Precision coating is an advanced operation that encompasses sophisticated and highly controlled functions, including the distribution of liquid across a conveying web and the control of coating thickness in the down-web direction. Coatings with the greatest uniformity are generally executed with precise fluid metering, precision coating heads and tight conveyance tension controls rather than with thickness feedback controls. An operation’s capabilities are thus partially defined by the quality and accuracy of solution delivery systems, so to maximize product quality, an advanced solution delivery system is required.

Solution delivery systems should be able to:

• Supply the solution free of contaminants;
• Deliver an accurate and measurable flow rate;
• Provide a homogenous solution by mixing and purging;
• Control temperature and viscosity;
• Maintain system cleanliness;
• Accommodate machine stops and starts with minimum waste;
• Minimize product changeover times.

Each of these goals is satisfied with an operationally sound process that insures technical efficiency. From pressurizing the solution and purging the system through to mixing and recycling the solution, the steps must be clear and well-established. As new products are introduced on the coating line, this attention to detail and procedure will keep changeover from adversely affecting product quality or production time.

Breaking Down the Process

Pressurize the Solution: Provide a driving force that enables the solution to flow through system components. Critical factors for success include:

• Optimization of delivery line materials, size and geometry
• Knowledge of solution properties such as rheology and flow rate
• Appropriate pump selection
• An understanding of pressure drops between system components

Purge the System: Remove cleaning solutions, air, and dilute product solution from the delivery system prior to coating starts. Critical factors for success include:

• Minimization of product solution waste, purge time and system volume
• Purging solutions with higher viscosity and density than resident solutions
• Sequencing of in-line mixed components requiring special treatment
• Pumping up to aid in the purging of air

Filter the Solution: Filtration methods vary widely, from rolled approaches to cartridges, bags, disks and more. Critical factors for success include:

• Offline filtration tests prior to coating to understand plugging rate, pressure drop and filtration effectiveness
• Protection against flow bypass via component selection, system geometry and materials selection
• Location of deaeration components downstream of filtration components if purging the filter is a concern

Remove Air: Both entrained air (bubbles) and dissolved air (air in the solution) can cause lines, streaks and other spot defects. Critical factors for success include:

• Optimization of mixing and other upstream processes to avoid air entrainment
• Avoiding poor line geometries (loops, horizontal/downward sloping hoses, etc.) that trap air bubbles or cause dissolved air to flash out of solution into bubbles
• Proper line diameter selection
• Use of a staged approach to deaeration by removing gross levels of entrained and dissolved air and then utilizing a debubbling device near the coater for residual air levels

Control Flow: Depending on the type of coating method, either pre-metered or self-metered coating, flow control methods will differ slightly. For pre-metered coating, critical factors for success include:

• Use of a flow feedback control loop
• Consideration of soft-walled delivery lines to absorb pressure pulsations
• Monitoring flow pulsations to identify root causes

Mix the Solution:  Insufficient mixing can cause longitudinal coating streaks, so a precise means for creating the solution is an imperative. Streaks from insufficient dynamic mixing change location and intensity with time, while those from static mixing will appear in fixed locations. Critical factors for success include:

• Consideration of the shear sensitivity of product solutions
• Use of mixing data from mixer’s manufacturer and conduction of experiments if necessary

Control Temperature: Good coating solution temperature control is necessary to controlling solution viscosity and ensuring smooth delivery. In a printing press, for example, ink viscosity control is critical to successful operation. Critical factors for success include:

• Isothermal operation of all solution delivery components, including the coating equipment, to minimize temperature-related defect generation
• Mixing of reactive components that have unique temperature – property relationships
• Solution temperature versus conveyed web temperature

Clean the System: Thoroughly cleaning the entire solution delivery system after each coating project is an oft-overlooked step, but is as important as any other piece of the process. Insufficient cleaning can lead to repellencies and streaks, or even worse, functional product failure (such as inadequate curing, poor alignment, desensitization, etc.), poor delivery system reliability for future operations and decreased filtration performance – all of which ultimately lead to higher costs.

In order to insure smooth introduction of new products, a commitment to each of these steps is required for optimum product quality and cost control. A precision, impeccably maintained, and well-characterized solution delivery system must always be part of the package for high-precision coating.