FEATURE Carton & Board Printing on carton and board
A winning packaging concept is made up of many elements, not forgetting what is physically possible with the chosen substrate and the print and production processes, says Tom Kerchiss of RK Print Coat Instruments
Packaging designers and converters regularly need to come up with a winning
formula if they want to keep consumers not only noticing their clients’ products, but also purchasing them. Something that always needs taking into consideration is the form in which the packaging will take (whether it’s a pouch, box or paperboard sleeve, etc.), its functionality and the image/appearance. Colour is a crucial factor, serving first as a
brand recognition marker for the consumer. Its appearance varies according to factors such as light, but also the substrates involved. In direct printed corrugated applications, the substrate plays a major role in dictating how the ink is going to appear on the finished job. Absorption and surface tension are the two main factors that influence print density, drying, trapping, and dot gain. When providing a customer with a colour match, it is important to use the same substrate as will eventually be printed on. Printing one colour on Kraft, mottled or even clay-coated sheets that have the same surface appearance will yield a wide range of shades from the same colour. The sheets with a high level of absorption produce a weaker colour due to the ink being absorbed; those sheets with a lower absorption, however, will produce stronger colours, and if clay-coated sheets are used the ink will not penetrate the surface. Surface tension therefore plays a role in determining colour strength. When printing on clay-coated board, insufficient coverage or poor trapping may
well be associated with poor adhesion, due to the materials in the ink having a combined surface tension that is higher than that of the substrate. Inks can often mix with water for greater effect; however, too much water causes an imbalance in the ink.
Monitoring rub resistance The rub resistance that printed ink exhibits has to be modified in some instances. For example, Kraft, mottled and bleach board tends to show more rub than a clay-coated board. Rub resistance is adjusted with waxes and specific polymers; colour communication devices are ideal for monitoring results as well as for product development (inks, adhesives, varnish, etc.). On a clay-coated board the ink supplier needs to be aware if the board is going to be overprinted with a varnish. Usually, complex graphic printing is undertaken on clay-coated board with the varnish added to serve barrier and gloss purposes. In this instance it would be counterproductive to add a wax additive, which would not only reduce the gloss but cause problems with solid over solid trapping.
Ink pH levels on board The pH of inks used for graphic printing on clay-coated board is generally higher than for Kraft board. The higher pH helps to stabilise the ink, makes it stay open and slows the drying of the ink on the printing die. A lower pH range is advisable for Kraft type boards because these are generally printed at faster speeds. Adjustment of pH is usually done with amines or ammonia. The former is more stable, while the latter poses health risks, requires more attention and leads to faster drying. Irrespective of the method, the ink pH must be checked occasionally to prevent changes caused by evaporation. pH stable ink is the best option, requiring less attention.
Printing on corrugated board The printing of corrugated board is subject to many processing variables, striping being one of them. Striping has print density and/or print gloss variations that appears as stripes on the printed liner parallel to the flutes of the corrugated medium. These print
www.convertermag.co.uk
The K202 Control Coater from RK Print
density variations are caused by the printing pressure inconsistencies associated with the board, while the print gloss variations are caused by surface distortion of the corrugated board, such as wash boarding. Whether direct printing or printing on linerboard, flexography is often the dominant print process. Colour communication devices and other product monitoring tools are now common in many production environments and play an important role in maintaining commercial and product viability.
An advantage of devices such as the
Control Coater or K Printing Proofer, offered by RK Print Coat Instruments, is that they can be used by everyone involved in the supply chain, from ink manufacturer and substrate producer, to printers and converters. This makes for greater collaboration, highlights problems quicker and speeds process solutions.
RK Print Coat Instruments T: +44 (0)1763 852187
www.rkprint.co.uk
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