Print drying

Inks are formulated to set rapidly on the paper surface and to allow work to be backed up or processed in other ways, with minimum delay. Setting time should not be confused with drying time, which can be significantly longer. Both setting and drying times are affected not only by ink formulation but also by substrate, how the print is made and the design and layout of images.

Problems associated with prints that are not sufficiently dry at the time of processing include:

• Print set off after transport to us but before we have processed the work
• Marking of the print during processing
• Reticulation and/or poor adhesion
• Print set off after processing by us

All these problems tend to affect heavily printed four colour builds particularly where they are backed up by similar images and are also likely to be variable in occurrence and severity throughout the job.

In each case allowing a little extra time for the print to drycan usually avert the problem.

Even when the print appears to have dried sufficiently it is possible for an effect, termed "sweat back", to soften an ink. This occurs when ink solvents, which would normally dissipate into the paper and then into the atmosphere, are driven back into the dried ink film causing it to soften to a point where set off can occur.

The presence of an impermeable laminate or varnish on one side of a sheet can hinder escape of ink solvents. Their only escape route is into and through the inks on the reverse of the sheet, thus increasing the risk of set off from the reverse side onto a laminated or varnished face side.

Recommendations

It is impossible, in any practical sense, to predict many of the problems caused by fresh, semi-dried inks and therefore preventative action needs to be taken at source, and applied to all work, if it is to have any positive impact on the problems.

Printers have ownership of the work and usually control the materials and techniques used in any job up to the finishing stage. Print drying, and the problems caused by the lack of it, are therefore solely within the control of the printer.

Finishers have to be aware of the potential risks when processing each and every job. They must be vigilant and maintain a system of regular inspection during processing so that print set off is found if it has already happened, and ink marking or varnish reticulation is identified and eliminated before it becomes a serious defect.

The most difficult problems tend to be those that happen after the job has been processed, e.g. set off after lamination. Often both the printer and laminator are confident that they have not done anything unusual in their own processing of the work and the resolution to the problem is usually confined to economics, with the result that both value and trust are stripped from our industry and little thought is given to preventative action for future jobs.

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Colour change/pigment choice

Light tints using the following pigments are particularly sensitive to bleaching when in contact with UV varnishes, water based laminating adhesives and water based coatings (including sealers).

• Reflex Blue
• Rhodamine Red
• Pantone Purple

Since all three pigments are integrated into the PANTONE matching system and they cannot be replaced within that system by alternative stable pigments, it has proven impossible to eliminate their specification or use on work to be varnished or laminated.

The varnish, coatings and adhesives industry has also been unable to manufacture materials that are guaranteed not to bleach these sensitive pigments. Given that the risk cannot be eliminated, the only option is to develop routines and practices that seek to minimise that risk.

Critical factors that we have found to be crucial to ink bleaching are:

Tint strength - Bleaching or colour change is often more severe if there is a small amount of sensitive pigment in the ink. This is the complete reverse of the natural assumption that it will not matter too much if there is only a little of the pigment present.

Print drying - Bleaching becomes visible more quickly and is more severe if the print is not hard dried at the time of varnishing or laminating. We have identified this factor because of the number of problems we have seen while bleaching has occurred as a ghost of images on the other side of the sheet. This is due to retarded ink drying in the backed up areas.

Stack temperature - Since most chemical reactions are promoted by warmth it is not surprising to find that high stack temperatures, such as result from UV varnishing are a factor in bleaching. We have seen badly stacked pallets where portions of sheets outside the body of the stack are not bleached while the remainder is severely affected. Also where our own processed sample sheets, which have not been exposed to stack conditions, do not match the bleaching found on the job.

Bleaching is promoted if the finisher uses highly aggressive varnish/adhesive formulations. Celloglas and their associate companies take exceptional care in selection of our raw materials and this is a prime requirement of any material before we approve it for use.

Varnish/adhesive quantity - It is also promoted when more varnish or adhesive is applied to the ink. We have found that spot UV varnishing - which applies 8 -10 gsm of varnish to the print - is more likely to bleach than rollercoat UV at 4-6gsm. Similarly prints on rough substrates that require high coating weights of adhesive are more likely to bleach than those on smooth paper where the quantity of adhesive can be reduced.

Time - An important feature of pigment bleaching is that it takes time to occur. If several or all of the previous factors combine then noticeable bleaching can occur within a few hours of processing. If circumstances are less extreme, bleaching may take months to appear and it is not unusual for a problem to remain unidentified until a file copy is exhumed for a repeat order.

Recommendations

Wherever possible avoid the use of these sensitive pigments. If you cannot avoid them, particularly in tints, then warn your finisher of their presence. Allow prints time to achieve a hard dried state (this differs from them being set enough to travel).

We have found that proofing a job to see if it will bleach seldom reproduces the fault because there are so many factors that have to conspire to produce the problem. Proofing does, however, delay production of the bulk job and this single action is usually successful in saving the job.

An apparent colour change can also occur to a print after lamination or varnishing because either the eye is tricked by the change in gloss of the print or by the slight milkiness of a matt film laminate or coating. There is no solution to this other than to make an allowance for the optical change at proofing. Applying either gloss or matt Sellotape to the print can reproduce these effects.

It should be noted that half-tone images will always show more colour shift than solid printed images – the ??? or varnish has the effect of “magnifying” the half-tone dots.

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Metallic inks

We actively try to dissuade designers from specifying metallic inks on lamination or varnishing work for the simple reason that their performance can be extremely erratic and often appears uncontrollable.

There are two common problems associated with metallic inks.

Poor cohesion between the flakes of metal near the surface of the print, caused by a lack of the resin that should be present to bind the flakes together. Laminates and varnishes applied to such a loosely bound surface inevitably show poor adhesion, which can often result in lift of a laminate at the trimmed edge or poor scratch resistance of the varnish or laminate. The failure occurs within the body of the ink, i.e. a cohesive split.

Lubricants, which are used when the flakes of metal are produced by grinding or milling, can migrate to the surface of the drying print in sufficient quantities to cause poor trapping or reticulation of varnishes in the same way as waxes in conventional pigmented ink.

Recommendations

If you have no choice but to use metallic inks we suggest that you choose an unabsorbent paper with good hold out and print using the minimum film weight of ink. Not only is this good economics but also it is effective in reducing the risk of both potential problems.

If the completed dry print shows poor rub resistance it is likely to give problems. The simplest test is to draw a finger over the print. If your finger removes quantities of metal we would suggest proofing the job either with a laminate or varnish. Proofs that give a defective varnish result can sometimes be laminated acceptably provided the metallic ink does not bleed over trims or fall in creased or embossed areas.

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Inks - Some general advice

Adhesion of laminates and varnishes to an ink can be seriously affected by the presence on the ink surface of waxes or other slip additives. These materials may come from the inks themselves or be applied to the ink surface during re-moistening of heat set web offset printed work.

All inks contain waxes or slip additives, however some have more than others and these are generally referred to as high wax inks. They are designed to offer improved scuff and handling resistance and do not need over laminating or coating.

The slip additives migrate to the ink surface during the drying phase of the ink. The amount that actually reaches the ink surface is largely dictated by the type and quantity added to the ink, the length of the drying time and the amount of ink applied to the paper.

Because ink drying is likely to be slower and the quantity of ink greater in four colour build areas of the sheet, this will promote migration and it is very common to find problems caused by slip additives or waxes only in these areas of the sheet. Infra Red drying of print can also promote migration.

UV varnishes may be difficult to "trap" or apply to an ink surface with a high wax content so much so that the varnish is unable to "wet" the ink resulting in reticulation of the varnish. Both problems produce, at best, a poor gloss in the affected area and, at worst, an extremely obvious defect.

Recommendations

All work to be laminated or varnished should be printed using low wax inks or sealed with coatings that also contain low levels of slip additives. The simplest way to ensure this is to specify to your ink supplier that inks be suitable for lamination and varnishing and to take the recommendations for their use seriously.

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Water based sealers

Water based sealers or coatings are increasing in popularity because they can provide a very useful reduction in the waiting time for backing up a job and also their reliability in preventing set off. They are of benefit to the finisher by reducing the use of spray powder and in providing a good clean surface to which both adhesives and varnishes can bond.

]As usual there are also some problems caused by sealers to counter balance the benefits.

The most important of these is that sealer coatings can severely retard the drying rate of the print it covers, with the result that a laminate, although bonding well to the sealer, may lift quite easily from the print due to a cohesive bond failure within the ink. UV Varnished sealed prints may exhibit poor adhesion and scratch resistance. This is also a major risk with sealed metallic prints because they will not show the poor rub resistance that we expect, and test for, on a potential problem job.

We have also seen a few problems where laminated jobs have been sealed on both sides over very heavy four colour builds. The sealer has, in effect, trapped all the print distillate solvents within the print. As these heavy solvents slowly percolate through the laminate they can attack the OPP film and cause problems of frizzing or piping.

Recommendations

Please use a sealer that is formulated to be used with UV varnishes and laminates. Some coatings are designed to be topcoats, and contain high levels of slip additives that will interfere with adhesion of the laminate or varnish.

Sealers may affect the initial bond when laminated – leave adequate grop, side and back edges free from sealer.

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