Settlements for General Problems in Flexo Printing
If the color is too rich
Phenomenon: The printing matters' color is heavier than the sample.
Reasons: (1) High viscosity; (2) The screen density of anilox roll is less, ink is more; (3) High color content, high water ink color saturation degree; (4) Plate-making problems (nodes of screen); (5) Maladjustment to the selected printed matters; (6) The temperature of printing workshop is not constant, which influences the use of ink.
Settlements: (1) Rationally add diluting agent to reduce the viscosity. The quantity of diluting agent influences not only ink viscosity, but also the color density of printing matters; (2) Rationally replace or increase the screen density of anilox roll; (3) Add reducer; (4) According to different printed matters, enlarge or decrease the screen nodes scaling value; (5) The actual printed matters are different from the designed; The surface of printed matters without dropwell, are too smooth to absorb ink, causing thick ink layer left on the surface, so that it reflects much colors; (6) Keep constant temperature and humidity level, guarantee the viscosity.
Stripes and mottles
Phenomenon: there are some unexpected spots on material reel.
Reasons: (1) Parts of the anilox roll do not carry ink; (2) Printing plate is not plain or level resulting in uneven coating. (3) Bubbles appear during printing process causing that ink splashes on to the printed matter. (4) Ink is too thin; (5) Bad viscosity; (6) The printed matters are uneven; (7) There are something wrong with the stamping roll or the dirt sticks to it.
Settlements: (1) Clean the anilox roll or check the ink quantity; add enough if it is not; (2) Check printing plate to find out what causes uneven coating; (3) Add antifoamer; (4) check the cap of ink fountain, connections of ink tub, ink gutter to prevent ink from flowing or sinking; (5) Add new adjusted ink; (6) Promote viscosity, change into plain and balanced printed matter; (7) Check the stamping roll, replace and clean the dirt.
If ink is too weak or thin
Phenomenon: the color intensity of ink is too low, saturation degree and color density not enough.
Reasons: (1) too much use of menstruum or low viscosity affects the ability to carry color elements; (2) The anilox distributor roller has been worn or it is not clean but stuffed. (3) Because of unbalanced stir, sedimentation occurs; Coloring strength is not enough, paint discrete; (4) The softness or hardness of printing plate is not suitable; screen density of anilox distributor roll has been too high, lack of ink; (5) The adjustment of ink roller is not proper leading to too weak force of impression; (6) Inappropriately use rubber distributor roller during ink distribution process; (7) In water-based ink, volatile compounds are beyond the standard resulting in thin ink layer; (8) Poor printability of printed matters; (9) Too much reducer; (10) Too high viscosity leads to bad ink distribution.
Settlements: (1) Add new ink and adjust it at proper viscosity. (2) Use new anilox distributor roller instead of the old one; (3) Fully stir the ink took from bucket before put it into ink fountain; (4) Replace with a proper printing plate; (5) Reduce the screen density, readjust the ink roller to make more ink attach to anilox roller and promote the force of impression; (6) Replace the rubber distributor roller in accordance with the printed matter (make use of the low degree of hardness); (7) Volatile compounds are over 3%; (8) Select printed matters according to printing matters' requirements. Choose those papers with strong ink absorption ability. It's generally believed that ink layer's thickness is, in letterpress 1-2 ¦Ìm, flexo printing 2-5¦Ìm, offset press 10¦Ìm; (9) Readjust ink; (10) Adjust ink viscosity reasonably.
The outline is unclear in process
Phenomenon: the outline of pictures is unclear.
Reasons: (1) Over printing pressure; (2) Too low viscosity; (3) Bad quality of printing plate; (4) The major color's ink is mixed with other colors; (5) Water ink on the printing plate dries too quickly; (6) The speed of machine is too fast while temperature is too low.
Settlements: (1) Make use of appropriate pressure and double-faced adhesive tape; (2) Increase viscosity; (3) Remake printing plate; (4) Use new ink instead of the old one; (5) Readjust reasonably; (6) Keep constant temperature and humidity level, increase machine's speed.
The small letters are not clear
Phenomenon: the small letters and dots are unclear (vague).
Reasons: (1) The printing plate is too hard, its definition and planeness are no good; (2) Ink property is not exquisite; (3) Inappropriate menstruum formulation, too quick volatilization, unsteady ink distribution, slow machine speed; (4) Operations' problems (not enough or uneven pressure); (5) Attached too much ink, pictures and letters become thick and vague; (6) Rough surfaces of printed matters have strong ink absorption ability; (7) The degree of overprint's accuracy is not enough which makes bad accuracy of offset overprint; (8) Plates stick to each other with inaccuracy; the thickness of double-faced adhesive tape is uneven; bubbles occur sometime; (9) There is dry ink left in anilox roller's gutter; (10) High viscosity leads to uneven ink distribution (under constant temperature).
Settlements: (1) Remake printing plate to make ink distribution balanced; (2) Replace ink, and test it in fine scraping plate apparatus; (3) Readjust the menstruum formulation; (4) Pay attention to appropriate operation; (5) Readjust the ink quantity; (6) Replace printed matters, reduce ink spread as much as possible, improve the degree of surface roughness; (7) Check to make circle jump less than 0.015 mm, parallel horizontal less than 0.03mm, and multi-overprint accuracy is within 0.1 mm; (8) Check, and proofread. Replace sticky plate's tape, operate to stick plates according to regulation; (9) Clean; (10) Readjust reasonably to promote ink distribution.
If overprint is not clear in special gold and silver ink
Phenomenon: After printing in gold and silver ink, the spot-color ink overprint still let out the bottom.
Reasons: (1) Too thick gold ink layer is hard to dry so as to high viscosity; (2) High pressure results in squeezing ink; (3) Little quantity of ink distribution causes that ink layer is not thick enough; (4) The degree of fineness of gold ink is not enough; connective materials are no good; there are quality problems within ink; (5) Rough surface of paper; (6) Too long time between the ground color printing and gold and silver ink printing; (7) Much different surface tensions of two ink layers result that the above (gold ink) layer can not cover the bottom layer balanced.
Settlements: (1) add proper quick-dryer in gold ink and slow-dryer ink spot color ink; Keep proper viscosity as much as possible if it does not influence printing effect; (2) Readjust reasonably; (3) In order to intensify metallic feeling and make effect turn out to be more rich and bright, promote the viscosity some high when you print, but keep the viscosity similar to that of general water-based ink when you print screen notes or thin line; (4) Replace with into another ink provided by another supplier; (5) Treat it or replace it; (6) Time should not be long; Print when ground ink is not totally dry; (7) Process corona treatment to change surface tensions, promote ink's adhesion force and reduce the second color's viscosity; Hollow out the plate of spot color ink to intensify metal's adhesion force.
If pictures and letters are out of shape or incomplete
Phenomenon: Screen nodes and letters are incomplete.
Reasons: (1) The roller's roundness is not enough; (2) Dirty surface of roller is not smooth; (3) unbalanced pressure of both sides; (4) Printed matter is wrinkle or tension is improper, the degree of smoothness poor, fiber's order uneven; (5) Distributing roller turns inflexibly; (6) Too high viscosity, poor flowing proper; (7) Volatilization is too quick, distributor rollers absorb ink a little, ink supply is not enough, interruption causes printing shift incompletely; (8) Bad quality of printing plates (high ozone concentration; parts of plate not plain); (9) Unbalanced sticked plate makes pictures and letters out of shape after press printing; Printing plates become out of shape after use over and over again leading to pictures and letters out of shape too; (10) Improper selection of printing plate's diameter causes printing plate greatly out of shape.
Settlements: (1) Replace with qualified accurate rollers; (2) Clean; (3) Readjust pressure; The transform of pictures and letters will not be completed if pressure is too small; Screen nodes are not clear, color is gray and light which intensify printing plate and paper's shortcomings; High pressure will cause pictures and letters distorted; Compared with rub condition under normal pressure, high pressure enlarge the printing plate's abrasion, decrease its pressrun, promoting machine's load and electric power consumption, at the same time, it influences parts' life span; (4) Replace printed matter; (5) Readjust to make distributing rollers flexibly; (6) Readjust viscosity properly; (7) Readjust ink; (8) Distribute ink reasonably; (9) Strictly check planeness and degree of completeness of pictures and letters; To the products giving priority to pictures, original pattern should be concerned; check them from aspects of pictures, layers, colors, definition and stereoscopic effect; To the products giving priority to letters, check them from aspects of letters' correctness, regulations and ink's balance; Plate making man recount deformation rate, and re stick plates; (10) Under the correct and standard condition of printing plates, select standard and correct double-faced tape and diameter.
VIII. Water flower pattern
Phenomenon: Along the direction of paper skip, there are obvious ink distribution marks. Printing is not smooth.
Reasons: (1) Poor smoothness of printed matter which has paper grooves; (2) Low viscosity makes ink thin, masking power poor; (3) Bad flowability; (4) The making of screen rollers' surface holes has problems; (5) Too much quantity of ink results that paper can not distribute or absorb ink which sedimentates on the surface of printed matter; (6) Problems of printing plate making; (7) Too dirt the rollers are.
IX. If ink become dry on the printing plate
Phenomenon: (1) During the process of ink printing, pictures are vague, small letters incomplete, printing areas uneven and water-based ink become solid on the printing plates.
Reasons: (1) Too slow speed of machine; (2) No clean for the printing plates after machine stops, or no clean regularly or irregularly in the printing process; (3) Ink goes dry too quickly; (4) Volatilization of ink is over standard; PH value decreases.
Settlements: (1) Increase speed properly; (2) Once machine stops, clean printing plates without delay, or clean them regularly or irregularly in printing process; (3) Add proper slow-dryer; (4) Add proper stabilizer to adjust PH value.
Application of UV Ink in Flexo Printing
Ultraviolet Ink, short for UV ink, is a kind of ink which makes use of ultraviolet radiation with a certain wavelength, through instant photochemical reaction, to change liquid UV ink into solid ink.
In around 10 years' rapid development, UV ink has come into a maturation period and been recognized as nuisanceless ink variety. In The 7 th International RadTech Asia Conference held in Malaysia in August of last year, lots of information has demonstrated that currently radiation curing materials in the world (including EB curing and UV curing) are mainly applied to printing field. Main products cover: UV varnish, silk screen printing UV ink, offset printing UV ink, flexo printing UV ink, UV printing ink for metal and UV varnish for electronic circuit, etc.. According to documents' introduction, flexo printing UV ink consumption reached 3000 tons in the USA in 1997, and progressive increases at the rate of 9-12% annually.
In recent years, domestic UV printing materials progress rapidly. According to documents' introduction, UV printing material consumption exceeded 2000 tons domestically in 1998 (including home-made and imported materials). Among these, the majority are applied to UV varnishing, printing ink for circuit and silk screen printing UV ink. Besides, there are some offset printing UV ink and letterpress printing UV ink, etc.. However, UV flexo printing ink is used to the least extent, which is in initial starting period.
UV flexo printing ink has attracted much attention abroad. The following characteristics are the reasons:
UV flexo printing ink is safe and reliable. No menstruum release; noninflammable; no environment pollution; apply to package printed matters such as food, drink, tobacco and medicine, and so on. UV flexo printing ink has been used in food package field for many years in foreign countries and there are no any problems.
UV flexo printing ink features sound printability and high printing quality. In printing process, physical property will not change, menstruum will not release, viscosity keeps stable, and it's hard to paste or pile the plates. It can print in high viscosity so as to strong inking force. It also features high screen nodes' definition, sound tone reproduction, bright ink color, good adhesion force. These features apply to fine product printing.
UV flexo printing ink goes dry instantly. It boasts high production rate, large sphere of application, and sound adhesion force to different printed matters such as paper, aluminum foil, plastics. Conglutination won't occur if products are folded at once after being printed.
UV flexo printing ink boasts sound physical and chemical performance. UV curing process is UV ink photochemical reaction, namely, a process from linear organization to network structure, which brings about many physical performances better than other types of ink such as water resistance, alcohol resistance, liquor resistance, rub resistance and age resistance.
UV flexo printing ink saves quantity. Because of no menstruum volatilization, with high effective constituent, nearly 100% shifts into ink film. Its used quantity is less than half of water ink or menstruum ink, decreasing clean times of printing plate and anilox roller, as well as the comprehensive cost. Concerning the above features, UV flexo printing ink has obvious advantages and prospects no matter concerning from environment protection, quality aspects or technical development.
There is still a long process for the popularization and application of UV flexo printing technology. On the one hand, people should make further recognition and comprehension, on the other hand, installations, light source, assorted equipments and technical skill need to be improved and perfected. Concluding international information, we will talk about the following issues which should be concerned in UV flexo printing technical application.
UV light source: Each color class of UV flexo printing need ultraviolet source. UV flexo printing inks with different colors have different ability of ultraviolet absorption. For examples: Yellow and fuchsine UV inks, with relatively low absorptive ability, are easy to cure and dry. While blue and black inks, having strong absorptive ability, are hard to dry. You may distribute UV light's power according to different colors to safeguard the dry of each color's UV ink rationally, save energy as well. Pay attention to UV lamp's age, and replace UV lighting tube to prevent halfway cure and adhesion. Generally, UV lighting tube's lifetime is 1000 hours. To frequently light off UV lamp will short its lifetime.
Anilox roller: It's better to set mesh angle of anilox distributor roller at 30 o or 60 o when you use UV flexo printing ink. The followings are several popular standard ceramic anilox rollers:
200 lines 9.5-11.0 BCM anilox roller: available to print opaque actual field map.
360 lines 4.5-6.5 BCM anilox roller: available to print actual field map and contour map.
550 lines 3.0 BCM anilox roller: available to print continuous-tone image products.
650 lines 2.5 BCM anilox roller: available to print continuous-tone image products.
800 lines 2.0 BCM anilox roller: available to print fine continuous-tone image products.
Rubber roller: Rubber roller must be able to prevent ultraviolet ink. The rubber rollers we generally use are made from EPDM rubber (Ethylene-Propylene-Diene Monomer) or silicon rubber.
Printing plate material: Though flexo photographic printing plate materials used currently are able to basically apply to UV flexo printing ink, you'd better to select printing materials with sound UV ink resistance, such as the Dupont Company's UVP-67 open type printing plate material.
Ink viscosity: Viscosity used in UV flexo printing is a little higher than in water-based ink and solvent based ink, and the viscosity's range in UV flexo printing is relatively bigger, which is between 200-1000 CPS. Whereas, general viscosity is between 300-500 CPS.
Savage: Keep out of light and hot source strictly. Storage temperature should be controlled at 5 o C-30 o C. Half year storage life.
Related additives: There are three common kinds of additives: 1. UV reactive diluent; 2. UV cure accelerator; 3. UV printing ink cleaner.
Safety protection: Two major points at present time: First, ultraviolet radiation light source is harmful to human body; Second, UV ink is harmful to human body, too. For the former, user needs to make use of lightproof material to shield and seal the light source, as well as to intensify ventilation. For the latter, user needs to ware gloves to prevent UV ink and skin from direct contacts. If UV ink spatters on one's skin, wipe up without water and then clean with soap and water.
UV flexo printing ink and its technical skill are in continuous improving process. New types and facilities appear unceasingly. All of these will promote UV flexo printing techniques to be more perfect.
There is a steady increase of flexo press to equip UV ink curing system in foreign countries. At the same time, flexo press is refitting UV ink curing system. Replacing water-based ink and solvent based ink as a development tendency, UV flexo printing ink is widely applied to wide and narrow flexo press. Without any doubts, there is a bright developmental perspective. The spread and application of UV flexo printing technique should be in accordance with our national condition and products' needs, and need the improvement of domestic facilities and equipments. With more and more strict requirements of environmental protection and quality, we are deeply convinced that in no distant future, there will be large development for UV flexo printing techniques in China.
(Extract from http://www.china-ink.net )
Attention to Some Problems When You Purchase and Use Flexo Printing UV Ink
(1) During purchases of UV flexo printing ink, you should be clear with the type of printed matter. Then, according to printed matter, select different flexo printing UV inks with different performances. For example, UV ink used to print paper should not be applied to print PE film.
(2) UV ink should be kept in the condition of ventilation and kept out of light. Better to store at constant temperature 16-28 o C. The light in workshop, especially the light around ink gutter, should not be too strong, so as to prevent UV ink from thickening and curing.
(3)During UV ink curing process, warm up the mercury lamp for several minutes after lighting off to completely gasify the mercury inside the lamp, so as to complete spectral output and prevent presswork from adhesion caused by bad UV ink cure.
(4) Before formal printing, let machines run idle for 5-10 minutes so that UV ink, anilox roller and printing plate become fully compatible, and UV ink's transferability gets promotion.
(5) Operator should wear gloves while operating to prevent UV ink and skin from direct contacts. If UV ink spatters on one's skin, wipe up without water and then clean with soap and water.