The specialized ink used in flexographic printing machines is water-based ink, which has the advantages of no pollution to the environment, no impact on human health, is not easy to burn, has good safety, stable ink performance, bright colors, does not corrode the plate material, is easy to operate, cheap, has good adhesion after printing, strong water resistance, fast drying, and a printing speed of 150-300m per minute. It is precisely because of the above characteristics that flexographic water-based ink is particularly suitable for printing and using packaging and decorative printing products such as food, medicine, cosmetics, etc. that require strict hygiene conditions. So what issues should be paid attention to when using it?
Main indicators of water-based ink for flexographic printing
1. Fineness
Fineness is a physical indicator that measures the size of particles such as pigments and fillers in ink, and is directly controlled by ink manufacturers. Users generally understand it and cannot change its size during use. The fineness of flexographic water-based ink is generally below 20 μ m, and the instrument used to measure fineness is a fineness scraper.
2. Viscosity
Viscosity is one of the main indicators of water-based ink, and its value directly affects the quality of printed materials. Therefore, in flexographic printing, the viscosity of water-based ink must be strictly controlled. The viscosity of water-based ink is generally controlled within the range of 30-60 seconds/25 ℃ (using a No. 4 coating cup), and it is better to control the viscosity between 40-50 seconds. If the viscosity is too high and the leveling is poor, it will affect the printing suitability of water-based ink, which can easily lead to phenomena such as dirty plates and paste plates; If the viscosity is too low, it will affect the ability of the carrier to drive the pigment.
If water-based ink is left for a long time, some inks with poor stability are prone to precipitation, layering, and some may exhibit false thickening. At this point, sufficient stirring can be carried out, and the above problems can naturally disappear. When using fresh water-based ink, be sure to stir it evenly before making dilution adjustments. During normal printing, the ink fountain should also be stirred regularly.
Water based ink is prone to freezing below 0 ℃. If the water-based ink freezes, it can be placed in a room with high temperature to dissolve naturally. After stirring evenly, it can continue to be used. When there is a large temperature difference between winter and summer, viscosity is most sensitive. When the temperature is high, water evaporates quickly and dries quickly. When operating, it is important to reduce the drying time or increase the machine speed; When the temperature is low, the evaporation of water is slow, and the drying of water-based ink is also slow. During operation, the drying speed of water-based ink can be increased or a drying device can be added. When used in winter, it is best to place the water-based ink stored in the warehouse in the workshop in advance to balance the viscosity.
3. Dry
Drying is one of the most important indicators in water-based ink, because the speed of drying, like viscosity, can directly reflect the quality of printed materials. Operators must have a detailed understanding of the drying principle in order to adjust the drying time of water-based ink reasonably according to the different products or substrates. While ensuring good drying of water-based ink, it is also necessary to consider moderate viscosity or stable pH value.
The drying of water-based ink includes three methods: volatilization, absorption, and reactive solidification. After thorough drying, water-based ink printed materials have strong resistance to water and friction. If the drying is too fast, the surface of the ink layer is prone to crust, and water-based ink is prone to dry up on the printing plate, causing dirty plates, unclear patterns around them, and inability to remove internal moisture, resulting in inaccurate overprinting. It can be adjusted with a retarder or solved by increasing the printing machine speed or turning off the drying system. When stopping the machine, it is best to clean the mesh roller or ink hopper. When stopping for a short period of time, the ink hopper should also be stirred frequently to prevent surface peeling; Drying too slowly can cause paper to stretch and reduce gloss, which can affect color stacking printing and may lead to adhesion, causing a lot of trouble for machine operators. At this point, it is necessary to first check whether the pH value is correct. Depending on the pH value, a suitable amount of stabilizer or ethanol can be added to adjust it, making the water-based ink dry faster.
4. PH value
Water based ink contains a certain amount of ammonium solution, which is used to improve its stability or enhance its water resistance after printing. Therefore, pH value is one of the important indicators. The pH value of water-based ink when it leaves the factory is generally controlled at around 9. The pH value of the machine can be adjusted or controlled between 7.8 and 9.3
It should be flexibly controlled according to the different substrates and temperatures. At the same time, the relationship between dryness and viscosity should be considered, and various additives should be added appropriately to adjust. The pH value of gold ink should not be too high and should be controlled between 8 and 8.5 to make it slightly alkaline. Generally, it is tested every 2-3 hours and adjusted accordingly. The pH value should be controlled within the optimal printing suitability range as much as possible.
Types, uses, and amounts of main additives for water-based ink
The main additives for water-based ink include diluents, stabilizers, blockers, cleaning agents, defoamers, and gold ink diluents.
1. Dilution agent
Dilution agent is a milky white slurry liquid with a certain viscosity and high pH value. It can be used to mix various colors and dilute four-color inks without reducing viscosity and pH value. It can be directly used on the machine to enhance the brightness of printed products. The dosage can be determined according to the user's color scheme and the depth of the color.
2. Stabilizer
Stabilizers are colorless and transparent liquids with a pH value of up to 11. They can increase the pH value of water-based ink and enhance its curing and drying time. If the printing is normal, the pH value is relatively stable, and the viscosity is too high, the viscosity of water-based ink can be adjusted after being diluted with 1 1 water, and the usage is generally controlled at 4%~6%.
3. Blocker
Blocker (slow drying agent) is a slightly yellow liquid with a pH value of around 5.5. It can inhibit and reduce the curing and drying time of water-based ink, prevent surface crust, and have a certain inhibitory effect on the phenomenon of dirty and paste caused by slow vehicle speed and fast drying of water-based ink. When the viscosity of water-based ink is too high and there is no need to use excessive blocking agents, it can be diluted with water before use, and the general dosage is controlled at 3% to 5%.
4. Defoamer
The main performance of defoamers is to eliminate bubbles in water-based ink and prevent problems such as white spots, sand holes, and water marks caused by the appearance of bubbles during printing. The dosage is generally 0.5%~1%. Too much dosage will affect the product quality. It is better to spray it out of the spray
5. Gold silver ink diluent
Gold and silver ink diluent is a milky white turbid liquid with a certain viscosity and slightly alkaline pH value. Its main function is to adjust the viscosity of gold and silver ink, improve printing suitability, and not oxidize or darken it, maintaining the original luster of gold and silver ink. The dosage is generally controlled between 2% and 8%, and should be flexibly used according to different temperatures, substrates, and vehicle speeds.
6. Cleaning agent
Cleaning agents are mainly used for machine cleaning of printing plates and mesh rollers, as well as for treating dry and crusted water-based ink.
Recycling and utilization of leftover ink
Because the ink fountain must maintain a certain amount of ink to ensure normal printing on each printing machine, it is impossible for each machine to run out of ink every time. At the end of printing each batch of products, there is usually a certain amount of remaining ink. Multi variety and multi-color unit printing, with a certain amount of ink remaining for each product. If not adjusted in a timely manner, continuing to use it will result in waste and increased costs. This requires that the remaining ink should be kept at the lowest possible point during allocation. The remaining ink must be stored separately, sealed, and free from ink or debris. If it is not clean, it should be filtered with a filter, and leftover ink should be used first for the next use. Light colors can be transferred to dark colors, and rough paper or products with medium to low printing quality can be prioritized for use.
Preservation of water-based ink
The shelf life of flexible water-based ink is generally one year. If it exceeds the shelf life or for a longer period of time, as long as there is no solidification gel, only surface layering or precipitation, it can continue to be used as long as it is thoroughly stirred and uniform. The storage temperature should be within the range of 5 ℃ -30 ℃. It is best to store in a warehouse, not outdoors, not exposed to sunlight, and not stored in a warehouse below 0 ℃. The shelf life of gold ink is six months or less.