There are so many requirements for digital printing paper! The evaluation, requirements, and methods are all here

There are so many requirements for digital printing paper! The evaluation, requirements, and methods are all here

In recent years, with the development of electronic publishing and cross-media publishing technology, the traditional printing market has been impacted, and digital printing has developed rapidly. Digital printing requires paper to have a smooth and delicate surface structure, reasonable and controllable ink fixation, vivid color graphic presentation effect, highly controllable paper shape stability and continuous printability.



Digital printing equipment is generally divided into electrostatic toner digital printing machine and inkjet digital printing machine, and different printing methods put forward different requirements for paper.



Performance requirements for digital printing paper



There is no unified standard for the performance requirements of digital printing for paper, and there is no standardized testing method and testing instrument, but some large foreign digital printing equipment manufacturers (such as HP, Canon, Epson, etc.) have a set of their own testing methods to strictly control the quality of digital printing paper, while there is very little research in this area in China. As a special paper, digital printing paper must have the following performance requirements:



(1) The ink is not completely absorbed by the paper and does not rub against the paper.



(2) The ink absorption speed and drying speed should be fast, the ink fixation effect should be good, and the horizontal diffusion and longitudinal penetration of ink droplets should be appropriate.



(3) The ink dots on the paper must have high contrast and bright tones.



(4) The ink dots are appropriately presented on the surface of the paper, and are close to circular, with high edge clarity.



(5) The printed image has high resolution, and the ink dots on the inkjet paper are intact and non-diffuse.



(6) The printed image has high color density, continuous gradation, bright and full color, full layering, and good reducibility.



(7) Color has a certain fastness. It does not change color or fall off for a period of time, that is, images and text must have water resistance, light resistance and indoor preservation.



(8) The paper has good conveying performance, good feel, friction resistance, no powder loss, etc.



Evaluation method of digital printing paper



Through relevant information and performance analysis of imported products, the performance evaluation of digital printing paper can start from several aspects, such as suitable whiteness, good ink absorption, bright and realistic colors, fast and uniform ink absorption, no seepage and rubbing, high image resolution and good image water resistance. Due to the limitations of detection methods, only more traditional methods are generally used to directly or indirectly reflect these properties.



01

Optical density



Optical density (also known as color density) refers to the density of ink droplets when ink is printed on paper to form patterns or handwriting, which is an important parameter to measure the quality of printed images, which affects the final visual effect, color gamut and color fidelity of printing. Generally, the higher the ink color density value, the more vivid the color of the image, so high ink color density can provide good image effects under the condition of ensuring other indicators. There are three detection methods.



(1) Spray red, yellow and blue solid color blocks on the paper sample to be tested, and measure the optical density and L value, a value and b value of different parts with CIELab optical density meter, and take the average density value as the optical density of the paper sample. A high average density value indicates good ink coherence and bright color of the printed image. The size of L, a and b values indicates the color reproduction: the larger the positive value of the L value, the brighter the color, and the larger the negative value, the darker it is; the larger the positive value of the a value, the redder it is; the larger the positive value of the b value, the more yellow it is, and the larger the negative value, the bluer it is. Therefore, the degree of color shift can be determined according to the size of L value, a value and b value.



(2) Print a black circle with an area of about 50px2 on the paper sample to be tested, seal and balance all the paper samples together for 24 hours, and measure the color density on the X-Rite Spectrodensitometer 528. If the experiment uses direct density (absolute value) measurement, the density display uses all display methods, that is, the color density of four colors: black, blue, red and yellow is displayed. The larger the measured value, the higher the density of the corresponding color in the image.



(3) Use an inkjet printer to print several black and yellow, pin and cyan blocks with a diameter of 3mm or 5mm, and test the reflection density of the color block.



02

Optical density is poor



Optical density difference refers to the change of optical density values of different parts of the pattern on the paper when the inkjet amount is the same and the color is the same. For patterns of the same color, there should be uniform inking or ink absorption density; if there is a change in density value, it means that the absorption of ink by the paper is uneven, which is intuitively manifested as different shades of color in the image.



Detection method: Measure the optical density at different parts of the same color block, subtract the lowest value with the highest value, and then express the ratio with the average value.



03

Image resolution



Image resolution is also called image resolution, which is mainly related to the surface performance of digital printing paper. In the process of inkjet recording, the ink is sprayed to the surface of the paper page in the form of tiny particles, and the expansion degree of ink droplets is mainly determined by the microporosity of the paper surface and the particle size of the pigment.



Detection method: First, determine the sample drawing, print the image with an electron microscope, and compare it with the sample drawing.



04

Seepage



Qualified digital printing paper requires that the ink droplets sprayed on the paper surface should expand evenly across the entire contour, that is, there should be the same expansion speed and penetration distance on the paper surface, otherwise there will be bleeding, also known as burrs. The occurrence of bleeding is mainly related to factors such as ink absorption, structural tightness, uniformity, pigment properties, etc. Bleeding is usually the main reason affecting the image resolution of digital printing paper.



Detection method: Since there is no scientific means to reflect this performance, the degree of permeability can only be determined by printing the sample drawing, and then using the electron microscope or the naked eye judgment method to determine the degree of permeability, which is commonly evaluated by the 5-point method. The sample diagram is selected from the comparison sample diagram commonly used in the printing industry (squares composed of lines of the same thickness and different arrangements, arranged according to different grades), 1~5, the degree of permeability is from heavy to light, and the first row of lines has a clear pattern, no bleeding, and no burrs is 5, and then arranged in order.



05

Ink absorbency



The ink absorption of paper refers to the absorption capacity of paper to ink, which directly determines the degree of ink absorption by the paper, restricts the amount of ink fed by printing, and determines the penetration and conjunctival of ink on the paper, thus affecting the color effect, ink layer density and dot expansion of the ink. Moderate ink absorption is the basic guarantee of clear printing and exquisite images. If the absorption capacity of paper for ink is too large, most of the binders in the ink are absorbed into the paper, and the pigment remains on the surface, and the ink gloss is greatly reduced, which will lead to dull printing, poor color effect, reduced ink layer density, and even transparent printing and powdering. If the absorption of paper to ink is too poor, the ink will dry slowly and the back side will be stained. The ink absorption of paper is determined by the capillary action of the paper fiber and the permeability of the filler, and the surface sizing degree can also change its ink absorption.



Ink absorbency test method: First, the 525px×150px specimen is measured on the whiteness meter to measure the green light reflection factor on its surface, the specimen is tested on the ink absorption tester, and then the green light reflection coefficient in the center area of the ink on the surface of the specimen after applying ink is measured. The difference between the reflection factor before and after the absorption of ink on the same surface of the specimen is divided by the reflection factor from the test sample to get the ink absorption value. The ink absorption value of the specimen is calculated as follows:



Ink absorption value (%)=[(R0-R1)/R0]×100+Rk



Where: R0-green light reflectance coefficient on the surface of the specimen before ink coating;



R1 - green light reflectance coefficient in the center area of the ink on the surface of the specimen after applying ink;



Rk-the correction factor of the ink (used to eliminate the difference between ink batches).



It should be noted that the ink of inkjet printing is quite different from that of offset and gravure printing inks in terms of form and properties, so the ink absorption value can only be used as a reference index for evaluating the coating of inkjet printing paper.



Electrostatic toner digital printing requirements for paper



01

Proper electrical performance



Digital printing machine, copier or desktop printing machine using the principle of electrostatic printing is a printing method that uses the electrostatic attraction of positive and negative charges to transfer ink powder, so it is required that the electrical properties of the paper - such as resistivity, dielectric constant, etc. should be controlled at an appropriate level, and paper with good conductivity such as aluminum foil liner paper will even produce arcing phenomenon (the phenomenon of unintentional charging of the non-conductive part or air accidentally), resulting in damage to the machine and poor printing quality.



02

uniformity and surface smoothness



The quality level of printed materials printed using the principle of electrostatic printing requires high uniformity of the printed paper: the distribution of fibers or fillers in the paper must be uniform, and the smoothness of the paper surface is also critical to the quality of color transfer. After the pigment reaches the paper surface, it is mostly fixed by hot pressing. Therefore, the smoothness, surface chemical properties and heat treatment properties of the paper surface must also be mastered at an appropriate level. In addition, paper containing wax, stearate and plasticizer will cause stains on the photoreceptor of the digital printing machine.



03

Temperature and humidity of the paper



The change of moisture content caused by the ambient temperature and humidity of printing paper in the process of transportation, handling and storage has a great impact on the quality of digital printing on the principle of electrostatic printing.



Paper requirements for inkjet digital printing



Digital printing using inkjet technology is very different from digital printing using the principle of electrostatic. Because inkjet is 50%~90% solvent-based, and the vast majority of pigments are solid polymer particles with pigments. The liquid properties of ink used in inkjet technology have requirements for surface absorption and absorption on paper, including: after the ink droplets are sprayed on the paper surface, the pigment should remain on the paper surface without diffusion, which requires controlling the speed and amount of solvent absorbed, and allowing the controlled solvent to finally volatilize from the paper to form a uniform and consistent imprint.



Therefore, inkjet digital printing machines have high requirements for the surface properties, pore structure, chemical composition and mechanical properties of paper, and not all inkjet printing machines or inkjet materials are the same. ...