Application of Laboratory Scraper Heating Coating Machine in Hot Melt Adhesive Coating

This article introduces the application of the laboratory blade heating coater in hot melt adhesive coating. It first explains the coating process and principles, including the three stages of heat conduction, fluid shear, and surface wetting, as well as the effect of temperature on adhesive viscosity. Next, it discusses key parameters of the equipment, such as the heating temperature range, blade gap accuracy, coating speed, and substrate tension. Then, it briefly outlines the operation steps, from preheating to stable operation, and introduces evaluation indicators for coating quality, such as thickness uniformity and peel strength. Finally, it mentions solutions to common problems and the expanded applications of the equipment in coating other materials.

Principle

Hot melt adhesive coating technology is widely used in packaging, labeling, tape, and nonwoven fabrics. The Laboratory Squeegee Heating Coating Machine heats the solid hot melt adhesive to a molten state by precisely controlling the temperature and squeegee gap, and then evenly coats it on the surface of the substrate. The process involves three physical links: heat conduction, fluid shear and surface wetting. The viscosity of hot melt adhesive decreases exponentially with the increase of temperature, and its rheological properties directly determine the uniformity of the coating. The relative motion of the scraper to the substrate causes the melt to form a thin layer, and the tension and speed of the substrate need to match the melt thixotropy to avoid wire drawing or glue breakage.

Device parameters

The core parameters of the squeegee heating coating machine include heating temperature range, squeegee gap accuracy, coating speed, and substrate width. The heating temperature is usually set between 80°C and 200°C, and the temperature control deviation should be less than ±1°C, otherwise the hot melt adhesive may degrade locally or cure incompletely. The scraper gap adjustment mechanism adopts a micron-level micrometer, and the linear error should be less than 2 microns. The coating speed is generally controlled from 0.5 m/min to 10 m/min, too high will cause jagged ripples in the thickness of the coating, and too low may cause adhesive accumulation. The substrate tension should be maintained at 0.3 N/cm to 0.8 N/cm to prevent wrinkling or slipping.

Procedure:

The operation process includes five steps: preheating, tool adjustment, glue supply, trial coating and stabilization. Start by placing the hot melt block into the heated hopper, setting the target temperature and waiting for the compound to melt completely (about 15 minutes to 30 minutes). The scraper gap is then preset to a target value, e.g. 50 microns, and then checked with a dial indicator. After starting the substrate conveying system, manually pull out the glue nozzle to allow the melt to contact the substrate. Observe the initial coating appearance: if horizontal lines appear, reduce the coating speed; If there is a leaking coating point, increase the scraper gap or increase the heating temperature. After stable operation, use a thickness gauge to measure at five points horizontally, and the thickness coefficient of variation should be less than 5%.

Coating quality evaluation

The coating thickness consistency, surface finish, peel strength with the substrate, and thermal stability were mainly evaluated. Thickness deviations are availableΔh = h_max - h_minindicated, Δh ≤ 5 microns is usually required. The peel strength was determined by the 180° peel test, in N/25 mm. The thermal stability is tested by differential scanning calorimetry, and if there is an obvious oxidative exothermic peak, it means that the process temperature is set high. The following table lists common hot melt adhesive types and their typical coating parameters:

Glue species	Recommended coating temperature/thickness
EVA-based hot melt adhesive	130-150°C / 30-80 microns
Polyolefin hot melt adhesives	160-190°C / 20-60 microns
Rubber-type hot melt adhesive	110-140°C / 40-100 microns

FAQs

If bubbles appear on the coating, it is usually due to water vapor or moisture adsorption on the surface of the substrate, so the compound can be vacuum dried at 80°C for 2 hours. If there is a drawing on the edge of the scraper, check whether the knife edge is worn or reduce the scraper pressure. When the ambient temperature is below 15°C, preheating the substrate to 40°C improves wettability. For high-temperature degradation-sensitive hot melt adhesives, it is recommended to pass nitrogen protection above the hopper to reduce oxidative cross-linking.

Application extensions

The device is not limited to hot melt adhesives, but can also be used for laboratory simulations of wax-based coatings, pressure-sensitive adhesives, and heat-shrink materials. By changing the scraper material (ceramic or stainless steel), it can be adapted to different viscosities. The coated samples can be further verified by aging tests and friction coefficient tests, providing a reliable process window for industrialization.