Ink printing suitability testing

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 gives the ink color, and the linking material guarantees 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 luster, etc.), tinting 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 viscometer", "Laray viscometer", "cone flat plate viscometer" 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.

1 linear slope sl: indicates the ink stringiness, that is, the length of the silk head, the greater the slope, the longer the ink head, the calculation method is as follows: d100 - 100 seconds ink spread diameter (cm);
D10 - ink spreading diameter (cm) at 10 seconds;
2 Interceptor I: Indicates the body of the ink, ie, the degree of softness and hardness of the ink. The smaller the interception, the harder the ink. The calculation method is as follows. The parallel plate viscosity meter can also determine the yield value, apparent viscosity, and plastic viscosity of the ink.
3 apparent viscosity: apparent viscosity is calculated using the following formula: Ï„-shear stress (dyn/cm2); D-shear rate (1/s); W-upper board weight, 115g; g-980cm/ S2; V-0.5 cm3 ink volume; R- spread radius in time t.
4 Yield value: Offset inks are non-Newtonian fluids and must be given a certain amount of external force before starting to flow. This fluid is also called plastic fluid. This force is called the yield value and can be calculated by the following formula: S0 - Yield Value (dyn/cm2); Maximum radius (cm) at which Rm-ink spreads at 30 minutes.

(2) Laray viscometer

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 ​​of shear stress τ and shear rate D can be calculated.

Where: weight of p-round bar, 130g; w-weight (g) on ​​round bar; Q-round bar radius of 0.6cm; g-gravity acceleration, 980cm/s2; b-round pipe radius of 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 ​​can be roughly measured by using graphing methods. Figure 1 is a diagram of a Larley viscometer. The abscissa shows the shear stress and the ordinate shows the time. Each set of weights (p+w) added to the rods and rods falling time (t) obtained during the measurement can be found on the corresponding points on the drawing, and at least 4 sets of values ​​should be tested.

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 flatness viscometer

The cone-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 40°. 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) Rotation Viscosity Meter

The rotational viscometer is based on the principle of torque generated by the rotation of the rotor in a viscous system to measure the viscosity of a 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 testing

In the printing process, the ink is frequently separated and transferred between the ink rollers, the printing plate and the blanket, and the blanket and the substrate in a thin state, and is continuously pressurized and forcibly separated. When separated, the ink molecules will inevitably produce a force that resists the separation of the ink thin layer, also called ink adhesion. The adhesion of the ink is related to the thickness of the ink layer and the speed of separation. The ink layer is thick and the viscosity is large; the speed is large and the viscosity is greater.

Adhesion can be measured by the ink stickiness meter. Its structure and working principle are shown in Figure 2. The viscometer consists of two synthetic rubber rollers and a hollow metal roller that can adjust the temperature with water circulation. A is a metal roller, B is a synthetic rubber roller, rotating around the O2 axis; C is a bracket, O2 axis is mounted on the upper end of the bracket, there is a balance lever L in the middle of the bracket, and the weight is W in the lower part of the bracket. The whole bracket can wrap around O1. Shaft rotation.

Figure 2 (a) is the instrument static state; (b) is the idle state when the instrument is not inked; (c) is the instrument is returned to the static equilibrium state after adding a weight P; (d) is after adding the ink The operating state of the instrument; (e) Adding a weight Q to the long position of the lever L causes the instrument to return to the static equilibrium again. The adhesiveness of the ink can be read out from L at this time.

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 32°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 printing. 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 ≤ 15μm; the fineness of high-grade fine ink requires ≤ 5μm.

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 μm or 100 μm, 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 30° angle to the light source, and the initial value of the dense particle spot is examined with a 5 to 10× 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 time "s" used for the total outflow of the fluid indicates the viscosity of the ink. The longer the outflow time, the greater the viscosity.

The fineness of the flexographic ink requires ≤ 20 μm. The color and coloring power were printed on the flexographic compliance tester and then tested with a densitometer and a colorimeter. Flexographic fitness instrument structure shown in Figure 3.

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 ink properties