Application requirements of the hottest plastic sc

Application requirements of plastic printing ink

1. Introduction

Plastic silk screen printing is a kind of reprocessing of plastic products. Due to the single dyeability and poor color fastness of plastics, secondary processing is needed in order to make up for these shortcomings and improve the appearance and decoration of plastic products. The ink used for silk screen printing has a great relationship with the quality of silk screen printing products. General thermosetting inks are not suitable for plastic screen printing, because the curing temperature of the ink is mostly above 100 ℃, and at this time, the plastic has already been deformed, so we can only choose solvent based inks according to the solubility of the plastic, that is, volatile dry inks

2. Requirements for the selection of printing inks for plastics

it is difficult to prepare plastic printing inks. Because the substrate is thermoplastics, if strong solvents are used, the plastic surface will be "over soluble", so a slightly soluble plastic solvent must be used, and its structure is similar to that of the resin of the printing plastic ink itself, so that their expansion coefficient can be close. Under the action of solvent, a miscible intermediate layer can be formed between the ink and plastic, which makes the ink have high strength and wear resistance after film forming

2.1 ink and solvent for plastic printing

when plastic printing, the inking performance should be tested first, and printing should be carried out under the condition of troubleshooting. When printing, the appropriate ink should be selected according to the use and requirements of the materials used and the finished products. Polyethylene plastic has excellent properties in water resistance, drug resistance, electrical insulation and other aspects, and its cost is also low, so it is widely used. Therefore, appropriate inks and solvents must be selected according to the different uses of the finished products. Polypropylene plastic has excellent mechanical strength and weather resistance, so it is used in the printing of high-grade commercial signs and lighting signs. Many printing products have high precision, so we should carefully choose pigments and solvents. The weather resistance of the signboard is very important, but with the passage of time, the solvent will change color, so we must pay attention to this. In synthetic resins, some resins will decompose and deteriorate under the action of sunlight, so processing treatments such as film pressing, spraying, lamination and so on are added in the post-processing process. Before printing, we must also understand the use conditions, bending degree, bonding performance, drug resistance and friction resistance of plastics. At the same time, we must also know what process is used in the post-processing of plastics

2.2 surface tension and fixing performance of ink

it is difficult to print silk on plastic sheets and films, because ink is difficult to fix on PVC, polyester, polypropylene, polyethylene and other plastics. This is because the surface tension of ink does not match the surface energy of plastic. Surface tension and surface energy are two extremely important factors affecting the quality of silk printing on various plastic sheets and films. The unit of measurement of surface energy and surface tension of solids is dyne/cm, which can be used as an indicator of ink fixation and wetting performance. In order for the ink to be successfully fixed on the substrate and film, the surface tension of the oiler must be equal to or less than the surface energy of the substrate and film. Generally speaking, when the difference between the surface tension of the ink and the surface energy of the plastic increases, the wetting and fixing properties of the ink will be improved

(1) when the surface tension of ink is actually applied, the surface tension of ink can be regarded as a fixed value. Some ink suppliers provide surface tension data, while others prefer to recommend smaller surface energy data in order to get better results. In this way, the supplier can ensure that the difference between the surface tension of the ink and the surface energy of the sheet and film will not be too large. By changing the solvent or adding surfactant and silicone resin, the surface tension of ink can be changed. However, improperly changing the ink surface tension may cause defects such as uneven imprinting and white spots

(2) surface energy of solids all solids have normal surface energy, but what we care about is the surface condition at the time of printing, that is, the so-called apparent surface energy. The apparent surface energy of solid surfaces polluted by grease, fingerprints, plasticizers and film removers will be significantly lower than the basic value. By improving the surface roughness of plastics or surface treatment (plasma treatment, corona treatment and flame treatment), the surface energy value of plastics can be significantly improved. The surface energy of plastic materials such as polyvinyl chloride (PVC), polyester, polypropylene and polyethylene is low. The reason is that plasticizers (to maintain softness) and film removers (to prevent mucous membranes) continue to move to the surface after the sheet and film are formed, which greatly reduces the surface energy. To determine whether the dissolution of additives has become a reality is the reason for the reduction of surface energy, the best experimental method is to wipe the test piece and film surface with alcohol. First, measure the surface energy of the plastic, and then wipe the surface of the substrate with ordinary alcohol and clean cotton cloth. If the value of surface energy changes (usually increases), there may be pollution

2.3 drying of ink

after printing, it is generally hoped that the ink can be dried faster to improve efficiency and save drying racks. But drying too fast or too slow will bring difficulties to printing. If it dries too fast, the ink on the printing plate will quickly change tone, which will appear in the plate phenomenon, reduce the fluidity of the ink and the wetting effect of the ink on the surface of the substrate, and the pattern printing is not firm. If it dries too slowly, it will bring difficulties to multi-color overprint, increase the space for drying or make the next version overprint impossible. Therefore, correctly mastering the drying speed is a problem that must be paid attention to in improving the printing efficiency and ensuring the printing quality. Apart from room temperature, humidity and air flow, the main factors affecting the drying speed are to reasonably select the type and proportion of solvent according to the specific drying conditions of the printing workshop, because the drying speed of ink depends on the volatilization speed of solvent. The volatilization rates of several commonly used solvents are listed below in the order from fast to slow: toluene acetone ethyl acetate ethanol butyl acetate cyclohexanone xylene n-butanol. When selecting solvents, in addition to considering the drying speed, we must also pay attention to their compatibility with various plasticizers in plastics. For example, most organic solvents can quickly dissolve with plasticizers in PVC films. If the solvent is not selected properly, when the ink is printed, the film will soon dissolve and swell and lose its flat appearance, making printing impossible. When this phenomenon occurs, the solvent formula should be adjusted in time or the film printing with less plasticizer should be replaced

3. Development trend of printing ink

while advocating environmental protection printing, environmental protection ink has also become an important development trend. Environmental friendly inks have the following requirements: (1) inks should be non-toxic and harmless to human bodies and organisms, and do not contain toxic elements, such as halogens, heavy metals, or their content should be controlled below the relevant standards. (2) In the whole life cycle of packaging products, ink should not pollute human body and environment and cause public hazards. In the late 1980s, some developed countries began to mass produce and use environmental friendly plastic inks, mainly water-based plastic inks and UV inks. 3.1 UV ink (1) UV ink is suitable for various printing materials, such as paper, coated paper, wood products, synthetic paper, polyvinyl chloride (PVC) 1 propylene (PMMA), thermosetting resin, etc. (2) UV printing ink has strong performance and is suitable for high-quality and diversified products and special-shaped products that need special processing. ① Physical property: liquid crystal display screen, anti-corrosion cathode welding; Electrical ones include insulation, magnetism, etc.: thermal temperature foaming, fluorescence, nocrystal, heat transfer printing, etc. ② Chemical and medicinal products include corrosion, ethanol stripping, etc; Biochemical ones include antibacterial and antifouling; Absorption, adsorption and reaction include water resistance, oxidation and corrosion, etc. ③ Surface decoration: crystal glue, anti metal etching, Matt wrinkles, non strong light, mirror metal, etc.: three-dimensional, thick film printing, Braille, crystal pattern, wrinkles, etc

3.2 water based inks

among volatile dry inks, water-based inks developed in the 1980s have attracted attention because they are conducive to environmental protection. However, due to the poor water demand of its template and the wrinkle easily caused by the water absorption of the paper when printing on the paper, it has not been popularized. After entering the 1990s, the above problems were gradually solved. For example: (1) drying of water-based printing ink. The biggest disadvantage of water-based ink is that its dryness is slower than that of solvent based ink. Now the composition of water-based ink is completely improved. (2) Water based inks are well fixed to absorbent objects such as cloth (cotton, linen, textile), paper (paper, paperboard), wood (wood, wood), leather (leather, artificial leather), but it is difficult to fix the printing materials with hard surface. Now because of water-based ink 5 The fixing agent with too large fit clearance between the working piston and the oil cylinder is made of, polyester and polyester coincidence resin. Its all-round connection function has excellent adhesion to the non absorbent substrate, the substrate with smooth and hard surface and the object with difficulty in fixing in the past. (3) The water-based ink not only realizes the full hydration, but also solves the problems of the softness, brightness and anti seepage of the printing ink. In Europe, water-based UV ink has long been used to print color large-scale road signs and posters. The increasing application of water-based ink and water-based UV ink is mainly due to the impact of environmental protection regulations and requirements to improve the working environment. Of course, there is another reason, four-color plus printing, point reproducibility is very good. Therefore, in order to give full play to the advantages of water-based ink in the future, the rapid expansion of water-based ink market is inevitable

3.3 thixotropic silk printing ink

thixotropic printing ink is both thermoplastic and cold printing, ensuring its leading position in technology. The thixotropic printing ink developed can be used not only for ceramic tiles, tableware, glass, but also for the printing of enamel products. Recently, a series of low melting point thermoplastic inks have been developed and reached the peak of research. The rheological properties of thixotropic ink are similar to those of cold printing ink or thermoplastic printing ink. These properties have the following advantages in printing. (1) Reduce waste products, use traditional ink, and restart will often cause dirty prints or other defects. Because thixotropic ink has good rheological properties, the use of this kind of ink is helpful to solve the above problems. When the scraper scrapes the ink, the fluidity of the ink increases, so that the ink is evenly deposited on the print. After printing, the fluidity of the ink decreases, thus preventing the ink from overflowing on the plate. It should be noted that the thixotropic silk printing ink after scraping has enough fluidity to meet the requirements of continuous high-speed printing. At the same time, thixotropic printing ink is also suitable for semi-automatic silk printing machines with slow printing speed. (2) Improve printing accuracy. Fine grained pigments and thixotropic binders form inks that can be used for higher mesh plates. Thick ink film can be printed with thixotropic printing ink. Using traditional ink, when passing through a thicker version, the sharpness of the image will be reduced due to the excessive accumulation of ink. But with thixotropic ink, the version will not be filled with ink, so the image will not be distorted due to paste. When printing high-precision multi chromatic images, the ink layer can maintain a certain thickness without affecting the fine level of the image after the thixotropic printing ink is directly printed on the utensils

4. Conclusion

to sum up, we see that in the future, traditional inks will slowly disappear in the printing ink market. Water based ink and UV ink