The growth of commercial digital printing is driven by companies expanding their portfolios with products not feasible, or complimentary, to sheetfed offset, there is also some substitution of offset presses by “high speed inkjet”.

Typical applications include direct mail, targeted advertising, greeting cards and photobooks where personalisation and customisation are taking centre stage. Digital printing is often used for short runs and print-on-demand production. But there are also hybrid models, where initial volumes are printed offset and additional copies are produced digitally on demand.

Choosing the right paper for specific print requirement

Different printing methods require different paper properties.

“Each type of paper is designed for a specific printing method. To achieve the intended print result, it is necessary to choose the appropriate paper for that method,” said Daniel Siegel, Technical Customer Service Manager, UPM Communication Paper.

For example, sheetfed offset requires higher surface strength to handle certain inks, while for heatset, the paper surface needs to allow hot air ink drying without blistering, and coldset requires a porous surface to allow ink drying by absorption.

Papers can be divided into three categories:

• Papers developed for digital printing: These are specifically designed for digital printing and often come with guarantees from their manufacturer. They are adjusted for inkjet or toner systems and some have certifications to provide consistent print performance.
• Papers with suitability indicated by the manufacturer: These are not specifically developed for digital printing, but the manufacturer can provide recommendations based on experience with certain digital presses but they are not guaranteed for all digital applications.
• Non-digital or wood-free uncoated (WFU) papers: These are intended for offset printing. They may work with digital printing, but no guarantee is provided and any issues arising from their use are the responsibility of the printer or customer.

Different printing technologies require different paper properties but use almost the same paper handling and logistics. Source UPM/OPHAL

“You have to evaluate whether it makes sense to pay more for the paper, since digital papers cost more, or if it is better to use a standard grade and accept the potential risk of issues,” he said. 

Paper properties and digital printing technology jointly determine achievable print quality. Optimal results require an application-specific trade-off between quality, performance and cost.

For example, digital ink has a much higher cost than offset but the use of papers treated for inkjet production can lower ink consumption to reduce total costs. So can ink coverage reduction strategies. Digital production often requires higher cutting accuracy and tighter tolerances. 

Right way to transport rolls within the print shop

“If you are entering roll-fed digital printing, your internal logistics must be able to transport narrow rolls,” Siegel said.

If there is no equipment for internal roll transport, special packaging solutions may be required, which can add recurring charges that increases the overall cost of paper.

From the paper manufacturer’s side, the preferred option is the use of standard roll specification that includes:

• Diameter: 1,250 mm
• Core: 76 mm
• Packaging without pallet
• Rolls loaded eye-to-the-sky on the truck
• Full truck loads

These specifications support efficiency for both sides. They allow the paper mill to combine orders and optimise production, while reducing costs for the customer.

Humidity affects paper stability and press performance

Paper is a hygroscopic material that absorbs or releases moisture very quickly, depending on the prevailing environmental conditions and equilibrium of moisture content. Dimensional stability and press performance depend on the paper being acclimated and stabilised to the press room conditions.

Dimensional variation of sheets is lowest at a relative humidity between 40 percent and 60 percent. 

Challenges associated with incorrect storage/handling:

Caused by humidity change

• Runability problems in the press
• Too loose or too tight edges on the paper roll
• Tensioning problems
• Paper curling
• Wrinkling
• Running properties can be negatively affected by static electricity, reduced bending stiffness & waviness.

Caused by conductivity

• Cold paper cannot be heated sufficiently
• Printing speed has to be decreased, lower dryer temperatures should be avoided

“For this reason, paper is produced with a moisture content of about 50 percent for offset printing, while for heatset printing it is lower at 38-45 percent,” said Andreas Wachter, Substrate Consultant at Canon Production Printing Germany.

The press room climate target is about 50 percent relative humidity at 23°C. Even if the climate is not controlled, it should be monitored for temperature and humidity fluctuations to determine the time required for acclimatisation.

If humidity is low, moisturising systems can help maintain good conditions in storage and press rooms by using a fine water spray. 

How to prevent condensation damage in rolls

“Condensation occurs when air is cooled below its dew point temperature,” Wachter said.

For instance, cold paper rolls stored in warmer, humid environments can develop condensation, that may lead to water damage. He recommends keeping rolls in their original, moisture-proof packaging and away from heat sources, open windows, damp walls, floors or direct sunlight.

“Paper does not transfer heat easily, so it needs time to equilibrate to the room temperature. The roll maintains its own microclimate inside the packaging, so it should not be removed during this period,” he said.

“Opening the packaging too early can cause wavy edges, which may lead to runnability issues. Very dry paper can also build up static electricity, which creates problems during finishing,” he added. 

Factors that cause roll damage 

“It is advisable to use a curved surface rather than a flat surface for transporting and storing rolls. A curved surface distributes the weight of the roll over a larger area to help prevent flat spots,” said David Croll, Senior Manager – Product Testing & Validation, Tecnau.

For printed rolls, inks or toners can have a tendency to stick together. If a roll rests on a flat surface for a long time, the weight concentrated in a small area and can cause the ink to stick, and when unrolled, it may lift the ink off the paper surface.

Rolls must be handled carefully during transport. Dropped rolls can become out-of-round, affecting the unwinder and causing misalignment in finishing. In book production, this can shift page numbers and produce unsellable products that become avoidable waste. Rolls and pallets should be stored carefully and clamp trucks use the correct pressure for the paper being lifted.

Correct winding tension is important. While paper mills manage this for unprinted rolls, digital printers may experience coneing or telescoping when rewinding roll-to-roll. A start/stop operation can worsen misalignment. Minor coneing can be corrected with an active web guide (±25 mm) but beyond that, the roll may be unusable.

Poorly slit edges can produce dust that accumulates in finishing equipment and can affect the optical sensors. “The more dust on the sensors, the more problems they have to identifying what they are supposed to read. This can include data matrix codes, cut marks or sensors detecting whether a sheet has moved from one part of the finishing equipment to the next,” Croll said.  

Curl can affect paper, and most products need to stay flat. Large rolls show less curl, but curl can appear near the core and end-of-roll. “Smaller cores make this worse. For papers over 150 gsm, larger 152 mm cores are recommended instead of 70-76 mm,” he added.

In digital finishing, some customers use adapters instead of standard cores after roll-to-roll printing. This allows the roll to run to the end with minimal curl. This method is recommended for consistent finishing quality, Croll said.

Nigel Wells, OPHAL editor, who moderated the session, said that following the right processes will save money, improve efficiency and reduce waste.

“This is also important for post press processing to ensure productivity with the importance of winding tension, slitting, roll and sheet edges. The costliest waste occurs at the end of the process, if a problem arises after printing, disposal costs are higher. Best practices should be applied throughout the entire process,” he emphasised.

About OPHAL

‘Optimised Paper Handling & Logistics’ is a best practice cross-industry collaborative project that brings together expertise from across the entire paper supply chain from the mill, through transport, storage, handling and printing. It is a generic best practice tool and reference for suppliers, transporters, converters and printers to improve their safety and efficiency. Substrates are the highest single cost for all printing applications and any reduction in their waste improves both economic and environmental performance. Over 30 industry associations and companies participated in the project. OPHAL is a cross-industry competence group hosted by the World Printers Forum. The 270-page FREE e-book is available from www.ophal.info.

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