1. What happens when the ink is over-cured? There is a theory that when the ink surface is exposed to too much ultraviolet light, it will become harder and harder. When people print another ink on this hardened ink film and dry it for the second time, the adhesion between the upper and lower ink layers will become very poor.
Another theory is that over-curing will cause photo-oxidation on the ink surface. Photo-oxidation will destroy the chemical bonds on the surface of the ink film. If the molecular bonds on the surface of the ink film are degraded or damaged, the adhesion between it and another ink layer will be reduced. Over-cured ink films are not only less flexible, but also prone to surface embrittlement.
2. Why do some UV inks cure faster than others? UV inks are 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 ink cures, the worse its flexibility after curing. As you can imagine, when the ink is cured, the ink molecules will undergo cross-linking reactions. If these molecules form a large number of molecular chains with many branches, the ink will cure quickly but will not be very flexible; if these molecules form a small number of molecular chains without branches, the ink may cure slowly but will definitely be very flexible. Most inks are designed based on application requirements. For example, for inks designed for the production of membrane switches, the cured ink film must be compatible with composite adhesives and be flexible enough to adapt to subsequent processing such as die-cutting and embossing.
It is worth noting that the chemical raw materials used in the ink cannot react with the surface of the substrate, otherwise it will cause cracking, breaking or delamination. Such inks usually cure slowly. Inks designed for the production of cards or hard plastic display boards do not need such high flexibility and dry quickly depending on the application requirements. Whether the ink dries quickly or slowly, we must start from the final application. Another issue worth noting is the curing equipment. Some inks can cure quickly, but due to the low efficiency of the curing equipment, the curing speed of the ink may be slowed down or incompletely cured.
3. Why does the polycarbonate (PC) film turn yellow when I use UV ink? Polycarbonate is sensitive to ultraviolet rays with a wavelength less than 320 nanometers. The yellowing of the film surface is caused by the breakage of the molecular chain caused by photooxidation. The plastic molecular bonds absorb ultraviolet light energy and produce free radicals. These free radicals react with oxygen in the air and change the appearance and physical properties of the plastic.
4. How to avoid or eliminate the yellowing of the polycarbonate surface? If UV ink is used to print on polycarbonate film, the yellowing of its surface can be reduced, but it cannot be completely eliminated. The use of curing bulbs with added iron or gallium can effectively reduce the occurrence of this yellowing. These bulbs will reduce the emission of short-wavelength ultraviolet rays to avoid damage to polycarbonate. In addition, properly curing each ink color will also help reduce the exposure time of the substrate to ultraviolet light and reduce the possibility of discoloration of polycarbonate film.
5.What is the relationship between the setting parameters (watts per inch) on the UV curing lamp and the readings we see on the radiometer (watts per square centimeter or milliwatts per square centimeter)?
Watts per inch is the power unit of the curing lamp, which is derived from Ohm’s law volts (voltage) x amps (current) = watts (power); while watts per square centimeter or milliwatts per square centimeter represents the peak illuminance (UV energy) per unit area when the radiometer passes under the curing lamp. Peak illuminance depends mainly on the power of the curing lamp. The reason why we use watts to measure peak illuminance is mainly because it represents the electrical energy consumed by the curing lamp. In addition to the amount of electricity received by the curing unit, other factors that affect peak illuminance include the condition and geometry of the reflector, the age of the curing lamp, and the distance between the curing lamp and the curing surface.
6. What is the difference between millijoules and milliwatts? The total energy irradiated to a specific surface over a certain period of time is usually expressed in joules per flat centimeter or millijoules per square centimeter. It is mainly related to the speed of the conveyor belt, the power, number, age, status of the curing lamps, and the shape and condition of the reflectors in the curing system. The power of UV energy or radiation energy irradiated to a specific surface is mainly expressed in watts/square centimeter or milliwatts/square centimeter. The higher the UV energy irradiated to the surface of the substrate, the more energy penetrates into the ink film. Whether it is milliwatts or millijoules, it can only be measured when the wavelength sensitivity of the radiometer meets certain requirements.
7. How do we ensure the proper curing of UV ink? The curing of the ink film when it passes through the curing unit for the first time is very important. Proper curing can minimize the deformation of the substrate, over-curing, re-wetting and under-curing, and optimize the adhesion between the ink and the humor or between the coatings. Screen printing plants must determine the production parameters before production begins. In order to test the curing efficiency of UV ink, we can start printing at the lowest speed allowed by the substrate and cure the pre-printed samples. Subsequently, set the power of the curing lamp to the value specified by the ink manufacturer. When dealing with colors that are not easy to cure, such as black and white, we can also appropriately increase the parameters of the curing lamp. After the printed sheet cools, we can use the bidirectional shadow method to determine the adhesion of the ink film. If the sample can pass the test smoothly, the paper conveyor speed can be increased by 10 feet per minute, and then printing and testing can be carried out until the ink film loses adhesion to the substrate, and the conveyor belt speed and curing lamp parameters at this time are recorded. Then, the conveyor belt speed can be reduced by 20-30% according to the characteristics of the ink system or the recommendations of the ink supplier.
8. If the colors do not overlap, should I be concerned about over-curing? Over-curing occurs when the surface of an ink film absorbs too much UV light. If this problem is not discovered and solved in time, the surface of the ink film will become harder and harder. Of course, as long as we do not perform color overprinting, we don’t have to worry too much about this problem. However, we need to consider another important factor, which is the film or substrate being printed. UV light can affect most substrate surfaces and some plastics that are sensitive to UV light of a certain wavelength. This sensitivity to specific wavelengths combined with oxygen in the air can cause degradation of the plastic surface. Molecular bonds on the substrate surface can be broken and cause the adhesion between the UV ink and the substrate to fail. The degradation of the substrate surface function is a gradual process and is directly related to the UV light energy it receives.
9. Is UV ink a green ink? Why? Compared with solvent-based inks, UV inks are indeed more environmentally friendly. UV-curable inks can become 100% solid, which means that all components of the ink will become the final ink film.
Solvent-based inks, on the other hand, will release solvents into the atmosphere as the ink film dries. Since solvents are volatile organic compounds, they are harmful to the environment.
10. What is the unit of measurement for the density data displayed on the densitometer? Optical density has no units. The densitometer measures the amount of light reflected or transmitted from a printed surface. The photoelectric eye connected to the densitometer can convert the percentage of reflected or transmitted light into a density value.
11. What factors affect density? In screen printing, the variables that affect density values are mainly ink film thickness, color, size and number of pigment particles, and color of the substrate. Optical density is mainly determined by the opacity and thickness of the ink film, which in turn is affected by the size and number of pigment particles and their light absorption and scattering properties.
12. What is dyne level? Dyne/cm is a unit used to measure surface tension. This tension is caused by the intermolecular attraction of a particular liquid (surface tension) or solid (surface energy). For practical purposes, we usually call this parameter dyne level. The dyne level or surface energy of a particular substrate represents its wettability and ink adhesion. Surface energy is a physical property of a substance. Many films and substrates used in printing have low print levels, such as 31 dyne/cm polyethylene and 29 dyne/cm polypropylene, and therefore require special treatment. Proper treatment can increase the dyne level of some substrates, but only temporarily. When you are ready to print, there are other factors that affect the dyne level of the substrate, such as: the time and number of treatments, storage conditions, ambient humidity and dust levels. Since dyne levels can change over time, most printers feel it is necessary to treat or re-treat these films before printing.
13. How is flame treatment performed? Plastics are inherently non-porous and have an inert surface (low surface energy). Flame treatment is a method of pre-treating plastics to increase the dyne level of the substrate surface. In addition to the field of plastic bottle printing, this method is also widely used in the automotive and film processing industries. Flame treatment not only increases surface energy, but also eliminates surface contamination.Flame treatment involves a series of complex 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 under heat and play a cleaning role; and its chemical mechanism is that the flame contains a large number of ions, which have strong oxidizing properties. Under high temperature, it reacts with the surface of the treated object to form a layer of charged polar functional groups on the surface of the treated object, which increases its surface energy and thus increases its ability to absorb liquids.
14. What is corona treatment? Corona discharge is another way to increase the dyne level. By applying high voltage to the media roller, the surrounding air can be ionized. When the substrate passes through this ionized area, the molecular bonds on the surface of the material will break. This method is usually used in rotary printing of thin film materials.
15. How does plasticizer affect the adhesion of ink on PVC? Plasticizer is a chemical that makes printed materials softer and more flexible. It is widely used in PVC (polyvinyl chloride). The type and amount of plasticizer added to flexible PVC or other plastics mainly depends on people’s requirements for the mechanical, heat dissipation and electrical properties of the printed material. Plasticizers have the potential to migrate to the substrate surface and affect ink adhesion. Plasticizers that remain on the substrate surface are a contaminant that reduces the surface energy of the substrate. The more contaminants on the surface, the lower the surface energy and the less adhesion it will have to ink. To avoid this, one can clean the substrates with a mild cleaning solvent before printing to improve their printability.
16. How many lamps do I need for curing? Although the ink system and the type of substrate vary, in general, a single lamp curing system is sufficient. Of course, if you have enough budget, you can also choose a dual-lamp curing unit to increase the curing speed. The reason why two curing lamps are better than one is that the dual-lamp system can provide more energy to the substrate at the same conveyor speed and parameter settings. One of the key issues we need to consider is whether the curing unit can dry the ink printed at normal speed.
17. How does the viscosity of the ink affect 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; the higher the ambient temperature, the lower the annual viscosity of the ink. Screen printing inks generally achieve good results on the printing press, but occasionally there will be problems with printability depending on the printing press settings and pre-press adjustments. The viscosity of the ink on the printing press is also different from its viscosity in the ink cartridge. Ink manufacturers set a specific viscosity range for their products. For inks that are too thin or have too low viscosity, users can also add thickeners appropriately; for inks that are too thick or have too high viscosity, users can also add diluents. In addition, you can also contact the ink supplier for product information.
18. What factors affect the stability or shelf life of UV inks? An important factor affecting the stability of inks is the storage of the ink. UV inks are usually stored in plastic ink cartridges rather than metal ink cartridges because plastic containers have a certain degree of oxygen permeability, which can ensure that there is a certain air gap between the ink surface and the container cover. 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 container is also crucial to maintaining their stability. High temperatures can cause premature reactions and cross-linking of inks. Adjustments to the original ink formulation may also affect the shelf stability of the ink. Additives, especially catalysts and photoinitiators, may shorten the shelf life of the ink.
19. What is the difference between in-mold labeling (IML) and in-mold decoration (IMD)? In-mold labeling and in-mold decoration basically mean the same thing, that is, a label or decorative film (preformed or not) is placed in the mold and the molten plastic supports it while the part is formed. The labels used in the former are produced using different printing technologies, such as gravure, offset, flexographic or screen printing. These labels are usually printed only on the top surface of the material, while the unprinted side is connected to the injection mold. In-mold decoration is mostly used to produce durable parts and is usually printed on the second surface of a transparent film. In-mold decoration is generally printed using a screen printer, and the films and UV inks used must be compatible with the injection mold.
20. What happens if a nitrogen curing unit is used to cure colored UV inks? Curing systems that use nitrogen to cure printed products have been available for more than ten years. These systems are mainly used in the curing process of textiles and membrane switches. Nitrogen is used instead of oxygen because oxygen inhibits the curing of inks. However, since the light from the bulbs in these systems is very limited, they are not very effective in curing pigments or colored inks.
Post time: Oct-24-2024
