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Blog Post By Inky Fingers

Printing Corrugated Without Pressure

Packaging designers and converters regularly need to pull more than a few white rabbits out of the hat if they want to keep consumers not only noticing their clients’ products but also purchasing them. A holistic approach is necessary, designers, marketing teams and packaging technologists must come up with a winning formula time after tine after time and before any novelty value wears off and the consumers move on to something else. A winning packaging concept is made up of many elements but always includes the form in which the packaging will take (pouch, box, paperboard sleeve, etc.,), functionality and image/appearance. – Never forgetting what is creatively or physically possible given the chosen substrate and the print and production processes chosen.

Colour is a crucial factor; serving first and foremost as a brand recognition marker for the consumer but colour also provides an objective indicator of what to expect and how we perceive the world around us. Colour and 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 reason for this is that the sheets with a high level of absorption produce a weaker colour due to the ink being absorbed into the substrate; 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. This produces a cohesive effect, causing inks to shy away from some areas of the substrate surface and produce an uneven appearance to the final print. Inks with a higher level of polymeric additives, and incorporating water can be used with great effect; in this instance; the water is often added at the press side to reduce viscosity for faster drying and faster press speeds. Care has to be taken though as too much water causes an imbalance in the ink, reducing the rub resistance and gloss characteristics. A diluting vehicle can be used to maintain ink balance.

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.). The FlexiProof designed and developed by RK Print Coat Instruments Ltd can be used for conducting tests associated with rub, wear resistance, scratch resistance, gloss and other parameters.

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.

The printing of corrugated board, in this instance post printing is subject to many processing variables, striping being one of them


The pH of inks used for graphic printing on clay-coated board is generally higher than for Kraft board. This is because the inks that print on clay-coated board incorporate more polymer and have a carefully balanced level of water. The higher pH helps to stabilize 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 and quick drying is necessary. 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.

The printing of corrugated board, in this instance post printing is subject to many processing variables, striping being one of them. Striping is 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 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, printing on linerboard, etc., flexography is often the dominant print process and as already indicated process inconsistencies affect colour quality and result in excessive waste, machine downtime and rejects.  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 FlexiProof, the Control Coater or K Printing Proofer or any one of the many other colour communication devices developed and manufactured by RK Print Coat Instruments is that can and are used by everyone involved in supply chain from ink manufacturer to substrate producer to printer/converters, etc. This makes for greater collaboration; highlights problems quicker and speeds process solutions.

The FlexiProof 100, the FlexiProof UV and FlexiProof UV/LED can be used for a wide variety of purposes. They enable users to colour match off-press resolving ink and other related issues, easily and quickly – reducing on-press waste and providing savings in other areas such as time, energy costs, press downtime, etc. They can be used to determine printability – gloss, flexibility durability and rub resistance, prior to full-scale production. How inks/substrates interact can be determined while users can trial new or unfamiliar materials on the FlexiProof rather than tying up an income generating production press.

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