Testing tools and methods for the suitability of t

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Testing tools and methods for the suitability of printing ink

ink is an indispensable and important material for printing. It is a stable and uniform mixture composed of pigment as dispersed phase and binder as continuous phase. Among them, pigments give the ink color, while binders ensure the transferability, transmissibility and dryness of the ink. According to the characteristics of printing process, inks can be divided into lithographic printing ink, gravure printing ink, flexographic printing ink and plate printing ink. In the printing process, in order to test the printability of the ink, some conventional performance tests need to be carried out, and the test items of different inks are slightly different

performance test of lithographic ink

the most commonly used lithographic technology is offset printing. Offset printing ink mainly tests its color performance (such as color deviation and gloss), coloring power, rheology (including viscosity, adhesion, thixotropy, ink dryness and fineness) and other technical indicators. According to the habits of the ink industry and the calibration unit of the testing instrument, the ink performance described in this paper adopts the centimeter gram second unit system

1. viscosity detection

viscosity is the main index to detect the rheological properties of ink, which indicates the size of friction resistance when the fluid flows. In offset printing, the viscosity of ink is related to the distribution and transfer of ink. The ink viscosity is inversely proportional to the printing speed. The printing speed is low and the ink viscosity is high; The printing speed is high, and the ink viscosity should be reduced accordingly. Due to the different types of inks, the viscosity test methods and test instruments are also different

generally, "parallel plate viscometer", "lalay viscometer", "conical plate viscometer" and "rotary viscometer" are used to test offset printing inks

(1) parallel plate viscometer

the parallel plate viscometer is composed of two parallel glass plates. Put 0.5cm3 of ink into the small hole recessed in the middle of the lower glass plate, drop the upper parallel plate freely from a fixed height, press it on the ink, and cut the ink to deform it

with the increase of time, the outward expansion diameter of the ink continues to increase. Finally, the ink characteristic curve can be made according to the expanded diameter of the ink and the corresponding time. From the curve, parameters such as the slope of the straight line, the cut-off moment and the fluidity can be obtained, and the expanded diameter of the ink at 60s on the viscometer is called the fluidity value

① straight line slope SL: indicates the drawing property of the ink, that is, the length of the wire head. The larger the slope is, the longer the wire head of the ink is. The calculation method is as follows:


where: d100-100 seconds ink spreading diameter (CM)

d10--10 seconds ink spreading diameter (CM)

② cut-off moment l: indicates the body bone of the ink, that is, the hardness of the ink. The smaller the cut-off moment is, the harder the ink is. The calculation method is as follows

I = D10 SL

the yield value, apparent viscosity and plastic viscosity of the ink can also be measured by using a parallel plate viscometer

③ apparent viscosity: the apparent viscosity is calculated by the following formula

η a= τ/D

τ= 2wgv/π2R5,D=6πR2/V × 0.4343sl/t

where: τ- Shear stress (dyn/cm2); D-shear rate (1/s); W-weight of upper plate, 115g; g-980cm/s2; V-0.5cm3 ink volume; R - spreading radius in t time

④ yield value: offset printing ink is a non-Newtonian fluid. Before starting to flow, an external force of GB 8626 (8) of the flammability test method for building materials must be given. This fluid is also called plastic fluid, and this force is called yield value. The following formula can be used to calculate

so=48wgv/π 2rm5

where: so yield value (dyn/cm2); The maximum radius of RM ink spreading at 30 minutes (CM)

(2) lalay viscometer

lalay viscometer is a commonly used instrument in ink quality management, also known as falling rod viscometer. When measuring, put the ink between the round rod and the short cylinder, use the falling of the metal round rod to shear the ink, and record the free falling time within a fixed distance. The falling time is proportional to the viscosity. Add weights with different weights, draw curves according to different shear rate, and calculate viscosity and yield value. When the round bar falls, the timer starts timing, and stops timing when it falls to 10cm. A group of values of (p+w) and T are obtained. Several groups of data can be obtained by adding weights of different weights to the round bar for testing. The shear stress can be calculated by substituting each group (p+w) and T into the following formula τ And shear rate D

τ= (p+w) g/2 π QL (dyn/cm2)

d=l/(B-Q) t (s-1)

where: p-weight of round bar, 130g; W- weight added to the round bar (g); Q-round bar radius 0.6cm; G-gravitational acceleration, 980cm/s2; B-the radius of the round pipe is 0.604cm; L-length of round pipe, 2.7cm; L-falling distance of round bar, 10cm; T-Time (s) for the round bar to drop 10cm

using the above calculation results, the viscosity and yield value can be roughly measured by the mapping method

according to experience, when the ink viscosity is greater than 400p, the weight added is 500 ~ 1500g, and when the viscosity is less than 200P, the weight added is 200 ~ 1000g

(3) cone plate viscometer

cone operation solid 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 conical rotor. The conical angle of the rotor is generally less than 40 °. The tested ink is cut by using the gap formed by the contact between the vertex of the upper conical rotor and the lower plate. Affected by the viscosity resistance of the ink, the torque is automatically recorded by the spring electronic test device connected to the conical rotor shaft, and the viscosity value of the ink can be calculated

(4) rotary viscometer

rotary viscometer is based on the principle of torque generated by rotor rotation in viscous system to measure the viscosity of liquid. General rotary viscometers are two coaxial cylinders, one of which is stationary and the other rotates at a certain speed, so that the annular gap between the inner and outer cylinders is filled with the tested liquid

during the test, select the appropriate rotor according to the viscosity of the ink. After the startup and preheating, install a certain amount of ink between the two cylinders according to the requirements of the instrument. During the test, the inner cylinder rotor rotates. Due to the influence of ink viscosity resistance, the spring of the instrument deflects, and the value can be read directly on the dial. Rotating at different speeds makes the belt tighten, and the resistance of the ink is different. The values read on the dial are also different. Using this series of values, the rheological curve of the ink can be drawn

2. adhesion detection

in the printing process, the ink is frequently separated, transferred, continuously pressurized and forcibly separated in a thin layer between the ink rollers, between the printing plate and the blanket, and between the blanket and the substrate. When separating, the ink molecules will inevitably produce a force to resist the separation of thin layers of ink, also known as the adhesion of ink. The adhesion of ink is related to the thickness of ink layer and the speed of separation. If the ink layer is thick, the viscosity is larger; If the speed is high, the viscosity will be higher. The adhesion can be measured by ink viscosity meter. The viscosity meter is composed of two synthetic rubber rollers and a hollow metal roller (which can adjust the temperature by water circulation)

when measuring, turn on the machine to adjust the balance without inking, fill the ink sample in the specified inking tube, and preheat the machine to raise the temperature to 32 ℃. After the machine is shut down, add the ink in the inking tube evenly to the synthetic rubber roller, close the inking roller, and then test. After inking for 30 seconds, hang the L-scale bar, and adjust the balance of the scale bar again with the weight on the L-scale bar within 30 seconds. Read the value on the scale, that is, the adhesion value (tack value) of the measured ink

the ink viscosity meter can also measure the adhesion increment by rotating the instrument for another 15 minutes on the basis of measuring the ink viscosity value. Subtracting the adhesion value of 1 minute from the adhesion value of 15 minutes is the increment of adhesion. The more value-added, the better the inkiness of the ink

3.fineness detection

the fineness of ink is an important index affecting the printing quality. Ink is a system in which fine particles are evenly dispersed in binder. The thickness of these solid particles is called the fineness of ink. The fineness of offset printing ink is generally required to be ≤ 15 μ m; Fineness requirement of high-grade fine ink ≤ 5 μ m。

the fineness of printing ink is generally measured with a scraper fineness meter. There is a deep to shallow groove in the middle of the fineness meter, and the deepest is 50 μ M or 100 μ m. The shallowest part is 0. The groove edge is engraved with a scale indicating the depth of the groove

first take 0.5ml of the ink to be tested and dilute it with No. 6 inking oil. Add 18 drops (0.36ml) if the fluidity is less than 24mm; Add 14 drops (0.28ml) if the fluidity is between 25 ~ 35mm; Add 10 drops (0.20ml) if the fluidity is between 36 ~ 45mm; No dilution is needed when the fluidity is above 46mm

pick up the diluted ink with an inking knife, place it in the deepest part of the fineness meter groove, place the scraper vertically and horizontally in the deepest part of the fineness meter groove ink, hold the scraper vertically, and scrape it evenly from top to bottom until it stops at zero. The fineness meter is tilted to the light source at an angle of 30 °, and the initial value of particle concentration points is examined with a 5 ~ 10 times magnifying glass. More than 15 particles in the scale range are the upper scale line, and no more than 15 particles are the lower scale line

performance test of flexographic printing ink

flexographic printing ink is determined by the short ink path of its corrugated roller. It is a fluid close to Newtonian fluid, with good fluidity and viscosity of only 0.1 ~ 1.0p. Flexographic inks are generally volatile and dry. When paper or paperboard is used as the substrate, they are fixed by infiltration

the viscosity of flexographic printing ink is mainly measured by Ford cup viscometer. During the test, block the small hole under the small cup with your hand, fill it with ink, release your hand to let the ink flow out when there is no bubble, and count with a stopwatch. The time "s" used for all the fluids to flow out indicates the viscosity of the ink. The longer the flow time is, the greater the viscosity is

the fineness of flexographic ink is required to be ≤ 20 μ m。 The color and tinting force are tested by densitometer and colorimeter after printing the sample sheet with flexographic fitness instrument

flexographic printability tester is composed of scraper, embossing roller, ink supply device and printing device, which can test the printability of flexographic printing ink. The printing pressure, the number of lines added by the embossing roller and the printing speed are adjustable, so the flexographic inks with different viscosities can be detected. The pressure of the corrugation roller of the flexographic printing suitability instrument is adjusted between 30 ~ 500N; The printing pressure is adjusted between 30 ~ 500N for convenient storage and transportation; The printing speed range is adjusted between 0.3 ~ 1.5m/s

intaglio printing ink performance test

intaglio printing ink viscosity is very small, close to Newtonian fluid, and has good fluidity. The testing method of gravure printing ink is basically the same as that of flexo printing ink

when testing gravure ink with flexographic printing suitability instrument, adjust the suitability instrument to gravure printing state. The method is as follows: at the same time of startup, "enter", a menu is displayed after the power is turned on, press 1 key to select "Settings", press 3 key to confirm, a menu appears, and select "gravure mode" Select "1" as gravure printing status and "0" as flexographic printing status; After selection, press the 3 key to confirm and enter the printing state to detect

source: printing industry channel

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