How to Improve Label Prepress Design and Platemaking Level

In the past 10 years, the field of label application has been continuously expanding, involving the food packaging industry, pharmaceutical industry, personal care products industry, electronics and home appliances, supermarkets and logistics, and anti-counterfeiting industries. There are also more and more types of labels, such as multi-layer labels for the pharmaceutical industry, double-sided labels for the cosmetics industry, film labels, electronic labels, in-mold labels, smart labels, and anti-counterfeit labels. Global label usage is growing at a rate of around 30% every year.

Label printing uses different printing methods according to different product needs, such as offset printing, gravure printing, flexo printing or screen printing. At present, the prepress production of domestic label printing mainly depends on the offset printing method. It is not known that printing methods other than offset printing, such as the increase in the number of flexo dots and the angle of the screen, are completely different from those of offset printing. Make fine colored labels. The exquisitely-printed proofs on the domestic exhibition site are mostly processed professionally or produced abroad. Therefore, the pre-press design and the plate-making level of the label printing play an important role in the quality of the label printing. This article briefly discusses how to improve the prepress design and plate making of label printing.

Manuscript design considerations

Half-tone originals should try to avoid selecting large-sized highlights and blurry originals or pictures, and try to select the first original, ie, reversal or reflection. For flexo, silk screen, gravure, and embossing, the design of large-area gradient-graded designs is minimized in the design so as not to detract from the quality and ideal state of the product caused by dot gain during printing.

Taking into account the characteristics of the printing press and whether the ink can be truly reduced, for example, the metallic ink in the flexo ink has no high glossiness of the metallic ink with other printing methods; in this case, the flexo printing and screen printing may be considered. The way to print gold and silver ink.

The original design must consider that the printer used can print a few colors at most, generally below 5 colors. Registration, cutting marks, and other information that will guide the production of the post process must be clearly traced on the plastic sheet. The design pattern fine line must be within the rated range: 4μm for the vertical line, 5μm for the vertical line, and the dot range varies according to different printing methods. Offset is between 2% and 98%, but the general control of flexo is 2%. Between -95%, offset printing generally uses 175 lines/inch screening, and flexo printing generally adds 150 lines/inch screening.

Selection of Separation Curve

After the color original is scanned and input into the computer, it is also necessary to perform the gradation compression processing on the image in order to realize the ideal dot restoration. Under normal circumstances, the high-contrast manuscripts are mostly denser in contrast to the gradation density range of the printed matter, and cannot be linearly proportionally compressed when adjusted for compression. The high school tune the level to compress some less, the secret tune level should compress some more, in order to make the high level section level density difference to pull apart, the intermediate tune theme level is deeper, and the dark tune section level density is flat. For the color separation curve, at the 50% screen point, the general product network needs to be compressed by 15%. Offset printing can be done at high light areas, and flexible prints that continuously adjust the image should generally be given 2% to 3% protective dots.

Prepress compensation

In all kinds of printing methods, dot gains will be more or less encountered. In particular, dot gains often occur in flexo and offset printing. In the case of flexo, there are not only dot enlargement problems, but also printing plates. The combination of bending deformations causes flexographic offset problems. According to this kind of situation, generally make the test printing plate, carry on the printing experiment, obtain the information that enlarges and distorts according to the experimental result, carry on the compensation treatment through pre-press processing.

In the prepress stage, "decrease" the size of the dots to compensate for dot gain in printing, there are mainly two kinds of pre-processing methods: one is the adjustment of the dot curve of the dot gain, completed before the film is out, the other is The calibration of the laser photodischarging is completed when the screen is screened. It is recommended to adopt the second method, which is to print samples of the film aligned with the laser imager according to the characteristics of the printing method. The dot gain is significantly reduced, and the halftone dots in the intermediate tone are 5% to 10% smaller than the ordinary printing plate. , And can make 0.5% ~ 3% of the dots are clearly reproduced on the film and plate.
Platemaking process

Label printing covers several major printing methods. During the plate making stage, different printing methods are selected according to different product properties. Different printing methods are used, and the plate making process is also different. This article gives a brief introduction to the flexography process.

The process flow of the flexographic plate making process is: manuscript → film (negative film) → exposure → rinsing → drying → post-processing.

Manuscript

The manuscript design for flexible printing should have the following characteristics: more colors, but less superimposition; does not require the reproduction of particularly small details; network cable is not too high, but can achieve color printing effect; can be packaged and processed online.

Film (negative film)

In accordance with the needs of plate making, graphic and clear, accurate size and size specifications; with frosted film, film density requirements for the same film; the use of positive film; measured by transmission densitometer, the density of white bits below 0.06; black density of 3.5 or more.

Exposure includes back exposure and main exposure

1 back exposure. The support film of the photosensitive resin plate is up and the protective film is laid down in the exposure drawer to receive exposure. Ultraviolet light penetrates the support film to cure the photosensitive adhesive layer to establish a stable substrate, and also to control the depth of platewashing and to enhance the bonding force between the support film and the photosensitive resin layer. The back exposure time is determined based on the required substrate thickness.

2 main exposure. Also known as front exposure, the photosensitive resin plate is supported with the film facing down and the protective film facing up. Tile in the exposure drawer, the protective film is peeled off once, then the film surface of the film is pasted on the photosensitive resin plate material, the vacuum film is flatly covered on the film (non-pharmaceutical film surface), vacuum is applied, so that the film is sensitive The resin layer is closely attached. Ultraviolet rays pass through the vacuum film and the translucent part of the film, so that the photosensitive part of the plate is polymerized and cured. The length of the main exposure is determined by the plate type and the light source strength. If the exposure time is too short, the graphic gradient will be too straight, the lines will be curved, and the small prints and small dots will be washed away. On the other hand, if the exposure time is too long, the print will be distorted. If there are large and small characters, thick and thin lines on the same printing plate, the separate exposure may be covered with a black film as the case may be. The fine parts will not be lost due to rinsing to ensure the plate quality.

rinse

The unexposed part is washed and dissolved, leaving the photopolymerized relief. The length of the washing process depends on the thickness of the printing plate and the depth of the impression. The washing time is too short. The printing resin will affect the plate-making depth. The plate-washing time will cause the plate to swell, resulting in deformation of the fine parts. Fall off.

drying

Remove the wash solvent and return the plate to its original size. Baking temperature between 50 ~ 60 °C. The baking time is determined by the thickness of the plate and the length of the washing time. The thick version is usually two hours and the book version is one hour. Too long baking time and too high baking temperature will make the printing plate brittle and affect the printing life. If the baking temperature is too low, the drying time will be prolonged, and the bake time will be too short. The printing will be rotted.

Post-processing

That is, in addition to sticking and post exposure. The photosensitive resin is completely hardened (polymerized) to achieve the required hardness index, and eliminate the tackiness of the printing plate to facilitate ink transfer. The post-processing time was obtained from the test with the aim of not cracking or sticking.