Recommendations for pre-printing
There are certain things that can be done in order to get the very best results in the offset printing stage. These allow for increased productivity, not to mention an increase of quality of the final product.
Key Factors To Keep In Mind
Feeding the paper into the machine
The pallet should be unpacked in line with how quickly it is actually needed. This doesn’t just have an impact on productivity, but the ambient conditions can have a negative effect on the flatness of the paper – certain temperature ranges and humidity are not kind to paper.
As an example, let’s say you have a pallet of 12,000 sheets and you are printing at a rate of 6,000 sheets an hour. The print job will, of course take 2 hours. During that two hours the sheets toward the bottom of that pallet are going to be exposed to the ambient conditions the longest and if the conditions are not ideal, they will be affected.
What if the whole pallet is not used? The remaining paper should be well protected (sides and top), using the original plastic from the pallet itself. The pallet should then be kept in the print room, rather being moved around, until it is next needed – this helps to preserve the quality of the remaining paper.
Dampening water usage should be kept to a minimum. This ensures not only print quality but also that absorption of humidity by the paper is kept as low as possible. For ink drying time optimisation, the pH of dampening water should be kept between 4.8 and 5.5.
After the paper is cut, it must be packed quickly – preferably in a packaging that is immune to the effects of humidity. This makes sure that the pre-printed material is ready for use at the laser, or inkjet, printer and it is totally flat and ready for printing again.
You should avoid exposing the paper, even when it is still packaged, to extreme temperatures and humidity levels. Ambient conditions in the print room should be kept consistent, and not subject to changes. Ideally, the conditions should be kept at the following:
● Temperature: 18 – 24 ºC
● Humidity (%): 30-55
No paper should be in the print room less than 24 hours prior to use, and with original packaging intact. This allows the paper, protected by the packaging, to acclimatise to the ambient conditions. The only time that the paper should be unpacked is immediately before being fed into the machine.
Between Offset printing and guillotine cutting, the paper needs to be protected against humidity absorption. This will help prevent the edges of the paper becoming wavy and leading to issues with curving paper after laser printing.
Anti-set-off powders should not be used. Whenever post-print personalisation is carried by laser printers, always use special inks for laser post-printing.
The majority of the time, paper will be cut into A4 sheets, therefore it is necessary to make sure that the grain of the fibre is parallel to longest side of the sheet. This is important because it is the only way of making sure that it performs properly in laser printers and post-processing operations (auto. envelope insertion, for example).