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Blog Post By Inky Fingers

Paper and Ink

Tom Kerchiss from RK Print Coat Instruments Ltd talks in-depth about paper and ink in another instalment of Inky Fingers

A pack not only contains and protects the product contained within, its also often regarded as the face of the product: its generally the closest the potential consumer will get to the handling and experiencing the actual product prior to purchase. For this reason the pack must be innovative and distinctive with all of the elements of the design coming together to help make the purchasing decision easier for the consumer.  

However, achieving the desired effect on press is not always easy. Colour for example is very subjective; literally it’s in the eye of the beholder - as apart from identical twins no two pairs of eyes have the same physiological construction and chemistry. The substrate may also presents challenges, affecting the selection of critical materials such as adhesives, inks and coatings.

Lets take paper as an example. The use of paper and paperboards for food packaging dates back to at least the early 17th century when cones of paper were used to hold sweetmeats and expensive product such as sugar. Today, Paper and paperboards are commonly used in corrugated boxes, folding cartons, bags and sacks and milk/juice gable top cartons, etc.

Colour is very subjective; literally it’s in the eye of the beholder

When used as primary packaging, paper is always treated, coated, laminated or impregnated with substances such as waxes, resins or lacquers. Paper for packaging includes sulphite, which is glazed to improve its wet strength and appearance. It can be coated to improve print appearance; sulphite papers can form part of a laminate structure and are used for biscuit wrapping and confectionary. Kraft papers are also widely used either as natural brown, unbleached heavy duty or bleached white. Natural Kraft provides for superior strength and can be used for packing flour and other dried goods. Other products include paperboard i.e., white board, solid and fibreboard offer superior strength per weight.

Some of the characteristics that must be taken into account when using paper include paper strength, base sheet uniformity, moisture content, smoothness, gloss, ink receptivity, brilliance, whiteness and coating type. The importance that each of these characteristics plays varies from job to job.

When printing the evenness of an ink film must be considered and is determined by a number of factors including the substrate. An ink film is approximately 1 micron thick, thus a paper with a surface roughness of less than 1 micron produces the smoothest ink lay down and the best gloss. Conversely, a paper with a rougher sheet surface is generally preferable for a process such as laminating, as the adhesive will adhere better to a less smooth surface.

 Ink receptivity is of course, also affected by the clay coating or the way in which the paper is finished.  If the fibres are well interlinked and are bedded in with the base stock, this will provide for a smooth coating, whereas if the fibres are orientated horizontally or in a random fashion, coating inconsistency occurs.

Ink density correlates closely to the percentage of pigment contained in the ink film, and this pigment loading accounts for most of the density, with paper absorption playing a lesser role. However, its ink film thickness, not ink density that ultimately determines ink gloss. Two ink films printed to the same ink thickness but with different levels of pigment loading will give the same gloss levels but will not be equal in density. This knowledge can be used in an attempt to overcome the roughness of some paper stock. By reducing the pigment loading for the ink, film thickness is increased and a desired density level achieved.

It is generally impossible to reproduce what one printer did with litho on a good white paper with flexo, gravure, or any other process if you don’t have the same level of whiteness

If the paper is glossy, normally it has a clay coat. More gloss gives the printer a better hold out of the inks and a better colour. One paper property that affects ink gloss is absorption. Ink films transfer from surface to surface by splitting, which produces a split pattern within the ink film and is present until the ink flows or levels. If the absorption rate of the paper is fast enough it can pull the thin oils from the ink film before the levelling out process takes place, when this happens the ink film retains the split pattern, causing a roughness within the ink film that scatters light and reduces the visual gloss.

The uniformity of an ink film may also be affected in other ways; for example, uniformity can be disrupted in the first printing unit by a phenomenon known as water interference. The ink, as it moves through the inking system to the paper, comes into contact with water, which is suspended within the ink film and on its surface. The water then connects with the paper before the ink, and if not removed from between the paper and ink film via absorption, or by other means, the ink film will be spotted with water resulting in a drop in density, poor uniformity and poor quality print.

Brilliance and whiteness of paper are essential for many print processes. Whiteness can affect the final colour, depending on the opacity of the ink. However, not only does whiteness of stock vary from supplier to supplier, it also varies across a single roll of stock. If a printer is using ink with a greater density such as letterpress, it will hide the substrate and any variation in whiteness. Other types of inks may not. It is generally impossible to reproduce what one printer did with litho on a good white paper with flexo, gravure, or any other process if you don’t have the same level of whiteness.  Problems associated with whiteness and brilliance is exacerbated by the use of recycled papers.

Some of the biggest changes in printing for all segments are coming about through standardisation of quality processes within the print and supplier chain operations. Substrate producers, ink manufacturers and printer/converters are endeavouring to work closer together to achieve a broader and deeper awareness, understanding and application of the technical parameters. Benchmarking and shared goal setting have become more prevalent as all involved try to find ways to bring process variables under control.

Colour communication systems play an important role in product development and product quality control; they enable ink, varnish, adhesive producers; paper, film and foil manufacturers, and printers and converters, etc., to developed new products, ensure product/batch consistency and repeatable print quality. Systems available from RK Print Coat Instruments Ltd include devices for gravure, gravure-offset and flexography, etc. Some devices, for instance the K Printing Proofer cover more than one print process, this device enables users to produce high quality proofs using flexo, gravure and gravure-offset inks while also allowing the operator to produce wet or dry laminating samples. Systems such as the Rotary Koater extend performance possibilities still further allowing organisations to select from almost two dozen print and coat head technologies. Hot air, infrared and UV curing possibilities adds to the versatility still further. For those converters, manufacturers and others with process aspirations that are out of the ordinary the bespoke VCM is an option.

Inconsistency of colour, poor rub and chemical resistance will mar print/pack presentation – unthinkable in a situation where penalty clauses may be evoked if quality is not up to scratch or where work may be rejected

Inconsistency of colour, poor rub and chemical resistance will mar print/pack presentation – unthinkable in a situation where penalty clauses may be evoked if quality is not up to scratch or where work may be rejected. For gravure users and producers of gravure inks the GP100 is worth considering. This device is for the production of high quality proofs using gravure inks of press viscosity. With the GP100 any flexible substrate can be printed; it incorporates a microprocessor controlled servo drive and offers a high degree of controllability with variable printing speeds of between 1 to 100 metres per minute.

The GP100 can be used for research and development, computer colour matching and for the production of presentation samples. Both producers of gravure inks and the user of such inks should find the GP100 invaluable when used for quality control purposes.

For those users and producers of flexo inks the bench top FlexiProof 100 and FlexiProof UV are off-line devices that can be used to determine ink/substrate interaction and for colour matching; trialling new or unfamiliar consumables and materials and for many other tasks. The FlexiProof continues year on year to build on sales with the latest incantation of the FlexiProof, the UV/LED FlexiProof being well received. This device utilizes a LED or Light Emitting Diode lamp system in place of the dichroic lamp heads. One of the significant advantages to LED is a massive reduction in power/energy and an ability to handle more sensitive substrates.

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