With the continuous development of the color inkjet printer market, its main consumables - color inkjet printing paper (also known as color spray paper, hereinafter referred to as inkjet paper) has also increasingly attracted the attention of users. There are two types of inkjet paper: ordinary inkjet printing paper (hereinafter referred to as ordinary inkjet paper) and coated inkjet printing paper (hereinafter referred to as coated inkjet paper). Ordinary inkjet paper is mainly used when the printing effect is not high. Coated inkjet paper is essentially a layer of porous paint with ink absorption on the surface of the substrate (paper, photo paper, transparencies, cloth, synthetic paper, etc.), and its printing effect is much better than ordinary inkjet paper. . The commonly referred color inkjet paper is a coated inkjet paper. There are two main types of inkjet paper: glossy inkjet paper and glossy inkjet paper.
There are no uniform standards for the performance requirements of inkjet paper, and there are no standardized testing methods and testing instruments. However, some large foreign manufacturers (such as Canon, Epson, etc.) have their own set of testing methods to strictly control product quality. However, there are few domestic research work in this area.
Through related data and analysis of imported products, it can be known that: Qualified non-glossy jet paper should have the following characteristics: low gloss (less than 10%, 75°) and suitable whiteness; good ink absorption; vivid color; Fast and uniform ink absorption; no bleed and imprint; high image resolution; good image water resistance. Therefore, the determination of the performance of inkjet paper can start from the above aspects. Due to the limitation of detection methods, the traditional methods can only be used to directly or indirectly reflect these properties.
Optical density
Optical density (also called color density) refers to the density of ink droplets when ink is printed on paper to form patterns or writings. It is an important parameter to measure the quality of printed images. It affects the final visual effect, color gamut, and color vividness of printing. degree. In general, the higher the ink density value is, the brighter the color of the image is, so the high ink density can provide a good image effect under the condition that other indexes are suitable. There are three kinds of detection methods.
1 The red, yellow and blue solid blocks were respectively sprayed on the tested paper samples, and the optical density and L value, a value and b value of different parts were measured with a CIELab densitometer. The average density was taken as the optical density of the sample. . A high average density value indicates good ink dot consistency and bright printed images. The values ​​of L, a, and b indicate the reproduction of the hue: the larger the positive value of the L value, the brighter the color and the darker the negative value; the larger the positive value of the a value, the larger the red value; the positive value of the b value The larger the value is, the more yellow it is. The larger the negative value, the more blue. Therefore, the degree of color cast can be determined based on the sizes of L value, a value, and b value.
2 Use a printer to print a black circle with an area of ​​about 2cm2 on the paper sample to be tested. After putting all the paper samples together for 24 hours, the color density was measured on an X-Rite Spectrodensitometer 528 spectrodensitometer. If the experiment uses the direct density (absolute value) measurement, the density display selects all display modes, ie, the color density of the four colors of black, cyan, red, and yellow. The larger the measurement, the higher the density of the corresponding color in the image.
3 Print a number of black and yellow, cyan, and cyan blocks of 3 mm or 5 mm diameter with an inkjet printer and test the color block reflection density.
2. Optical density difference
The difference in optical density refers to a change in the optical density value at different portions of the pattern on the paper when the ejection amount is the same and the colors are the same. For the same color pattern, there should be a uniform ink or ink density; if there is a change in the density value, the ink absorption of the coating is not uniform, intuitively manifested in the image color shades.
Detection method: The optical density is measured in different parts of the same color patch, and the lowest value is subtracted from the highest value, and then the ratio to the average value indicates that the larger the value is, the more uneven the ink absorption is.
3. Ink absorption
Ink absorption, also known as ink absorption speed, is an important indicator to measure the printability of inkjet printing paper. Moderate ink absorption is the basic guarantee for clear prints and beautiful images. The simplest way to observe the ink absorption of inkjet paper is the amount of time it takes for the ink to fade after the ink is sprayed onto the paper to form a pattern. If the ink absorption is too low, pearling may occur due to the amount of ink, drying time may be long, the ink flow and the printed surface may be dirty.
(1) It is recommended to use the method for detecting the absorption of coated paper ink (Note: The ink used in ink jet printing is different from the ink used in offset and gravure printing in terms of form and nature. Therefore, the ink K&N value can only be used as an evaluation of ink-jet printing paper. Layer reference index): measured on the IGT printing machine, immediately after pressing a drop of ink on the uniform roller with a syringe, press the paper at a fixed pressure speed, and then determine the length of the trace of the color ink on the paper surface, Expressed in cm. The large numerical value indicates that the ink absorption rate of the pattern is slow and the ink absorption is poor; the small value indicates that the ink absorption speed is fast and the ink absorption is good; if the complete trace cannot be formed, the ink droplet cannot be infiltrated into the paper immediately after being pressed by the roller and the paper surface Non-directional flow, that is, poor ink absorption.
2 Ink detection method: As with the method of assessing infiltration, the sample was determined by a 5-point method. The template drawing is taken from the comparison chart of the printing industry evaluation. It consists of 24 color blocks with different ink ejection amounts. The more ink ejection amount, the easier the ink flow is. In the same printing mode, the boundary of each color block is completely clear and without The ink flow appears to be 5, and the ink flow is reduced by one level for each of the four color patches.
3 printing ink drying time detection method: In the temperature (23 ± 2) °C, humidity 65% ​​conditions, use a printer to print a line of Chinese characters, repeatedly swab the cotton ball to the stopwatch timing, rub no less than dry.
4. Penetration
Qualified inkjet paper requires that the ink droplets sprayed onto the paper should be evenly spread over the entire contour, that is, there should be the same expansion speed on the paper surface, the same penetration distance in the coating, and vice versa. Phenomenon, also known as burrs. The appearance of bleed is mainly related to factors such as ink absorption, structural tightness, uniformity, and pigment properties of the coating. Peeling is often the main reason that affects the resolution of an unbleached inkjet image.
Detection method: As there is no more scientific means to reflect this performance, only by printing a template, and then using the electron microscope or naked eye to determine the degree of infiltration, commonly used five points assessment. The template map is selected from the comparative pattern diagrams commonly used in the printing industry (blocks made of the same thickness and different arrangement of lines, arranged in different grades), 1 to 5, the degree of seepage from heavy to light, the first line of line pattern Clear, no bleed, no burrs 5, followed by the order.
5. Print through
The image of the inkjet paper should be formed mainly on the top layer of the coating and the substrate, ie, the ink cannot pass through the coating and the base paper to the back. If this phenomenon occurs, it is called through-printing. The printout is usually caused by the small amount of ink absorbed by the coating and the tightness of the base paper, which causes the ink to penetrate into a shorter path.
Detection method: After determining the whiteness of the paper sample, print the blue block on the paper and measure the whiteness (no back coating) on ​​the back of the printed page. Use the whiteness of the paper sample and the difference in whiteness of the printed image to indicate the degree of penetration. The greater the difference is, the more severe the print is. The smaller the difference is, the lighter the print is.
6. Image resolution
The image resolution is also called the resolution of the image, which is mainly related to the performance of the inkjet paper coating. In the inkjet recording process, the ink is sprayed on the surface of the paper sheet in the form of tiny particles. The extent of the ink droplets in the coating mainly depends on the microporosity of the coating and the particle size of the pigment. Generally, the particle size is smaller. The better the microporosity is, the closer the ink droplet diffusion is to the theoretical value, so it can be said that using a pigment with a small particle size (silica, etc.) can improve the resolution of the printed image under certain conditions with certain properties.
Detection method: First determine the model diagram, print out the image and observe the image with an electron microscope, and compare it with the model diagram. The high resolution must first ensure that no drainage and seepage occurs.
7. Water resistance
Ink jet paper is generally required to have good image water resistance, that is, the paper is wetted with water after printing into an image, and the image is not damaged. The use of cationic fixing agents in paints is often used to solve this problem.
Detection method: Inkjet paper measures the ink density after printing using an X-Rite spectral density meter, and measures the ink density again after water treatment. The post-print paper was immersed in water at 25°C for 4 minutes to evaluate the water resistance, and then taken out and dried. Then compare the ink density measurements before and after soaking.
Reprinted from: Digital Printing