Viscosity, also known as internal friction, is a layer of fluid to another layer of fluid relative to the movement when the resistance. It is a characteristic of the fluid which obstructs its flow. Ink viscosity is generally "park", "centipoise" to represent. Screen printing inks have a viscosity of between about 4,000 to 12,000 centipoise.
Viscosity is too large Ink on the substrate wettability is poor, difficult to transfer to the substrate through the silk screen. Resulting in printing difficulties, imprinting lack of ink.
Viscosity is too small, will cause the imprint to expand, resulting in the merger of printed lines, as waste.
The viscosity index can be measured using a viscometer.
Viscosity change and printing of the relationship is: ink in the screen, the viscosity of the more stable the better, but transferred to the printed piece, the viscosity becomes larger the sooner the better. Thixotropy of the former is not conducive to the latter, so the appropriate thixotropy is desirable, and the shear tone on the printing harmful.
Add solvent, diluent or plasticizer, can reduce the viscosity; plus fillers, pigments, silicides, can increase the viscosity.
Thixotropy is the ability of a liquid to regain its original viscosity due to a decrease in stress viscosity. In the silk screen process, the performance of the ink after a certain time in the static thickening, viscosity becomes larger, stirring and then thinning, viscosity is also a small reversible phenomenon. Because the ink particles in the shape of the pigment is irregular, despite the adsorption of a layer of binder, is also an irregular ball. Therefore, after a certain time at rest, the pigment particles will contact or close to each other, resulting in mutual attraction, hinder the free movement of particles, ink thickening, sticky. However, this kind of temporarily stable structure, after being stirred by external force, is quickly destroyed, lifting the mutual attraction between the particles, the free movement of particles has been restored, mobility improved, ink thinning, viscosity decreased. Screen printing ink thixotropy smaller the better. In order to eliminate this disadvantage, before printing, to fully stir the ink to make it back to normal, and then printing.
The more irregular the pigment particles in the ink, the more polygonal, such as black ink, the thixotropy is large. On the contrary, such as yellow ink, its thixotropy is small. Ink in the link material, less pigment, thixotropy is small, otherwise thixotropy. In addition to the different materials on the impact of thixotropy is also great, such as polymerization of vegetable oil produced by the ink, the thixotropy small, such as polymer resin as a link material, thixotropy.
3. Yield value
Yield value refers to the fluid plus a certain external force, from elastic deformation to flow deformation of the boundary stress, is also the ink to start laminar flow must be applied to the minimum stress. Yield value is too large, the ink is hard, not easy to open, inconvenience ink, leveling poor; yield value is too small, print thin lines and poor dot reproduction. Silkscreen ink layer thick, so the yield value can not be too small. Screen printing ink according to different requirements, the yield value can be 1000dyn / cm2 to 3000dyn / cm2, fine lines when printing, the yield value should take high value.
Fluidity is the reciprocal of viscosity. Namely: viscosity, fluidity small; viscosity is small, fluidity is large.
Ink flow can be seen as no external force, a certain amount of ink in a certain period of time and a certain degree of smooth surface of the natural flow. Ink flow can measure the thin thick ink. In the silk screen printing ink its fluidity generally controls in 30 ~ 50mm. Measurement is to take one milliliter of ink, at 250 grams of pressure after 15 minutes, measure its diameter can be.
Ink flow is large, easy to expand the imprint, so that the small gap between the thin line so that the merger is not clear; mobility is small, easy to break the lines in the ink line, printing is also difficult.
Plasticity refers to the deformation by external force, can completely or partially maintain the nature of its deformation.
Silk screen printing ink is between the fluid and semi-solid thick suspended colloid, so it is both fluidity, but also plasticity. Pigment and other solid content is low, the ink and thin, plasticity is small, silk screen printing ink requires a certain degree of plasticity, in order to maintain the accuracy of printing, or printed lines of the pole Easy to expand.
6. Surface Tension.
The surface tension of the ink is related to the transferability of the ink and the stability of the imprint on the substrate. For example, when the surface tension of the ink is greater than the surface tension of the printing surface is more, the print will shrink, Surface tension, the printing will appear blocking phenomenon.
Fineness is an indication of the size of the pigment and other solid raw material particles in the ink and the uniformity of the distribution of these particles in the binder.
Ink fineness and the fineness of the solid powder is not the same. The latter refers to the basic particle size of the powder, which is very small, in units of micrometers, and in fact the pigment is added to the ink binder in an aggregated state in which a plurality of primary particles are adhered together. After grinding, the aggregates are dispersed, the better the dispersion, the smaller the ink particles (fineness). Fineness will affect the gloss of ink and ink leveling.
Viscoelastic refers to the ink by Lee board pressure after shearing fracture, screen version of the pop-up, ink rebound performance. Ink and substrate bonding, and screen from, there is the phenomenon of rapid retraction, is a typical ink viscoelastic phenomenon.
The viscoelasticity of the ink, the greater impact on the silk screen is the phenomenon of drawing. Drawing phenomenon is when the scraping scraping the board, the screen version of the pop-up moments in the screen and the substrate between a lot of ink filaments. This is a screen printing in the most sensitive to the phenomenon, not only easy to print and screen sticky dirty, and even make printing impossible.
Drawing length and the viscosity of the ink. Viscosity, ink silk length; otherwise short. To this end often reduce viscosity or reduce the molecular weight of the resin to reduce ink viscosity, improve ink viscoelasticity, reduce the drawing phenomenon.
Drawing phenomenon also with the force of time. The same ink, if the separation rate was quickly split elastic, then the ink is short; separation speed is very slow, the ink as pure liquid, like the full shed, not silk.
Screen printing requires both ink on the screen can not dry the conjunctiva for a long time, but also in the printing, the substrate drying in the sooner the better. This will keep the seal clean, save space, speed up the printing speed, improve quality.
Silkscreen on ink drying requirements, in the multi-color continuous overprint is particularly prominent. Drying methods commonly used are natural drying, hot air drying, ultraviolet drying, electron beam irradiation drying, infrared drying, microwave drying and other forms.
To measure the degree of drying of the ink, a simple method can be used. That is to produce a wide 37.5px, 100px long, good image of the screen printing plate, the ink printed on the substrate. To observe the ink, non-stick paper as a standard, calculated from how long after the non-sticky printing, so that the ink can be measured in such substrates on the drying time.
For screen printing, the online slow dry and fast-drying ink, is the ideal screen printing ink, light solid, heat-curing and hot-stamping, such as cold-formed ink is generated.
Refers to the degree of deformation of ink compression, paint and coatings industry can be used to measure the hardness. From the printed application, the hardness can also be used scratch-resistant measurement, such as with the same force, with a different hardness (H) of the pencil to fully dry (48 hours after printing) ink film to be able to carve the ink film pencil The H value of the measured ink of India. Hardness of metal, hard plastic and rigid rigid body printing printing is very important.
11. Light resistance
Refers to the ink imprinted in sunlight under the color stability of a performance. It depends on the light resistance of the pigment, the thickness of the ink layer, the properties of the binder and the filler. Packaging, printing and outdoor advertising on the requirements of high light fastness, often require outdoor advertising lasted 3 to 5 years, no significant change in color printing products. Irradiation of light causes chemical reactions (such as oxidation or reduction) and physical (crystal) changes in the pigment, leading to darkening, fading, and complete withdrawal of the pigment.
12. Chemical resistance
Ink resistance to water, acid, alkali, alcohol and other solvents, collectively referred to as chemical resistance, the determination of standards and methods from the application target, the current commonly used method of immersion and wet rub method.
Soaking method is to dry the printed matter, soaking in selected water, acid, alkali, alcohol and other solvents, after a certain period of time out, compare the degree of change before and after printing products soaked.
Wet wiping method is to wet the cloth with the selected solution, the printed matter on the dry wet and dry after a number of times (50 ~ 100), and then compare the difference before and after printing wet rub.
Other colors such as ink color, gloss and transparency (or hiding power), respectively, with colorimeter, reflection gloss meter and densitometer (or hiding power meter) to be determined.
13. Fixation fastness of screen printing inks
Ink is the most important material in screen printing, in the entire printing process occupies an extremely important position. Because the screen printing object is quite extensive, such as paper, metal, plastic, glass, wood, ceramics and various textiles and so on. So the correct grasp of ink performance, is a smooth screen printing essential conditions.
Silk screen printing ink is one of the most prominent problem is the ink after printing on the substrate fastness. There is no fixed fastness, it means no printing, but also cause great waste. Fixation fastness issues related to the ink and substrate bonding mechanism. At present, generally believed that the impact of ink fixation fastness for the following reasons:
Resin is the main constituent of ink binder, ink fixation effect and resin properties are closely related.
a. Molecular weight. If the molecular weight of the resin and the molecular weight of the substrate is too close, the ink solvent is not easy to choose. If the molecular weight of the resin is too large, in order for the ink to achieve the required viscosity of silk screen, the ink in the solid (resin) content is too small, resulting in solvent waste. High molecular weight, melting point is also high, difficult to dissolve in the solvent, the adhesion effect. If the molecular weight of the resin is too small, the cohesive force and the compatibility are poor, and the attachment effect is poor. The choice of molecular weight of the resin is important and must be appropriate.
b. Glass transition temperature. Polymer chain segment movement only in the glass transition temperature above, it has a significant diffusion rate. The whole chain motion can only be carried out at higher temperatures. The room temperature curing ink its glass transition temperature of the resin should be very low, which is obviously beneficial to the adhesion effect. For heat curing inks, the heating temperature should be higher than the glass transition temperature, lower than the substrate heat distortion temperature. The attachment effect is related to the operating (drying) temperature.
c. The composition of the resin. Resin molecules in the functional group, branched-chain structure directly affect the ink on the substrate fixation effect. So screen printing ink for different substrates more and more specific. A single resin often can not meet the needs of special screen printing ink, in fact, the use of two or three copolymer resin, resin mixed resin modified methods can be prepared with excellent quality ink. Choosing the proper resin component is beneficial to the fixation effect.
d. Elemental content of the resin monomer. Synthetic resin monomer content of an element changes, so that the performance of the resin also changes. Design ink formulations, the synthesis of some special resin, will greatly improve the ink performance. A change in the content of an element in a resin will result in a prominent change in a particular property of the resin.
e. Resin polarity and crystallinity. The existence of polarity conducive to the effect of fixation, but the high polarity of the linear polymer, high crystallinity, copolymerization and grafting can be used to modify the resin to destroy the crystallinity and maintain polarity.
Practice has proved that epoxy resin on wood, metal, some plastic stools can produce many chemical bonds, so many special screen printing ink often used epoxy resin to formulate in order to achieve the desired fixation effect.
Solvents that make up the ink also affect fixation. Resin and solvent used in the solubility parameter is appropriate, for the substrate solubility parameters must also consider similar. But for the plastic screen printing ink from the other screen printing effects to consider (other special ink should be the case).
a. Resin and solvent solubility parameters are similar, conducive to dissolution.
b. High boiling point solvent use too much fixation effect decreased.
c. Improper selection of solvent fixing effect is poor, the use of solvents from the multi-printing to consider, so the ink solvent often use more than three kinds of mixed solvents.
d. The solvent and binder in the ink (polymer) and the substrate contact with the substrate surface swelling and dissolution of the substrate and the ink in the inter-diffusion material to form a mixed layer without boundaries after the middle layer , Leading to the disappearance of the interface, the formation of ink and a solid combination of substrates.
e. Diffusion process is the ink solvent on the substrate surface of the process of dissolution, and the dissolution of the substrate is determined by the ink and the solvent in the polarity of the material. Only the ink and the substrate polarity is similar to dissolve only with a fastness to adhesion.
Ink in the link material is generally linear structure of macromolecular polymers, the molecular ends of the easily diffuse to the substrate, the short side chain is not conducive to the proliferation of large branches in favor of diffusion. Linkage of molecular weight, chain length, its penetration is slow, diffusion is also slow. Molecular weight is small, the penetration of fast, bonding force, but their cohesive strength is small. Therefore, as a binder polymer needs to have an optimal molecular weight, both for diffusion, but also a certain degree of cohesion. The binder is sensitive to temperature in the process of diffusion to the substrate. The temperature can promote the Brownian motion of the macromolecule, the speed and the ability of diffusion, and the bonding fastness of the ink.
A wide range of additives, will greatly improve the performance of ink. Effect on the fixation effect of powder additives are:
a. Crosslinking agent. Crosslinking agent can not only improve the fixing effect of ink, but also can greatly improve the hardness of ink, solvent resistance, water resistance and drying speed.
b. Coupling agent. Coupling agent to promote the ink in the resin and polymer printing materials produce coupling effect, can improve the fixing effect of ink.
4) Ink performance.
The wettability of the ink is good, the fixation effect is favorable, if the substrate has not infiltrated, the ink has been dried or cured, the fixation effect is poor, which involves ink flow performance problems. Flow performance and other factors are closely related to silk screen, where only focus on the fixation effect.
a. Preparation of ink, the solvent must control the rate of evaporation, the presence of high boiling point solvent leveling.
b. The use of appropriate molecular weight resin to formulate the ink, the higher the viscosity of the resin is not conducive to the flow of ink.
c. Add the appropriate leveling agent to the ink.
Glass directly affects the screen printing ink fixation effect. Through the substrate modification, surface treatment and other methods, can also improve the ink in the substrate fastening fastness.
a. Through the modification of the substrate to improve the fixing effect of the ink.
b. The surface of the substrate cleaning process, but also can improve the fastness of ink on the fixation. The surface of the substrate dust, grease, dirt, the presence of ink is difficult to fix, must be removed.
c. The substrate surface oxidation, change the surface of the substrate, etc., will greatly improve the fixing effect of the ink.
With the coupling agent for pretreatment of the substrate to promote the activation of surface groups, but also t
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