UV flexo printing

UV ink curing treatment and process configuration

In recent years, there have been some major changes in the packaging industry. Affected by regional promotions, short-lived jobs have become the main livelihood for many packaging and printing. The scope of the packaging industry involves food and food packaging, pharmaceutical packaging, beauty and health product packaging, and folding box production. field. The more fierce competition on the shelves of goods shows that sellers and printers must try to innovate and launch visually impactful labels and product packaging.

Product packaging is the last salesperson of a product. Only through the attraction of customers or the specific commitments made by a certain product manufacturer can one realize their value. Suppliers responded to the needs of market cities by developing new substrates, UV inks, and coating materials. Therefore, there are many new methods for producing labels. It is also common to use several printing processes in combination, so that printing quality comparable to plain printing and gravure printing can be obtained.

Supplier response

Printing equipment suppliers responded in a timely manner and worked hard to innovate to move forward in a constantly changing industry. The devices that have been introduced are able to handle a wide variety of substrates, with fast job conversions and one pass, which can be printed using more than one printing process. At present, several printing processes are commonly used for printing. The color stability and physical properties of products printed with UV inks and coating materials are greatly improved.

Printing press with multiple configurations

Currently manufactured presses can have a variety of process configurations including flexo or UV flexo printing, gravure printing, screen printing, rotary embossing, offset printing, and rotary bronzing. Recent changes due to the need to gain market share show that narrow web presses are becoming wider, while wide web presses are narrowing. This creates an interesting new market that overlaps, for example using UV flexo technology to produce folding boxes on linkage printers, which is a challenge for traditional sheet-fed offset printers.

UV Curing System - Needed (Cold) UV

Both existing and future printed materials require unique heat treatment methods. As the heat-sensitive unsupported film label and packaging printing industries are used more and more, the need for high-efficiency, fast-curing UV inks and coatings (without damaging the substrate, does not cause alignment errors) has increased. UV curing equipment manufacturers have taken steps to introduce UV curing equipment that can be used flexibly with existing or new presses. Currently, narrow, medium, and wide web presses can be individually equipped with UV curing devices, and can also be equipped with hot air and UV drying curing devices.

Variable factors affecting the UV curing of substrates and inks

The variable factors that affect the UV curing of substrates and inks include the depth of cure, surface temperature, and different properties of the substrate. The requirements for each curing are also different due to the accuracy of the image, the level of detail, and the substrate. Cured inks must impart different ink film weights, color densities, adhesion forces and flexural elasticities to different products. Printing on a crate with UV inks requires the use of unique Western inks. This ink is easy to transfer, but also take into account the different characteristics and properties of the substrate.

UV ink and UV lamp interaction

Many interactions between UV inks and UV lamps will not only affect the characteristics of the final product, but also determine that this will not only affect the characteristics of the final product, but also determine the efficiency of UV curing equipment. We know that light consists of a bald head, which is the basic unit of light energy and usually travels straight at the speed of light. Each photon has a certain wavelength, and its energy size is related to the same wavelength. When photons pass through an object, they are reflected, refracted, or absorbed. After the photons are absorbed, the photon hardly transforms into another form of energy, usually thermal energy. The chemical changes it causes are called photochemical changes.

Chemical Properties of UV Inks - Photo Initiators (Active Ingredients) UV inks are photochemical compounds that typically consist of four components: monomers, oligomers, pigments (and other additives), and photoinitiators. The only optically active ingredient is a photoinitiator that reacts with certain wavelengths of ultraviolet light. When the photons of the ultraviolet light collide with molecules of the photoinitiator, a curing reaction is initiated.

Photon absorption efficiency

When using UV inks, there are two important things to keep in mind:

Photons that have not collided with photoinitiators are wasted and photons with certain wavelengths that do not activate photoinitiators are also ineffective. All components of the UV ink have a light-shielding effect, in particular blocking UV light. The degree of reduction in the intensity of light passing through an absorbing substance obeys Beer's Law of Lamb waves. The law describes how the intensity of light is reduced due to the effects of ink film thickness and absorption characteristics. From this we can draw two additional basic conclusions:
The photoinitiator molecules are uniformly dispersed in the UV ink. The intensity of the photons scattered on the surface of the UV ink print is unevenly distributed on the ink layer (UV irradiance)

Irradiance, sometimes called the energy intensity of a UV lamp, refers to the irradiation power per unit area of ​​the printing surface at any point in the curing process. Irradiance is measured in watts or milliwatts/cm2. The irradiance value is the maximum irradiance value of the UV lamp and can usually be obtained at the focus of the UV light on the surface of the substrate. The irradiance is determined by factors such as the performance of the reflector of UV energy and the like for several decades. The greater the illuminance, the greater the irradiation power at any level in the ink film.

Ultraviolet irradiation dose

The term “dose” refers to the amount of time taken by the camera, or the amount of energy released on the surface within a specified time, measured in Joules or millijoules/cm2 (1 watt/cm2 X1 is 1 Joule/cm2). Does the 1 second 1W/cm2 dose produce the same curing effect? The answer is no, because the dose does not account for the depth of radiation and does not adequately describe the emancipation of the amount of UV. We used UV inks to print on polycarbonate substrates with a line number of 154 lines/cm. Curing under different UV lamp irradiation conditions compared the curing effects and found that it was the key to the curing effect. The effect is UV irradiance, not radiation dose.

Under the condition that the UV lamp irradiation conditions are not changed, let the printing image pass once or twice under the UV lamp with different irradiation power peaks, and then compare the curing depth of the UV ink with the next irradiation irradiance and irradiation dose. The impact. Passing printed text under UV lamps with lower power actually increased the depth of cure, but it did not double as expected. In fact, if the printing image is passed under the UV lamp for the third time, the increase in cure will be further reduced. Increasing the irradiation time does not make up for the insufficient irradiation intensity of the UV lamp.

Spectral selection

The ultraviolet wavelength is 200-450 nm. The limit of the short wavelength is 200 nanometers, because the short wavelength can not be transmitted in the air. At about 450 nm is the starting point of the spectrum of visible light. In the case of screen printing UV inks, both bands function simultaneously. Short waves act on the surface of the ink, while long waves act more deeply on the interior of the ink. Loss of shortwave ink surface becomes sticky. The loss of long-wave ink loses its adhesion.

UV curing light

UV curing lamps use mercury as the main source of light. Some metal halides are also often added inside the lamp in order to obtain UV curing lamps with different UV spectra. Since all UV lamps have mercury in them and produce a variety of wavelengths, including short wavelengths, the surface of the ink can be cured to a certain degree by short-persian energy. The added metal halide can increase the output of a specific longer wavelength ultraviolet light, so that the degree of dry curing and adhesion of the properly formulated UV ink is enhanced.

Some of the frequently used UV lamps include:

H-type lamp for transparent light inks and inks

D-type lamp, facing some color screen printing ink

V-lamp, difficult to pass ink/white ink

Past defects

In the past, when a UV lamp with a metal halide was added, the drawback was that the power regulator was often adjusted. Now with solid-state variable-voltage power supply, UV light can be started without adjustment.

Reduce heat and surface temperature, control infrared

When the substrate can not withstand excessive heat, or UV coating may be damaged due to excessive heat, the temperature should be a problem. For example, in color silk screen printing, the temperature must be controlled to be lower in order to minimize the increase in outline points when UV inks are used. During the use of the UV lamp, infrared (IR) energy (not generated by mercury or metal halide) is generated due to temperature increase and emitted by the quartz lamp.

The air-cooling device in the dry curing system can be used to lower the surface temperature, and the print can also pass through the curing chamber at a faster speed to obtain the surface temperature.

Customer and market demand

Consumer goods distributors will continue to demand greater room for printing images in order to maintain their existing market share. The above-mentioned needs of dealers can be met by a wide variety of printer configurations, an ever-increasing variety of printing consumables, and continuous advancements in UV curing technology.

Reprinted from: Paper Arts