Due to the unprecedented fierce competition in the market, many manufacturers in order to save costs and generate profits, as much as possible the use of lower prices of varnish, will often result in printed products appear sticky, uneven coating, orange peel, appear needle Holes, pits, poor adhesion, hair growth, poor gloss, insufficient brightness, etc.
The quality of printed products depends largely on the varnish, varnish drying, and production process. Only the actual conditions are flexible and good for these three points. Together with good glazing equipment, the quality control Great help.
Varnish
UV Varnish
UV varnish is mainly composed of oligomer, reactive diluent, photoinitiator and other additives
one. Oligomers
Oligomers, also known as prepolymers, are the most basic components in UV coatings. It is a film-forming substance whose properties play an important role in the curing process and the properties of the cured film. Structurally, the oligomers are all low-molecular resins containing C=C unsaturated double bonds, mostly acrylic resins.
Two: reactive diluent
Also called cross-linking monomer, it is a functional monomer. Its role in polishing oils is to adjust viscosity, cure speed, and cured film properties.
Three: Photoinitiator
Photoinitiators are substances that absorb radiant energy and produce chemical intermediates that have the ability to initiate polymerization. They are the main components required for any UV curing system.
Four: Additives
Used to improve the performance of the ink. Auxiliaries commonly used in UV varnish are stabilizers / used to reduce thermal polymerization during storage, improve the stability of varnish storage, leveling agent / used to improve the leveling of the varnish film surface, prevent the occurrence of shrinkage cavity , but also increased the gloss of the coating coating
Antifoaming agent / used to prevent and eliminate the bubbles produced during the manufacture and use of coating
Waterborne coating
Waterborne varnish is composed of synthetic resin and water 45% synthetic resin and 55% water
One: The main agent, the film-forming resin, and the film-forming substance of the polishing agent are usually synthetic resins that affect the quality of drying, adhesion, glossiness, and other glazing.
Two: adjuvants:
1: Curing agent, improving the film forming property of the aqueous main agent, and enhancing the poly-strength of the film forming agent
2: Surfactant, lower surface tension, improve leveling
3: Defoamer, control foaming, needle eye, orange peel, pitted
4: Desiccant, increase the drying speed of glazing, improve the adaptability of printing products
5: Adhesion promoter to improve the adhesion of varnish prints
6: Wetting and dispersing agent to prevent sticking and improve wear resistance
7: Plasticizers to improve flex resistance
III: Solvent: Disperse and dissolve synthetic resin, auxiliaries, the main component is water (can add a certain amount of ethanol, improve drying performance)
Varnish viscosity:
UV Varnish is generally adjusted according to the speed of the polishing machine
60-200 meters/minute control in no more than 28 seconds
20-50 meters/minute control in 80 seconds, water-based varnishes with offset press, 30-60 seconds, when the polishing machine 20-50 seconds
dry
UV Drying In general, the main supporting UV drying, infrared drying, cooling the exhaust water-based Varnish drying mainly infrared, hot air drying UV drying affect the performance of UV curing lamps, there are several factors: UV spectral distribution, radiation, radiation And infrared radiation.
Most UV rays contain two UV wavelengths, both of which work simultaneously. Short waves work on the surface, and longer waves act on the deep layers of the ink. This is because short-wave energy is absorbed by the surface and cannot enter deep layers. Short-wavelength underexposure can cause the surface to become sticky, while insufficient long-wave energy can cause adhesion difficulties.
The efficiency of a UV lamp depends on how easy it is to launch photons into the curable material to initiate light triggering molecules. UV curing is determined by photon-molecule collisions. Light can trigger molecular interaction. In addition, the cured film has optical and thermodynamic properties. They interact with radiant energy and have a major impact on the curing process.
Spectral absorption energy is the absorption of a substance into a wavelength within a gradually increasing thickness. The more energy absorbed near the surface, the less energy is obtained in the deep layer. However, this situation varies with the wavelength. The total spectral absorbance includes all effects from light triggers, single molecules, oligomers, and additives including pigments.
The following should also be noted in actual production:
UV light oil viscosity is too small, the coating is too thin; Ethanol and other non-reactive solvent dilution excess; UV oil coating is not uniform; Paper absorption is too strong; Will cause poor luster, brightness is not enough according to the different conditions and according to the paper The speed of the polishing equipment is appropriately increased to increase the viscosity and coating amount of the UV varnish. For the paper with strong permeability and absorbency, a primer layer can be applied first. It can also add certain short-wave tubes to increase the leveling UV lamp aging, weakening the intensity; UV light oil storage time is too long; non-reactive thinner add too much; machine speed is too fast. It will cause poor drying, incomplete photocuring and sticky surface. The curing speed requirement is less than O. In the case of 5S, it must be ensured that the power of the high pressure mercury lamp is generally not less than 12O W/cm, the lamp should be updated in time, and if necessary, a certain amount of UV varnish accelerator can be added to accelerate drying.
UV light oil part is small, coating is too thin; Ink contains too much ink or dry oil; Ink surface has crystallized; Ink surface anti-adhesive material (silicone oil, dusting) will cause print surface UV light oil coating No, the flower coating is too thin; non-reactive diluents (such as ethanol) add too much; the surface of the printed matter is more dusty. It will cause white spots and pinholes in the UV coating. Care should be taken to ensure that the production environment and the printed surface should be kept clean, and that the thickness of the coating be increased. A small amount of smooth additives can also be added. The dilution is preferably a reactive diluent that participates in the reaction.
UV light oil viscosity is too high; pressure is not uniform; coating volume is too large; UV light oil leveling is poor. May cause uneven surface coating, streaks, and orange peel
In addition, in actual production, we must also pay attention to the maintenance of the temperature of the polishing production at about 20 °C, the characteristics of the paper, the heat resistance and alcohol resistance of the printing ink, the resistance to alcohols, ester solvents, and good acid and alkali resistance. These have a great influence. Polishing quality.
Reproduced from: Wins