Definition
A wirewound coating rod is a laboratory tool used to prepare wet film coatings of specific thicknesses on a flat substrate. It is typically made of precision-machined stainless steel or other corrosion-resistant materials, with a cylindrical rod body wrapped around a specific diameter of wire. Two parallel wound wires create a gap with the rod surface, which accommodates and transfers the paint when the rod is pulled, leaving a wet film coating of uniform thickness determined by the wire diameter on the substrate.
How it works:
The working principle of wire wound coating rods is based on gap metering. When the operator places an appropriate amount of paint on the substrate in front of the rod and pulls the coating rod at a constant speed and pressure, the paint is filled into the cavity formed by the wire with the rod and the substrate. As the rod moves, excess paint is scraped off, and only one layer of paint that matches the diameter of the wire is evenly applied to the surface of the substrate, forming a wet film. Wet film thickness (WFT) is theoretically equal to the diameter of the wire, and the relationship can be approximated as:
WFT ≈ d
where d represents the nominal diameter of the wound wire.
Measurement method
Coating preparation using wire wound coating sticks is a straightforward and efficient method. First, select a coating rod that meets the target wet film thickness specifications. The paint sample to be tested is poured or dripped along the substrate starting line. Then, hold both ends of the coating rod with both hands and place it steadily in front of the coating, ensuring that the wound wire is in full contact with the substrate. Pull the bar smoothly towards the operator at a uniform, moderate speed (usually between 0.5 and 1 m/s) without pausing or tilting halfway through. After the coating is completed, immediately check whether the wet film is continuous, uniform and free of streaks. The prepared wet film can be used to evaluate the subsequent drying time, adhesion, hardness, gloss, and final dry film thickness.
Influencing factors
The quality of the coating film is affected by a variety of operational and conditional factors. The operator's pulling speed and applied pressure are key factors, and uneven speed or excessive pressure can lead to variations in coating thickness or streaking. The rheological properties of coatings, such as viscosity and thixotropy, directly affect their ability to fill gaps and level, with high viscosity leading to application difficulties and low viscosity leading to sagging. The surface flatness, cleanliness and material of the substrate also affect the adhesion and appearance of the coating. Environmental conditions, including temperature and humidity, can alter the paint's application properties and drying process. Additionally, the manufacturing precision of the coating rod itself, the degree of wire wear, and the cleanliness after use are also important conditions to ensure consistent results.
Applications:
Wire wound coating rods are widely used in the testing of coatings, inks, and related materials in multiple industrial and R&D fields. In the coatings industry, it is used for new product formulation development, color matching, masking testing, and initial evaluation of the physicochemical properties of coatings. The printing ink industry is commonly used to test the printability, color strength, and abrasion resistance of inks. In the field of surface treatment such as automobiles, furniture, and metal anti-corrosion, it is used to simulate the construction effect of topcoat and primer. In addition, it is used as a basic coating preparation tool for quality control and comparative studies in laboratories in industries such as electronic materials, paper coatings, textile finishing agents, and adhesives.
Key points of selection
Choosing the right wire wound coating rod requires a comprehensive consideration of multiple technical parameters. The primary parameter is the nominal wet film thickness, which is determined by the diameter of the wire wound, and the user should choose a series of rods with the appropriate thickness range according to the test criteria or their own needs. The length of the rod should be greater than the width of the substrate to be coated. In terms of material, high-grade stainless steel is commonly chosen for its corrosion resistance, easy cleaning, and wear resistance. Manufacturing precision is crucial, focusing on the uniformity and firmness of the wire winding, as well as the straightness of the rod. For special coatings, such as highly corrosive or viscosity, specific materials or specially designed rod shapes may need to be considered. At the same time, ensure that the selected coating rod meets the requirements specified in relevant international or industry standards (such as ASTM, ISO, etc.) to ensure the comparability and reliability of the test results.