Enclosed doctor blades clean excess ink from the anilox roll and return it to the ink sump.
Partially-cleaned anilox roll.
The objective of the flexographic printing process is to transfer ink to a substrate.
In its simplest form, the flexographic printing system consists of four basic parts: fountain roll, ink metering also known as the anilox roll, plate cylinder and impression cylinder. There are two traditional types of ink feed systems used in flexography, the two-roller systems and the doctor blade system.
The early, basic two-roll flexographic printing system consists of a smooth rubber fountain roll rotating in ink in an open pan. The partially submerged fountain roll transfers the ink to a mechanically engraved, chrome plated anilox roll. The anilox roll, in turn, transfers the ink to the plate cylinder. The plate cylinder then deposits the ink to the substrate.
The amount of ink transferred to the anilox roll is dependent on the anilox cell volume. This is determined by the depth of the individual cells engraved on the roll. An anilox roll with deep cells transfers larger volumes of ink than an anilox roll with shallow cells. The anilox roll has always determined the amount of ink transferred to the plate cylinder and, eventually, the thickness of the ink that is printed on the substrate. Therefore, in the end, amount of ink that is printed on a substrate is dependent on the anilox cell volume. The problem with early methods of ink delivery is the lack of control of the amount ink deposited during the transfer process, causing the print quality to rapidly deteriorate.
Subsequent transfer methods placed the anilox roll in the pan and employed a rubber roll to wipe excess ink off the anilox in an attempt to reduce the ink film before it moved onto the plate. While this method offered some improvement, it was speed sensitive. In addition, hydraulic forces pushed the meter roll away form the anilox roll allowing more ink to transfer to the plate.
Along with the ink -metering action of the nip (define) between the fountain and anilox rolls, it is possible to add a reverse-angle doctor blade to shear the ink from the surface of the anilox roll just beyond the ink metering nip. The doctor blade is usually made of spring steel, plastic or other synthetic material. In this type of system, the purpose of the single doctor blade is to increase the removal of surface ink and ensure a more controlled inking of the printing plates. On a single-blade doctor blade system, the amount of ink transferred to the anilox roll depends only on anilox volume. Ultimately less ink is used to print with the use of reverse angle doctor blades, which saves costs and raw materials and reduces VOM and HAP emissions as compared to the traditional two-roll system.
The two-roll system or the adaptation of the two-roll system to be used with doctor blades, use large fountain pans to hold the inks. These open pans, because of their large surface area, allow for significant evaporation of solvents into the drying system or surrounding area. Even with the use of fountain covers, considerable amounts of solvent still evaporate in a two-roll ink delivery system. However, converting a press from a two-roll system to the alternative reverse angle doctor blade system is quite costly.
In an enclosed system, two doctor blades are used. The reverse-angle blade is typically made of steel and the trailing blade is made of plastic. The blades are set about two inches apart, but this may vary with different manufacturers. The reverse angle blade acts as the true doctor blade and wipes excess ink from the anilox roll. The trailing blade acts as a capture or containment blade and holds the ink within the confines of the chamber. The blade oscillates back and forth to reduce wear on the rollers.
The blades are connected in a box-like enclosure with flexible material at both ends. With an enclosed doctor blade system that seals at both ends of the roll, ink is delivered from the ink pump to the doctor blade system and then back to the pump – essentially, a closed loop. Ink is usually pumped into the system at the middle of the chamber, but can be pumped at several locations on wide presses. A pan is generally placed beneath the anilox roll to capture ink splashes and cleaning fluids during cleanup at the end of a press run.
Flow Through System
Flow through doctor blade systems provides few of the environmental benefits of an enclosed doctor blade system. In a flow through system, the ink is pumped into the chamber, however, without end caps. The ink flows through the ends onto an open ink pan. Much like the two-roll system at this point, there is increased exposure of the ink to air promoting evaporation. The ink then drains back into the ink bucket. In terms of emissions, the flow-through system may approximate the emissions of single blade doctor blade and two-roll systems since, in each of these systems; ink is spread over the ink pan.
Best Management Practices & Pollution Prevention
Enclosed doctor blade systems (sometimes called chambered doctor blade systems) improve print quality and provide environmental benefits, along with environmental compliance advantages. Adding an enclosed doctor blade assembly to an older press or buying new equipment with the systems as standard equipment can reduce emissions of air pollutants. Use of enclosed doctor blades systems has been recognized a pollution prevention technology by the U.S. EPA.
While the press is idling idle, comparing a traditional two-roll ink feed system to an enclosed doctor blade system VOC emissions can be reduced by as much as 50 percent using the enclosed doctor blade system. During press operation, this difference is somewhat less, but still significant, at nearly 20 percent reduction in VOC emissions while using the enclosed doctor blade system. The enclosed system also reduces worker exposure to VOC emissions.
Some doctor blade systems are automated for wash-up while the doctor blade remains in the printing position. Automated cleaning systems can further reduce waste in several ways. Most systems utilize a first stage recirculation wash mode where used dirty water/solvent is used to do the initial cleaning. This dirty water/solvent is returned to a common tank after each wash and can be reused many times. If done properly, this first stage wash actually can do most of the cleaning. Clean water/solvent is used for a final rinse only.
For access to vendors who may supply alternative materials and equipment, see the PNEAC Vendor Directory.
VOCs and HAPs may be emitted from the inks being used. For more information see the Ink (hyperlink) section. Waste Water
If using water based inks, water containing ink and cleaning compounds are generated during the process of cleaning the ink metering systems parts. The wastewater may be restricted from discharging into the sanitary sewer depending on the type of inks and cleaning products used. Depending on the pigments in the waste (regulated metals) the waste may be classified as hazardous). The regulated metals differ among each state.
If using solvent, UV or EB curable inks wastes containing ink and cleaning solvent are generated during the process of cleaning the ink metering systems parts. The waste may be considered a hazardous waste due to flash point, F-listed solvents, or pigments, which may contain regulated metals. The regulated metals differ among each state.
Health & Safety
Many flexographic printing presses are equipped with metal doctor blades. This thin metal strip is sharp when new, and can become razor sharp after being used. Extreme care must be taken when cleaning or installing new blades in the blade holders. Wearing steel or fiber mesh gloves will prevent cuts. Plastic gloves are a poor alternative.
Doctor blades should be properly disposed of after removal. Since they are extremely sharp, blades should never be put in any container that a co-worker may reach into unknowingly.
Extreme caution must be used during press wash-ups. Usually wash-ups are done with cloth or paper rags that are easily cut by doctor blades.