Discussion on Quality Control of Offset Printing (I)

      In order to maintain a stable print quality for prints, the first thing to do is to inspect the prints. Generally, the prints are compared with the proofs to see if they are printed well. The mass prints are also used to see if the quality is consistent with the contrast method. However, this traditional method of quality control will be replaced by data management, standardization and standardized quality management. That is, the print quality signal strips are placed on the side of the printed sheet or printed along with the printed image. The densitometer is used to measure the optical densities of various image patches, which are calculated and converted into quality data that need to be controlled. The manual method is used. Or use "print quality control device" to control the print quality.

A signed proof of offset printing

The color printing sign plays a guiding role in printing color. It is the basis for guaranteeing consistent ink quality and stable quality. It is also a necessary measure to control printing quality.

Signing is a work that requires high technical requirements. Since most printers in our country rarely use densitometers in their pressrooms, and because of the small size of paper in our country, it is difficult to produce signal bars. Therefore, many printing companies rely on technology. Experience to do this work, so the quality requirements for all aspects of the sampled personnel are even higher.

The sample proofs made with the proofing machine mainly look at the effects of the plate making and provide reference proofs for the official printing. There are many differences between it and the printing proofs. Therefore, according to the proofing of the printing proofs, it is necessary to carefully analyze the proofs that have been printed, and they must be aware of the following questions.

A time for signing

When it is best to sign a sample, this is the first consideration for the signer. Generally, it is not the signing of the printing proofs that is officially opened. Instead, during the test printing process, the ink correction is basically the same as the manuscript or the type of proof played, or close to it. The first one is used as a reference, and the notice is opened again. India. At the same time, the amount of water on the layout was closely observed and adjusted, and the ink fountains were fine-tuned. When the ink was printed to about 1,000 sheets, the ratio of ink and dampening fluid on the ink roller was stable. At this time, only one printed proof was printed. After printing the product, the ink color can basically keep consistent. This practice has two advantages. First, it stabilizes the ratio of ink and wetting fluid. Second, it prevents a large number of prints from being inconsistent with the ink color of the sample. Its purpose is only one, to prevent color mischief that may occur when the sample is mishandled. Of course, if the number of impressions is 10,000 or tens of thousands or more, then it can be signed. If it is a short-lived color job with less than 1,000 prints, it is even more cautious. Obviously experience is the main thing here.

II. Influence of the nature of paper

The main aspects of the nature of paper caused by the nature of the sample, such as ink absorption, smoothness, whiteness and so on.
1. Paper Absorption The absorption of ink by paper is mainly to absorb the link material in the ink. Imprinted ink is excessively absorbed by the paper and absorbed on the surface of the paper. It does not exhibit the degree of saturation that the color should have. On paper with large pores, it will even penetrate the paper together with the binder, and the density value after the print is dried. It will be significantly reduced. If the ink absorbency of the paper is greater, the density value of the color of the print is reduced, and the density value between dry and wet inks is changed more. For example, the same image is used in the coated paper and uncoated paper. Differently, this is a special consideration when signing a sample.

2. Paper Smoothness The paper has high smoothness, small surface pores (micropore structure), and low ink absorption. Paper with low smoothness absorbs ink, and it can affect the difference in density between blotted wet ink and dry ink. On paper with high smoothness, the wet density of the blotted ink and the dryness of the blot after drying is very small. On the other hand, paper with low smoothness has a large difference between the wet density of the imprinted ink and the density of the dried field after imprinting. This means that the wet ink printed on paper with high smoothness (such as coated paper with a smoothness of 1000S or more) is dried. After that, the size of the ink has hardly changed. The wet ink printed on paper with low smoothness will become smaller after drying. Different inks, the amount of change is also different, the general blue ink change is greater than the red ink, the red ink change is slightly larger than the yellow ink, light ink change is smaller than the dark ink.

3, the whiteness of the paper is not white paper, the general paper has a different color, such as partial blue, reddish, yellowish, partial gray and so on.

After the color prints are printed on the paper, the hue is affected, and the partially green paper slightly biases the print to blue. When printing yellow ink, it is easy to produce a feeling that the amount of ink is not enough, and often it will increase the amount of ink, thereby increasing the ink layer thickness of the yellow ink, resulting in too many yellow ink at the time of the signing, and the final print will be yellowish; When printing blue ink, it will produce the illusion of too much ink. If you reduce the amount of ink, prints will lack blue. Therefore, in the case of partial-green papers, attention should be paid to the prevention of yellow ink and red ink, and the lack of blue ink.

The reddish paper is printed with yellow ink and red ink. It will create an illusion that the amount of ink is high. If the amount of ink is reduced, the final printed product may lack yellow and may lack red. When printing blue ink, there is a feeling that the amount of ink is not enough. It will inevitably increase the amount of ink underneath, and the final print will be blue and purple. . Therefore, for reddish paper, yellow ink and red ink cannot be reduced at the time of signing, and blue ink cannot increase arbitrarily.

The yellowish paper is often encountered. For this type of paper, how do you sign the sample and how to print the product? The key problem is to use color to use brains, and the proof sheets are cold (blue and green are more), and can be used in yellow ink. Add appropriate amount of yellowish ink, because the color of yellowish ink is cooler than that of yellow ink, so it can squeeze out part of the warmer hue of the paper. When printing red ink, try to use pure pink ink or magenta ink. The blue ink may use sky blue ink, and the sky blue ink is relatively pure in the blue ink, containing red and containing less yellow. Black ink should also use blotting ink. Yellowish paper should use as little wetting fluid as possible.

Three other issues

1, the depth of the printing plate For the printing plate, the proofing version is much deeper than the upper printing plate, because the dot gain rate in the printing process is larger than the dot enlargement rate of the proofing, so the printing plate on the machine should be basally lighter. Different printing paper, the depth of the printing plate requirements are not the same, generally used in the printing plate offset printing plate than the printing plate used in the printing plate shallower, this is because the offset paper on the dot gain ratio than Coated paper has a larger dot gain rate.

In order to prevent the sign color deviation caused by the depth of the printing plate, the signer should identify the depth of the printing plate and observe three areas with a magnifying glass:

High-light areas: If the characters are in the area, the bright spots around the eyes, and the fine dots on the printing plate should be smaller than the dots on the proofs.

Middle tone: 50% of the ideas can not take a corner.
Dark tone area: hollow white dots are not paste, not together.

2. The ink is thin, and the sample-taking personnel should go deep into the machine site to check the thinness of the ink. If the ink is thin, the color deviation of the printed matter just printed out and dried is particularly serious.

3, before the network deformation and virtual reality sign, apply a magnifying glass to carefully observe the imprint has no distortion or false hair problems, if there is any deformation of the network, such as stretching, expansion, ghosting, etc., the color tone of the print screen becomes dark, then thousands It cannot be satisfied with the need to reduce the thickness of the ink layer to meet the requirements of the hue. This wrong practice will inevitably lead to print color deviation. There are two real situations in the network. One is the problem of printing, and the other is the pressure on the machine or other failures. Here, you must first troubleshoot and then sign the sample.

4. How to sign a printing sample based on a full color sample or original In general, for a four-color offset printing press, the printing proof is relatively easy to determine. The two-color machine is a bit harder, and the monochrome offset press is more troublesome. If the yellow plate is placed in the first color sequence of the monochrome machine, and the color-resolving cells on the retina of the human eye have a low resolution for printing yellow on white paper, this will bring difficulties to the signing of the yellow printing plate. . For this reason, people now break the traditional color sequence when they print color jobs on a single-color offset press, changing to blue, yellow, red, black or red, yellow, blue, black, etc., which is not inconsistent with the traditional color sequence. The reason for this is because the paper, ink and wetting fluid used today are much better than they used to be.

If there is no monochromatic sample and blue or red is printed in the first color, be sure to capture the characteristics of the picture. For example, if you want to use blue, you must capture the features of the sky, sea, and leaves that are directly related to blue, and red. It is necessary to grasp the features of the character's lips, cheeks, clothes, the sun and so on. Of course, there are spot color blocks that can't be missed and can be compared and signed.

The yellow version of the signing mainly observes whether the fine dots in the high light area are flush with each other, whether the depth of the printing plate meets the requirements, and yellow is mainly used as the background color. Therefore, the size of the yellow ink must be strictly controlled and must not be taken lightly.

5, to correctly understand the customer's annotations on the proofs, after the general proofs are laid, first send the customer to approve the signature, and some customers often have proofs on the proofs such as "When the printing is slightly lighter" or "blue and then deeper" and other words . The sample-taking staff must repeatedly scrutinize and accurately understand the customer's intentions before they can sign satisfactory samples according to the customer's wishes.

Drawings and photographic works of art express their artistic connotations in terms of color and tone. Therefore, the sample-taking staff should look at the entire picture of the manuscript, carefully analyze and combine the characteristics of the colors used in printing, so that the printed matter and the manuscript are almost the same, even more than the original This requires the signatories to constantly improve their artistic accomplishments, observe more, think more, and accurately sign print samples.

Four how to sign printing proofs according to digital proofing

Digital proofing is an important part of the digital process of printing. In this sense, digital proofing is not comparable to traditional proofing. Traditional proofing will be eliminated. In the face of this indisputable fact, how to print proofs based on digital proofing and print products What?

In general, traditional proofing belongs to analog printing. There are many man-made factors, and the stability of color is not necessarily satisfactory. However, the long-established printing habits of offset printing operators are deeply entrenched. They always think that the traditional proofing is closer to the actual printing. It is difficult to accept digital proofing. This is because digital proofing uses ink-jet (water-based ink) method to sample. Sample proofs cannot form traditional printing outlets. In addition, the printing materials used in digital proofing belong to special materials, which are different from the printed materials. Far, but the color of the digital proofing is stable, generally 6 color printing, wide color gamut, the gamut space that can be expressed basically covers all of the gamut space, in the face of digital proofing to sign printing proofs must pay attention to the following point:

(1) Accuracy of Hue No matter how the digital proofing changes, the colors of the printing proofs and the digital proofs will remain the same. If the color of the proofs shifts, the quality of the whole batch of printed matter will be affected.
(2) The amount of ink should not be large The digital model is water-based ink. The printing material not only has high smoothness, but also has poor absorption of water-based ink. The color of the graphic and text is basically totally reflected, which makes the picture and text beautiful and clean, and any paper on ink is Absorption and reflection of the absorption, so, according to digital proofing when the ink should not be small.
(3) Grasping the characteristics of the screen When pressing digital sample printing samples, you must grasp the screen characteristics. In particular, the field or spot color blocks must be analyzed and compared, and you must not let go of them.
(4) Be familiar with the printing materials. Be familiar with the printing materials used.
(5) It is necessary to diligently compare the printed samples and the printing proofs within half an hour after the printing samples are signed to compare the stability of the ink transfer during the printing process.

[Case 9-1]
There is no formal signing staff in a printing factory, and the general captain can replace it. The captain used a four-color offset press to print two batches of the same job. The first batch of ink was too small, and the second batch of ink was too large. Then the third batch of ink was in the middle of the first two colors. When printing the third batch of job, the captain placed the first two color proofs on the sample table. When the machine had printed more than 10,000 copies, the captain had not confirmed the sample sheets.

[Expert Tips]
The above example shows that the captain has numerous hearts. This is a taboo for the sample. In addition, the captain does not understand the industry terminology “larger, smaller, lighter, and deeper” in printing.

[Case 9-2]
The hospital has to print 16 400 academic papers.