Flexographic Printing and VOC Control

The printing industry reaches across many products familiar to everyone. Products consumers use every day are made utilizing some type of printing including food packaging, labels, and tapes.

The main industrial printing processes are:

Offset lithography
Flexography
Digital printing: inkjet & xerography
Gravure
Screen printing
Package Printing

Flexographic Printing

Flexography is a process used primarily for printing on a variety of different substrates including plastic, foil, acetate film, and paper. Typical products printed using flexography include brown corrugated boxes, flexible packaging including retail and shopping bags, food and hygiene bags and sacks, milk and beverage cartons, flexible plastics, self-adhesive labels, disposable cups and containers, envelopes and wallpaper.

A flexographic print is made by creating a positive mirrored master of the required image as a 3D relief in a rubber or polymer material. This flexographic plate is inked and the substrate is finally sandwiched between the plate cylinder and the impression cylinder to transfer the image. The roll-fed substrate is then fed through a dryer which allows the inks to dry before the surface is touched again. Flexography uses very fluid (low viscosity) inks with a high volume percent of organic solvent. These inks contain volatile organic compounds (VOCs) such as alcohols, glycols, esters, hydrocarbons and ethers which when evaporated during the printing process, react in the atmosphere to form ozone which can cause adverse effects on human health.

The major emission sources in flexographic printing operations are:

Coating application lines
Flash-off areas
Drying ovens

For both new and existing sources, technologies available to achieve the emission standards include:

Applying materials (coatings and thinners) that meet the emission limit, either individually or collectively,
Using a capture system and add-on control equipment to meet the emission limit, or
Using a combination of both approaches.

The most typical add-on control equipment is an oxidation solution to eliminate VOCs emitted from metal decorating applications. Oxidation of VOCs and other air pollutants works by simply oxidizing the air pollutants with oxygen and heat. In this environment the VOCs are converted to harmless inert byproducts like CO2, water vapor (H2O), and usable heat. These harmless by-products are released to the atmosphere or used within primary or secondary energy recovery techniques to further lower the operational costs.

A Regenerative Thermal Oxidizer (RTO) is recognized as the most effective and cost-efficient way to destroy VOCs from these processes. Each Regenerative Thermal Oxidizer (RTO) system incorporates specialized ceramic media in the regenerator (heat transfer bed) to allow thermal efficiencies up to 97%. RTOs operate by preheating the process air as it passes upward through the first ceramic heat exchange media. This pre-heated air then exits the heat exchange media and enters a combustion chamber where it is heated to the setpoint temperature and oxidization of the VOC’s is completed. Hot, clean air is then passed downward through the second ceramic heat exchange media bed where it is cooled. This regeneration of the heat exchanger allows for much higher thermal efficiencies than traditional recuperative style heat exchangers.

Additional systems such as floor sweeps, hoods, and close capture systems are often used to collect solvents that evaporate from other parts of the printing press (ink room, distillation system, etc), in addition to those collected from the dryer.