Ink is an important and indispensable material for printing. It is a stable and homogeneous mixture consisting of a pigment as a dispersed phase and a binder as a continuous phase. Among them, the pigment imparts the color of the ink, and the connecting material ensures the transferability, the transferability, and the drying property of the ink. According to the characteristics of the printing process, the ink can be divided into four kinds of lithographic printing inks, gravure printing inks, flexo printing inks and screen printing inks. In the printing process, in order to test the printability of the ink, some routine performance tests are required, and the detection items of different ink species are also slightly different.
Lithographic ink performance testing
The most commonly used offset printing technology is offset printing. Offset printing inks mainly detect its color properties (such as color shift and gloss, etc.), color strength, rheology (including viscosity, adhesiveness, thixotropy, dryness and fineness of ink), etc. Technical indicators. According to the ink industry's habits and calibration units for test instruments, the performance of inks described herein is measured in units of centigrams of grams.
1. Viscosity detection
Viscosity is the main indicator for detecting the rheological properties of an ink, and it indicates the magnitude of the frictional resistance when the fluid flows. In offset printing, the viscosity of the ink is related to the distribution and transfer of the ink. The ink viscosity is inversely proportional to the printing speed, the printing speed is low, the ink viscosity is large; the printing speed is high, and the ink viscosity must be reduced accordingly. Due to the different types of inks, the viscosity testing methods and the test instruments used are also different.
Test offset inks generally use "parallel plate viscosity meter", "Laray viscosity meter", "cone flat plate viscosity meter" and "rotation viscometer" and other methods.
(1) Parallel plate viscometer Parallel plate viscometer consists of two parallel glass plates. Put 0.5cm3 of ink in the concave hole in the middle of the lower glass plate, drop the upper parallel plate from a fixed height, press it on the ink, cut the ink, and deform it.
As the time increases, the diameter of the ink expanding outwards continuously increases, and finally the ink characteristic curve can be made according to the diameter of the ink expansion and the corresponding time. From the curve, parameters such as slope, intercept, and fluidity can be obtained, and the extended diameter of the ink at 60s on the viscometer is called the fluidity value.
1Linear slope sl: Indicates the ink drawability, ie the length of the wire, the greater the slope, the longer the ink head, the calculation method is as follows sl=d100-d10
In the formula: ink spreading diameter (cm) when d100--100 seconds;
D10--10 seconds ink spreading diameter (cm);
2 Interceptor l: indicates the body of the ink, that is, the degree of hardness and hardness of the ink. The smaller the interception, the harder the ink, the calculation method is as follows I=d10-sl
Using the parallel plate viscometer can also determine the ink yield value, apparent viscosity and plastic viscosity.
3 apparent viscosity: apparent viscosity is calculated using the following formula Î·a=Ï„/D
Where: Ï„-shear stress (dyn/cm2); D-shear rate (1/s); W-upper plate weight, 115g; g-980cm/s2; V-0.5cm3 ink volume; R- at t The spread of time within the time.
4 Yield: Offset inks are non-Newtonian fluids and must be given a certain amount of external force before they begin to flow. This fluid is also known as a plastic fluid. This force is called the yield value and can be calculated using the following formula: So=48wgv/Ï€2Rm5
Where: So-yield value (dyn/cm2); maximum radius (cm) at which Rm ink spreads at 30 minutes.
(2) Laray viscosity meter Laray viscosity meter is commonly used in ink quality management equipment, also known as drop stick viscometer. During the measurement, the ink is placed between the round bar and the short cylinder, and the ink is sheared by the drop of the metal round bar, and the time of free fall within a fixed distance is recorded, and the fall time is proportional to the viscosity. Add weights of different weights, and draw graphs based on different shear rates to determine viscosity and yield values. When the round bar is falling, the timer starts timing, and it stops at 10cm to stop counting. The value of a set of (p+w) and t is obtained. After adding weights of different weights on the round bar for testing, several sets of data can be obtained. . By substituting each group (p + w) and t into the following formula, the values â€‹â€‹of shear stress Ï„ and shear rate D can be calculated.
Where: weight of p-round bar, 130g; w-weight (g) on â€‹â€‹round bar; Q-round bar radius 0.6cm; g-gravity acceleration, 980cm/s2; b-round pipe radius 0.604cm; L-round tube length, 2.7 cm; L-round rod drop distance, 10 cm; t-round rod drop 10 cm time (s).
Using the above calculation results, the viscosity and yield values â€‹â€‹can be roughly measured by using graphing methods.
According to experience, when the ink viscosity is greater than 400P, the added weight is 500-1500g, and when the viscosity is less than 200P, the added weight is preferably 200-1000g.
(3) Cone-face plate viscometer Cone-face plate viscometer is a precision viscometer used to study the rheological properties of non-Newtonian fluids. The sample is placed between a fixed plate and a replaceable rotating cone rotor. The conical angle of the rotor is generally less than 40Â°. The ink to be tested is cut using the gap formed by the apex of the upper cone rotor contacting the lower plate. Under the influence of ink viscosity resistance, the torque is automatically recorded by the spring electronic test device connected to the cone rotor shaft and the viscosity of the ink can be calculated.
(4) Rotating viscometer Rotary viscometer is based on the principle of torque generated by the rotation of the rotor in the viscous system to measure the viscosity of the liquid. A typical rotational viscometer is two coaxial cylinders, one of which is stationary and the other is rotating at a constant speed so that the annular gap between the inner and outer cylinders is filled with the fluid being tested.
According to the viscosity of the ink, the proper rotor is selected during the test. After the preheating is started, a certain amount of ink is placed between the two cylinders according to the requirements of the instrument. During the test, the inner drum rotor rotates. Due to the influence of ink viscosity resistance, the spring deflection of the instrument can directly read the value on the dial. When rotating at different speeds, the resistance of the ink is not the same, and the value read on the dial is also different. Using this series of values, the rheological curve of the ink can be drawn.
2. Adhesion Test In the printing process, the ink is thinly separated between the rollers, the plate and the blanket, and the blanket and the substrate are frequently separated, transferred, continuously pressurized and forced to separate. When separated, the ink molecules will inevitably produce a force that resists the separation of the ink thin layer, also called ink adhesion. The adhesiveness of the ink is related to the thickness of the ink layer and the speed of the separation. The ink layer is thick and the viscosity is large; the speed is large, and the adhesiveness can be determined with the ink stickiness meter. The viscometer consists of two synthetic rubber rollers and a hollow metal roller (water circulation temperature can be adjusted).
During the measurement, the balance is adjusted by turning the power on without ink, and the ink sample is filled in the specified ink filling tube. The preheating is started to raise the temperature to 32Â°C. After shutting down, the ink in the ink injection tube is evenly added to the synthetic rubber roller, and the ink roller is closed and tested again. After 30 seconds of uniform ink, the L weighing rod is hung up, while the weighing rod is adjusted again with the weight on the L weighing rod within 30 seconds. Read the value on the rod, which is the tack value of the tested ink.
The ink stickiness meter can also allow the instrument to rotate again for 15 minutes on the basis of measuring the viscosity of the ink to measure the adhesion value. Subtracting the one minute adhesion value by the 15 minute adhesion value is the increase in adhesion. The smaller the added value, the better the ink's ink property.
3. Fineness detection The fineness of the ink is an important indicator of the quality of the print. The ink is a system in which fine particles are uniformly dispersed in the binder, and the degree of the thickness of these solid particles is called the fineness of the ink. The fineness of offset ink generally requires â‰¤ 15Î¼m; the fineness of high-grade fine ink requires â‰¤ 5Î¼m.
The fineness of the printing ink is generally measured by a scraper fineness meter. In the middle of the fineness meter, a deep to shallow groove is provided, and the deepest point is 50 Î¼m or 100 Î¼m, and the shallowest point is 0. Grooves are marked with a scale indicating the depth of the groove.
First take 0.5ml of the test ink and dilute it with No. 6 varnish. 18 drops (0.36 ml) with a fluidity of 24 mm or less; 14 drops (0.28 ml) with a fluidity of 25-35 mm; and 10 drops (0.20 ml) of a fluidity of 36-45 mm; fluidity Do not dilute above 46mm.
Pick the diluted ink with a dip knife and place it in the deepest part of the groove of the fineness meter. Place the squeegee vertically across the deepest part of the groove of the fineness meter. Hold the squeegee vertically and use it uniformly from top to bottom. Scratch to zero to stop. The fineness meter is inclined at a 30Â° angle to the light source, and the initial value of the dense particle spot is examined with a 5 to 10Ã— magnifying glass. More than 15 particles in the scale range are the upper scale marks, and no more than 15 particles are the lower scale marks.
Flexographic ink performance testing
Flexo printing ink is determined by the short ink path of its anilox roller. It is a fluid close to Newtonian fluid with good fluidity and a viscosity of only 0.1-1.0P. Flexo inks are generally volatile and dry. When paper or cardboard is used as a substrate, they are fixed by the action of permeation.
The viscosity of flexographic printing inks is mainly measured with a Ford cup viscometer. During the test, the small hole under the cuvette was blocked by hand, and the ink was poured. When there was no air bubble, the hand was let go and the ink flowed out. The stopwatch was used to measure the time. The viscosity of the ink is indicated by the time "s" used for the total flow of the fluid. The longer the flow time, the greater the viscosity.
The fineness of the flexographic ink requires â‰¤ 20 Î¼m. The color and coloring power were printed on the flexographic compliance tester and then tested with a densitometer and a colorimeter.
The flexographic printability tester consists of a doctor blade, an anilox roller, an ink supply device, and a printing device to test the printability of a flexographic printing ink. The printing pressure, the number of anilox roller screens, and the printing speed are adjustable, so flexographic inks of different viscosities can be detected. The pressure of the flexographic printability anilox roller is adjusted from 30 to 500N; the printing pressure is adjusted from 30 to 500N; the printing speed is adjusted from 0.3 to 1.5 m/s.
Gravure printing ink performance testing
Gravure inks have a low viscosity and are close to Newtonian fluids with good fluidity. Gravure inks are tested in the same way as flexo inks.
When using a flexographic printability tester to test gravure ink, adjust the adaptability meter to a gravure state. The method is as follows: At the same time when the power is turned on, â€œENTERâ€ is displayed. After the power is turned on, a menu is displayed. Press 1 to select â€œSettingsâ€, press 3 to confirm, and a menu appears. Select â€œGravure modeâ€. Select â€œ1â€ as gravure. Status, "0" is the flexo status; after selecting it, press 3 key to confirm, entering the printing state can be detected.
Source: Printing Industry Channel
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