Selection of Drum Screen Printing Ink in Different Environments

At present, the screen printing technology used by domestic barrel makers has become more and more extensive, but due to the printing aspect, the barrel makers should be said to be laymen. Therefore, quality problems, process problems, and efficiency problems often occur in actual production. Especially in winter, when the temperature of the environment is low, many steel drum manufacturers in the North will have more or less problems in quality when they screen-print graphics on the surface of steel drums, such as slow drying of ink and poor adhesion. Brushed, cracked, and umbilical. These phenomena directly affect the sales of steel drums and the economic benefits of enterprises. At the same time, it also caused many bad influences for the company. What are the causes and influencing factors of these phenomena? How can we prevent it in production? To answer these questions, we must first understand the structure and properties of drum screen printing inks, so as to understand how to choose. Therefore, we first discuss the performance of drum screen printing inks.

First, the performance of steel screen printing ink

To make the drum screen printing ink have good performance and quality, regardless of the type of ink used by the drum manufacturer in the screen printing, the ink must meet the conditions that the color cannot be easily wiped or stripped. The request. The main properties of the ink for screen printing on steel drums are represented by the following parameters. Such as viscosity, thixotropy, fluidity, plasticity, viscoelasticity, dryness, hardness, light resistance, chemical resistance, adhesive strength.

1, the viscosity of the ink

Viscosity is the resistance produced when a layer of fluid moves relative to another fluid, ie, the internal resistance to fluid flow. The ink viscosity is generally expressed in terms of poise and centipoise. The screen printing ink has a viscosity of about 4-12 Pa·s. The viscosity of ink is closely related to printing. The faster the printing speed is, the lower the viscosity of the printing ink is. According to the actual production experience, in the winter, when the steel drum is screen printed outdoors, the viscosity of the printing ink should be correspondingly lower; the viscosity index of the ink can be used with the viscosity. Calculate to measure. Excessive viscosity of the ink on the wettability of the drum is poor, when the ink is not easy to transfer to the drum through the screen. Difficult to print, the printed image after the lack of ink; viscosity is too small, the print will cause the print to expand, resulting in overlapping lines of printed graphics, resulting in waste.

There are several factors that affect the viscosity of drum screen printing inks:

1 Currently, steel screen printing inks are mostly monochromatic. For such inks using the same pigment, the greater the viscosity of the binder, the greater the viscosity of the resulting ink.

2 For the same binder, the greater the amount of pigment and filler, the greater the viscosity of the resulting ink;

3 For the same binder, if the pigment and filler are used in the same amount in both inks, the larger the particles of the pigment and filler, the smaller the viscosity of the produced ink and the smaller the viscosity of the particles;

4 The better the pigments and fillers are dispersed in the same ink binder, the lower the viscosity of the ink; conversely, the viscosity of the ink is high.

2. Thixotropy of ink

Thixotropy refers to the ability of a liquid to recover its original viscosity due to a decrease in the stress viscosity. In the screen printing process, the ink is thickened after standing for a certain period of time, the viscosity becomes large, and the ink becomes thin after stirring, and the viscosity also becomes a reversible phenomenon. Because the shape of the pigment particles in the ink is irregular, it is an irregular sphere even though it adsorbs a layer of binder. Therefore, after a certain time of standing still, the pigment particles will come into contact with each other or come close to each other, causing mutual attraction, hindering the free movement of the particles, and the ink will become thick and sticky. However, this temporarily stable structure, after being stirred by an external force, is quickly destroyed, releasing the mutual attraction between the particles, recovering the free movement of the particles, improving the fluidity, thinning the ink, and lowering the viscosity.

The thixotropy of the drum screen printing ink should be as small as possible. In order to eliminate this unfavorable factor, the ink must be fully agitated before printing to return it to its normal state and then printed. The more irregular the pigment particles in the ink, the greater the thixotropy, and conversely, the less thixotropy. Such as black ink, thixotropy is large; while the white ink and yellow ink is less thixotropic. In addition, there are more binders in the ink, less pigment, and less thixotropy, and vice versa.

3, the ink flow

The degree of fluidity of the drum screen printing ink is the reciprocal of the viscosity. That is, the viscosity is large, the fluidity is small, the viscosity is small, and the fluidity is large.

The degree of fluidity of the drum screen printing ink can be regarded as the degree of natural flow of a certain amount of ink in a certain period of time and a certain leveling surface without external force. The fluidity of the ink can measure the thickening of the ink. In the drum screen printing ink, its fluidity is generally controlled at 30~50mm. The measurement method is to take one milliliter of ink and measure the diameter after 50 minutes at a pressure of 50 grams.

Practice has proved that when the steel drum screen printing, the greater the degree of ink flow, the greater the degree of printing ink after printing, so that the small gaps of the thin lines are not clear so that the merger; ink flow is small, after printing There are broken lines and lack of ink in the graphic ink, printing is also difficult.

4, ink plasticity

Plasticity refers to the property of deforming completely or partially after being deformed by an external force. Screen printing ink has both fluidity and plasticity. High pigment and other solids content, the ink appears thick, plasticity is large; pigments and other solids are low, the ink is thin, and the plasticity is small. Screen printing ink requires a certain degree of plasticity in order to maintain the printing accuracy.

5, the viscoelasticity of ink

The viscoelastic property of the drum screen printing ink is the property that the ink is sheared and fractured after being pressured by the squeegee, and the screen plate rebounds and the ink quickly rebounds. The sticking of ink and steel drums and the detachment of wire meshes have caused rapid retraction, which is a typical phenomenon of ink viscoelasticity.

The viscoelasticity of the ink, which has a large influence on screen printing, is the phenomenon of drawing. The drawing phenomenon is that when the squeegee is scraped and the screen is bouncing, many ink filaments appear between the screen and the steel drum. This is the most bogey phenomenon in the drum screen printing. It is not only easy to make the steel drum and screen plate dirty, and even make the drum screen printing impossible.

The length of the drawing is related to the viscosity of the ink. The viscosity is large and the ink is long; otherwise it is short. For this purpose, the viscosity of the ink is reduced by increasing or decreasing the viscosity of the adhesive or reducing the molecular weight of the resin, thereby improving the viscoelasticity of the ink and reducing the phenomenon of drawing.

The drawing phenomenon is also related to the time of action. In the same ink, if the separation speed is rapidly split by elasticity, the ink is short; the separation speed is very slow, and the ink flows completely like a pure liquid and does not become filamentous.

6, the drying of ink

Barrel screen printing requires that the ink does not dry the conjunctiva for a long time on the screen plate, and that after printing, the drying on the steel tub should be as fast as possible. In this way, the printed images can be kept clean and clear. Not only can the printing quality of steel drums be guaranteed, but they can also meet the requirements for timely delivery.

7, the hardness of the ink

The hardness of the drum screen printing ink coating refers to the degree of deformation of the ink film, which can be divided into penetration hardness and scratch resistance hardness. From the actual application of the steel drum, the scratch resistance is measured. If the same force is used, the fully dried ink film is marked with a pencil of different hardness, and the H value of the pencil that can break the ink film is the hardness of the measured ink. . The hardness of the ink during drum printing is an important parameter for drum screen printing.

8, the lightfastness of ink

The light resistance of the drum screen printing ink is the strength of the ink resistant to light. Ink fading is caused by the absorption of light energy by pigment molecules. The lightfastness of inks depends mainly on the lightfastness of the pigments or dyes. The lightfastness of the graphics printed on the drum surface depends on the lightfastness of the pigments, the material of the drum, the thickness of the ink layer, the degree of adhesion of the binder and the ink.

9, the chemical resistance of ink

The ability of steel drum screen printing inks to resist water, acids, alkalis and solvents is collectively referred to as chemical resistance. The measurement standards and methods are based on the actual application. At present, there are commonly used soaking methods and wet rubbing methods. The soaking method is to immerse the dried steel drums in the selected water, acid, alkali and solvent, and remove them after a certain period of time, and compare the degree of change before and after soaking in the steel drum printing graphics; the wet wiping method is a cloth. After soaking in the selected solution, wet and dry several times on the dried steel drum after printing, and then compare the difference between the wet and the rubbing of the steel drum after printing.

Second, steel drum screen printing ink adhesion

Drum screen printing requires the ink to have good adhesion, and the problem of adhesion of the screen printing ink is also the most prominent problem of the ink after the drum screen printing. If there is no adhesion of the ink on the steel drum after the printing of the steel drum, it means that there is no printing, and it also causes great waste. The problem of ink adhesion involves the adhesion mechanism of ink and steel drums. At present, it is generally believed that the following factors affect ink adhesion:

1, the impact of resin

Resin constitutes the main component of the ink binder. The adhesion of the ink is closely related to the resin properties.

1 Molecular weight. If the molecular weight of the resin is too close to the molecular weight of the drum coating, the ink solvent is not easy to choose. If the molecular weight of the resin is too large, in order for the ink to reach the required viscosity for screen printing, the solid (resin) content in the ink is too little, resulting in a waste of solvent. It has a high molecular weight, a high melting point, and is difficult to dissolve in solvents, which is detrimental to the adhesion effect. If the molecular weight of the resin is too small, the cohesive force and compatibility are poor, and the adhesion effect is poor. The choice of the molecular weight of the ink distribution resin is very important and must be appropriate.

2 Glass transition temperature. The polymer segment movement only has a significant diffusion rate above the glass transition temperature. The whole chain movement can only be carried out at high temperatures. The glass transition temperature of the resin for room-temperature curing inks should be very low, which is obviously advantageous for adhesion.

3 resin components. The functional groups and branching structures in the resin molecules directly affect the adhesion of the ink on the steel drum. Therefore, the printing inks used for the steel drums with different coatings in the steel drum screen printing are also different. For example, the coating on the surface of steel drums is coated with amino baking varnish and galvanized coating, chrome-plated coating and tin-plated coating.

4 Elemental content of resin monomer. Changes in the content of a certain element of the synthetic resin monomer, so that the performance of the resin will also change. Therefore, when designing a drum screen printing ink formula, synthesizing some special resins will greatly improve the ink performance. Changes in the content of a certain element of the resin monomer will lead to a prominent change in a certain characteristic of the resin.

5 The polarity and crystallinity of the resin. The presence of polarity facilitates the adhesion of the ink, but the highly polar linear polymer also has high crystallinity, and copolymerization and grafting can be used to modify the resin to destroy the crystallinity and maintain polarity.

Practice has proved that epoxy resin can produce chemical bonds on steel drum surface coatings. Therefore, drum screen printing inks are often formulated with epoxy resin in order to achieve the desired adhesion effect.

2, the influence of the solvent

The ink formulation solvent also affects its ability to adhere. As we all know, the solubility parameters of the resin and the solvent used are similar, and the solubility parameters of the steel drum surface coatings must be considered at the same time.

1 The solubility parameters of resin and solvent are similar, which is conducive to dissolution.

2 Excessive use of high-boiling solvents can reduce the adhesion of the ink.

3 The improper choice of solvent, ink adhesion is poor. The use of solvents must be considered from a variety of factors, so ink solvents often use more than three mixed solvents.

4 The solvent and the binder (high molecular polymer) in the ink are in contact with the coating material on the surface of the steel drum to swell and dissolve the coating material on the surface of the steel drum, so that the binder in the coating material and ink of the steel drum surface are mutually dispersed. The formation of a layer of mixed, unbounded intermediate layers leads to the disappearance of the interface and the formation of a strong bond between the ink and the surface coating of the drum.

The diffusion process is the process of the ink solvent dissolving the paint coating on the surface of the drum, and the dissolution of the paint coating on the steel drum depends on the polarity of the binder and the solvent in the ink. Only when the polarity of the ink and steel drum surface coatings are similar, can they be dissolved and have adhesive fastness.

The binder in the ink is generally a molecular polymer with a linear structure, and the molecular ends of the molecule are easily diffused into the paint coating on the surface of the drum, and the short side chain is disadvantageous