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FLEXO Magazine : February 2009
TECHNOLOGIES & TECHNIQUES THE SELECTION PROCESS In order to identify the optimum substrate for a given job, the printing and converting characteristics of each substrate must be considered along with the substrate's ability to achieve the required packaging and product function characteristics. FIRST 4.0 outlines a systematic approach to achieving this objective. 1. Identify the packaging and product function requirements. 2. Determine the optimum substrate category-Le., pa- per, corrugated, film, foil, etc.-to achieve the product requirements. 3. Identify the appropriate group and subset of the substrate category. For example, corrugated, bleached white, E-flute. 4. Identify potential suppliers of defined substrate. 5. Request samples, specifications and pricing from each supplier. 6. Identify important substrate attributes influencing printing/ converting quality and compare them to the available prod- ucts. Suppliers should be able to provide specifications and tolerances for key printing/converting attributes. FIRST 4.0 lists many substrate properties that influence print quality, along with recommended procedures, specifications and tolerances. The specifications and tolerances listed in FIRST should be viewed as generally acceptable limits; tighter specs may be required for specific applications. If the appropriate specification and tolerance for a given application is un- known, it can be determined through optimization trials. 9. Implement a CoA (Certificate of Analysis) or CoC (Certificate of Compliance) program. This is an im- portant part of the process control system. A CoA/CoC report should be included with each substrate ship- ment; it should verify that the shipment conforms to defined specifications. If a substrate supplier has multiple manufacturing locations (mills, extrusion plants, etc.), determine which manufacturing locations are approved locations. Substrates produced by the same company at different manufacturing facilities must conform to the same specifications. This is critical if the variability intro- duced by the substrate, which influences print quality and product function, is to be minimized. PROPERTIES INFLUENCING PRINT QUALITY FIRST 4.0 separates substrate properties that affect print qual- ity into three categories: structural properties, surface properties and chemical properties. The properties identified below have a significant impact on print quality. STRUCTURAL PROPERTIES Clarity/Haze. Haze is the scattering of light by a transparent or translucent substrate that results in a milky appearance or reduced clarity of objects when viewed through the substrate. Variables influencing haze include the crystallinity and mo- lecular weight distribution of resins, using copolymers, gauge, processing temperatures during extrusion, and additives and coatings. The selection of a substrate for a particular job is based primarily on the finished product requirements, such as strength, flexibility, and protection. 7. Shorten the list of potential suppliers by comparing sup- plier specifications and tolerances against customer and printer requirements. 8. Utilize optimization trials, sample testing, and process vari- ability data (provided by substrate suppliers) to identify the supplier(s) whose product meets the necessary print and end-use requirements with the least amount of variability. The printer may choose to request process variability data both from within a production run and between produc- tion runs (for example: sampling shipments of the substrate for several months) to assess both types of variability. If the printer is considering using more than one supplier, or multiple mills/extrusion plants from a single supplier, the printer may choose to evaluate product and data from all potential sites simultaneously to ensure product compat- ibility and to identify any problems due to site-to-site prod- uct variability. - Dimensional Stability. Dimensional stability reflects the ability of a substrate to hold its original size, or constant dimension, in all directions when exposed to physical stress or variable mois- ture. Substrates with poor dimensional stability will not hold col- or-to-color register. Dimensional stability is especially important when water-based inks and unit print stations are used. Dirt/Gels. This defect appears as apparent dirt on a substrate influencing its aesthetic appearance. Dirt and gels can create print defects and voids. Size, frequency, color and location are typical criteria used for measuring visually. Flatness. Flatness, curl, or warp is defined as the departure of a substrate from a flat plane. If severe, it can contribute to registra- tion problems and other print defects. Formation. Formation is a structural property of paper prod- ucts. It is a measure of the uniformity of the fiber distribution in the paper. The higher the number, the more uniform the sheet. FEBRUARY 2009 www.flexography.org FLEXO
Sustainable Winter 2009