Study on flexo printing technology of coated paper labels

1. Problems Related to Design Platemaking 1. Layout Design In flexographic printing, since the plate cylinder is a cylinder, the entire plate is to be covered on the cylinder to form a closed cylinder, so the layout design There is a need for some consideration. The length of the printing plate must be less than the circumference of the plate cylinder.

For example: printing a kind of square mark, the specification is 103mm * 103mm, between the square mark has 3mm draws a knife, there is a kind of 102 tooth edition roller (the arc length of 8 teeth is 1 inches), Is it suitable?

via caculation:
103×103+3×3+3=321 (mm)
321/(1×25.4÷8)=321/3.175=101.102 (tooth)
The conclusion is that this type of 102-tooth plate cylinder can be used completely.
In addition, the design of wine labels should also give full play to the advantages of flexographic printing, as far as possible to abandon the shortcomings of flexographic printing. We know that in the flexo printing, the phenomenon of dot enlargement is very easy to occur. Therefore, in the design of the wine logo, halftone images should be avoided as much as possible. In general, color labels, lines, and text should be used to design the wine labels.

2. The film we know, flexographic printing plate has a certain degree of flexibility, so in the version of the roller paste, the plate is easily deformed, that is to say, the plate will be extended along the circumferential direction of the platen roller, so when the film out The length of the film cannot be exactly equal to the circumference of the plate cylinder, but it should be smaller than the actual circumference of the plate cylinder. This is what we usually call the reduction rate. The general reduction ratio can be calculated using the following empirical formula:

Film reduction ratio = (1-K/S) × 100%
Where: K = 2π × (plate total thickness - thickness of the support layer), S = 2π × (plate cylinder radius + double-sided tape thickness + total plate thickness).

Problems related to printing processes and printing operations <br> When performing flexographic printing, the matching of the printing color sequence and the number of anilox roller lines is a critical link. Since the advantages and disadvantages of flexo printing have been fully considered when designing the wine labels, the design of the wine labels is based on the field and lines, so the selection of the anilox rolls does not have to blindly pursue the high screen number. Generally, in the field and line printing, an anilox roller of 400 to 800 lines/inch is sufficient to meet the requirements.

In the field of large-area printing, it is appropriate to select anilox rollers with a low number of screen lines. When printing small letters and thin lines, it is necessary to select an anilox roller with a higher number of screen lines; When glazing, you can choose an anilox roller with a higher number of screen lines, because as long as the light on the surface of the print is uniform; when printing gold and silver ink, because the metal ink particles are coarse, so you should choose a lower cable Number (eg, 400-440 lines/inch) anilox roll.

3. Problems Associated with Surface Finishing <br> To increase the aesthetics of the wine labels and protect the wine labels, the surface finishes of the wine labels, such as glazing, hot stamping, and gold and silver ink printing, are often required. The printing of gold and silver ink is used as an example.
1. The main role of glazing is to increase the abrasion resistance of printed products and in some cases also increase the water resistance of printed products.
At present, the glazing methods used in flexographic printing are mainly water-based glazing and UV glazing.
When applying waterborne coatings, the main issue to consider is the drying of waterborne varnish. Generally, the drying speed of the water-based varnish can be accelerated by adjusting the drying position of the infrared drying light box and adding some quick-drying agent to the varnish.

When UV polishing, it is important to consider whether the UV varnish is completely cured. Since the curing of the UV varnish is related to the UV excitation wavelength of the UV lamp, the intensity of the UV light emitted by the UV lamp after a certain period of use will gradually decrease. Therefore, the UV lamp needs to be replaced periodically. The service life of general UV lamps is 800 to 1000 hours.

2. Gold-silver ink printing Water-based metal inks are processed from copper powder, aluminum powder, binders and additives through special processes. Since the proportion of copper powder and aluminum powder is relatively large, it is easy to precipitate, so it must be stirred before use. Uniform.
In the printing process, waterborne metal inks will increase in viscosity due to volatilization of the solvent over time. At this time, a small amount of new ink and an appropriate amount of metallic ink thinner or a small amount of water are required to be diluted.

In addition, the optimal ambient temperature for printing metallic ink is 25±3°C; the viscosity of the ink should be determined according to the speed of the printing press and the requirements of the printed pattern, generally 40 to 70 seconds (25°C, coated with 4# cups).

Since the metal ink particles are relatively thick, an anilox roller with a low number of screen lines is required for printing.

IV. Problems in post-processing <br><br><br><br><br><br><br> The on-line follow-up processing of flexo labels is usually broken and die-cut. When breaking a sheet online, sometimes semi-finished products stick together and are not easily separated. This is mainly caused by excessive static electricity. The general solution is to install several static eliminators in the second half of the paper path, or to install humidifiers in the subsequent units of printing.
Since flexo printing is a brand-new printing method in our country, it is not standardized in many aspects. For this reason, many people always require flexographic printing with other printing standards. This is a misunderstanding because other printing methods Standards are not universally applicable. Compared with other printing methods, flexo printing has both its advantages and its shortcomings. Therefore, we must pay attention to the advantages of flexographic printing in flexographic printing, and abandon or overcome its shortcomings.

V. Other related issues 1. Regarding printing equipment When applying flexo printing machine for printing coated paper labels, due to the long paper path, when performing multi-color printing, especially when applying water-soluble ink to print paper After multiple infrared drying, overprinting is very likely to occur, resulting in some unnecessary waste. The control measure for this phenomenon is to add a humidifier so that the pull-up rate of the printed product can be reduced as much as possible.

When printing with a satellite flexographic printing press, the impression cylinder will be thermally expanded to increase the pressure, affect the printing quality, and damage the printing plate. Therefore, a cooling device is generally installed in the impression cylinder. In addition, due to the short distance between the printing cylinders in the satellite flexographic printing presses, it is difficult to dry them. Therefore, it is necessary to add a high-efficiency drying device in the satellite flexographic printing machine.

2. About printing ink In the printing process, the viscosity of the ink will directly affect the ink transfer characteristics and stability of the ink transfer. In general, after the temperature rises, the viscosity of the ink will decrease, and the stability of the ink transfer amount can be easily guaranteed.

At present, the inks used in flexographic printing mainly include water-soluble inks, UV inks, and solvent-based inks. Here, the use and preservation of water-soluble inks and UV inks are briefly described.
(1) Water-soluble inks Water-soluble inks are mainly composed of pigments, binders, defoamers and anti-foaming agents, and are generally alkaline.
In the use of water-soluble ink should be done: before printing, add ink to the ink tank should be diluted to the best viscosity of the ink, the thixotropic ink to be fully stirred; in the printing process, should make the ink's pH The value is maintained at 8.5 to 9.5, to ensure that the ink has a suitable pH value is conducive to the ink to maintain proper viscosity, but also makes the ink drying rate tends to be reasonable, thereby reducing the paste version failure.

The auxiliary agent for adjusting the pH value of the ink is a pH stabilizer, and the amount of the stabilizer added is appropriate. The addition of too much amount will affect the hue of the printed product and lead to a reduction in the friction resistance of the graphic part of the printed product.
The preservation of water-soluble inks is also critical, and reasonable storage will create good conditions for re-use of the ink. Water-soluble inks contain ammonia and amines that maintain the basicity of the ink, as well as auxiliary solvents such as ethanol or isopropanol. These are very volatile substances. Therefore, the preservation of water-soluble ink must be sealed. Since the remaining ink contains some additives that regulate the performance of the ink, directly injecting it into the original ink will inevitably cause deterioration of the original ink. Therefore, the remaining ink should be stored separately and used up as soon as possible to reduce losses. . Before adding ink, use a certain amount of original ink and make appropriate adjustments. In addition, some dark-colored or inter-color inks (such as black ink, green ink, etc.) have a large thixotropy, especially at low temperatures. Therefore, the storage temperature of water-soluble ink should be controlled at 20 to 25°C.

(2) UV inks The following points should be used when using and storing UV inks:
1UV ink should be strictly dark, heat storage, storage temperature should be controlled at 5 ~ 30 °C, shelf life is generally six months.
2 It needs to be evenly mixed before printing on the machine. Dilution can use special reactive diluent or UV varnish and a small amount of ethanol.
3 cleaning can use a dedicated UV cleaning agent or ethanol.
4Be sure to pay attention to the luminous intensity of the light source. The aging UV lamp should be replaced in time to prevent the printing ink from being cured due to incomplete curing of the printing ink layer. UV lamp life is generally 800 ~ 1000 hours, pay attention to frequent start will significantly reduce the lamp life.

5 It should avoid direct contact between the ink and the skin to prevent irritation and allergies. It should be promptly cleaned after contact.
3. The printing materials suitable for flexographic printing of printing materials are very broad, generally based on paper, film, stickers, etc. Here, only the printing of a paper label is described as an example. Generally when printing paper, especially when printing with water-soluble inks, overprinting is very likely to occur. The main reason is that paper is an absorbent material, and water-soluble ink is printed on paper. One of the water will be absorbed by the paper, and then infrared drying, so repeated several times will inevitably lead to overprinting of the paper is not allowed to register. The solution to this type of failure is to locally humidify the prints and reduce the infrared drying stalls as much as possible. In addition, the appropriate speed increase can also alleviate such problems to some extent.

Reprinted from: empiresoft.net