Testing method for raw material performance in the

  • Detail

The standard testing methods for raw materials in the printing industry mostly focus on the testing of paper mimeograph ink. The literature workers of the International Association of printing research institutes have compiled the printing standards of 15 countries and two organizations. This publication lists 88 standards or specifications on paper and other printing materials issued by Germany and 66 standards on ink and other related materials issued by Poland. Some standards are about adhesives and other book binding materials, as well as photosensitive materials. But in general, the printing industry does not pay enough attention to materials other than ink and paper

a large number of testing methods, testing instruments and testing images can be evaluated by the printing factory, which means that almost all the properties of paper or ink can be tested to comply with the set standards. The key question now is what testing methods should be used and how many tests should be done in various situations

when testing paper or other substrates, it is difficult to obtain representative samples. For a stack of papers, especially large ones, it is very difficult to randomly sample them without re stacking them. For web paper, except for the (less representative) outer layer, no test can be made

generally speaking, the sampling of ink and photosensitive materials is not a big problem. In the sampling of raw material inspection, another problem that should be considered is that some inspection methods are destructive. Such as the strength test of paper and paperboard and the sensitivity test of photosensitive materials, their samples will be damaged and consumed

when selecting testing methods for a certain material, it should be considered to use those tests that can obtain data related to printing image defects or printing production and speed control inaccuracies

the frequency of testing a certain performance shall be determined according to the tolerance of the performance change, which is related to the allowable quality defect level of the final print. The more stringent the quality requirements of printing products, the more frequent testing of raw materials is required

for most printing enterprises, they should not spend so much time, money, material resources and manpower on testing raw materials as government departments or large printing enterprises. The benefits of testing may not be enough to cover the costs of testing. However, this situation refers to large-scale inspection, while in fact, most printing enterprises can carry out small-scale inspection. All printing enterprises can invest properly to do those tests with high economic benefits

the following tests are used to control the factors affecting color reproduction:

1. Paper detection

some tests can be implemented with very simple equipment

(1) thickness: in order to maintain correct printing pressure on the printing machine, use a cylinder diameter gauge. The thickness of the paper should also be measured with a micrometer. Generally, ten pieces of paper are used to measure five points

(2) opacity: the opacity visual scale is an effective measuring tool. Print a group of characters on the gradually darkened dot tone background, and cover the tested paper on it. The characters begin to be invisible at some place of the ruler, which indicates the opacity of the paper

(3) absorbency: k n ink is probably the most commonly used and cheapest test method. The ink is composed of gray dye and oil base. Apply it to the paper sample and wipe it off after two minutes. The density of the gray is the absorption of the paper

(4) gloss: measure along the paper grain direction with a glossometer and take the average value

(5) paper efficiency: this test combines the gloss and absorption of the paper, and can measure the influence of the printing material on the color of the ink film. Non absorbent paper with 100% gloss can produce the most saturated colors. On the contrary, 100% absorbent but matte paper (such as newspapers) will produce very poor colors

(6) color and lightness: the approximate characteristics of paper color and lightness can be obtained by measuring the density of red, green and blue filters after calibration with the reflection densitometer. For an ideal paper, the density under each color filter should be the same. A low density value indicates a high lightness. The visual comparison method of paper sample is more meaningful to detect the color and lightness of paper

(7) peeling: Although Dennison wax stick peeling test has its shortcomings, it is still a suitable method to determine the anti peeling ability of a specific substrate. It has been adopted by many manufacturers. Melt the end of a wax stick and press it onto the substrate to be tested. After 15 minutes, pull it up quickly and check the removed fiber or coating on the end face

like many other raw material tests, the relative comparative significance of wax stick test is greater than the predictive significance. In other words, if peeling does not occur, the paper with a final peel strength higher than 10 will not be peeled under the same conditions

2. Ink detection

the first step to evaluate the ink is to scrape the ink sample, preferably on the paper to be printed. It is usually operated with a wide ink knife, or a small ink roller can be used to roll out a thicker ink layer. When comparing two inks, it is generally required to scrape two ink samples on the same piece of paper in parallel. There is often a field ink bar in the middle of the scraping sample paper

(1) opacity: some inks, such as yellow ink, are not as transparent as other inks. In color printing or overprint, this performance must be known. If the last ink is not transparent enough, the color after overprint will be biased towards the last ink. The opacity can be obtained by checking the ink color on the ink bar of the scraping sample

(2) color: parallel scraping can be used to compare the colors of various color inks, especially for magenta ink in color printing. The pigments of this ink include mineral red, rhodamine yellow, and a mixture of the two. Unless the two printing inks have the same color, it is impossible to have the same color

ink color can be quantitatively expressed by Preucil ink formula, that is:

where l, m and h respectively represent the lowest reading value, intermediate reading value and highest reading value measured by the reflection densitometer through the red, green and blue filters. The hue gray value can be marked on the circular or triangular chromaticity diagram of GATF. The advantage of triangle chromaticity diagram is that it is easy to get the representation domain of a certain group of color inks. Connect a straight line between the three ink coordinate points to form a triangle area, which represents the two dimensions of the three-dimensional color space, and its edge is the representation domain limit of the group of ink

(3) ink strength or pigment concentration: the problem often encountered in color printing is not whether the pigments contained in the ink are consistent, but whether the pigment content percentage is consistent. The latter problem is that in order to obtain the same color saturation, the ink with low pigment content needs a thicker ink film than the ink with high pigment content

the detection of pigment content was as early as 2016, sometimes referred to as the whitening test. One part of ink was mixed with 50 parts of opaque white ink, and the same proportion of white standard ink was used as parallel scraping samples to visually check the difference in the intensity of the two inks. The red, green and blue channel readings of the densimeter can be used to quantitatively measure this difference. Continue to add white opaque ink to the darker ink with parallel scraping samples until the scraping samples of the two inks are consistent. In this way, a relative quantitative description of the pigment concentration relationship between the two inks can be obtained

(4) ink adhesion: ink adhesion affects ink adhesion and paper peeling. A simple test can be used to compare the adhesion of the two inks. Take a little ink sample from each of the two inks and apply it on the plane. Press the index finger of both hands on each ink sample and quickly lift the finger. The disadvantage of this method is that it can not obtain quantitative absolute adhesion, but only the relative value of adhesion. Of course, now we can use electronic ink adhesion instrument or other adhesion instruments to quantitatively measure the adhesion value under certain conditions

the quality of raw materials has a great impact on the final quality of printed matter. The quality of raw materials can be ensured by finding manufacturers or sellers who can provide high-quality raw materials, or by rejecting unqualified products through testing the arrived raw materials and goods. On the issue of raw material testing, although testing is inconvenient, large printing houses should realize that the investment in testing is much less than the resulting income. Small printing plants can also get obvious benefits from testing, as long as they purchase small testing equipment with less investment and use existing employees to conduct testing operations

as a manager of an enterprise, when you plan to implement raw material quality control, you should first consider two aspects, and weigh whether the investment in testing equipment is less than the benefits it can bring. Moreover, the records on quality problems and their causes should be summarized and analyzed in detail, so as to better control the quality of raw materials in the printing process

Copyright © 2011 JIN SHI