressed by the parameter resistivity, which expresses the time it takes for a static charge to decay. However, this parameter may not correlate fully with print quality performance, and other parameters such as electrostatic charge decay may be more useful. The maximum charge the paper can hold, and the rate of decay of that charge, will be related to the efficiency of toner transfer and the runnability on a press.
Some level of electrostatic non-uniformity in paper is inevitable and can be tolerated. Image noise (related to uniformity of toner transfer) and optical density levels (related to transfer efficiency) have been correlated with a “characteristic length” which describes the typical scale of voltage variations experienced at the surface. In effect, the demands on paper are specific—that the paper is able to allow charge transfer to exactly the right extent, followed by limited decay and holding that charge for long enough for the transfer step to take place. This sophisticated balance is a critical property of substrates for digital printing.
Appearance Properties
Rendering near-continuous tone, photo-quality images requires high-integrity, sharp dot placement onto bright papers to provide the expected high contrast and image resolution. Overall the standard of acceptable image quality is increasing. Printers are becoming more sophisticated at colour management, and new presses offer internal and closed-loop colour calibration. There is a general tendency towards brighter paper shades to add apparent snap to colour digital images; this is driven primarily by marketing initiatives, but there is no doubt that a high brightness paper offers print quality advantages. However, optical brighteners, or fluorescent whitening agents will degrade in time, limiting the shelf-life of high brightness papers.
Whiteness and brightness are frequently confused in the world of paper specifications. Whiteness and shade refer to light reflection properties; a truly white paper reflects all colours of the visible spectrum evenly. Paper that absorbs some frequencies in the visible range may appear to have a colour cast or hue to the human eye. A paper with a “cool” blue cast may appear to make blue and black printed colours snap more from the page. Paper with a neutral or warm white hue tend to bring out reds, yellows and oranges, and can be a suitable choice for rendering skin tones. Overall, high whiteness may be linked to the appearance of greater contrast. Overall, brightness affects the contrast, colour values, and attractive appearance of a printed product. Opacity is an important consideration with duplex (two-sided) printing. Although toners do not penetrate as deeply as offset and inkjet inks into the structure of the paper, highly toned areas can lead to show-through in two-sided printing. This is more of a challenge to colour printing in which toner levels in some systems may approach 300% coverage and above.
Hybrid Printing
Digital printing, with its variable data capability, may be used to print variable content onto shells or forms that have been printed using traditional, fixed-plate technologies. This means that a substrate is subjected to two sets of stresses, for example, high moisture levels from offset exposure, followed by high heat and charging levels in a digital press. Dimensional instability is a key failure mode, often managed by controlled environmental conditioning between printing stages. Minimizing both ink density and fountain solution level may reduce dimensional instability in subsequent digital runs. Waterless offset processes therefore are advantageous if electrophotographic printing is to follow.
Printing toner onto offset-printed areas can result in poor toner adhesion, so designers need to be aware of the need to leave sufficient space between digital and offset printed areas to avoid overprinting. Coated papers printed digitally after an offset run can cause blistering if moisture is trapped beneath the surface. This moisture can boil in the fusing cycle and burst through the coating if it is unable to escape through pores. This also occurs in areas of high ink or toner coverage if fusing temperatures are too high. Inkjet grades will exhibit blisters when exposed to high fusing temperatures, so specialty inkjet papers are not appropriate for digital
Toners
The recent trend in toner technology is towards smaller, more tightly controlled particles with more sophisticated surface additives. Smaller toner particles now available with lower resin-to-pigment ratios perform best on the smoothest papers. Lower toner resin levels are improving image quality, reducing differential gloss, enabling close to off-set appearance, and resulting in lower toner coverage. Chemically-prepared toners or Polymerized toners result in a more uniform and more precise shape and particle size distribution than toners prepared by extrusion/grinding methods. This increased level of size control coupled with the use of sophisticated charge control agents and other surface additives has brought toners into a new age of functionality, and is largely responsible for the significant improvement in image quality in third generation digital systems. Cleaning, transfer and toner charging are now more efficient and image quality improvements are shown in halftone rendering and fine line reproduction. Digital papers with textures are now accessible to some systems with these new toners. Synthetic substrates can be used for a wide range of applications (packaging, signage, labelling, security documents, etc.) if their surface energies are carefully balanced with qualified inks/toners.
Finishing
In-line finishing capabilities available with the high-end production digital presses challenge the runnability of papers, which may be subjected to multiple stresses. Both tighter sheet dimension tolerances and more uniform thicknesses are essential as more sophisticated finishing options become available. Systems with inline cutting and trimming need to manage dust in order to minimize static problems in the digital print engine. Dust can arise from front-end trimming, and also from loose material such as fibres and fillers that become detached from paper surfaces, particularly at high production speeds and with friction feeds. Dust can also be attracted to the photoreceptor resulting in point image defects and discharging.
Some inline booklet finishing equipment may require folding across the paper grain, which is a problem for highly toned areas. Toned areas may crack when folded against the grain and this should be taken into account when designing documents. For such applications short-grain papers are necessary, but the stiffness may not be appropriate for high-speed transport and may require lower run speeds.
Recycled Papers in Digital Printing
There are many challenges in manufacturing high quality printing papers using recycled fibres. This is a growing segment of the digital papers market, especially for transactional and business applications for which a company’s environmental policy may dictate materials choices. “Stickies” and contaminants in recycled papers are a particular hazard for electrophotographic printing because non-uniformities in charging characteristics may interfere with toner transfer. Defects may be exaggerated by a surrounding charge field, and even small contaminants can result in deletion spots that are easily visible in highly toned areas. New technologies are developing to increase the efficiency of recycling, and to manage contamination levels, and it is anticipated that improved quality in uncoated recycled grades will increase the use of recycled papers in digital printing. |