2023 The Complete Guide To UV curing ink in use in the problem
I. What happens when the ink is over-cured?
There is a theory that when the surface of the ink is exposed to too much UV light, it will become harder and harder. And when one prints another ink on this hardened ink film and dries it a second time, the adhesion between the top ink and the bottom ink becomes poor.
The other theory is that excessive curing causes photo-oxidation of the ink surface. Photo-oxidation occurs by breaking the chemical bonds on the ink film surface, and if the molecular bonds on the ink film surface are degraded or damaged, then the adhesion between it and another ink layer will be reduced. Over-cured ink film is not only poor flexibility, but also prone to surface embrittlement.
2, why usually UV ink curing speed than other inks?
UV ink is generally formulated according to the characteristics of certain substrates and the special requirements of certain applications. From a chemical point of view, the faster the curing speed of the ink, the less flexible it is after curing. As you can imagine, when the ink curing occurs, the ink molecules are cross-linked, if these molecules form very many molecular chains and have many bifurcations, then the ink will be cured very fast, but not very flexible; if these molecules form fewer molecular chains and have no bifurcations, then this ink may be cured very slowly, but it will definitely be very flexible. Most inks are designed to meet the needs of the application. For example, for inks designed for the production of membrane switches, the cured ink film must be compatible with laminating adhesives and be flexible enough to accommodate subsequent processes such as die-cutting and embossing. It is worth noting that the chemical materials used in the ink must not react with the surface of the substrate, otherwise it will cause phenomena such as cracking and breaking or delamination. The curing speed of such inks is usually slow. In contrast, those inks designed for the production of cards or hard plastic display boards do not need to have such a high degree of flexibility, and depending on the needs of the application, they dry more quickly. On whether the ink dries fast or slow, we have to start with the *end application in mind. Another issue worth noting is the curing equipment. Some inks can originally be cured very quickly, but due to the curing equipment does not work efficiently, it may also lead to slowing down the curing speed of the ink, or curing incomplete.
3、Why does the polycarbonate (PC) film yellow when using UV ink? How to avoid or eliminate the yellowing of polycarbonate surface resistance?
Polycarbonate is more sensitive to UV light with wavelength less than 320nm. Yellowing of the film surface is caused by the breakage of molecular chains caused by photo-oxidation. The plastic molecular bonds absorb UV energy and produce free radicals, which react with oxygen in the air and change the appearance and physical properties of the plastic.
If UV inks are used to print on polycarbonate film, then the yellowing of its surface can be reduced, but not completely eliminated. The appearance of this yellowing can be effectively reduced by using curing bulbs with added iron or gallium, which reduce the emission of short wavelength UV light to avoid damage to the polycarbonate. In addition, the proper curing of each ink color also helps to reduce the exposure time of the substrate in the UV light and reduce the possibility of discoloration of polycarbonate film.
4. What is the relationship between the setting parameter (watts/inch) on the UV curing lamp and the reading we see on the radiometer (watts/cm2 or milliwatts/cm2)?
W / inch is the unit of power of the curing lamp, which is based on Ohm’s law volts (voltage) x amps (current) = watts (power); and watts / cm2 or milliwatts / cm2 indicates the peak illumination (UV energy) per unit area when the radiometer passes under the curing lamp.
The peak illuminance depends primarily on the power of the curing lamp. We use watts to measure peak illuminance primarily because it represents the electrical power consumed by the curing lamp. In addition to the power received by the curing unit, other factors that affect peak illuminance include the age of the curing lamp, the condition and geometry of the reflector, and the distance between the curing lamp and the curing surface.
5、What is the difference between mJ and mW?
The total energy irradiated to a particular surface in a given period is usually expressed in J/cm2 or mJ/cm2. It is mainly related to the age of use, the power of the curing lamp, the number, the speed of the conveyor belt, the state and the shape and condition of the reflector in the curing system.
And irradiation to a particular surface of UV energy active radiant energy power is mainly expressed in watts / cm2 or milliwatt / cm2. The higher the UV energy irradiated to the surface of the substrate, the more energy that penetrates into the ink film. Whether it is milliwatt or millijoules, are to be measured only if the wavelength sensitivity of the radiometer reaches certain requirements.
6、How do we ensure proper curing of UV inks?
The curing of the ink film at the first pass through the curing unit is very important. Proper curing minimizes substrate distortion, over-curing, rewetting and under-curing, and optimizes the adhesion between ink and ink or between coating and coating.
Screen printers have to define production parameters before production starts. To test the curing efficiency of UV inks, we can first start printing at the *lowest* speed that the substrate will allow and cure the sample sheet that was printed first. Subsequently, the power of the curing lamp is set to the value specified by the ink manufacturer. For colors that are not easily cured, such as black and white, we can also adjust the parameters of the curing lamp upwards. After the sheet has cooled, we can use the bi-directional shadow line method to determine the adhesion of the ink film. If the sample sheet can successfully pass the test, then the paper transfer speed can be increased by 10 ft/min, and then printing and testing can be performed until the ink film loses its adhesion to the substrate, and the transfer belt speed and curing lamp parameters at this time can be recorded. Next, the conveyor speed can be reduced by 20-30% according to the characteristics of the ink system or the recommendations of the ink supplier.
7. Should I be concerned about over-curing if the colors are not overlapping?
Over-curing occurs when the surface of an ink film absorbs too much UV light. If this problem is not detected and solved in time, the surface of the ink film will become harder and harder. Of course, as long as we don’t do color overprinting, we don’t need to be too concerned about this problem. However, there is another important factor to consider, and that is the film or substrate being printed on. UV light can affect most substrate surfaces and certain plastics that are sensitive to specific wavelengths of UV light. This sensitivity to specific wavelengths combined with oxygen in the air can lead to degradation of the plastic surface. The molecular bonds on the substrate surface may be broken and cause a failure of adhesion between the UV ink and the substrate. The degradation of the surface function of the substrate is a gradual process and is directly related to the UV light energy it receives.
8、What is the unit of measurement of the density data displayed on the densitometer? What factors will affect the density?
Optical density has no unit. The densitometer measures the amount of light reflected or transmitted from a printed surface. A photoelectric eye attached to the densitometer converts the percentage of reflected or transmitted light into a density value. In screen printing, the main variables that affect the density value are the thickness of the ink film, the color, the size and number of pigment particles, and the color of the substrate. The optical density is mainly determined by the opacity and thickness of the ink film, which in turn is influenced by the size and number of pigment particles and their light absorption and scattering properties.
9、Dainty level of printing substrate and change the dainty level?
Dain/cm is the unit used to measure surface tension. This tension is caused by the intermolecular gravitational force of a particular liquid (surface tension) or solid (surface energy). For practical purposes, we will usually refer to this parameter as the dyne level. The Dain level or surface energy of a particular substrate represents its wettability and ink adhesion. Surface energy is a physical property of a substance. Many of the films and substrates used in printing have low print levels, such as polyethylene at 31 dynes/cm and polypropylene at 29 dynes/cm, and therefore require special treatment.
Flame Treatment : By nature, plastics are non-porous and have inert surfaces (low surface energy). Flame treatment is a method of pretreating plastics to increase the dyne level of the substrate surface. In addition to the plastic bottle printing field, this method is also widely used in the automotive and film processing industries. Flame treatment not only improves the surface energy, but also eliminates surface contamination. Flame treatment involves a complex series of physical and chemical reactions. The physical mechanism of flame treatment is that: the high temperature flame transfers energy to the oil and impurities on the surface of the substrate, causing them to evaporate by heat and playing a cleaning role; and its chemical mechanism is that: the flame contains a large number of ions with strong oxidizing properties, and the oxidation reaction occurs with the surface of the treated material at high temperature, causing the surface of the treated material to form a layer of charged polar functional groups, which improves its surface energy and thus also This increases the surface energy and thus the ability to adsorb liquids. Proper treatment can increase the Dainty level of some substrates, but this is only temporary. When you are ready to print, there are a number of other factors that can affect the substrate’s dyne levels, such as the time and number of treatments, storage conditions, ambient humidity, and dust levels. Because Dain levels change over time, most printers find it necessary to treat or re-treat these films prior to printing.
Corona treatment: Corona discharge is another method of increasing Dain levels. By applying high voltage to the dielectric roll, it can ionize the surrounding air, and when the substrate passes through this ionized area, the molecular bonds on the surface of the material are broken. This method is usually used in rotary printing of film materials.
10、How does the plasticizer affect the adhesion of the ink on PVC?
Plasticizers are chemicals that make printing materials softer and more flexible, and their use in PVC (polyvinyl chloride) is very common. The type and amount of plasticizer added to flexible PVC or other plastics depends largely on the mechanical, thermal and electrical properties one requires from the printed material. Plasticizers have the potential to migrate to the surface of the substrate and affect the adhesion of the ink. Plasticizers left on the surface of the substrate are a kind of contamination, which will reduce the surface energy of the substrate. The more contamination on the surface, the lower the surface energy, and the less it will adhere to the ink. To avoid this, one can clean the substrate with a mild cleaning solvent before printing to improve their printability.
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11、How does the viscosity of the ink affect the printability?
Most inks are thixotropic, which means that their viscosity changes with shear, time and temperature. In addition, the higher the shear rate, the lower the viscosity of the ink will be; the higher the ambient temperature, the lower the annual ink will be. Screen printing inks generally achieve good results on the press, but occasionally have problems with printability depending on press settings and pre-press adjustments. Also the viscosity of the ink on the press is different from the viscosity it has in the cartridge.
Ink manufacturers will set a specific viscosity range for their products. For too thin or low viscosity ink, the user can also add appropriate thickening agent; and for too thick or high viscosity ink, the user can also add thinner.
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12、What are the factors that affect the stability or shelf life of UV ink?
An important factor affecting the stability of the ink is the storage of the ink. UV ink is usually stored in plastic cartridges rather than metal cartridges, because the plastic container has a certain degree of oxygen permeability, which ensures that there is a certain air gap between the ink surface and the container lid. This air gap – especially the oxygen in the air – helps to * minimize premature cross-linking of the ink. In addition to packaging, the temperature of the ink containers plays a crucial role in maintaining their stability. High temperatures can cause premature reactions and cross-linking of inks.
Adjustments to the original ink formulation may also affect the stability of the ink on the shelf. Additives, especially catalysts and photoinitiators, may shorten the shelf life of the ink.
13、What is the difference between in-mold labeling (IML) and in-mold decoration (IMD)?
The basic meaning of in-mold labeling and in-mold decoration is the same, that is, the label or decorative film (prefabricated, unprefabricated) is put into the mold, and the molten plastic will support it when the part is formed. The former uses labels that are produced using different printing techniques, such as gravure, offset, flexographic or screen printing. These labels are usually printed only on the upper surface of the material, while the unprinted side is attached to the injection mold.
In-mold decoration is mostly used to produce durable parts and is usually printed on the second surface of the transparent film. In-mold decoration is usually printed on a screen printer, and the film and UV inks used must be compatible with the injection mold.
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