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| Maillefer topography scanner |
The topography is a novel solution for cable quality control during production. The original idea to develop such a device was sparked many years ago during development of new generation of extruder screws for XLPE insulation in CV lines.
Collaboration since 2014
New versions of screws were first evaluated with standard output tests for various sizes. Usually, after initial tests, Maillefer collaborate with its customers. One of the first SM7 screws was sent to NKT Karlskrona for evaluation. The VCV lines in Karlskrona are designed for very high output and were optimal for evaluating the screw performance at extreme conditions.
The SM7 was developed to push the boundary of productivity, offering increased output with lower melt temperature when compared with previous generation SM5 screws.
During the initial testing in Karlskrona it became evident that the design goal was slightly too ambitious and a better balance between output, melt temperature and melt homogeneity was to be found.
Initial measurements and roadmap for screw development
An in-house measurement device just for studying the cable core samples was developed. The lab device was based on mechanical gauge running on the cable surface. At the same time the Pilot Vertical Line in R&D Center of Maillefer was commissioned, which could be used to recreate the production conditions as well as the surface irregularity seen in insulated cable cores. The Pilot Vertical Line was extremely helpful during the next phase of iterative screw designs.
Several development versions of the SM7 screw with step-by-step changes to critical dimensions were made. Each version was tested in the Pilot Vertical Line by extruding a model cable sample with identical process conditions. This is something that would have been nearly impossible without such a unique platform for development. Each of the model cable samples were then measured with the device Maillefer had developed. This way an actual measurement data of the surface irregularity from comparable samples made with different screw versions was obtained.
Typically XLPE screw designs are a balance between extruder output, melt temperature and melt quality. Output and melt temperature are easy to measure, but melt quality is not so straightforward. The issues with melt quality are only critical when aiming for the highest possible productivity at the maximum speed of the extruder.
Maillefer´s way of studying the actual end product and irregularities in cable samples proved to be very helpful to reach the design goals. A version of the SM7 screw was thus achieved that has improved output and lower melt temperature when compared to SM5, but that also has slightly better melt quality.
Topography Scanner concept
Topography Scanner is a concept for online geometry and surface measurement of the insulated cable core. This device was developed as a part of Maillefer´s continuous strive for better and smarter extrusion lines. The online measurement of the cable core roundness with the lengthwise waviness measurement were combined. Automated surface defect detection was also included. With the help of such a powerful tool, it is possible to monitor production quality and optimize the production conditions of CV lines.
Increased output and better quality
The cable samples from NKT Karlskrona were proving to be quite helpful. They provided Maillefer with one meter long cable samples produced with all of the screws, the prototype version SM7, SM5 and current version of SM7. These samples were analyzed with both Topography Scanner and the mechanical lab measurement device. Both of the measurement principles showed similar results in regards to lengthwise variation on the surface.
Now that the complete surface map of each sample was measured, it was evidently clear that initial assessment of NKT regarding the prototype SM7 was correct. The cable surface had significantly more lengthwise waviness than the SM5 screw. And what is more important, the new version of SM7 produced the best result! At equal line speeds the surface quality was best with the new SM7 screw. This improvement made it possible to increase the production speed by roughly 8 % without sacrificing geometrical quality of the insulated cable core.
