Flexo Coater Printer Precision Coating: Gravure vs. Flexo Coating Heads for PSA and Barrier Layers
Within a flexo coater printer, the coating head can utilize either a flexo-like anilox system or a gravure coating system (engraved roll). Both methods have distinct advantages for different coating applications. This article provides a technical comparison to guide the selection of the appropriate coating head for specific products like pressure-sensitive adhesives (PSA) and barrier coatings.
Flexo coating heads use an anilox roller and a doctor blade to meter the coating, similar to a printing deck. The coating is transferred from the anilox to a rubber applicator roller or directly to the web (if the web is in contact with the anilox). This method is simple, easy to clean, and can handle a wide viscosity range. The coating thickness is controlled by the anilox volume and the transfer ratio. However, the pattern of the anilox cells may create a faint "rosette" pattern on the coating, which is acceptable for adhesives but may be undesirable for clear barrier layers. The maximum coat weight is limited by the anilox volume; heavy coatings require very low line screens, which can produce a non-uniform surface.
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Gravure coating heads use an engraved cylinder (similar to a gravure printing cylinder) that rotates in a coating bath, and a doctor blade scrapes off excess, leaving the cells filled. The coated web contacts the engraved cylinder directly, and the coating is transferred via a nip. Gravure coating offers superior uniformity and smoothness, as the cells are finer and more regular, and there is no transfer step that introduces variability. It is the preferred method for high-quality barrier coatings (e.g., PVDC, EVOH) where pinholes are unacceptable. However, gravure cylinders are expensive to engrave and are dedicated to a specific coating pattern (continuous or pattern); they are less flexible for quick changeover.
Pattern coating capability: Flexo heads can be easily set up for pattern coating (e.g., only applying adhesive to specific areas) by using a plate with raised areas that pick up coating from the anilox and transfer it to the web. This is ideal for "strip adhesive" or "spot coatings." Gravure heads can also be engraved with patterns, but the cost is higher, and the pattern is fixed. Thus, flexo heads are more versatile for short-run pattern jobs, while gravure is better for long-run continuous coatings.
Speed and viscosity range: Flexo heads can operate at higher speeds (up to 600 m/min) with water-based and solvent-based coatings, and can handle very high viscosities (hot-melt up to 50,000 cps) when using a heated anilox. Gravure heads are limited to lower viscosities (up to 5,000 cps) and speeds up to 400 m/min for thick coatings, though they are excellent for low-viscosity solvent systems. For hot-melt adhesives, flexo heads are almost universally used because the anilox can be heated to maintain the melt viscosity.
Cleaning and changeover: Flexo heads are easier to clean because the anilox roller and chamber can be removed and washed; the doctor blade is inexpensive and replaceable. Gravure heads require cleaning the engraved cylinder, which is more time-consuming and may damage the engraving if harsh cleaners are used. For plants that change coating types frequently, flexo heads are preferred.
Cost: Flexo heads have lower initial cost and lower tooling cost (anilox rollers are cheaper than engraved cylinders). However, for very long runs (millions of meters), the higher durability of gravure cylinders (which last 5-10 times longer than anilox) may offset the initial cost.
Application-specific recommendation: For PSA label production with pattern adhesive, a flexo coating head is the standard. For high-barrier transparent films, a gravure head is often chosen. Some advanced
flexo coater printers offer both heads in a single machine, with a quick-change mechanism to swap between them, providing maximum flexibility. By understanding the strengths of each method, converters can select the optimal coating head for their specific products, balancing quality, cost, and productivity.
