TACKLING PROBLEMS

Surface Picking	Static
Paper Mottle	Curl
Set Off / PoorDrying	Cracking on the fold
Blade Lines	Ghosting
Wavy edges / Tight Edges

When problems occur on press it is quite reasonable to suspect that the subtrate being printed may either be contributing to the problem, or in some cases may even be the cause of the problem. It is also true to say that very often the substrate is a victim rather than a culprit. However, it is also wise to raise your concerns with your merchant (please see COMPLAINT HANDLING section).

Nonetheless time pressures often mean that a 'quick fix' is preferable to a partial or full reprint. The purpose of this section is to suggest ways in which problems can be solved or their impact lessened.

Surface Picking

This is a problem usually associated with the laying down of solids. It is unusual for a sheet to equally prone to picking on BOTH sides. In many cases, therefore, printing on the reverse of a seemingly problematic sheet may see a picking problem reduce or even disappear altogether. Another remedy is to use the first unit to "scavenge" by running on impression with fount but no ink. This should remove most of the debris, and allow you to achieve a better result. Finally, reducing ink-tack, altering pressures and adjusting the ink film density may all alleviate picking.

Paper Mottle

Again, the tendancy to mottle is rarely identical for both sides of the sheet. If possible, when you experience mottle, try printing the reverse side and compare the results.

Set Off / Poor Drying

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Such problems are as often due to the inks used as the paper itself. Widespread use of so called "stay-fresh" inks have many advantages however there can be downsides. Such inks avoid "skinning" because their tendancy to oxydize has been chemically reduced. Many paper stocks (especially those with smooth hard surfaces such as Ivory Boards, and Cast Coateds) rely heavily upon oxydation of the ink for the image to dry properly. This is because, the surface is so "closed" it is difficult for the ink to penetrate and thereby to dry by absorption alone. Where you encounter poor ink drying/set-off, check to see if the inks are of the "stay-fresh" variety. If they are, try switching to conventional inks which will oxydize; it is likely that the drying performance of the ink on paper will improve significantly.

Blade Lines

These lines are usually caused by small hard particles that become trapped between the paper and the coating blade as the base paper is being coated at the paper mill. Such defects are almost impossible to detect until an ink solid is printed on the affected area of the sheet. Usually, blade lines are intermittent and only a very small proportion of the pallet will be affected. Indeed, only one side of the sheet is normally affected. Depending upon the design of the job, it may be worth printing on the reverse side when blade lines are discovered on the top side.

Wavy Edges / Tight Edges

Where the edges of the paper have become wavy, the ambient conditions in the press room have a significantly greater relative humidity than the paper itself. Because the paper is "dryer" than the environment it is exposed to, the paper will try to equalise itself with the surrounding air. It will therefore absorb moisture along its egdes and swell/stretch. This will cause major difficulties in feeding on press, and if the paper has already been subjected to one pass prior to the edges swelling, then misregister will be inevitable on subsequent passes. Where you discover wavy edges you may be able to retrieve the situation by securely wrapping the stack in moisture resistant material, and then leaving the wrapped stack at a constant temperature for 36-48 hours. In many cases the sheet will revert close to its original mill condition.
Beware that prolonged exposure to a higher RH environment will see the return of the edge wav The perfect solution is to have the facility to adjust your press room conditions (ie temperature AND humidity). Paper is normally manufactured to be in balance at an ERH of 55 at 21C and the ERH range at 21C is usually 40-60. Only where the paper falls outside this range will a mill accept that the paper is the primary cause of a wavy edge problem. Best practice would be to keep the paper WRAPPED and at the press room temperature and to only unwrap it immediately before use; where multi-pass/process work is involved, ALWAYS wrap the paper in moisture resistant material between processes.
The converse of wavy edges are TIGHT EDGES; here the press room conditions are dryer then the paper. Exposed edges will therefore give up moisture and dry out and shrink as the paper seeks to equalise itself. The remedies and solutions are exactly the same as those described for WAVY EDGES. ide air is cold and press rooms are heated-this causes the air to expand and become dryer as a result; WAVY EDGES are more common in summer where outside air temperatures are much higher and generally more humid.

Static

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The "sudden" appearance of static within a stack of paper can be a nightmare. It is true to say that the causes are not fully understood, what is clear is that it is rarely a problem due entirely to the paper alone. All stacks of sheeted paper will contain redidual electrostatic energy. It is rarely noticed, because in "normal" conditions it naturally discharges into the air. However, hot press rooms in winter months mean the surrounding air is very dry and will therefore not act as an effective conductor. Sheets will be prone to sticking together and even sticking to metal parts of the press.
The best solution is to raise the press room humidity and lower the temperature; unfortunately, this is not always possible. The next best solution is to fit static eliminator bars/ionising bars to the press. Some success may be possible through fitting foil strips/tinsle above the feed and allowing this to a degree but it is just what it seems-a "Heath Robinson" solution which is largely unreliable. Vigorously fanning the stack can also help, but the discharge of static will only be temporary because if the surrounding air remains warm and dry, the static will soon build up again. It is UNLIKELY that a replacement of fresh paper will help if the press room conditions remain the same.

Curl

No paper will ever be perfectly flat due to the way it is formed and then dried on a paper machine. So all mills have a defined tolerance for acceptable curl for each of their products. If the edge of a sheet is lifting by over 5mm above the plane, this will generally be considered to be excessive curl.
Curl can be a symptom of paper out of balance with its environment (see wavy/tight edges above); it can also be a deliberate paper property such as 1-side coated boards, where a down curl across the grain is induced to ensure the long edge is flat when gripped on the press. More usually, however curl is attributed to a fault during the manufacture of the paper. Where paper has an excessive curl, the best course is to replace it. However, where time does no permit, and the job is printed one side only, the paper will feed better if presented curl down. For duplex jobs there is really no effective remedy, although rolling the paper and slowing the press may help. Using wedges/chocks can help angle the paper to ease feeding; reducing the stack height at the feed and the delivery could also lessen the inconvenience.

Cracking on the Fold

A common problem where boards, espectially coated boards, are folded into covers. Surface coating will have quite different elasticity and fold resistance to the base paper beneath, so cracking on the fold is an inevitable fact of life when folding such products. The key is to limit the extent to which it cracks Prevention begins during the planning of the job. As far as possible, avoid folding over a solid of ink or varnish because the cracking will be far more obvious where the white base is exposed within a dark solid. When folding it is essential that the three variables of mechanical creasing machines are optimally combined; they are: the WIDTH of the creasing channel, the DEPTH of the creasing channel, and the BREADTH of the creasing knife. Considerable dividends will be gained by exercising the same degree of care with creasing that was given when originally printing the job.

Ghosting

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Ghosting is an intermittent phenomenon that is caused directly by the gaseous bi-products (vapour) emitted when solvent based inks dry as the ink carrier evaporates. The ghost image is created as when the vapour is trapped between adjacent sheets on the press delivery. Normally the vapour will escape, but if the airflow is restricted some of the vapour will be absorbed by the reverse side of the sheet falling on top of the inked area. Usually the side absorbing the ink has yet to be printed and so the "vapour image" will be invisible to the naked eye.
When the surface contaminated with "vapour images" is later printed, the invisible image "develops" and appears as a ghost within the freshly printed area. This is because the vapour image is a barrier and consequently it will affect the absorbency of the ink laid on top of it. The problem is exacerbated by the weight of the paper stack. Ghosting is far more common when printing board weights and is usually more prominent in sheets nearer to the base of a stack in comparison to sheets nearer to the top.

What is not fully understood by technologists is why, when a job is reprinted with "all things being equal", the problem does NOT reoccur. Many theories exist relating to air temperature, ink temperature, dwell times, press settings, fount levels but none are conclusive. What is certain however is that the problem cannot originate from the paper being printed because none of the components used in constructing the sheet have any petrochemicals present.

However there are several techniques for reducing and/or eliminating ghosting when it is encountered on press:

1) Set-off spray will enable air to flow between the sheets as the stack builds enabling the vapour to escape.

2) Reducing stack heights will lessen the weight and in turn the prominence of the ghost.

3) If a solvent based sealer is being used change to an aqueous based product as the vapour from the sealer can promote ghosting.

4) If a solid is to be printed on one side and text/half tones on the other, print the solid first. Ghosts are always more visible in solids.

5) If ghosting is suspected put the unprinted contaminated side through the press on impression only (no ink) as this will assist in dispersing the vapour. Clean the blankets very regularly to avoid a ghost building up on the blanket (if not it could then transfer back to the sheet).

When ghosting occurs it is usual for a culprit to be sought. The problem is that no one, as such, is toblame. Ghosting is widely recognised as a phenomenon and should be treated as such. The interested parties therefore must take a commercial view rather than a technical one.

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