Cooling and Rewinding in Hot Melt Coating: Chill Roll Design, Winding Strategies, and Blocking Prevention
After hot melt adhesive is applied and the composite is laminated, the coated web must be cooled sufficiently to solidify the adhesive before rewinding. Without adequate cooling, the adhesive may be tacky, causing the layers to stick together (blocking) when wound under pressure. The cooling process typically uses one or more chill rolls (also called cooling rolls) through which chilled water (5-15°C) circulates. The web is wrapped around the chill roll for 180-270° of contact to maximize heat transfer. The cooling rate determines the crystallization or solidification of the adhesive. For semi-crystalline polymers like EVA and polyolefin, rapid cooling produces smaller crystals and a clearer film but may reduce immediate bond strength; slower cooling allows larger crystals to form, increasing bond strength but risking blocking. The balance is typically achieved by setting the chill roll temperature to 10-20°C for most PSAs and 20-30°C for thicker coatings. The chill roll surface is chrome-plated and polished (Ra <0.05μm) to prevent adhesive sticking, and a doctor blade may be used to clean the roll surface. Hot Melt Coating Machines comprise A.C digital drives for main drive and rewind, a powder brake for unwind, and two chill rollers for dry coating, providing maximum line speed of 100 MPM.
Chill roll design parameters: The heat transfer rate Q (W) required to cool the adhesive is Q = C × M × ΔT, where C is specific heat of adhesive (typically 1.5-2.5 J/g·K), M is mass flow rate (gsm × width × speed), and ΔT is temperature drop (from application temperature to required winding temperature, e.g., 150°C to 30°C = 120°C). For a 1600mm wide line coating 20 gsm at 200 m/min, M = (0.020 kg/m² × 1.6 m × 200 m/min × 1/60) = 0.107 kg/s. Q = 2.0 × 0.107 × 120 = 25.7 kW. The chill roll must remove 25.7 kW of heat. A single 500mm diameter roll with 270° wrap and water flow of 50 L/min may achieve this. For higher speeds or thicker coatings, multiple chill rolls (e.g., two 400mm rolls in series) or a larger diameter roll (600-800mm) are used. The coolant flow rate determines the ability of a roll to remove the required heat and temperature variation across the roll face. The chilled water temperature is controlled by a chiller unit with accuracy ±1°C. For energy-efficient operation, Howie Machinery‘s high-speed coating line uses spiral water cooling for composite and cooling rollers (Φ215mm and Φ600mm) to maximize heat transfer with minimal water consumption. The hot melt extrusion coating machine is mainly suitable for high viscosity hot melt adhesive coating such as PA, PES, PU, EVA, and TPU. After coating, the prepared hot melt adhesive film is cooled rapidly through a cooling device to achieve the required mechanical properties.
Hot Melt Coating Machine - Hot Melt Adhesive Coating Machine
Multiple chill roll configurations: For thin coatings (under 10 gsm) on high-speed lines (over 200 m/min), a single chill roll may suffice. For thick coatings (over 50 gsm), two or three chill rolls in an “S” wrap pattern are used to increase cooling capacity without excessive roll diameter. Chill rolls are essentially heat transfer rollers that run a cold fluid; multiple chill rolls in an “S” pattern are frequently required to deliver thorough cooling. Each roll may have independent temperature control; the first roll may be set to 20°C to solidify the surface, the second to 10°C to cool the core, and the third to 5°C for final cooling. For PUR adhesives that cure by moisture, the cooling rolls may be eliminated or set to a higher temperature (30-40°C) to allow the adhesive to remain in a reactive state longer. However, even for PUR, some cooling is needed to prevent blocking in the winder. For applications requiring a matte or textured surface on the adhesive, the chill roll may be engraved with a pattern; the adhesive solidifies against the pattern, picking up the texture. The CL350 narrow width hot melt coater laminator features water-cooled coating rollers, a precision die mount, and motorized adjustment for die positioning, designed for a range of coating technologies.
Rewinding process: After cooling, the web is wound into rolls. The rewinding station must handle the coated web without damaging the adhesive or creating defects. For products with a release liner (e.g., label stock), the liner prevents direct adhesive-to-adhesive contact; the winder can use conventional tension control. For linerless products (e.g., single-sided tape), the adhesive is on the outside of the roll, touching the back of the previous layer. To prevent blocking (permanent sticking), the adhesive must be cooled below its softening point (typically <40°C) and the winding tension must be minimized. The rewinding tension is controlled using taper tension: tension decreases as roll diameter increases. The recommended linear taper setting range is a tension decrease of 3-5% for every 10 cm increase in roll diameter. The thicker the material, the larger the required taper; the taper adjustment should be proportional to the material thickness. For a 400mm diameter roll, the tension at the core (e.g., 100 N total) may taper to 60 N at full roll (40% reduction). For very tacky adhesives (e.g., high-tack PSA), the roll may be wound with interleaving paper or film between layers; this requires an additional unwind for the interleaf. For double-sided tapes (adhesive on both sides), the tape is wound with a release liner; the rewinder must handle two webs simultaneously (the tape and the liner) and maintain alignment.
Taper tension control algorithms: Modern rewind drives use a formula: T_setpoint = T_base × (D_core / D_roll)^K, where K is taper exponent (typically 0.2-0.5). For K=0.5, tension drops as the square root of diameter. The drive calculates D_roll from line speed and roll rotation speed (derived from an encoder on the rewind shaft). During acceleration and deceleration, inertia compensation adds extra torque to overcome roll inertia, maintaining constant tension. For high-speed turret rewinders, the PLC controls the transfer from a full roll to a new core. A lay-on roll (also called a nip roll) applies pressure to expel air between layers and ensure a tight, block-free roll. The lay-on roll pressure also tapers down as the roll diameter increases. For stretchy substrates, the lay-on roll may be covered with soft rubber (Shore A 30-50) to avoid marking. If the rewind tension is too high, the roll may become “tight” and cause blocking or telescoping; if too low, the roll may be loose and telescoping (edges protruding). The observation of the end face shape helps determine if taper tension is properly adjusted: a smooth end face without “chrysanthemum pattern” or “star shape” deformation indicates correct tension. It is recommended to reduce the tension to 70% immediately during roll change and restore it after 2 seconds to prevent film breakage.
Defects related to cooling and rewinding: “Blocking” (layers sticking together) - increase cooling (reduce chill roll temperature or add a second chill roll), reduce winding tension, or apply a release coating to the back side of the substrate. “Telescoping” (roll edges protruding) - taper tension too low or too high; adjust taper factor, check alignment of rewind shaft and lay-on roll. “Wrinkles” in rewound roll - incoming web has wrinkles; add spreader roll before rewind, or reduce winding tension. “Adhesive squeeze-out” at roll edges - coat weight too high near edges; correct die profile, or trim edges before rewinding. “Roll hardness variation” (soft and hard spots) - uneven winding tension; check dancer roll operation and rewind drive calibration. For all defects, log the rewind parameters (tension setpoint, actual tension, lay-on pressure, roll diameter) for analysis. Preventive maintenance includes cleaning the lay-on roll weekly, checking the rewind drive belt tension monthly, and calibrating the roll diameter sensor annually. By properly designing the cooling section and optimizing rewinding parameters, hot melt coating machines produce finished rolls that are ready for slitting, converting, or shipping, with no defects and maximum customer satisfaction.
