Tags:
Self-bonding varnish is a functional coating on electrical steel. In addition to insulation properties to prevent eddy current losses between the laminations, the quality of the varnish coating is primarily determined by the following factors:
While bondability is crucial for the quality and stability of the package, varnish leakage is an undesirable effect and thus a sorting criterion for the finished component. A visual assessment is performed of the coating discharge.
Bondability is determined by means of standardized testing methods.
The defined method used to assemble the stack is required as a basis for reproducible results when using each of the testing methods.
The roller peel test based on DIN EN 1464 determines the peel resistance of adhesives bonds. Two test specimens are bonded together, whereas one of the joined bodies must be flexible. A deflector roll on the tensile test machine separates the test specimen at a defined angle.
The floating roller test is an established testing method that provides useful results pertaining to the adhesive force as well as the strength and thickness of the steel strip used in the test setup.
The tensile lap-shear test based on DIN EN 1465 also provides information about the strength of adhesive bonds. Once the test specimens are adhesively bonded, The tensile test provides information about the quality of the adhesive bond. In contrast with the roller peel test, in this test the adhesive strength not dependent on the strength and thickness of the steel strip.
The defined method used to assemble the stack is required as a basis for reproducible results when using each of the testing methods.
A visual assessment of varnish squeeze-out between the laminations after bonding depends on several factors and always includes subjective factors in the assessment, including lighting and the subjective assessment of testing personnel. That is now a thing of the past because voestalpine has developed a method for objective assessment and automated detection of varnish squeeze-out on bonded test specimens.
Laser triangulation is used to create a 3D point cloud on the front side of the bonded test specimen. Protrusions between the laminations that may be indications of varnish squeeze-out are visualized and can be automatically detected and identified as defects.
The major advantages of these test methods compared to visual control by operating personnel are as follows:
Test specimens are produced in compliance with EN 1464. The test specimen must measure 25 mm x 200 mm and be at least 3 mm high (regardless of steel strip thickness). The long side of the individual laminations is positioned transverse to rolling direction.
Exactly one lamination is peeled from each test specimen, regardless of steel strip thickness, using the roller peeling device outlined in EN 1464.
The geometry of test samples is selected pursuant to EN 1465. The tensile lap-shear test is performed in reinforced design for all strip thicknesses, whereas the actual component to be tested is reinforced with two layers of the same material. This serves to assess the actual quality of the bond and to prevent any plastic deformation or fracture of the joined component (see Figure 4). The overlap area is 12.5 mm by 25 mm (sample width). To avoid specimen deformation during the test, support plates must butt against each other. Any gap is not permissible.
