Intermittent hot melt coating machine
An intermittent hot melt coating machine applies adhesive in a non-continuous manner – starting and stopping the adhesive flow to create coated patches, spots, or stripes separated by uncoated gaps. This is essential for products that require adhesive only on specific areas, such as diaper front tabs, envelope seal flaps, medical patch backing, or product samples attached to a card. Intermittent coating can be achieved by several methods: a slot die with a fast-acting pneumatic or servo valve that opens and closes the flow; a roll coater with a segmented applicator roll; or a spray gun with on-off control. The machine must have a control system that synchronizes the adhesive timing with the substrate position, often using a web encoder and programmable logic controller (PLC).
The working principle: In a typical intermittent slot die system, the die is supplied by a gear pump running continuously, but a valve is mounted between the pump and the die. The valve opens for a specified time (e.g., 50 milliseconds) to allow adhesive to flow, then closes. The moving substrate passes under the die, and each valve opening produces a discrete patch of coating. The length of the patch is determined by the valve open time multiplied by line speed. The gap between patches is determined by the valve closed time. For intermittent spray systems, a similar valve controls the air and adhesive flow. High-speed intermittent coating requires valves with response times under 10 milliseconds. Some machines use a "suck-back" mechanism to retract adhesive at the die lip to prevent drooling during the closed phase.
Hot Melt Coating Machine - Hot Melt Adhesive Coating Machine
Typical applications: 1) Diaper manufacturing – intermittent coating of front tape pads (where the diaper tab adheres) and leg elastic attachment. 2) Envelopes – applying adhesive only on the seal flap, not the entire envelope. 3) Medical patches – coating adhesive only on the patch area, leaving a non-adhesive handling tab. 4) Product sampling – applying a spot of adhesive to attach a sample sachet to a magazine page. 5) Bookbinding – intermittent glue application for spine reinforcement at intervals. 6) Labeling – spot labeling for removable labels. Intermittent coating saves adhesive and allows functional design where only specific zones need bonding.
Key parameters: The most critical are valve response time (must be fast enough for desired patch length at line speed), adhesive viscosity (should be low to medium, 500-5000 cP, for fast valve closure to avoid stringing), and temperature stability (to maintain consistent viscosity). The minimum patch length is limited by valve dynamics – for a pneumatic valve, typical minimum open time is 15-25 ms, giving at 300 m/min a patch length of 75-125 mm. Servo valves can achieve 5 ms open time. For very short patches (e.g., 10mm), a rotary valve or a screen-based intermittent system is used. The repeatability of patch length and spacing is typically within ±1mm for well-tuned systems.
Control systems: Modern intermittent hot melt coaters use a PLC with encoder feedback from the web drive. The operator programs patch length, gap length, and number of patches per revolution. The PLC outputs a signal to a solenoid valve (for pneumatic) or directly to a servo valve. Some systems include a "pattern verification" sensor (optical or capacitive) that checks that each patch is present and correctly positioned, triggering a reject if not. The die or spray head is often mounted on a pivot or lift mechanism to move away from the web during machine stops or splices.
Maintenance and troubleshooting: The valve and nozzle are the most critical components. Adhesive charring or contamination can cause the valve to stick open or closed. Regular cleaning of valve seats and needle tips is necessary. If patches have tails or strings (adhesive trailing beyond the intended patch), increase the valve close speed or reduce viscosity. If patches are shorter than expected, check if the valve is opening fully or if the pump pressure is too low. If patch spacing drifts, verify the encoder signal and web slip. Intermittent coating machines are essential for converting lines that produce multiple discrete products from a continuous web, especially in hygiene, packaging, and medical industries.
