Pattern Coating with Hot Melt Adhesives: Rotary Screen, Slot Die Shim, and Gravure Technologies
Pattern coating—applying adhesive in a non-continuous pattern such as dots, stripes, grids, or custom shapes—enables products that require breathability, reduced adhesive usage, or selective bonding. The three dominant technologies for hot melt pattern coating are rotary screen printing, slot die with shim plates, and engraved gravure rolls. Each has distinct advantages in pattern complexity, resolution, speed, and coat weight control. Rotary screen printing uses a cylindrical mesh screen with open holes in the desired pattern; a heated squeegee forces adhesive through the holes onto the substrate. Slot die pattern coating uses a thin stainless steel shim with cut-out slots or holes clamped between the die halves; adhesive exits only where the shim is open, creating precise stripes or discrete dots. Gravure pattern coating uses an engraved roll with cells arranged in a pattern; the roll picks up adhesive from a pan, is doctored clean, and transfers the adhesive to the substrate. The choice among these methods depends on the required pattern geometry, coating weight, line speed, and budget. Universal Converting Equipment’s CL1600 wide web machine can apply hot melt adhesive using proximity coating slot dies or non-contact spray technology such as ITW Dynatec Dynafiber, with precise tension control for processing non-woven, paper, and plastic materials.
Rotary screen pattern coating is the most widely used method for high-speed, high-volume hygiene products such as diapers and sanitary napkins. The screen is a cylindrical frame with a fine mesh (typically 20-80 mesh) coated with a photosensitive emulsion. The desired pattern (dots, lines, swirls) is exposed and developed, creating open areas where hot melt can pass. The screen rotates in contact with the substrate, and a heated squeegee inside the screen pushes the adhesive through the open mesh. Dot patterns (e.g., 1mm diameter, 2mm spacing) are common for breathable lamination. Rotary screen can achieve pattern resolution down to 0.5mm and coat weights from 5-150 gsm. The open area (typically 15-40% coverage) allows moisture vapor to pass through while providing sufficient bonding. Key advantages include continuous high-speed operation (50-200 m/min), ability to produce very fine patterns, and relatively low tooling cost for pattern changes—a new screen must be made, but the process is standard. The rotary screen is heated to maintain adhesive fluidity, and the squeegee pressure controls dot height. For hygiene lines, the pattern is often applied intermittently (on-off) to match product length, requiring the screen drive to start and stop quickly. A novel hot-melt and other viscous fluid coating screen-printing method has been patented for applying patterned deposits for moving web and other substrates, including room temperature coatings.
Hot Melt Coating Machine - Hot Melt Adhesive Coating Machine
Slot die with shim plates offers the highest precision among pattern coating methods, with coat weight accuracy of ±2-3% and extremely sharp pattern edges. A shim is a thin stainless steel sheet (0.05-0.3mm thick) placed between the two halves of a slot die. The shim has cut-out areas where adhesive is allowed to exit; the rest of the shim blocks the slot. This creates a pattern of stripes or discrete patches. For example, a shim with 5mm open slots spaced 10mm apart produces 5mm-wide adhesive stripes. Dual pattern shim assemblies have been developed that permit various different overlapping or overlying deposition or application patterns, allowing complex multi-stripe configurations from a single die. The thickness of the shim determines the height of the slot opening, controlling the coating weight for a given pump flow and line speed. Slot die pattern coating is ideal for producing tapes with adhesive-free zones (e.g., double-sided tape with a center void), or for edge-coating only the edges of a web. The pattern edges are extremely clean, with no feathering or overspray. However, pattern changes require disassembling the die and replacing the shim—a 30-60 minute operation that is acceptable for long production runs but not for frequent changeovers. The die must be heated uniformly across its width; any temperature variation will cause uneven flow through the shim openings. Slot die pattern coating can run at very high speeds (up to 400 m/min) and achieve coating weights as low as 2 gsm. It is preferred for precision applications like medical patches and optical films.
Gravure pattern coating is the simplest and lowest-cost method, using an engraved roll with cells that form a pattern. The roll picks up adhesive, a doctor blade removes excess from the roll surface, leaving adhesive only in the cells, which then transfer to the substrate. Pattern resolution is limited by the cell engraving; typical dot sizes range from 0.5-2mm. Gravure is best suited for coarse patterns like random dots or large-area coverage with low precision requirements. It can achieve coat weights from 2-100 gsm, but accuracy is lower (±10-15% variation) compared to slot die or rotary screen. Gravure also suffers from “feathering” at pattern edges due to the way adhesive empties from cells, and the pattern may change over time as cells wear. For low-volume production or applications where appearance is not critical (e.g., basic nonwoven lamination for industrial wipes), gravure is a viable option. It offers the fastest pattern change—simply replace the engraved roll. Engraved rolls are typically made of steel or ceramic and may be chrome-plated for wear resistance. A rubber backing roll provides pressure to ensure transfer. Some hot melt coating lines combine multiple pattern methods: for example, a gravure pre-coater applies a thin primer pattern, followed by a slot die pattern coater for the main adhesive. The rotating bar slot die combined with a rubber backing roll provides the most efficient and controllable method for precision hot melt coating in the current state of the art, and the rubber roll compliance allows small gels to escape, preventing streaks.
Defect prevention in pattern coating is specific to each technology. For rotary screen, the most common defect is “missing dots” due to plugged screen holes. Regular cleaning with hot melt solvent and compressed air, or ultrasonic cleaning, is required. Screens have a limited life (10,000-50,000 impressions) and must be replaced when dot size becomes inconsistent. For slot die with shim, “smearing” occurs when the adhesive spreads beyond the shim opening because the temperature is too high (viscosity too low) or the die gap is too large relative to the coating thickness. Reducing temperature by 5-10°C usually resolves smearing. Another defect is “weeping,” where adhesive leaks from the closed portions of the shim because the die bolts are not tight enough or the shim is damaged. Ensure uniform torque on die assembly bolts (e.g., 20 Nm for M8 bolts). For gravure, “cell plugging” occurs when adhesive dries in the cells; use a heated doctor blade and keep the roll at operating temperature even when stopped. For all pattern coating methods, registration—aligning the pattern to printed marks or product features—may be required. A registration sensor (optical or ultrasonic) detects a mark, and the PLC adjusts the rotational position of the screen, shim die, or gravure roll accordingly. Achieving registration accuracy of ±0.5mm is typical. By selecting the appropriate pattern coating technology for the application and implementing robust cleaning and registration procedures, manufacturers efficiently produce breathable, cost-effective, and functionally superior coated products.
