Intermittent Hot Melt Coating: On-Demand Adhesive Application for Discrete Parts
Intermittent hot melt coating machines apply adhesive in a start-stop manner to coat discrete parts such as labels, medical patches, gaskets, or carton flaps. Unlike continuous web coating, the substrate may be fed as individual sheets or as a web that stops or marks for pattern application. Common technologies include electrically or pneumatically actuated valves that open and close rapidly, reciprocating slot dies, or rotary cutters. The key challenge is achieving clean cutoff without stringing or dripping. Intermittent coating is essential for products requiring adhesive only on certain zones, reducing waste and preventing adhesive on die-cut waste.
Valve-based intermittent coaters use multiple nozzles or a single slot die with a shut-off valve. When the valve opens, pressurized hot melt adhesive flows onto the substrate; when closed, flow stops. The valve response time is critical: typical open/close times are 5-20 milliseconds. For high-speed intermittent coating (e.g., 200 cycles per minute), valves with short stroke and low-mass needle are used. The adhesive pressure must be regulated (2-10 MPa) to ensure rapid shut-off without drool. After closing, a slight suck-back or reverse pulse may be applied to pull the adhesive back from the nozzle tip, preventing stringing. The nozzle tip is heated to keep adhesive molten but should not contact the substrate.
Hot Melt Coating Machine - Hot Melt Adhesive Coating Machine
Reciprocating slot die coating: The die is mounted on a linear actuator that brings the die into contact with the substrate (or near-contact) for a defined time, then retracts. This method applies adhesive in a precise rectangular patch. The coating thickness is determined by pump flow and dwell time. It is used for coating gaskets onto housings or adhesive patches onto medical dressings. The actuator must have fast acceleration (1-2 m/s²) and precise positioning (±0.1 mm). Cycle times can be 1-3 seconds per patch. To prevent stringing during retraction, the die may have a spring-loaded lip that closes slightly. This method is slower but gives high coat weight uniformity.
Applications: In labeling, intermittent coating applies adhesive only to the label area on a liner, not between labels, allowing easy die-cutting. The machine coats a pattern, then the web advances, stops, and coats again. This requires precise registration using a mark sensor. For medical patches, intermittent coating deposits a precise amount of pressure-sensitive adhesive onto a release liner, then the patch material is laminated. For carton sealing, intermittent coating applies glue to flaps at intervals. In each case, the machine must handle the web start-stop dynamics, which cause tension spikes. An accumulator (dancer) or a servo-driven pull roll with controlled acceleration/deceleration ramps is needed to smooth tension.
Defects in intermittent coating: "Stringing" (adhesive thread between coated areas) is common when valve closure is too slow or adhesive viscosity too low. Increase suck-back pressure or reduce temperature (increase viscosity). "Drooling" (adhesive leaks when valve closed) indicates worn valve seat or insufficient back pressure. Replace valve or adjust. "Inconsistent patch size" may be due to pressure fluctuations; install a pressure regulator and accumulator near the valve. "Missing coating" if valve fails to open; check solenoid and pilot pressure. Regular maintenance includes cleaning valve needles and seats, replacing O-rings, and checking electrical response time. Use a strobe light to observe the adhesive jet during operation.
Control systems for intermittent coating: Programmable logic controllers (PLC) with high-speed counters trigger the valve based on encoder position. For web stop-start, the system uses a servo motor with precise indexing. The coating recipe includes on-time, off-time, and delay from registration mark. Advanced systems use vision feedback to adjust timing dynamically. For multi-lane intermittent coating (e.g., several label strips), each valve can be independently controlled. Operators must ensure that the adhesive remains stable during idle times; a recirculation circuit keeps adhesive flowing when valves are closed to prevent degradation. By understanding these principles, intermittent hot melt coating machines can apply adhesive precisely and cleanly to discrete products.
