High-Speed Hot Melt Coating for Diapers and Sanitary Napkins: Construction and Elastic Attachment
Disposable diapers and sanitary napkins are assembled using multiple hot melt adhesive applications. A typical diaper line includes: (1) construction lamination – bonding the top sheet (nonwoven) to the acquisition layer and backsheet, (2) elastic attachment – gluing elastic strands to leg cuffs and waistbands, (3) core stabilization – spraying adhesive on fluff pulp or superabsorbent polymer (SAP), and (4) tape attachment. Each requires a specific hot melt coating machine configuration. Construction lamination uses full-width pattern coating (e.g., spiral spray or dot pattern) at low coat weight (2-5 gsm) to maintain breathability. Elastic attachment uses high-speed intermittent coating (up to 1200 cycles/min) with small nozzles. The machines must run at 300-600 m/min and cycle with product length (e.g., 60 products/min on a 0.5 m pitch). Reliability is paramount; a stoppage of just 1 minute wastes hundreds of products.
The dominant coating technology for hygiene is the spiral spray (also called swirl or omega spray). A heated nozzle uses compressed air to atomize the hot melt into a fine spiral pattern. The pattern width can be adjusted from 10 to 100 mm. The coat weight is controlled by pump speed and line speed. Spiral spray gives excellent coverage with low adhesive usage (2-5 gsm) and maintains breathability. The nozzle has an electric heater and a thermocouple. The valve opens and closes rapidly (5-15 ms) to create intermittent patterns. The air pressure for atomization is 0.5-2 bar. The adhesive temperature is 140-160°C, viscosity 1000-3000 mPa·s. For elastic attachment, a smaller nozzle (e.g., 0.3 mm orifice) applies a thin bead or swirl directly onto the stretched elastic strand. The strand is then laid onto the nonwoven.
Hot Melt Coating Machine - Hot Melt Adhesive Coating Machine
Slot die pattern coating is also used for hygiene, especially for dot patterns. A slot die with a shim creates discrete dots (e.g., 1 mm dots at 3 mm spacing). The die is mounted close to the web, and a vacuum roll holds the nonwoven flat. Dot pattern reduces adhesive usage further (1-3 gsm) and gives even softer feel. Rotary screen coaters are used for larger dots or stripes. Each method has trade-offs: spiral spray is versatile and forgiving, but produces some overspray; slot die dots are cleaner but require precise alignment. Many hygiene lines use a combination: construction lamination with spiral spray, elastic attachment with small slot die or spray, and core stabilization with spray.
Adhesive selection for hygiene: The adhesive must be skin-safe, hypoallergenic, and have low odor. SBC-based adhesives (SIS or SBS) are common, with softening points 70-100°C. For elastic attachment, the adhesive must have high creep resistance (hold the stretched elastic without snapping back). Usually a higher molecular weight SBC or adding tackifiers. For core stabilization, the adhesive must be compatible with SAP and not agglomerate. The hot melt coating machine’s melt tank should be designed for continuous feeding from bags or boxes. A grid melter is typical to melt adhesive from solid to liquid without overheating. The tank capacity may be 100-200 kg to run for 8 hours.
Control system: The hygiene hot melt coating machine is integrated with the production line PLC via high-speed fieldbus. The encoder tracks the line speed and product position. The pattern is triggered by a product length counter or a registration mark. For elastic attachment, the adhesive pattern is applied while the elastic is stretched; after relaxation, the pattern compresses. The control system must calculate the stretch ratio. For example, if the elastic is stretched 3:1, a 10 mm adhesive stripe on the stretched elastic becomes 30 mm on the relaxed product. Many systems have a “stretch compensation” parameter. Operators can adjust the pattern length via HMI. A vision system checks for adhesive presence and pattern integrity; missing glue triggers rejection.
Defects and solutions: “Glue ear” – adhesive build-up on nonwoven edge; adjust pattern width or clean nozzle. “Stringing” – adhesive threads; increase nozzle temperature or reduce air pressure. “Low bond” – insufficient adhesive or contamination; check filters. “Elastic pop-out” – adhesive fails to hold; increase coat weight or change adhesive. “Nozzle clogging” – carbonized adhesive; clean nozzle with solvent or replace. Preventive maintenance includes daily nozzle cleaning, weekly filter changes, and monthly calibration of temperature sensors. The hygiene industry demands high efficiency; typical hot melt coating systems achieve 98-99% uptime with proper maintenance. By mastering these technologies, manufacturers produce comfortable, reliable hygiene products efficiently.
