High-Speed Intermittent Pattern Coating for Hygiene Products: Rotary Screen Optimization
In disposable hygiene manufacturing (diapers, sanitary napkins, adult incontinence products), pattern coating is applied intermittently—turning on and off at frequencies up to 1200 cycles per minute—to match product length while the web runs continuously at 200-500 m/min. Rotary screen pattern coating is the dominant technology for these high-speed intermittent applications. The screen rotates only when adhesive is needed; during the “off” period, the screen stops or the adhesive flow is diverted. The challenge is to achieve clean starts and stops without stringing (adhesive threads between dots) and to maintain consistent dot geometry across millions of cycles. The pattern typically consists of dots or lines for elastic attachment (leg elastics, waistbands) and spiral or dot patterns for construction lamination (bonding top sheet to acquisition layer). The rotary screen is driven by a servo motor that can accelerate from zero to full speed in milliseconds, synchronized with the product length encoder. The squeegee inside the screen applies pressure only during the “on” cycle; a valve may control adhesive flow to the squeegee. The combination of precise servo control, heated screen, and optimized adhesive rheology enables reliable intermittent pattern coating at hygiene line speeds. Nordson’s Control Coat QE applicators are designed for high-speed intermittent operations, delivering outstanding cutoff performance even at the highest line speeds.
Key process parameters for intermittent rotary screen pattern coating include: screen mesh count (20-80 mesh for SBC-based PSAs), emulsion thickness (typically 100-300μm, which determines dot height), squeegee pressure (0.2-0.6 MPa, controlling adhesive throughput), and adhesive temperature (140-170°C for hygiene-grade PSAs). Dot geometry—diameter, height, and spacing—must be consistent product to product. For elastic attachment, dot diameter is typically 0.8-1.5mm, dot spacing 1.5-3mm, and coverage area 15-25%. The adhesive’s open time must be short (0.5-1 second) because the elastic strand is applied immediately after the adhesive dots are deposited. If open time is too long, the adhesive may bleed through the nonwoven or lose tack before the elastic contacts it. If too short, the bond strength is insufficient, causing “elastic pop-out.” The viscosity of the adhesive at application temperature is typically 1000-3000 mPa·s for spray and 2000-8000 mPa·s for rotary screen. Lower viscosity allows finer dots but increases the risk of strike-through (adhesive penetrating the nonwoven). The screen must be heated to maintain consistent viscosity; usually, the screen is heated via internal oil circulation or cartridge heaters embedded in the screen frame. Temperature uniformity across the screen width is critical; a 2°C cold spot will cause missing dots in that zone. The squeegee blade (polyurethane or metal) wears over time; dot height decreases as the blade wears, requiring periodic replacement (every 1000-2000 hours). Some advanced systems monitor dot height with a laser sensor and automatically adjust squeegee pressure to compensate for wear.
Hot Melt Coating Machine - Hot Melt Adhesive Coating Machine
Intermittent operation introduces additional dynamics: when the screen stops, the adhesive in the open mesh may begin to cool and solidify, potentially clogging the holes. To prevent this, the screen may have a “standby temperature” slightly lower than operating temperature, or the squeegee may be retracted when not coating. Some systems use a recirculation pump to keep adhesive flowing even when the screen is stopped, preventing stagnation. The start-up sequence is critical: the screen must accelerate to match web speed before the adhesive is applied; otherwise, the pattern will be smeared or stretched. The control system uses an encoder on the pull roll to measure web speed and position. The screen servo motor is slaved to the encoder with a programmable acceleration profile. When the encoder reaches a position trigger (e.g., product start), the screen accelerates, the squeegee engages, and adhesive is applied. At the end of the product, the squeegee retracts, the screen decelerates to a stop, and the pattern ends cleanly. The delay between triggers must be adjusted to compensate for mechanical response times (typically 5-10 milliseconds). For some hygiene lines, the pattern length may be shorter than the screen circumference; the screen rotates only a portion of a full revolution, then reverses to the starting position for the next product. This “reciprocating” mode requires even more precise servo control and is used for applications where the pattern is much shorter than the screen, such as small patches for sanitary napkins.
Adhesive properties for intermittent pattern coating are specialized. The adhesive must have good “suck-back” characteristics—it should not continue to flow after the screen stops. This is achieved by using a valve with a reverse stroke (suck-back) that pulls adhesive back from the screen holes at the end of the cycle. The suck-back volume and speed must be tuned; too little suck-back leaves adhesive protruding from the screen, causing stringing; too much suck-back pulls adhesive out of the holes, causing missing dots on the next cycle. The adhesive should also have a high “green strength” (tack immediately after deposition) to hold the elastic in place before the bond fully develops. SBC-based PSAs with a high styrene content provide excellent green strength. For core stabilization (bonding superabsorbent polymer (SAP) to the fluff pulp), a lower-tack adhesive is used to prevent blocking of the pulp. The adhesive is often applied as a fine spray (not rotary screen) for core stabilization because the pattern must be random, not structured. Many hygiene lines combine rotary screen for elastic attachment, spiral spray for construction lamination, and spray guns for core stabilization—all controlled by the same PLC.
Troubleshooting intermittent pattern coating defects: Missing dots are often caused by clogged screen holes; clean the screen with hot melt solvent or replace the screen. Stringing (adhesive threads between dots) indicates excessive suck-back or temperature too high; reduce suck-back volume or lower temperature. Inconsistent dot size across width suggests uneven squeegee pressure or temperature variation; adjust squeegee blade leveling and check zone heaters. Dot smearing (elongated in the web direction) indicates screen speed mismatch with web speed; check encoder calibration and servo tuning. Adhesive strike-through (dots visible on the opposite side of the nonwoven) indicates viscosity too low (temperature too high) or coat weight too high; reduce temperature or adjust squeegee pressure. Elastic pop-out (the elastic strand detaches from the adhesive) indicates insufficient bond strength; increase adhesive temperature for better wetting, increase coat weight (dot size), or change to a higher-tack adhesive. For sanitary napkins, which are thinner and more delicate than diapers, even lower coat weights (1-3 gsm) and smaller dots are used. By optimizing these parameters, hygiene manufacturers achieve reliable intermittent pattern coating that meets the demanding requirements of high-speed diaper and sanitary napkin production lines, with product quality that ensures comfortable wear and reliable performance.
