Introduction
Knitted fabrics have long been a staple in classic apparel and home textiles, but their role is rapidly expanding into technical applications. The growing use of automation in weft knitting is changing how manufacturers handle machine setup, product design, and quality control. This shift is especially visible in flat knitting and large circular knitting systems. It is also pushing the industry closer to flexible production, lot size one, and smarter manufacturing methods.
Fundamentals of Weft Knitting and Applications
Knitted fabrics are defined as structures made by interlooping one or several threads or thread systems through stitch formation. The most common form of interlooping is weft knitting, followed by warp knitting. Both techniques are separated by the movement of the loop-building yarn. In weft knitting, all needles are sequentially supplied with a single weft yarn during one knitting cycle. In warp knitting, the yarn feeding and loop forming occur along all needles in the needle bar at the same time in one knitting cycle. All needles are lapped by different warp guides during this process.
These fabrics are used in classic apparel and clothing, such as pullovers, underwear, or bathing suit fashions, as well as home textiles like curtains or furniture covers. As knitting technology has advanced possibilities to manufacture technical yarns like aramid, carbon, glass, polyamide, and polyester, the field of knitted technical textiles is growing. By varying process parameters, the porosity of the structure and the stress and strain behavior of the fabrics can be designed according to the application. Typical applications can be found in geotextiles, automotive interiors, sport industry products, filter materials, and nets in addition to apparel textiles. In this area, automation in weft knitting is becoming increasingly relevant because it helps maintain consistency while supporting more complex product requirements.
Types of Knitting Machines
In the industry, three types of knitting machines are mainly used: flat knitting machines, large circular knitting machines with diameters larger than 165 mm, and small circular or body size machines with diameters smaller than 165 mm. Flat knitting machines provide high flexibility in product design. Because of the special needle-thread arrangement, the range of pattern and color variation is greater in flat knitting machines compared to circular knitting machines. Using flat knitting machines, whole garments like pullovers can be produced within a single knitting process.
This technology of whole garment production is either called Stoll-knit and wear by H. Stoll AG & Co. KG in Germany, or WHOLEGARMENT by SHIMA SEIKI MFG., LTD. in Japan. Thus, no additional sewing process is needed afterward. This high flexibility comes with a reduction in knitting speed. The stable realization of complex pattern commands, such as a loop transfer from one needle to another, requires more time than realizing standard loop formation like a single jersey t-shirt pattern. The construction of the flat needle bed, fed with yarn by a horizontally moving carriage, also results in a discontinuous knitting movement.
In general, circular knitting machines are more productive in square meters per minute than flat knitting machines because of the continuous circular movement of the needles. The needle movement speed of circular machines can reach up to 2.2 m/s, whereas the carriage speed of flat knitting machines is about 1.3 m/s. Large circular knitting machines can knit up to 144 threads simultaneously. Flat knitting machines can handle up to four threads at the same time. This is one reason automation in weft knitting has become such an important topic for manufacturers looking for both speed and consistency.
Pattern Elements and Production Parameters
In knitting, patterns such as t-shirt or polo shirt structures are realized using pattern elements or cams, such as tuck, miss, or knit elements. By combining these pattern elements, manufacturers can influence not only the haptic and optic properties but also the mechanical properties like stress and strain behavior of the knitted product.
The knitted fabric is produced by defining fabric structure information in pixel images, where each pixel refers to a single knitted loop. This information is then transferred to the knitting machine. The general input parameters for knitted products cover several areas. Product requirements include color design and knitted pattern structure based on desired mechanical and optical properties. Machine status involves the position of yarn bobbins in the bobbin creel, the number and position of used knitting units or systems, the diameter of the cylinder for circular knitting machines, or the production width of flat knitting machines. It also includes machine gauge, electronic or mechanical needle selection, sinker height change mechanisms, type of take-down, and equipped feeders like positive, storage, or elastane feeders. Process parameters include the yarn tension of each feeder given by yarn length per feeder revolution, sinker height, knitting speed, take-off tension, and spreader width. Material information covers material type, filament or staple fiber yarn, yarn fineness, stress and strain behavior of the yarn material, and the amount of filaments.
Industry 4.0 and Lot Size One Production
Circular knitting machines are initially designed for mass production. Originally, this technology was used to manufacture a large batch of the same fabric to be cut and sewn afterward to create the final product like a t-shirt. Increasingly, technical products such as geotextiles, home textiles, automotive interiors, and mattresses are produced using large circular knitting machines. All these application markets provide a high sales volume. The market volume combined with high productivity and high material costs makes production in Europe interesting to knitting companies.
The trend in Germany is to implement automation up to Industrie 4.0 standards within the knitting process. In 2016, a publicly funded project called Strick 4.0 was launched at the DITF-MR research institute in Denkendorf, Germany. The project aimed to analyze new perspectives and future approaches regarding Industrie 4.0 in weft knitting. The potential of Industrie 4.0 in textiles includes the efficient production of customized products, notably producing lot size one using smart factories, and establishing new dynamic business and engineering processes. In this context, automation in weft knitting is not only about machine control, but also about making production more adaptable.
At the Institut für Textiltechnik of RWTH Aachen University, researchers are investigating the use of large circular knitting technology for lot size one applications. The researchers have implemented a programming process pipeline that enables quick product changes. The first step in enabling flexibility and lot size one production in the textile industry is making the large circular weft-knitting machine more adaptable to allow subsequent production of different designs. This step is to digitalize the entire process by setting up a virtual bobbin creel and logging the positions of each yarn bobbin into the process pipeline. The pipeline processes the input data by scaling the knitted fabric in course and wale directions, factoring in the knitting and downstream heat setting parameters. The process parameters setup necessary for production is also included in the product programming. All information is sent to the machine via Wi-Fi, and the machine immediately starts running.
To increase lot size one possibilities using large circular knitting machines, the Institut für Textiltechnik invented a way to manufacture three-dimensional weft-knitted fabrics according to desired target geometries using common large circular machines. This methodology represents a next step toward increasing the flexibility of circular knitting machines for lot size one production.
Cross-linking between production steps can save material, human resources, and machine capacity. Two major German manufacturers of large circular knitting machines have invented processes that interlink yarn processing with the weft knitting step. Mayer & Cie. calls its technology Spinit technology or Spinitsystems, while Terrot GmbH has launched a cross-linked technology called Corizon. In flat knitting, complete near-net shaped production of fabrics for technical applications in the sport shoe industry, medical fields, or 3D covering and cushioning can be achieved. This approach is often underestimated in practice, especially when discussions around automation in weft knitting focus only on speed and miss the value of process integration.
Quality Monitoring of Knitted Fabrics
Quality in knitting is interpreted as the uniformity or homogeneity of the fabric. Texture imperfections are patches that locally break the homogeneity of a texture pattern. A defined deviation in quality is called a defect. The defective point differs optically in its appearance or materially with a changed machine density from the rest of the product. Fabric quality is evaluated by measuring four main parameters: weight per area, mesh density, fabric feel including softness or stiffness, and elasticity. Fast, efficient, and precise quality monitoring of yarn-based textiles is crucial for the industry. In 2010, waste caused by dead times in warp knitted structure production led to a loss of 160,000 euros per year per enterprise. Due to high quality requirements for the knits, only one defect within 4000 square meters of produced fabric is allowed. As production speed increases on large circular weft-knitting machines, the probability of defect production also rises, creating a conflict with quality requirements.
Optical quality monitoring can be executed off-line or online. Inspecting samples after they are produced, often in a downstream process, is called off-line quality monitoring. Inspecting during fabric production is called online quality monitoring. Because online monitoring leads to higher defect detection rates than trained personnel, the online system is preferred. The detection rate for trained personnel is around 70%, while off-line monitoring systems can deliver rates up to 100%. The Institut für Textiltechnik, in cooperation with the Institute for Imaging and Computer Vision at RWTH Aachen University, has shown that horizontal stripe defects in weft-knitted fabrics caused by uneven yarn tension can be detected off-line with a 97% rate.
Online defect detection for knitted structures is more challenging than monitoring woven fabrics due to loop geometry and the overall mesh structure. Consequently, fewer investigations have been executed in the field of online monitoring for weft-knitted structures. There are no sources discussing online defect measuring in knitting. Defect measuring is different from defect detection. Defect detection shows significant differences in the structure surface or appearance. Defect measuring displays yarn dimensions like length and diameter within the fabric. Several sources refer to online defect detection during the production of knitted fabrics. This is where automation in weft knitting has a clear advantage, since repeatable monitoring is difficult to maintain by hand over long production runs.
Common Defect Categories in Knitting
The majority of knitting defects can be divided into two categories: bands and streaks, and stitch defects.
The first category includes bands and streaks. This category encourages the barré effect, a continuous horizontal visual barred or striped pattern. These defects are mainly caused by physical, optical, or dye differences in yarns, errors introduced during yarn spinning, or inefficient fabric formation. Inefficient formation can result from a wrong sinker height setting on at least one feeding system, incorrect yarn tension, or variations in fabric take-off tension. Skew distortion is the angular display from the ideal perpendicular angle of wales and courses in the knitted fabric. This defect is mainly caused by errors in yarn twist parameters and twist direction. Bowing, stop marks, and needle lines also belong to this first category of knitting defects.
The second category involves stitch defects. Dropped stitches, also called the laddering effect, occur when loops are cast off during production. This error occurs when yarn is fed wrongly to the needle due to loose yarn tension or a yarn break. Cloth press-off and holes are also stitch defects caused by yarn breaks or weak places in the yarn. Knitting undesired pattern elements like tuck or miss elements are also classified as stitch defects.
Defects in weft-knitted fabrics are currently detected by a knitting expert during the process on a random basis. Roll material from large circular knitting machines is inspected in an additional downstream process, meaning there is no possibility of error prevention, only error detection and waste disposal. Defects are caused either by inhomogeneous yarn material mixture, fineness, and tension, or by false settings of knitting process parameters like speed, sinker height, yarn tension, take-off tension, or spreader width.
Automated Vision Detection Systems
Optical properties like surface evenness and quality are important parameters for knitted fabrics. The human eye is very sensitive and can detect deviations in patterns as fine as 20 micrometers. For technical applications like flame-retarding underwear, fencer skiing clothing, or sheer pantyhose, a small selective fabric mistake can disqualify the fabric from meeting mechanical and optical requirements. Errors with spatial extension in the production direction, such as color mistakes, pattern failures, or laddering, are critical because they result in many wasted running meters before detection.
Repetitive errors like stripe defects from uneven yarn tension are hard to detect even by trained operators, because the repetitive surface change is difficult to differentiate from a desired pattern. These tasks are increasingly being substituted by automated vision detection systems that count or measure fabric characteristics. Despite practical advantages, there are few applications in weft-knitting technology so far. Investigations for online quality monitoring on large circular knitting machines exist at the research level. Industrial settings do not use them yet. For circular weft-knitting machines, two main setups exist where the camera system can be installed either inside or outside the cylindrical knitted tube. Ring lights are often used in these setups. Outside the tube, the camera can be installed on a stand to prevent perspective distortion caused by machine vibrations during production.
Conclusion
Weft knitting is evolving from simple mass production to a highly flexible, digitized process capable of producing lot size one. By integrating Industry 4.0 concepts, manufacturers can cross-link production steps and utilize advanced programming pipelines to quickly change fabric designs. At the same time, automation in weft knitting is helping improve machine flexibility, process control, and quality monitoring. As these technologies move from research to industrial floors, they will continue to shape efficient and competitive knitted fabric production.
