Gravure Hot Melt Coating: Principles, Gravure Roll Design, and Process Parameters
Gravure coating is a traditional method adapted for hot melt adhesives, especially when a patterned or controlled low-volume coating is needed. In gravure hot melt coating, a heated engraved roll (gravure roll) picks up molten adhesive from a heated reservoir (tray or chamber). A doctor blade scrapes excess adhesive from the roll surface, leaving only the adhesive in the engraved cells. The adhesive then transfers to the substrate as the substrate is pressed against the gravure roll by a backup roll. This method is widely used for solvent-based and water-based coatings, but hot melt gravure is more challenging due to the need for precise temperature control and the higher viscosity of hot melts. However, it offers advantages in cost and simplicity for certain applications like tape and label primers.
The gravure roll is the heart of the machine. It is made of steel or ceramic, typically 150-300mm in diameter, with a surface engraved with cells. Cell shapes can be pyramid, quadrangular, trihelical, or honeycomb. The cell density is specified in lines per inch (LPI), ranging from 20 to 250 LPI. For hot melt adhesives (viscosity 1000-20000 mPa·s), lower LPI (20-80) and deeper cells are used because higher-viscosity fluids need larger openings to fill properly. The cell volume (measured in cm³/m² or BCM) determines the theoretical wet coating weight. For example, a 50 LPI roll with 60 BCM might deliver about 15-20 gsm of a 1 g/cm³ adhesive. The actual transfer efficiency is typically 60-80% because not all adhesive leaves the cells. Therefore, gravure coating weight is less precise (±5-10%) than slot die but sufficient for many applications.
Hot Melt Coating Machine - Hot Melt Adhesive Coating Machine
The doctor blade system is critical for uniform coating. The blade is a flexible steel strip (0.15-0.3mm thick) pressed against the gravure roll at an angle (typically 30-60°). It removes excess adhesive, leaving only the cells filled. For hot melts, the blade and the roll must be heated to prevent adhesive from solidifying on them. Blade wear is a major issue because hot melts can be abrasive; hardened steel or ceramic-coated blades are used. The blade pressure must be adjustable (typically 0.1-0.5 N/mm linear load). Too low pressure leaves a film on the roll surface (causing “whiskers”); too high pressure wears the roll and blade quickly. Some gravure coaters use a closed-chamber doctor system with two blades (doctor and sealing) for high-speed operation.
Temperature control in gravure hot melt coating is complex because the adhesive must be kept molten in the tray, on the roll, and during transfer. The gravure roll is internally heated by oil or electric cartridges. The backup roll may also be heated (to promote transfer) or cooled (to set the adhesive after transfer). The adhesive tray has heating elements and a circulation pump to maintain uniform temperature. The dwell time of adhesive in the tray is longer than in a slot die system, increasing the risk of degradation. Therefore, a “small-volume tray” with continuous replenishment is recommended. The transfer nip pressure (between gravure and backup roll) is typically 1-4 N/mm; higher pressure improves transfer but may deform the substrate.
Common defects in gravure hot melt coating and solutions: “Orange peel” (rough surface) occurs if the adhesive does not level before solidifying; increase temperature to lower viscosity or reduce line speed. “Missing dots” (voids) indicate that some cells did not fill; increase tray level or reduce doctor blade pressure. “Scumming” (streaks along the web) is caused by a damaged doctor blade; replace blade. “Gravure haze” (overall high coating weight) is due to the doctor blade worn or set too light; increase pressure or replace blade. “Pattern repeat” (periodic marks) indicates a damaged gravure roll cell; repair or replace roll. The gravure roll should be cleaned ultrasonically every week and inspected for wear. Because of the mechanical contact, gravure rolls have a limited life (500,000-1,000,000 meters) before re-engraving.
Despite lower precision compared to slot die, gravure hot melt coating remains popular for applications requiring high coat weights (50-200 gsm), for extremely low coat weights (2-5 gsm with fine rolls), or for patterns (e.g., stripe coatings using engraved channels). It is also simpler and cheaper for small runs. Many manufacturers use a gravure coater as a primer station before slot die coating. When selecting a gravure hot melt adhesive coating machine, consider the viscosity range, desired coat weight accuracy (±5% typical), and production speed (up to 200 m/min for gravure, higher for chambered systems). Proper maintenance of the roll and blade is essential for consistent quality. By understanding these principles, operators can effectively use gravure coating for hot melt adhesives.
