Flexographic ink drying mechanisms are divided into two broad categories. The first is evaporation where either organic solvents or solvents mixed with water are evaporated using infrared heat or forced heated air. The second mechanism involves curing the ink film with either ultraviolet light (UV) or an electron beam. The type of drying system largely depends on the type of inks being run on the press, which is directly tied to the substrate being printed. For example, solvent-based inks are typically run on nonabsorptive substrates such as plastic film, while water-based inks are used to print paper or paper board.
Most flexographic printing presses are equipped with an integrated drying system, which most commonly consists of infrared or heated, forced air drying methods. There are two types of integrated drying systems found on flexographic printing presses; interstation (a.k.a. between-color) and tunnel dryers. Interstation dryers are individual dryers after each print station except the last one. The dryer after the last print station is called the main tunnel or overhead dryer.
The supply fan takes air, from either the roof or inside the plant, past a gas-fired burner and directs this air to the substrate to dry the ink or coating. The heated air is pushed onto the substrate through a series of air nozzles, narrow slots running perpendicular to the substrate, across the width of the substrate. The heated air causes the liquid component of the ink, which is either water or solvent, to vaporize, the exhaust fan pulls the air from the dryer and pushes it to either the atmosphere or a pollution control device. When the air is exhausted, ambient air is pulled into the dryer at the web entrance and exit. In all cases, the exhaust volume is greater than the supply volume. Therefore, the dryer is kept under a slight negative pressure, which prevents volatile air from escaping into the plant.
Ultraviolet (UV) or Electron Beam (EB) curing systems can only be used in conjunction with UV or EB curable inks. Waterbased or solvent based inks can not be dried with UV or EB curing equipment. For UV inks to cure, light systems with special shielding for safety, are provided. The same is true for EB inks. The press is equipped with a drying system that emits an electron beam onto the ink causing it to cure instantly.
Best Management Practices & Pollution Prevention
When utilizing forced, heated air drying systems, drying efficiency can be improved by incorporating high velocity air impinged against the web. The impingement acts to break the vapor barrier above the ink so that the solvents can be more readily vaporized. Infrared heating elements can also be added to a dryer system to improve the efficiency of the dryer, and therefore, the speed of the press.
Variations in drying efficiency can be controlled by regulating heating elements or by modifying the air flow to the print station dryer.
The velocity of the impinged air has a great influence on drying, especially for water-based inks.
However, it is important to recognize that excessive heat causes a web to change length resulting in ink register error.
Beyond a certain point, additional heat or additional air flow will have very little effect on drying capabilities. When printing with inks that contain a significant portion of solid material, including pigments, etc. as is commonly the case with water based inks , drying requirements will reduce the speed at which the press can be run. This is one significant factor that as affected the adoption of water based inks.
Most press manufacturers offer a recirculating airflow system on heated forced air dryers that recycles air containing volatized solvents. The recirculation principle utilizes a supply fan sized to provide the necessary volume and pressure of hot air to the dryer and to exhaust the majority of the air containing the vaporized solvents. A smaller exhaust fan removes the filtered air, products of combustion and leakage into the dryer. In all cases, this exhaust must meet the safety standards and guidelines for lower explosive limits (see Health and Safety).
For access to vendors who may supply alternative materials and equipment, see the PNEAC Vendor Directory.
Ink dryers are a source of VOC and HAP emissions from solvents in the in ink volatilizing during the ink drying process.
U.S. EPA has established maximum achievable control technology (MACT) standards for the emission of hazardous air pollutants (HAPs). HAPs are emitted from the dryers as a result of the ink drying process. These emissions must be considered when determining air permitting requirements. Recording keeping requirements apply to HAP emissions from the dryers and depending on the permitting and state and local requirements reporting may be required as well.
Nitrogen Dioxide (NOx) and Sulfur Dioxide (SOx) are emitted from fuel combustion in the gas fired drying systems. Records of the amount of NOx and SOx emitted from the facility must be maintained. Depending on the permitting requirements that apply to the facility, the emission of NOx and Sox may be required to be reported to the state environmental regulatory agency and/or U.S. EPA
Also See Oxidizers
UV Drying Systems generate spent lamps that contain mercury, which is a regulated waste under the U.S. EPA Universal Waste rules.
Health & Safety
The exhaust airflow volume from the press dryer is designed according to National Fire Prevention Association (NFPA) guidelines that dictate that the ventilation or exhaust rate must be designed and maintained to prevent the vapor concentration in the dryer from exceeding 25% of the lower explosive limit. The NFPA states that an estimated rate of ventilation or exhaust cannot be less than 10,000 cubic feet (measured at 70°F) per gallon of solvent evaporated in the dryer. This limit is based on ink/solids/solvent concentrations, types of solvents, press speed, substrate width and the number of printing units. All these factors must be considered to determine maximum solvent conditions or the most solvent the dryer will have to evaporate.
Where UV and EB ink curing equipment is used, care should be given to handling the inks. The inks can be a skin irritant. Personal protective equipment such as gloves, safety glasses and aprons should be provided.
Employees should be informed and protected from UV light exposure from the curing equipment. Generally the light system is equipped with shields, etc. to protect employees from exposure. However, employees should be informed of the dangers associated with exposure and accidental access to the lights.