The homogeneity of the Backlack coating plays an extremely important role in the production process of companies that punch electrical steel strip as it effects the adhesive strength and emission values of the Backlack. Up until now, monitoring compliance with the relevant customer specifications involved a complex testing process. Reliably testing the entire length of the core of a coil was nearly impossible due to the fact that taking samples destroys the material. As a development partner, Waelzholz has now integrated the innovative LILIT© system – as a contactless, complete inspection – in ongoing operations.
Backlack offers clear advantages when used to bond electrical steel strip lamination stacks, especially for electric motors used in electric vehicles, because it offers both outstanding performance and high comfort. Daniel Dunker, electrical steel strip specialist in materials technology at Waelzholz in Hagen, explains: “Backlack really shines in applications where the motor manufacturer’s specifications need to be met exactly.” This involves questions surrounding the ability to process the electrical steel strip with a Backlack layer both during the punching process as well as subsequent baking. For example, Backlack’s adhesive strength and emission values are relevant in this regard. According to Dunker: “The manufacturers want the Backlack layer to be highly uniform over the entire length of the core of a coil and across batches. After all, this is the basic requirement for achieving consistency in further processing steps.”
Reliable testing without sampling
The term polymerization describes how long the molecular chains in the Backlack coating are. It is a key factor that has an effect on Backlack’s adhesive strength and emission values as mentioned above. When used to coat electrical steel strip, however, Backlack’s degree of polymerization during the hardening process may vary due to local variations in drying. The conventional analysis of random material samples is not enough to make a comprehensive statement about the degree of polymerization over the entire length of the core of a coil. According to Dunker: “This is why we were looking for a way to reliably test the entire length of the core.” The company has now achieved this with the innovative LILIT© system, which the developer TAU Industrial Robotics has adapted to Backlack testing together with Waelzholz. The main advantage of this testing method is that it can be used during ongoing production without the need to take samples.
The LILIT© testing system operates based on the principle of UV spectroscopy, in which a light source emits UV light that is reflected or partially absorbed by the Backlack. The reflected light spectrum is then measured, and the degree of polymerization is determined based on this information. According to Dunker: “Using the ‘light in-line insulation test,’ we are able to very accurately determine the uniformity of the hardening process. To achieve this, we previously determined the correlation of the reflected light spectrum to the material properties in a series of tests, and we can now use this knowledge to make very accurate statements about the properties of the Backlack’s layer.”
This allows Waelzholz to ensure that the electrical steel strip’s Backlack coating has an optimum degree of polymerization with consistent homogeneity – over long periods of time and numerous batches. Dunker concludes: “We make sure that our customers can fully leverage Backlack’s potential for their products. This applies both to the performance and comfort of the future electric motor and to the cost-effectiveness of the manufacturing processes. We strive to achieve total reliability here, which we implement to the benefit of our customers through the maximum level of quality characteristic of a complete inspection.”