How to Get High Quality Digital Printing Paper (I)

Although paper manufacturers may not be very clear about the true meaning of "digital technology." But they do know that digital printing needs their help to get products like offset quality...

A variety of digital printing processes have appeared for several years, and we have seen the direction of short-run printing in the industry. Laser and inkjet printers have become necessary tools for small offices or SOHO families. Xeikon and Indigo's digital presses have quickly expanded into the short and medium-volume print markets. The advent of these technologies presented new challenges to paper manufacturers and paper applicators to ensure the normal operation of these systems and obtain acceptable print quality.

Maybe in the next few years, there will be good dry toners, liquid toners appear, so that the ability to run inkjet printers. However, for paper producers, the improvement of paper performance is also a key part of the work. Its biggest technical problem is undoubtedly that the paper manufacturer must be able to produce coated paper for production that the user can accept, and that the paper also needs to be cost-effective, easy to accept, and can be used simultaneously on various printing presses. This goal may soon be realized. This article discusses the key issues that are manifested in the printing performance of each digital process.

The production of such coated papers must be developed in accordance with new printing techniques and ink transfer requirements (eg, ink absorbency and surface strength). The current modern digital printing technology is different from the traditional printing technology, so the current coated paper cannot meet the requirements in production. Therefore, there are new regulations on the physical and chemical properties of the coated paper used. At present, the use of coated paper for offset printing machines has a wide range of applications, and to make it a universal paper for inkjet, laser, and other various digital presses is just a matter of course.

Type of digital press

Digital presses have the following very important features:

1) Directly use digital data to generate printed surface information;

2) It is very easy to reimage the printed surface;

3) Information with changing content can be printed on each sheet.

Although this definition is an acronym for various forms of digital printing presses, this article mainly focuses on inkjet and toner-type printing systems. Currently, these two devices occupy a large proportion of the market. These systems, unlike the DI type printers, have new requirements on the paper surface, and therefore pose new challenges to paper producers and coating companies.

As technology advances, expectations for print quality will inevitably increase. And now, we may already be able to accept the quality of inkjet printers printing on uncoated paper, and we may expect the next generation of such printers to be able to get the quality of print magazine effects on coated paper. In order to obtain satisfactory digital printing effects on coated paper, further improvements in various aspects are required, and the price of this paper should be comparable to that of conventional paper.

Dry powder printing machine

In paper consumption, non-contact printing is mainly based on dry powder electrostatic printing. Each office will have a monochrome laser printer. With the advent of Xeikon and related products, this market has expanded. As far as paper, office copying, laser printers and short-run digital printers are concerned, the same requirements can be met; paper specifications for toner transfer and melting technologies are new. In most systems, the transfer of toner mainly depends on electrostatic effects, while the melting condition is the effect of thermal effects.

These differences result from the paper feeding and stacking requirements. Some laser printers demand more paper for better performance. This type of paper requires a short cycle of operation and is subjected to drag and push in the printer. If the office's laser printers are sheet-fed, the problem is mainly with paper, double or multiple sheets.

For paper manufacturers, one of the most important parameters to control is resistance performance (Keithley resistance gauges can be used). If the resistance of the surface is very high, the static electricity absorbed by the paper will be high, and the paper sheets will stick together, which will cause misfeeds or double sheets, and even interfere with the control signals of the printer. If the resistance of the paper is very low, the transfer performance of the toner is poor, so that the density of the printed image is low. Resistance performance is usually adjusted by the paper manufacturer using salt, but the resistance of the paper is very sensitive to changes in humidity. For this reason, office laser printers or Xeikon-type devices are used to control the humidity of environmental conditions.

The uniformity of moisture content in paper is also a key issue. If the paper is too dry when it is packed, the edge of the paper quickly absorbs the moisture in the air when the bag is unwrapped, causing uneven distribution of moisture in the periphery and inside. At the same time, if the paper is too moist when packaged, the surrounding will lose water when opened. If the water content of the paper is too large, the toner will be charged with moisture during heating and melting, and blistering may occur, especially on short-run printing presses. The unbalanced humidity affects the print quality of digital presses like Xeikon.

If the quality of the paper is not very good, the occurrence of surface fuzzing will cause the density of the fibers in the printed part to be different, which will result in uneven transfer of the toner and eventually produce unnecessary print spots.

The thickness of the paper is a factor that affects the print quality of another type of Xeikon printer. The rougher the surface of the paper, the higher the surface that the print head touches for the print head, which can cause the toner to concentrate on the top of the printed surface, causing unevenness. Rough surfaces theoretically produce the same phenomena as uneven humidity and improper processing when printed. However, the print stain it produces is the most serious.

In addition, the coated surface of the paper surface is very tight due to the characteristics of the specific paint and paste. This can also produce blistering and coating separation during the heating and melting phase, just like moisture in fibrous tissue. In addition, when the surface of the coated paper is heated, the closedness of the surface is so good that water vapor and heat cannot be released, resulting in overheating of the surface of the paper. These factors cause the toner to separate from the surface of the paper and adhere to a portion of the press.

Wet powder type digital printing machine

The Indigo E-Print 1000 is another short-run color press that differs from dry powder presses in the use of wet inks during printing. Indigo's device can be considered as a digital offset press, so it must have different requirements for the printing performance and performance of the paper.

For this device, the main printing performance is mainly ink and paper compatibility. If the paper used in the production of pulp is acidic, the final paper is rarely on this Indigo press. This is because Europe's paper is generally neutral or alkaline. It is better to use than North American paper. This fact actually masks the production of more complex chemical changes.

The paper's ability to absorb ink depends, in fact, on the paper's Lewis acidity (release and absorption of protons) and the polarity of the sheet's surface energy and the amount of dispersive force.

When printing on Indigo presses, the chemical properties of the surface of the coated paper are generally normal, because there are a lot of calcium carbonate particles on the surface of the paper, and this kind of particle is a very good Lewis base. Non-coated papers must sometimes be treated with polyethylenimine (sapphire) prior to printing to ensure correct surface properties. Problems with printing performance often indicate incompatibility between paper and ink. Most of the ink's absorption of paper is caused by chemical incompatibility.