Problems that can bite back, faced by designers, printers, buyers and specifiers. Ink techniques, paper storage, recycling, office printing, digital printing and the paper industry today. In this short information series, we offer advice and solutions on the topics.

Precautions you can take to avoid ink rub

Even though designers, printers, buyers and specifiers are more knowledgeable today than they have ever been, there is always a place for a little extra advice and support. Here, we take a look at the eternal problem of ink rub on silk and matt coated stocks.As designers become ever more imaginative (and mistakes increasingly unaffordable) a little foresightcan avoid ink rub spoiling a beautiful print job.As you probably know, it is the coating of calcium carbonate that prevents glare and makes these papers suitable for large areas of text. This coating is in fact, made of very uneven and sharp edged microscopic particles. Pressure and scuffing can break off these particles making them act as an invisible irritant to ink and cause ink rub to occur on unprinted areas. In addition, the pH and conductivity of machine fount solutions must be considered, otherwise, a chemical reaction will occur, resulting in the paper coating being deposited on the offset blanket.There are a number of precautions you can take to avoid ink rub. These include:

Performing seals
It is essential to use inks specially formulated for silk/matt coated stocks. Specifying the proposed paper and supplying samples of the actual stock to your supplier can help at the outset.Ideally, all silks and matts should be sealed with an oil-based varnish or water-based sealant. Breathable emulsion varnishes work extremely well. Under-varnishing does not minimize ink-rub, but overall varnishing or varnishing of unprinted areas in contact with print helps enormously.You will need to consider whether work should be film laminated or varnished after initial sealing. Although sealants containing wax helps reduce print rub, the UV varnishing and field lamination processes work best on inks with reduced wax. UV varnish will not readily key to oil-based varnishes, but water-based sealants can be used satisfactorily.

An in-line varnishing unit will also reduce ink rub problems, as well as improve turn-around time.

Here are some recommendations you may not yet know:

  • Ideally, pages should be designed so that large areas of print face each other, to reduce to likelihood of ink rub ? especially if no seal or varnishes are to be used. Solid and heavy tone areas will also be more effective if designed back to back to minimize show-through.
  • Standard Reflex Blue is particularly prone to ink rub and should be avoided. Substitutes are available.
  • Keep handling to a minimum and ensure that paper transport on the machine is adjusted precisely. If marking does occur, a silicon spray can be used in the final stages on the areas creating ink rub.
  • Avoid heavy print areas which bleed off the page, as these can cause ink rub when trimmed.

Improving your print performance

Apart from the more obvious effects of sunlight and dust ? which can be prevented with good stack covers ? print performance is dependent on the effects of temperature and humidity on paper.This is a particular problem in cold weather. When cold material is exposed in a warm environment the surrounding air quickly condenses and the excess moisture is absorbed by the material. This causes wavy edges. It is therefore, essential to equalize temperatures before the moisture-proof covers are removed.Here is a chart giving recommended recovery times to allow incoming material to reach as near a room temperature as possible. Recommended recovery times for a 750kg pallet of material (times based ontemperature difference between paper and surrounding atmosphere):

Temperature Recovery Time (Hrs)
10c 24
15c 40
20c 75
25c 90

The relative humidity (RH) of both the material and the press room is also critical. High humidity can cause wavy edges. For example, if the outside atmosphere is damp, with relative humidity of 75% and the relative humidity of the paper is 45%, the fibres will expand as moisture is absorbed from the atmosphere.The opposite effect, tight edges, results when the humidity of the room is too low and paper contracts. This is a common problem in winter, particularly in freezing temperatures, when factory heating is increased. Relative humidity drops significantly (to about 20%) and edge shrinkage will occur as the material (at about 45% RH) gives off moisture to the atmosphere, causing fibres to contract.To protect your material from the effects of relative humidity variation, you need to ensure that all material remains moisture- proof wrapped until conversion is protected at each stage of production and/or conversion. It may also be worth considering investing in humidity control machinery, which is relatively inexpensive.Your paper need not be a hostage to fortune. If you take the precautions described, you should avoid such problems.

Here are some recommendations you may not already know about:

  • Ensure that all material remains moisture proof protected at each stage of production and/or conversion.
  • Invest in machinery to control humidity
  • In a small room, increase relative humidity locally with buckets of water, wet rags or sawdust.

Office printing tips

The choice of paper used in the office depends on more than weight, colour finish and technical compatibility. Personal taste is obviously important and most requirements can now be satisfied from the wide range of papers available.Which papers perform best is determined by the types of printers you use. All printers require paper that will feed well, run freely and not jam. So the maximum requirement is for paper that is dimensionally stable (i.e. it won’t expand or contract), dust free, accurately cut to size, with an appropriate rigidity and smoothness to ensure good imaging.Many papers are suitable for use in a variety of printers, but it is necessary to check compatibility first. Even the most expensive printer will deliver poor results if the wrong paper is used.Photocopiers and lasers
Photocopier and laser printers all run at very high temperatures, so there must be strict control of the moisture content to prevent curling, misfeeding and dirty images.Whether a laser printer uses heat pressure or freezes the ink onto the page by gas fusion, paper for either process needs a particularly smooth surface to guarantee a good image reproduction.Ink-jet
Ink-jet printers fire tiny droplets of ink onto the paper and so require a correctly sized surface which allows the ink to be readily absorbed and to dry quickly without feathering. Too soft and the ink will show through. Too hard and it will smudge.Dot Matrix
Dot matrix printers are well suited to printing multi-part forms. They may be less sophisticated, using impact technology and inked ribbons, but the paper requirements are no less stringent. Heavier weight paper is necessary to avoid unsightly impressions on the back of the paper. A smooth surface helps give a good image with minimal smudging.

Here are some recommendations:

  • Always check that the paper you are using complies with the manufacturer’s specifications for your printers. Office printers are generally set up to take 80g/m2 paper (e.g. copier paper), although most should work with weights up to 160g/m2.
  • Conduct trials to test compatibility
  • Keep paper protected, somewhere dry and at the recommended temperature (not too hot or too cold) to ensure it remains flat.
  • Use inks or toners that are specified for your particular printers.

 

Digital printing ? which paper is best?
Digital printing is the most rapidly developing technology in printing today. It may be leading edge stuff, but it still uses paper. Here we clarify some of the processes and what they mean for paper selection.

All digital printing involves converting computer generated data ? usually carried via ISDN or disk ? into an image. Some machines employ laser scanners first to transfer the image to a plate and then to offset and impression cylinders. Other types convert the data into a bitmap image (via a Raster Image Processor) and reproduce the image directly onto the substrate.

It is not clear cut which is best ? they are generally designed for different purposes. All offer increasingly exciting opportunities for economic short run colour printing, with some offering the facility to print variable data in full colour on each sheet. The potential is fantastic, allowing designers to design specifically for short runs and printers to meet client requirements more economically than ever before.

To understand what digital technology means for paper selection, we need to look at the different technologies a little more closely. Put simply, most digital printers are either electrostatic ? fusing toner onto the substrate (like photocopiers or laser printers), or they use ink in a variety of different ways e.g. ink-jet (continuous stream, phase change or drop-on demand), dye sublimation to thermal wax transfer. The Indigo machine is electrostatic in part, but also uses special inks and litho technology.

The various electrostatic processes require paper that has a lower moisture content

  • To prevent curling from the intense heat in the printer and
  • To improve toner fusion

Paper used for most of the ink-jet processes needs to have a softer surface to allow the ink to penetrate and aharder body to prevent feathering of the print. A specialist coating may also be needed to ensure fast drying which makes for smudge-free, high definition, high quality printing.

We can only give some broad guidelines here, but the key points are that:

  • Different processes are suitable for different purposes and
  • Each process demands particular performance characteristics from the paper used.

Here are some recommendations:

Before printing

  • Data must always be supplied in specified file formats
  • Always supply a marked hard copy, together with any original artwork or logos
  • Treat large, flat background tints with care. Lighter colours are best and over a large area, it is preferableto add a pattern to vary the tone and pitch of the colour. Avoid cheap software!

Electrographic (electrostatic or die-electric)
Widely employed in the wide-format market, paper is selectively charged and a multi-pass process then attracts toner to build up a colour image.

Ink-jet
Drop on demand: liquid ink is heated and tiny droplets are then forced onto the paper. Coated papers are best.

  • Phase change: solid wax inks melt for printing and solidify on contact with the paper. Faster delivery and brighter colours that drop on demand. Plain papers or pigmented films are suitable.
  • Continuous stream: a constant stream of small ink droplets form a continuous tone effect. Comparatively slow and expensive, generally used in large formats for proofing.

 

Dye sublimation 

Generally used for visual and positioning proofing. Employs a special paper to receive coloured dyeswhich are vaporised by a print head.

Thermal wax transfer 

Wax coated ribbons transfer pigment to the substrate by a heating process. Some machines demandspecial papers.