Guide: According to the needs of product glazing, local UV glazing can apply local glazing to the prominent parts of trademarks and packaging prints. Compared with the surrounding patterns, the glazing pattern is bright, bright and three-dimensional, which can produce unique Artistic effect. This article briefly describes the relevant knowledge of the local UV coating process, the content is for the reference of friends:


Local UV coating process

Local UV is a kind of surface finishing technology of printed matter. It is named because it uses UV varnish with high brightness, transparency and abrasion resistance to selectively glaze printed graphics. While highlighting the theme of the layout, it also improves the surface decoration effect of printed products, which is mainly used in the finishing of book covers and packaging products.

UV effect
At present, the common local UV effects include: local bright, local extinction, local matte, local colorful local refraction, local wrinkles and local ice, etc.

Local UV can be implemented after coating or directly glazing on the printed matter, but in order to highlight the effect of local glazing. Generally, it is done after the printed matter is coated, and the matte film is mostly used, accounting for about 80% of the local UV coating products.

The biggest difference between partial glazing and full-page glazing lies in that one is the need to make a printing plate and the other is that there are certain registration requirements for the glazing equipment. There are three main elements of local UV coating, namely coating plate, equipment registration and curing equipment. In order to ensure that the local UV glazing part is consistent with the size of the printed image and text, the photosensitive imaging method should be used when making the glazing plate.

Screen printing partial glazing

Screen printing has the characteristics of high ink transferability, thick ink layer and strong three-dimensional effect. The screen for glazing should generally be 200~300 mesh screen (for partial frosting and partial colorful glazing effect, the number of meshes is lower), and the tension of the stretch screen is 20-25N/cm. Because the equipment investment is not large. Many books and magazines and packaging factories prefer screen printing when increasing local glazing business.

Partial glazing of flexo

The characteristic of flexographic printing is the ability to form a thicker coating with a simpler process. Therefore, it is very popular in UV coating. The partial glazing of flexographic printing is divided into ordinary flexographic printing glazing and special flexographic printing glazing, and some simple graphics are still using engraved flexographic printing.

The partial glazing of flexographic printing is suitable for off-line glazing and can also realize online glazing. The flexographic printing and glazing method, with anilox roller and cavity doctor blade, can accurately control the amount of varnish, and the glazing quality is stable.

Gravure spot coating

Gravure printing also has advantages in local UV glazing due to the thicker printing ink layer and the high printing power of the gravure cylinder, especially the appearance of the sheet-fed gravure printing machine has contributed to the development of gravure local glazing. However, due to its high cost of plate making, it is mainly used for long plate glazing.

Other local glazing methods
Before UV varnish appeared, no one would use offset printing for glazing because its printing ink layer was too thin. However, UV varnish has the advantages of thick film layer, high gloss and high overprinting accuracy. It also has some applications in local UV coating, but it is mainly used for online coating.

In terms of production methods, local UV coating machines can also be divided into offline coating and online coating. Off-line glazing refers to printing. The glazing is performed on special equipment and requires the use of a special glazing machine or a calender. In off-line glazing, since the printed matter can be sufficiently dried, higher gloss can be obtained than in-line glazing. In addition, the flexibility of off-line coating production is great.

The equipment investment is small. On-line glazing means that printing and glazing are completed on one production line, that is, glazing and UV curing devices are added to the printer. Make the printed matter directly glaze after printing.

New local UV technology

①Mixed ink technology

Because UV printing or UV glazing is not suitable for the surface of the printing material with strong absorption, and the adhesion of the printing ink surface is not good, most of them need online UV glazing. If all UV inks are used for printing, the cost will be increased. : If you use ordinary ink to print, laminating or glazing before glazing will undoubtedly hinder the application of offset printing on-line glazing.

The mixed ink technology effectively solves this bottleneck problem. In terms of performance, it combines the advantages of UV inks and traditional inks, that is, the advantages of photocuring and oxidative polymerization drying, which improves the synergistic effect of the ink surface and UV varnish, paving the way for on-line coating of offset printing.

②Reverse upward light
Pro-Cure is relative to the traditional partial glazing. The traditional glazing method (flexographic printing screen printing or gravure printing) is adopted for local glazing, and finally local glazing plate printing must be used to achieve the high contrast effect of local graphics and text. Therefore, the overprint accuracy of glazing and printing becomes glazing The key to the process. The reverse-up light process breaks through the above-mentioned laws and solves the registration problem.

③Online UV coating
Online glazing is no longer a new technology for flexographic printing and gravure printing, but for offset printing with a wide range of applications, higher printing accuracy and speed, it is launched under the voice of differentiation and value-added services The glazing technology with innovative ideas.
There are four main types of online UV coating methods that have been applied:
UV ink + UV coating;
Mixed ink + UV coating;
UV ink+OP oil+UV glazing (backward glazing);
Ordinary ink + double coating (water-based coating + UV coating).

1) Requirements for printing ink additives.
If the local UV coating is directly applied to the surface of the printed product, do not add or add as little as possible various additives to the ink during printing, such as de-tackifiers, ink-adjusting oil, etc., so as not to affect the surface polarity of the ink layer and cause the coating time oil The adhesion fastness to the ink layer is poor, and the varnish layer is easy to fall off, which seriously affects the glazing effect.

2) Requirements for printing powder spraying.
The amount of powder spraying is required to be small. A large amount of powder spraying will form a layer of powder on the surface of the printed product, causing the UV varnish layer to not firmly adhere to the ink layer, and it is easy to fall off, appearing in a cloud-like shape, which seriously affects the glazing effect. If the amount of powder sprayed is not well controlled during printing, the powder must be removed by manual dusting if necessary.

3) Requirements for the surface tension of laminated products.
For products that require partial UV coating after coating, it is best to use a film that has been corona treated on both sides, that is, the surface tension of both sides of the film must reach 38mN/m or more, so as to ensure that the UV varnish can be firm The ground is attached to the film. At the same time, it is necessary to check whether the surface tension of the film used is uniform, so as to avoid local peeling of the varnish layer.

4) Requirements for the surface tension of varnish.
It is required that the surface tension of the varnish must be less than the surface tension of the ink, so that the varnish can well wet, adhere, and penetrate the graphic part of the surface of the printed product. Otherwise, after the varnish is applied, the varnish layer will shrink to a certain extent, and even blisters and other abnormal phenomena may occur.

5) Requirements for the viscosity of varnish.
It is necessary to adjust the viscosity of the varnish and the thickness of the varnish layer to make it have good coating properties. To avoid problems such as too thin varnish layer leading to surface blooming and poor gloss, or too thick varnish layer to show orange peel effect or difficult to solidify. The viscosity of the varnish can be adjusted by adjusting the UV curing temperature, and a small amount of alcohol can also be added for adjustment if necessary.

6) Requirements for UV lamps.
Adjust the intensity of UV light according to the characteristics of the printing paper and the area of ​​the UV coating pattern. Generally, the curing equipment is equipped with 1 to 3 sets of UV lamps for adjustment. The corresponding light intensity can be achieved by selecting the number of UV lamps to be used, so that the varnish layer is easy to cure, and not easy to become brittle or break.

7) Application of UV primer.
For full-page glazing products, water-based primers are often used to improve the smoothness of the paper surface and the gloss of UV varnish. This method is also applicable to the local UV coating process.

8) Requirements for design.
During indentation and die-cutting, the varnish layer is more likely to crack and fall off, resulting in white marks at the indentation or incision, making the product unsightly. It is recommended to avoid designing UV coating patterns at the indentation or incision during design, or The color of the above position patterns should be designed as light as possible to avoid unsightly effects caused by the shedding of the varnish layer.

9) Consideration of paper deformation.
As the paper is easily affected by the environmental temperature and humidity to produce expansion, so for some products that require high overprinting accuracy, the expansion of the paper size must be taken into account before plate making, and the deformation rate must be calculated to ensure local UV coating Overprint accuracy of patterns and printed patterns.

10) Inspection method for the firmness of the varnish layer.
Use a transparent tape with good viscosity to stick and pull the varnish layer or scratch the varnish layer with your nails. Use the standard that the varnish layer does not fall off after sticking or scratching to check whether the varnish layer is firm.