Flexo Ink Viscosity Range: Your Comprehensive Guide

परिचय

Flexographic printing, a versatile and widely used method, particularly in the packaging industry, relies on the transfer of ink from a flexible relief plate to a substrate. This process, known for its efficiency and ability to print on a wide array of materials, hinges on numerous factors, with the viscosity of ink standing as a cornerstone of print quality. The consistency and flow properties of the ink directly dictate the success of ink transfer, influencing everything from color density and uniformity to the sharpness of printed details. Maintaining the correct viscosity range is therefore paramount to achieving optimal results in flexographic printing operations. This guide contains important information for achieving just that.

The Ideal Flexo Ink Viscosity Range

स्याही का प्रकार	Viscosity Range (Zahn Cup #2)	Viscosity Range (cP)	सामान्य अनुप्रयोग
वाटर बेस्ड	18-25 seconds	80-200 cP	Paper, paperboard, some films
विलायक आधारित	20-30 seconds	100-300 cP	Films, foils, some papers
UV-Curable	30-50 seconds	200-500+ cP	Labels, films, specialty substrates
High Solids Water-Based	25-35 seconds	150-350 cP	Corrugated board, kraft paper
Specialty Inks	Varies	Varies	Metallic inks, varnishes, coatings

Accurate Measurement of Flexo Ink Viscosity

It is therefore important to measure the viscosity of flexo ink in order to achieve the best results in the printing process. There are several tools and methods that are used in the industry as follows:

• चिपचिपापनकप (Dip Cups): These are simple and inexpensive tools that are used for making rapid comparative measurements. A cup with a calibrated orifice at the bottom is placed in the ink and the time taken for the ink to flow through the orifice is recorded. The result is usually given in terms of time, which is normally measured in seconds.
• Rotational viscometers: These are more accurate and absolute viscometers and the units of viscosity are centipoise (cP) or Pascal-seconds (Pa·s). They operate by determining the amount of force needed to turn a spindle at a fixed rate in the ink.
• Automated चिपचिपापन Control Systems: These are incorporated in the current flexo presses and they help in the monitoring and control of the ink viscosity in real time. They usually employ sensors that are capable of determining viscosity through vibration, pressure drop, or other characteristics.
• Density कप: Density is not a measure of viscosity but it can give some idea about the variation in the composition of the ink which in turn affects viscosity. Density cups are used to determine the specific gravity of the ink, which is helpful in checking the evaporation of the solvent or contamination.

Viscosity Ranges by Flexo Ink Type

Water-Based Flexo Ink Viscosity

Water based flexo inks are preferred because they are eco friendly and safe for use in food packaging since they do not contain hazardous solvents. These inks generally have Newtonian or slightly shear thinning characteristics. Their viscosity typically ranges from 80 to 200 centipoise, which is determined using the Zahn Cup #2 with a drain time of 18 to 25 seconds. This range is important to sustain in order to get clean and sharp prints with good color strength on the substrates such as paper and paper board.

Solvent-Based Flexo Ink Viscosity

Solvent based flexo inks which are characterized by their fast drying time and good adhesion to films and foils have a slightly higher viscosity than water based inks. It has a viscosity of 100 to 300 centipoise, which may be equivalent to a Zahn Cup #2 of 20 to 30 seconds. These inks are very sensitive to solvent evaporation and hence require close monitoring during the printing process to avoid changes in viscosity that may affect the quality of the prints in the middle of the run.

UV Flexo Ink Viscosity

UV-curable flexo inks are different from the traditional flexo inks as they dry through the process of curing under the ultraviolet light. These inks are generally the most viscous among the common flexo ink types, with viscosities ranging from 200 cP and above, sometimes even exceeding 500 cP for some formulations. A Zahn Cup #2 measurement could be 30 to 50 seconds or even more in terms of drain time. The viscosity of UV inks depends mainly on the choice of monomers, oligomers, photoinitiators, and any filler or additive.

Key Factors Influencing Flexo Ink Viscosity

Temperature’s Impact on Ink Flow

Temperature plays a crucial role in determining the viscosity of flexographic inks. When the temperature is high, the kinetic energy of the ink molecules is high and hence the viscosity of the ink is low and the ink will flow easily. On the other hand, when the temperature is low, the molecular activity is low and this makes the ink to become thick and have a high viscosity. In the case of flexographic printing, it is very important to control the temperature of the ink. Variations can cause uneven ink deposition, which results in variations in color density, dot gain, and print uniformity. Temperature control measures such as the ambient temperature may need to be put in place to maintain a steady flow of ink and achieve a consistent print quality throughout the run.

Resin and Pigment Effects on Viscosity

The type and concentration of the resin used in a flexographic ink formulation are the key factors that define the viscosity of the ink. The resins which are responsible for holding the pigment together and fixing the ink to the substrate are of different molecular weight and structure. An increase in the molecular weight of the resins or an increase in the concentration of the resin will lead to an increase in the viscosity of the ink. Also, the nature of the pigments such as size, shape and dispersion of the pigments in the vehicle of the ink can also influence viscosity. When pigments are not well dispersed or when the pigment concentration is high, the internal friction in the ink increases and the viscosity of the ink may be affected, which may cause problems with ink flow and printing quality.

Solvent Evaporation and Viscosity Changes

Solvent evaporation is a critical factor that influence the viscosity of both solvent borne and water borne flexographic inks. During the process of printing, the solvents evaporate and the concentration of solids (resins and pigments) rises. This results in a higher concentration of solids and therefore an increase in the viscosity of the ink. In solvent-based inks, the volatility of the specific solvents used will determine the rate of evaporation. In water-based inks, the rate of evaporation of water depends on factors such as temperature, humidity, and the air surrounding the press. Excessive solvent or water loss can make the ink too viscous and result in issues such as poor ink transfer, blocked anilox cells, and low print quality. This can also impact on the ink consumption.

Shear Rate and Printing Speed Considerations

The shear rate that a flexographic ink undergoes during the printing process, which is directly related to the speed of the press, can affect the apparent viscosity of the ink, especially for non-Newtonian inks that are shear thinning. Shear-thinning inks are those that have a tendency to reduce their viscosity when subjected to shear forces like those experienced when the ink is transferred from the anilox roll to the printing plate and then to the substrate at high speeds. This reduction in viscosity under shear is beneficial for ink transfer and leveling. On the other hand, at low shear rates, the ink will revert to its higher resting viscosity to avoid problems such as ink misting or splatter. Hence, when choosing and regulating the flexo ink viscosity, it is crucial to take into account the printing speed.

Ensuring Consistent Print Quality: The Role of Reliable Flexo Presses by KETE

The quality of the prints in flexographic printing is directly dependent on the reliability of the printing press. A dependable press guarantees that all the parts of the ink transfer process, including the ink fountain and the impression cylinder, are precise and consistent. This is important in reducing fluctuations in the ink film thickness and transfer efficiency since these are directly influenced by ink viscosity. A reliable press will apply a constant pressure and speed, which will help to minimize the variations in viscosity due to mechanical variations.

KETE flexo presses are designed to maintain stable and accurate conditions for ink viscosity control and, therefore, high quality of the prints. With over a decade of experience in the industry, we incorporate state-of-the-art inking systems in its range, which includes models for water-based, solvent-based, and UV inks. For instance, the high-speed CI series, which can reach up to 300 m/min in models such as the KTFP-CI300, has a central impression cylinder that reduces mechanical fluctuations that affect the ink’s viscosity. In addition, some of our presses are equipped with automatic operating systems to ease the operations of the machines and may include features for checking and regulating the ink quality. This emphasis on sound engineering, compatibility of inks, and easy-to-use automation reflects KETE’s mission to provide dependable printing solutions that enable companies to produce high-quality packaging and printed materials consistently.

Contact us:

• Address: 20th Floor, Xinchao Building, Anyang Street, Ruian City, Wenzhou City, Zhejiang Province, China 325200
• Phone: +86-577-66873227 / 66689036 / 66689037
• WhatsApp / WeChat: +86-18806770163
• E-mail: sales@ketegroup.com

Troubleshooting Common Flexo Ink Viscosity Issues

High Viscosity Problems and Solutions

High ink viscosity can present itself in several forms; poor ink transfer, which leads to weak or faint prints; plugged anilox cells, which cause uneven ink deposition and streaking; and high dot gain due to the ink’s inability to spread properly on the substrate. Also, high viscosity can put pressure on the press and even cause some of its parts to break. In order to reduce viscosity, the best way is to add the right thinner or solvent to the ink in a proper manner. For water-based inks, this would be water and for solvent-based inks, it would be the solvent or solvents that the manufacturer of the ink recommends. Thinner should be added slowly and the mixture should be stirred well while checking the viscosity from time to time until the required range is obtained.

Low Viscosity Problems and Solutions

On the other hand, low ink viscosity can also lead to poor print quality. These may include; excessive ink flow that leads to bleeding or feathering of fine details, thin ink films that lead to poor color density and coverage and high chances of ink misting or splashing that leads to contamination of the press and the environment. There are several ways to increase low ink viscosity as follows; For water-based inks, the ph value might also be changed, but direct thickening is normally needed. In the case of solvent-based and water-based inks, it is possible to increase the concentration of solids and therefore the viscosity by allowing some of the solvent or water to evaporate. This process should be carefully monitored to avoid over-thickening.

Maintaining Consistent Viscosity During Printing

It is a preventive measure that requires the following practices in order to ensure that the ink is of a constant viscosity throughout the run:

• Monitoring and Adjustment: It is recommended to set a routine for checking the ink viscosity with the right measuring instrument. According to the readings, make small, controlled changes as required by adding more water or thickener as recommended by the ink manufacturer.
• Clean the ink fountain and any related parts: It is essential to clean the ink fountain and any related parts to avoid the accumulation of dried ink or other debris. These can change the viscosity of the ink and affect the flow of the ink and may cause a jam in the ink system in the long run if not rectified.
• Avoid Contamination: Ensure that the ink fountain is not contaminated by foreign materials like paper dust or debris. Ink is a very sensitive medium and any contaminants that may be present will have an impact on the viscosity of the ink and the quality of the prints.
• Solvent and/or water loss: To reduce solvent or water evaporation, always cover the ink fountains when possible, maintain the pressroom environment at the proper temperature and humidity, and consider the use of humectants or retarders in the ink.
• Use of Automated चिपचिपापन Control Systems: If the press has an automated viscosity control system, then it should be properly set and working efficiently. These systems offer constant surveillance and control, which greatly improve the stability of viscosity.

निष्कर्ष

The ideal viscosity range for flexographic inks is not a fixed value but rather a range that has to be achieved and maintained depending on a number of factors peculiar to each particular job. Thus, understanding the principles of ink viscosity, using the correct methods of measurement, identifying the factors that can affect it, and applying preventive and corrective measures, flexographic printers can provide high-quality prints. It is therefore recommended that operators use the information contained in this guide to improve the flexographic printing process and aim at achieving the best ink viscosity to improve on the quality of the prints as well as the efficiency of the operation.