Metallic yarns are hard to knit. They are less elastic than elastane, they kink under tension and they shed fine particles that build up over time in the feed system. These limitations are used to build a metallic leggings jeans machine, but the machine itself does not solve the problem, the preparation steps and operating parameters are equally important.
Preparing the Yarn Before It Goes to the Machine
Metallic yarns are seldom run from the cone directly into the machine. First, the metallic core is covered with nylon or polyester filaments on a separate covering machine. This step of covering has two functions: it protects the metal layer from being abraded as it passes through the feed system and it adds just enough resistance to stretching so that the yarn can withstand the tension changes that happen during knitting. The metallic part is prone to breakage and peeling off its base film without a covering. The feed path on the jeans machine itself is maintained as straight as possible. The metallic layer degrades from friction, and every sharp bend increases friction. Some operators install ceramic-coated guides at the feed point to specifically reduce this friction and the yarn tension is set lower than would be used for standard elastomeric fibers.
Machine Settings That Affect Breaking
Most directly affecting whether the yarn breaks there are two settings on a metallic leggings jeans machine. The first is velocity. There are many machines that can run at 900 rpm, but metallic yarns are better off at slower speeds – around 700 rpm is a good practical starting point for most metallic legging production. The faster you run, the bigger the spikes in tension at the needle's entry point, and the metallic yarn can't stretch enough to absorb them. The second is the inlay architecture. Metallic yarn is often not knitted into the main loop in metallic leggings. Rather it is incorporated into the fabric and the base yarns form the main structure. This technique shows the metallic thread on the surface and also means that the metallic yarn does not have to undergo the action of forming a loop, which would stress it even more.
Monitoring and Maintenance Currently in Production
Metallic yarns throw off fine particles as they are run. These particles accumulate on the feed wheels and guide points, gradually altering the effective tension. Usually operators check the yarn feed for tension variations or visible dust build-up every 15 to 30 minutes. Defects are often caused by worn guides. The guides create friction points that damage the metallic layer, resulting in visible gaps or broken filaments in the finished fabric. To keep production steady the practical way is to replace guides on a regular schedule rather than waiting for a defect to appear. There are common defects in the production of metallic legging. The gaps in the metallic thread can be seen, the broken filaments on the surface of the fabric and the uneven color reflection. Any of these can be traced to one of three causes: uneven tension on the metal feed, wrong timing of the inlay to the base knit, or worn guides causing friction.