Protective sleeves made of cut-resistant materials are used to avoid injury in industries where glass and metals are processed. For improved comfort and ergonomics, various materials are often combined in sophisticated sewing processes: yarn made of aramid fibers on the exterior help protect against cuts, natural protein fibers on the inside make the item more comfortable and ergonomic to wear. Flexitex GmbH, the Steinbeis Research Center for Automation in Lightweight Construction Processes (ALP) and the chair of the department of Lightweight Structures and Polymer Technology at Chemnitz University of Technology came together on an R&D project that involved creating a process for manufacturing protective gear on a circular knitting machine. These items are made of highly durable cut-resistant yarns and natural fibers, offering ultimate comfort for the wearer. They can now be manufactured in an endless production process. Based on the results of the research project, the specialist machine builder Sotex Sondermaschinen GmbH produced a fully automated material assembly machine.
Several important elements come together in the prototype machine: the cutting technology used to process the cut-resistant fabrics, the airflow-based doubling process, and the sewing technology used to seam the semi-finished product, which is open-ended due to the nature of the knitting process. An innovative conveyor system featuring pneumatically driven hand-off stations links up the individual process modules. For the project, tests were conducted with various sizes of cut-resistant sleeves and forearm protectors made of different fiber-based materials with ultrahigh molecular weight. A material made of natural protein fibers was added in the stitching process (avoiding an additional sewing step), thus ensuring that the product is comfortable to wear thanks to its interior, simultaneously guaranteeing optimal protection against cuts on the exterior. To meet all of the many requirements involved, the automated process produces both single- and double-layered cut-resistant gear.
To test and enhance the main qualities of the protective sleeves, the project partners created a bonded fabric made of aramid and natural fibers. They then analyzed the product for comfort and protective properties. As an alternative to aramid, a yarn made of ultra high molecular weight polyethylene (UHMW-PE) was used, as it offers excellent stiffness. Processing cut-resistant items made of bonded fabrics by means of circular knitting meant analyzing and modifying the systems used to feed in the yarn. Ensuring constant tension on the threads was a significant challenge in this respect because consideration had to be given to the greatly varying levels of firmness, stiffness and friction properties of the materials to be bonded. The optimized bonding process developed during the project makes it possible to manufacture semi-finished products by combining innovative materials, opening up new application areas for circular knitting technologies.
The ratio of the fabric’s strength to its stretchiness is central to why the protective sleeves are so comfortable. A highly stretchy structure allows for elastic deformation of the cut-resistant sleeve on the arm. The fabric has to be less stiff in the cross direction of the weave. To allow for a greater range of motion in the wrist for versions with a thumb hole, it is important that the structure of the fabric is also stretchy lengthwise. The pilot machine developed for the project makes it possible to manufacture numerous options without extensive retrofitting. There is also no need to use product-specific combinations of individual processing steps. Similarly, there is no need for system add-ons later down the line. The protective sleeves can be manufactured from the semi-finished product in a flexible, fully automated process. The material assembly machine was designed to reduce downtime by using a magazine to quickly mount spools. As a result, it can process up to ten spools at a time, automatically taking care of spool changes.