Printing on Uncoated Paper and Board

Sealing

Sealing using a breathable machine varnish can help prevent solvents getting trapped in the paper, reducing problems such a gas ghosting and ink rub – particularly when heavily inked areas come into contact with white paper. However, water based (aqueous) sealants can cause excessive curl in the paper owing to the added moisture. Please also note that sealing changes the surface character of a paper, an important factor when choosing to print on uncoated paper. Retaining detail in darker shadow areas Conventional halftone printing has a variable dot size which can close up in the three-quarter and shadow tones because of the spread, or ‘gain’, of the larger dots. One way to avoid this is to use a repro technique called Stochastic or FM screening where the dots are all the same size, relying on a greater or less number of dots to create the various tonal areas. Because the dots are all the same size there is no detail loss in the darker shadow areas.

Scan format

If scans are to be supplied to the printer they should be as RGB files rather than CMYK files. This way the printer has access to them and has the possibility of adjusting the scan curves. Scans that have been previously archived as CMYK files can be converted to RGB but this risks detail loss. This also applies if the image has been generated as a digital photograph rather than as a scan.

Spot varnish

A gloss litho printed varnish works best on a smooth polished surface, so the natural texture of an uncoated sheet reduces this effect. An alternative way of creating the highly reflective ‘spot gloss’ effect is to use a transparent printing foil.

Folding and cross-folding

All papers can crack when folding unless simple rules are followed. Always crease the paper before folding and whenever possible fold with the grain of the paper. Paper is usually bought with the grain running along the longest dimension (Long Grain) but sometimes it is better to have the grain running across the sheet (Short Grain). The heavier the paper weight the more relevant this decision. Whenever you cross-fold a broadsheet there is always the danger of creasing or ‘crows footing’ where the two creases overlap. This can be greatly reduced by keeping the paper weight – and consequently the bulking – down.

Bright colours

We see colour as reflected light so the softer, more textured surface of uncoated paper reduces this reflection. Colour strength or brightness can be enhanced by the use of high density pigment inks, or simply by double-hitting colours. An extreme example of this is the use of fluorescent inks.

Contrast

Instead of keeping the entire image ‘open’ at repro stage, which can produce a flat result, increase the visual contrast by keeping shadows strong (90-95%), the mid- and three-quarter tones open and the highlights bright (0% in the catch light areas).

Ink drying time

In theory it would be possible to print 100% (solid) of each of the four colours, giving coverage of 400%. When printing in four process colours the amount of ink on the paper should not exceed a maximum coverage of 240% in any one area. Keeping overall ink coverage within these parameters reduces the risk of problems associated with rubbing and transference of ink to another page (set off). The way in which ink dries on uncoated paper can be improved by the common practice of turning and airing paper stacks. Even better performance can be achieved by the use of Fully Oxidizing inks. Press Open inks should be avoided.

Choice of weight

Paper weights are the same for Coated and Uncoated paper (measured in g/m2 – grammes per square meter); however, the greater bulk or thickness of uncoated paper means that, generally, a lighter weight can be used to achieve the same volume. Printing on a lighter grammage of uncoated paper has the added benefit of saving costs on distribution and mailing.

Printing detail

The amount of detail in a picture is more to do with the screen resolution used than the paper it is printed on. In the past, printers have been inclined to use coarser halftone screens (as low as 133 dpi) when printing on Uncoated papers. A screen ruling of 175 dpi will give better detail, provided the repro has been adjusted correctly. Stochastic screens, when available, will give even better detail retention because of the way they map the image more closely.

Suitable images

Illustrative techniques such as water colour, crayon and pastels are far more complementary to uncoated paper than Coated as the natural paper texture retains the subtlety of tone better.

The use of one of several under colour removal repro techniques such as UCR, GCR or PCR helps to improve both the contrast and shape of metallic and highly reflective subjects such as glassware. Keeping the shadows dense, the mid-tones open and the highlights bright increases the illusion of contrast.

Large solid areas of colour will look different on uncoated paper, but this is part of its surface character and not a printing problem. If you want a totally smooth flat colour, then you have to use a totally smooth or coated paper. Uncoated paper is neither better nor worse than coated paper, but it is different in character.

Choosing images that are best suited to the paper is one way of achieving exceptional printed results.

Further information please contact your local Finwood representative.

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