In the process of web fed printing, the metal parts of the printing press, including the rollers which pull the substrate through the press, begin to heat up due to friction, conductivity of the metal, and convection drying. When heat builds up on the rollers the ink can become tacky (begin to dry) and cause print quality problems. Additionally the heat build up can weaken both paper and film substrates. When subjected to heat, polymeric films can soften, lose strength and stretch. Although heat is required to complete some printing operations, equipment temperatures must be carefully controlled, and this necessitates the use of cooling rolls.
To combat the problem of heat build up many flexographic printing presses are equipped with liquid cooled, self-contained chilling systems. These chilling systems include a refrigeration unit, separate from the press, which cools the liquid (typically water) and circulates the chilled water through the rollers that pull the substrate through the press. The partially warmed water, due to contact with the metal rolls, is then recirculated back to the chilling unit.
In a cooling roll, coolant flows in one end, fills the roll about half full, and flows out the opposite end. If more substrate cooling is required, a self-venting roll may be used. In this design, most of the air is automatically exhausted as the roll rotates and most of the roll body is in contact with the coolant.
Best Management Practices & Pollution Prevention
An improvement in substrate cooling and roll efficiency is the double-bodied roll. The coolant is forced to flow in an angular or ringed space. Consequently, more flow velocity control can be designed into the roll, resulting in more heat transfer from the substrate. A refinement to the double-bodied roll is the spiral wound coolant chamber.
Cooling rolls can be devised by using other rolls as substitutes, such as uncovered fountain rolls, plate cylinders or impression cylinders, in positions following the dryers between the last dryer and rewind or where a temperature reduction is necessary.
The best way to minimize the amount of energy needed to cool substrates is to minimize the amount of heat needed to dry the substrate (see Dryers).
Alternatives to previously-used chlorofluorcarbon (CFC) refrigerant products are now available. These are much less harmful to the ozone layer. If the refrigerant must be replaced in the chiller unit, the properly trained and licensed technician performing the replacement will use one of these products.
For access to vendors who may supply alternative materials and equipment, see the PNEAC Vendor Directory.
Additives, similar to those added to boilers, to control bacteria and algae growth may be added to the chilling water. Glycol based chemicals may also be added to prevent freezing of the fluid if overcooled. These additives may be hazardous. When replacing water in the chilling unit, the waste water may be prohibited from discharge into the sanitary sewer if the water has been treated with hazardous additives. The MSDS of the additive products and the local discharge regulations should be evaluated prior discharging any of this water.
Chillers may contain refrigerant chemicals that contribute to the depletion of the ozone layer which encompasses the Earth in the upper atmosphere. The use and handling of these chemicals is strictly regulated by US EPA and state environmental regulatory agencies. According to 40 CFR Part 82, if your organization has equipment, including vehicles that contain more that 50 total pounds of some refrigerants, you may be covered by this rule.
If the equipment contains a Class I or Class II ozone-depleting substance it must be labeled. See 40 CFR Part 82, Subpart E.
If maintenance must be performed on the chiller (regardless of the amount of refrigerent), it must be completed by an individual who has been properly trained and licensed according to EPA handling and recovery requirements. At no time is refrigerent allowed to be simply vented to the atmosphere; it must be recovered for reuse. See 40 CFR Part 82 Subpart F.
Halon fire extinguishing systems including permanent and portable fire extinguishing systems are covered by these regulations as well. Halon fire extinguishing systems were commonly provided in areas where electrical or computer equipment is permanently stored. This could include areas of the facility such as the computer room or press electrical panels.
Lists of ozone depleting substances
Health & Safety
Formaldehyde and vinyl acetate are hazardous materials and, if possible, should be eliminated from the adhesive formulation. If the formaldehyde and vinyl acetate cannot be eliminated from the glue formulation, employees should be monitored for exposure to excessive levels of formaldehyde and vinyl acetate, especially since these materials can lead to long term health affects.