Definition
Hot melt adhesive coating wire rod is a metering tool used to evenly apply hot melt adhesive to the surface of a substrate at a specific thickness in laboratory or industrial production. It is typically made of metal with a precision-wound stainless steel wire on the surface to create a series of uniform grooves. Its core function is to precisely control the thickness of the wet film, which is a key tool for evaluating the coating performance of adhesives and studying the coating process.
Principle
The working principle of hot melt adhesive coating wire rods is based on metered transfer. When the hot melt adhesive heated to a molten state is placed on the substrate, the operator pushes the wire rod to roll forward at a uniform speed and pressure. The groove formed by the steel wire winding on the surface of the wire rod carries and measures the glue, the excess glue is scraped off, and only the amount of glue in the groove is transferred to the surface of the substrate, thus forming a wet film of uniform thickness composed of rubber strips. The thickness of the wet film is mainly determined by the geometric depth of the groove of the wire rod, which is theoretically equal to the diameter of the winding wire. Its basic relationship can be expressed as:
Wet film thickness ≈ wire diameter
Measurement methods and standards
Coating with hot melt adhesive coating wire rods is a standardized operation. First, the hot melt adhesive is heated to its recommended application temperature so that it fully melts and maintains a constant temperature. Then, place an appropriate amount of glue on the starting end of the substrate. Place the wire rod smoothly in front of the glue and push the wire rod to scrape in one direction at a constant speed and with moderate pressure vertically downward. Immediately after completion, the formed wet film is observed or measured. Wet film thickness can be measured with a micrometer and converted in combination with the solids content of the adhesive, or instant measurement with a professional wet film thickness gauge. For related operations, please refer to ASTM D823 and other standard practice methods for the preparation of uniform coating films.
Influencing factors
The coating effect is affected by a combination of factors. The wire bar parameters are the basis, the diameter of the steel wire directly determines the thickness of the theoretical wet film, and the uniformity and spacing of the wire winding affect the stripe consistency of the coating. In terms of operating processes, the speed at which the rod is pushed and the uniformity of the pressure applied is crucial, as uneven speeds can lead to fluctuations in coating thickness. The viscosity, leveling and thermal stability of the molten hot melt adhesive will affect its transfer efficiency in the rod groove and the flatness of the final film. Environmental and substrate conditions, such as substrate flatness, surface energy and temperature, can also affect the spread and adhesion of the adhesive.
Applications
The application of hot melt adhesive coating wire rods is focused on areas that require precision coating testing. In the packaging materials industry, it is used to test the sealing performance and dosage of hot melt adhesive on cartons and labels. In the field of textiles and nonwovens, it is used to study the coating process of hot melt adhesives for compounding. In automotive interiors, furniture finishes, footwear and other industries, it is used to evaluate the coating uniformity and bonding effect of adhesives before material compounding. In addition, in adhesive R&D and quality testing laboratories, it is a common tool for formulation researchers to evaluate the sizing performance of new products and determine the optimal coating thickness.
Selection guidance
Choosing the right coating wire rod requires a comprehensive consideration of multiple technical parameters. The primary factor is the target wet film thickness, and the wire rod model corresponding to the wire diameter is selected according to the required thickness. Secondly, the coating width should be considered, and the length of the wire rod should be slightly larger than the width of the substrate. Adhesive properties also need to be matched, and for high-viscosity hot melt adhesives, it may be necessary to choose a model with a groove design that facilitates adhesive transfer. In terms of ease of operation, the wire rod with a handle facilitates the application of force evenly. In terms of material, corrosion-resistant stainless steel is usually chosen to ensure long-term use accuracy. It is recommended that users determine the most suitable wire rod specification through a series of tests according to the actual application of the rubber type, substrate and process requirements.