flexography, exploring the significance of ink and its transfer system in this printing technique.
We'll take a closer look at the key components involved, including the ink fluidity, viscosity, and transfer properties.
Additionally, we'll discuss the role of ink formulation and its compatibility with stereo materials.
aspects of flexography and its applications in the printing industry.
Why Was Flexography Named ?
- Flexible Printing Plates : The process uses flexible rubber or elastomeric printing plates, which allow for versatility and adaptability in printing on various materials.
- Dominance in Flexible Materials : Flexography is the primary printing process for a wide range of flexible materials, including papers, films, foils, wall coverings, floor coverings, and flexible packaging.
- Versatility : Beyond flexible materials, flexography can be applied to rigid substrates like wood, plastics, glass, and metals, making it suitable for diverse printing applications.
- Reflecting the Process : The name "flexography" accurately represents the printing process's flexibility, adaptability, and suitability for various printing and converting applications, particularly in the flexible packaging industry.
Ink Transfer Systems in Flexographic Printing
- Fountain Roller : This roller is responsible for transferring ink from the ink pan to the anilox roller, ensuring a consistent and controlled ink supply.
- Anilox Roller : The anilox roller meters the ink using engraved cells, each carrying a specific amount of ink to be distributed. This helps achieve uniform ink thickness on the printed material.
- Doctor Blade : To prevent ink buildup, a doctor blade may be installed. Its purpose is to scrape excess ink from the anilox roller, ensuring no ink is transferred beyond the engraved cells.
- Print Cylinder : The ink travels from the anilox roller to the print cylinder, which carries a flexible polymer printing plate responsible for transferring the image to the substrate.
- Stereo : The stereo is made from polymer resin, which is poured to replicate the artwork. The stereo is then hung onto the printing cylinder.
- Impression Cylinder : The substrate is carried on an impression cylinder, which presses the material against the printing cylinder, resulting in the transfer of the desired image.
- Double-sided Adhesive Tape : The flexible polymer printing plate is held in place by a precision-engineered double-sided adhesive tape, consisting of various layers to ensure a secure hold and easy removal after printing.
- Roll Web Tension Control : Flexographic machines precisely control roll web tension through the press, ensuring smooth and efficient printing on various substrates.
- Drying Systems : Efficient and isolated drying systems are used to dry inks quickly, preventing interference with the immediate printing area or inks held within.
General Properties of Inks
There are several fundamental characteristics required for flexible inks, determined by the nature of the printing process and the conditions and forces encountered during printing, in addition to the requirements of the printed product and its intended use.
Inks consist of three main components - pigment, binder (binder material), and solvent.
Ink Fluidity
Ink is a liquid that must remain in this state until it is applied to the substrate.The ink may require viscosity reduction upon supply by adding a solvent to adjust it for printing.
In most machines, ink is stored in a tank beneath the printing unit.
The ink is pumped into the designated interior, which is equipped with a drain to maintain the ink level at a specific level corresponding to the position of the color cylinder.
Consequently, the excess ink is constantly circulated through pumping and gravity flow, necessitating a liquid state.
The fluidity is also required for the operation of color cylinders, dampening rollers, and processing discs, and in many machines, the movements can be substantial.
During the recycling of ink through the unit, contact with the atmospheric air inside the ducts and on the cylinders causes some solvent evaporation, resulting in an accompanying increase in ink viscosity.
This leads to an increase in the ink layer's weight on the print, and it becomes necessary to replace this loss by adding an appropriate solvent.
This can be done manually by viscosity tests during operation and solvent addition for correction, or preferably, using automatic viscosity control equipment.
Ink Viscosity
Ink viscosity control is important in flexography, particularly as a means of maintaining the print density during the run.The actual viscosity chosen for a specific job depends on several factors, including press speed , substrate, type of metering , temperature , solvent mix and print thickness required.
In practice, satisfactory printing can be achieved over a wider range of viscosity compared to the gravure process.
Effect of Printing Speed
Printing speed also influences the ink viscosity requirements.
As the speed increases, shear forces also increase, leading to a higher amount of ink being carried.
Consequently, a larger volume of solvent needs to be added to maintain the same print density.
This is well-known by printers and can cause issues, as slow running during proofing at the start of a job may require the use of thicker ink than what is used during actual printing to achieve the same print density.
Effect of Low Viscosity
Printers typically use viscosity adjustment and solvent addition as a means of controlling color strength in the print.As solvent is added to the ink there are two effects:
first, a reduction of colour concentration due to the increased volume,
secondly, the lower viscosity of the diluted ink reduces the shear forces applied during metering, resulting in a decrease in the ink film weight.
This second effect becomes more apparent with simple roller nip metering. Excessive dilution may lead to a deterioration in the appearance of the print and a loss of abrasion resistance.
Effect of High Viscosity
At very high viscosities, print distortion and uneven working on fine text can occur.Viscosity control becomes challenging due to the wide variation as solvent is lost during the printing process.
Transfer
Ink transfer properties are influenced by the sophistication of ink metering techniques, such as reverse angle bladed anilox rollers.
Press speed and sheer forces have a lesser impact on film weight with advanced metering.
However, simpler metering methods can alter the weight carried, depending on the ink's ingredients, tack properties, and wet adherence to rollers, stereo, and substrate.
The binder component of ink has the most significant influence on these properties, with higher concentrations and molecular weights leading to increased transfer.
For successful transfer, the substrate's surface energy should be higher than that of the ink.
In the laboratory, ink formulations and resins' transfer properties can be compared by applying two inks side by side using a hand anilox proofer, ensuring equal strength and viscosity.
This assessment is essential for ink formulation, and using the hand anilox applicator for color matching proves to be a better method than other hand applicators not related to flexographic ink metering.
Colour and strength
The color and strength of a flexographic print depend on the thickness of the ink applied to the material and the type and concentration of the colorant used in the ink.
The wet ink thickness can vary significantly, typically ranging from 2 to 15 mm,
depending on the printing press, the way the ink is metered, the material used, and the printer's practices.
Flexographic ink usually has a thinner film compared to gravure, so it needs to be stronger to achieve satisfactory coverage.
Since the ink film thickness varies widely,
it's challenging to determine the exact color concentration during formulation.
The usual approach is to provide ink with higher viscosity and greater strength than what's likely to be needed on the printing press.
This allows adjustments to be made using solvents and reducing agents. Experience and knowledge of different printers' equipment and practices can also be helpful in formulating inks with the right strength and color.
Stereo composition
The materials used in manufacturing stereos have a significant impact on the choice of ink solvent.
These materials include natural and synthetic rubbers, photopolymer compounds, and sometimes plastics.
Typically, both water and ethanol are suitable solvents for all materials.
However, in flexographic printing, the diverse substrate requirements and various printing uses have led to the utilization of other solvents to expand the range of resins beyond those that are soluble in water and ethanol alone.
Certain solvents can cause swelling or dissolution of stereo composition materials.
A table provides a general guide on the compatibility of different solvents with commonly used materials in stereo composition.
Nevertheless, it's essential to consider that solubility properties may vary among different photopolymer and rubber compounds.
Inks are often composed of a blend of different solvents, and a particular solvent may be used in low concentration in a formulation without adverse effects on the stereo material.
Stereo manufacturers are usually willing to provide information on solvent resistance for their compounds.
However, limitations on the use of certain solvents, particularly with highly soluble photopolymer materials, can restrict the available choices for ink binders.
This may conflict with the requirements for ink adhesion on challenging substrates or end-use resistance.
Resolving these conflicting demands is not always straightforward through ink formulation alone.
Adhesion and end use
Adhesion and end use are crucial factors in ink formulation, especially when dealing with non-permeable substrates where ink adhesion is critical.
For absorbent surfaces like paper, adhesion is achieved through ink penetration into the paper.
However, on non-absorbent surfaces, adhesion relies on chemical and physical bonds and wetting between the substrate and ink.
The binder, a component of the ink, plays a vital role in adhesion.
A suitable resin must be chosen to ensure proper adhesion to the specific substrate.
Different substrates present various challenges, and sometimes a combination of resins is needed to achieve adhesion and maintain a balanced ink performance.
Generally, the more inert the substrate's surface, the harder it is to achieve good adhesion.
The end-use conditions of the printed material are essential when formulating inks for packaging.
Requirements may include resistance to heat sealing, non-adherence to reverse printed coatings or adhesives, and specific surface slip characteristics.
The printed material should withstand storage conditions, considering the various components present in packaged products, such as water, oils, fats, soap, and detergents.
Special storage conditions like deep-freezing can also impact the print.
The choice of colorant depends on bleed and fastness specifications, and in high-demand applications, dyes and soluble pigments are typically excluded.
The selected binder must possess suitable resistance properties since some packaged products can soften certain resins, leading to adhesion loss or color transfer.
Solvents usually don't affect resistance properties, but resistant binders may require stronger solvent mixtures for solubility.
Conclusion
flexography is a highly adaptable printing technique known for its versatility and suitability for various applications.
The significance of ink and its transfer system in flexography cannot be understated. Proper ink fluidity and viscosity are essential for maintaining print quality and density during the printing process.
Ink formulation, including the choice of solvents and binders, is crucial for achieving adhesion and end-use resistance on different substrates.
With its dominance in flexible materials and ability to print on various substrates, flexography continues to be a valuable and widely used method in the printing industry.
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