‘Matt’, ‘Silk’ or ‘Dull’ finishes have become very popular with designers and Graphic Artists. Matt coated stock is ideal for brochures, annual reports, advertising material, corporate literature, books or just about any print job where a softer result is desirable in preference to a higher gloss finish. Fine screen four colour process reproductions achieve excellent lift and good print gloss. This is combined with text matter that is easy for the eye to read without undue reflection as if often the case with gloss papers.
Scuffing and ink rub
Matt coated papers have a special appeal and are very popular, however, there are some traps we should be aware of. All matt coated stocks are prone to ‘ink rub’ or ‘scuffing’ in varying degrees. It is important that designers, graphic artists, planners, estimators, printers and binders are aware of situations that surround producing work on matt coateds. The problem occurs when dry printed images mark unprinted areas that they come into contact with. When there is pressure and movement between the printed and unprinted surfaces, scuffing or ink rub is a possible problem.
During the process of putting ink on paper and producing the finished article, whether it be a brochure, book, annual report etc., it is possible that scuffing will appear during various stages of the job. At the printing press, scuffing may occur at the feeder when printing a second pass for additional colours, or when backing up. Pressure from feedback drive wheels and brushes can create friction of one sheet on another. Suction slow down wheels in the delivery can scuff work already printed on the reverse side. Both scenarios can be avoided if suitable ‘gutters’ are provided at the design and layout stages.
Bindery managers and operators need to be fully aware of this situation as binding is the process where most problems occur. During guillotining, pressure applied by the clamp and knife can cause ink rub, especially when trimming through a heavily inked area.
Folding machines have many functions that may cause scuffing problems. This is due to the mechanical action of feeding, transporting through the folding stations and delivery of the folded sections.
Binding lines, whether they be saddle stitch and trim, perfect bound and trim or section sewn hard case bound, are also an area for concern. All binding lines have feeder mechanisms that remove folded sections one at a time, making them vulnerable to scuffing against the surface of other sections on the feed hopper.
Designers and graphic artists, in particular, need to be aware of scuffing on matt coateds. Careful planning at an early stage can alleviate problems later on in the finishing process.
It is advisable not to have heavy ink coverages bleeding off the edge of the finished size job. This situation will nearly always cause scuffing when trimming either on the guillotine or the three knife trimmer. When designing work with pages that have heavy ink coverage, try to position them so that they face pages that also have heavy ink coverage, never opposite a blank page.
Avoid designing jobs that combine reflex blue or special colours of predominantly reflex blue with matt coated stock. Where possible, design jobs that allow for gutters.
Planners and estimators should also be fully aware of the intricacies of matt coated paper. It may be necessary to allow extra time between printing of sides and before guillotining, folding and binding. It may also be advisable to apply a sealing varnish to areas with heavy ink coverage. These things have to be taken into account when costing and planning work flow. Interaction between planning and production is critical to achieve best results, in terms of both quality and efficiency.
Why do Matt Coated Scuff?
The coating mixture for matt coated papers contains a high percentage of calcium carbonate. The granular particles in calcium carbonate assist in reducing the gloss. Due to this granular structure of the calcium carbonate particles the coating applied to the paper takes the form of peaks and valleys to diffuse the light hitting the surface of the sheet.
Investigation has shown that the main feature of the matt coated surface which affects print abrasion is the brittleness of the peaks. Small, unbound fragments may break off and become trapped on the surface of the paper. These will be invisible to the eye, but an irritant to the printing ink. It is these aspects which paper mills have been and are addressing, hence the reduced levels of ink rub in recent years, despite the dramatic increase in sales and specification of matt coated papers.
There are other factors that may influence ink rub and should be considered. One is the pH and conductivity of the fountain solution. It is known that an acidic fountain solution will attack calcium carbonate and create an effervescent-like effect. This reaction will soften the coating and give rise to a white powder deposit on the offset blanket.
Low pH fountain solution is also likely to cause emulsification which will reduce the ink’s ability to fully harden. It is therefore, recommended that a minimum pH 5.5 at the correct conductivity value be achieved. (We recommend that printers have suitable instruments to accurately monitor the fountain solution).
Ink is a fundamental element in the problem of scuffing and ink rub. Overseas research clearly indicates that the interaction between ink and the surface of the paper is also significant in rub resistance. A relatively rough paper surface will encourage uneven uneven ink absorption. Large quantities of binder can be removed from the ink film, leaving the pigment sitting on the surface of the sheet without being suitably bound to the stock. It was found that ink types with a high content of oxidisable/poltmerisable material in the binder were the most suitable. The investigation also showed, however, that inks behaved differently on different papers.
One ink is therefore, not necessarily suitable to all papers. It also concluded that the use of course grained spray powders added to the abrasiveness of the ink/surface paper and therefore, exacerbated the problem of scuffing.
Recommendations
- It is essential that inks specially formulated for matt coated materials are used.
Do not contaminate the printing inks with additives.
- When ordering inks indicate the type and name of the paper and if possible, submit samples to your printer.
- Spray powder acts as an abrasive. Run minimum amounts and avoid coarse grained powders. It may be necessary to run smaller stacks or rack piles into the delivery.
- Consider over varnishing if a problem situation is likely to occur. Contact your ink supplier for advice on the most suitable overprint varnish or sealer for your particular application. Overall varnishing, spot varnishing or varnishing of the unprinted areas in contact with other printed surfaces will help enormously.
- In line coating will not only reduce ink rub problems, but will greatly reduce the turn around time of a printed job through the printing and finishing processes.
- Arrange the layout so that print faces print and not plain paper. If possible, include gutters to allow for wheels and brushes etc.
- Avoid illustrations that bleed off the edge as these can cause scuffing when trimmed.
- If possible, avoid using reflex blue.
- Keep handling of printed stock to a minimum and precisely adjust paper transport on machines i.e. rolls, belts, tapes, wheels etc.
- With feeding sections where a section is removed from the bottom of a stack, it is vulnerable to ink rub. Reduced speed and minimal pile height can reduce the marking effect.
- It has been found that waiting 24 to 48 hours before folding reduces the potential to rub.
Manufacturers of both paper and ink will endeavour to continually improve their product performance in relation to their resistance to rubbing. In the meantime, it is advisable to carefully select the combination of materials used. Consider and pre-plan all processes (specific to matt coateds) to enhance the result and take extra care in the binding and finishing department.