Study on flexo printing technology of coated paper labels

First, problems related to design and platemaking

Layout design

In flexo printing, since the plate cylinder is a cylinder, and the whole plate is covered on the cylinder to form a closed cylinder, the layout design needs some consideration, and the length of the plate must be less than The circumference of the platen roller.

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. Out of film

We know that the flexographic printing plate has a certain degree of elasticity. Therefore, when it is pasted on the platen roller, the printing plate is easily deformed, that is, the printing plate is elongated along the circumferential direction of the platen roller. Therefore, the film cannot be produced when the film is discharged. The length is just 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 miniature 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).

Second, issues related to printing processes and printing operations

When performing flexo printing, the matching of the printing color sequence and the number of anilox roller lines is a crucial 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.

Third, issues related to surface finishing

In order to increase the beauty 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 above light and gold and silver ink printings are used as examples for illustration.

1. Polishing

The main role of glazing is to increase the abrasion resistance of the prints and, in some cases, increase the water resistance of the prints.

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 and silver ink printing

Water-based metal inks are made of copper powder, aluminum powder, binders, and auxiliaries that are specially processed. Since copper powder and aluminum powder have a large specific gravity and are easily precipitated, they must be mixed well before use.

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.

(to be continued)