Sewing Process

The basic process of sewing involves fastening of fabrics, leather, furs or similar other flexible materials with the help of needle and threads. Sewing is mainly used to manufacture clothing and home furnishings. In fact, sewing is one of the important processes in apparel making. Most of such industrial sewing is done by industrial sewing machines. The cut pieces of a garment are generally tacked, or temporarily stitched at the initial stage. The complex parts of the machine then pierces thread through the layers of the cloth and interlocks the thread.

Industrial Sewing
Although it seems to be a simple process, industrial sewing is quite a complex process involving many preparations and mathematical calculations for the perfect seam quality. Good quality sewing also depends on the sound technical knowledge that goes into pattern designing and making. Flat sheets of fabric having holes and slits into it can curve and fold in three-dimensional shapes in very complex ways that require a high level of skill and experience to manipulate into a smooth, wrinkle-free design. Aligning the patterns printed or woven into the fabric also complicates the design process. Once a clothing designer, with the help of his technical knowledge, makes the initial specifications and markers, the fabric can then be cut using templates and sewn by manual laborers or sewing machine.

While handling the fabric and in the process of sewing, the cloth must be held stiff and unwrinkled. The seam quality is very sensitive to cloth tension that varies from time to time in the whole sewing process. These undesirable variations in the cloth tension affect the product quality. Therefore, there arises the need of strict control over the whole process. The work of sewing is focused on the handling of fabrics lying on the working table and guide them towards the sewing machines needle along the seam line. The attention is equally focused on the control of appropriate tensional force so as to maintain high quality seam.

Pre sewing functions
Before the actual task of sewing begins, there are certain other tasks that have to be taken care of which can be termed as fabric handling functions- Ply separation; Placing the fabric on working table; Guiding the fabric towards sewing needle; and tension control of fabric during the sewing process.

While ply separation, stacks of fabric plies are sequentially positioned with the help of some feeding apparatus in an unloading position. The uppermost ply in such a stack is individually and sequentially separated from the stack. In the process, an edge of the separated fabric is presented between the jaws of a gripping device. A proximity switch determines the spacing between the gripper jaws. When this spacing confirms that only one fabric ply is in place between the jaws, the single ply is transferred over onto the receiving end of a conveyor for further processing. In aerodynamic technique, the uppermost ply of fabric is lifted by suction from the remainder of the stack.

When the fabric is placed on the working table, the tasks that are performed before the sewing process include - recognizing the fabric's shape, edges that will be sewn, planning of the sewing process and identification of the seam line.

Recognizing the fabric's shape: The appropriate tensional force depends on the fabric properties. So the fabrics have to be identified into categories like knitted fabrics, woven fabrics etc. depending on their physical properties. Various shapes of the fabrics such as, convex, non-convex, with straight or curve edges, also have to be considered and each of them require different handling strategies. In brief, the sewing methods done by automatic systems require classification of fabrics into various categories and certain preliminary scheme of the path that the fabric must follow so as to produce the required stitches.

Fabric Edges to be sewn: There are two basic types of stitches- one is that are for joining two parts of cloth together and the second one is done for decorative purposes. Sometimes, both types of stitching have to be done on some parts of cloth, for example, a pocket has to be joined on three sides with the apparel as well as it may be given some decorative stitches too. At what points and which type of stitching has to be done- all such information is stored digitally on automated devices through Computer-Aided Design (CAD) and accordingly sewn.

Planning of sewing process: Sequence of seams to be stitched is determined before the sewing starts. Which part will be joined first, what stitches will follow one another, etc. are decided. However, some stitches have to be necessarily done before or after another stitch. In the example above, the decorative stitches must be done first followed by the joining stitches. Embroidered patterns also follow the same sequence but sometimes in clothing items like hats, decorative stitches or embroidery is done after the production of hats and with the help of embroidery machine.

Identification of seam lines: Sewing process is performed on seam lines situated inside the fabric edges, some millimeters inside the fabric's outer line. For the straight lines, the seam line is found by transferring the outer lines inside the fabrics and the intersection of these lines makes the vertices of the seam line. Therefore, the seam line is parallel to the outer edge and the distance between the two has to be determined as it is different for different parts of the cloth. This distance is greater for trousers legs than for a shirt sleeves. Seam allowance is the area between the edge of the fabric and the line of stitching. It is usually 1.5 cm away from the edge of the fabric. Seam allowance is usually 2.5 cm or more for standard home dressmaking. Industry seam allowances vary but they are usually 0.6 cm.

Sewing Fabrics
The sewing process consists of mainly three functions- guiding fabric towards needle; sewing of the fabric edge; and rotation around the needle. The fabric is guided along the sewing line with a certain speed that is in harmony with the speed of sewing machine. The orientation error is either manually monitored or if monitored automatically then error is fed to the machine controller so that the machine corrects the orientation of the fabrics. When one edge of seam line is sewed, the fabric is rotated around the needle till the next edge of the seam line coincides with the sewing line. The sewing process is thus repeated until all the edges of seam line planned for sewing, are sewed.

Significant Aspects of Sewing
There are certain aspects that have to be carefully considered while the sewing process as they are very crucial for high quality sewing.

Thread tension and consumption: Correct balancing of the stitch and the tension given to the threads is very important for quality stitch formation. Thread consumption, which is closely associated with correct stitch geometry and thread tension, is usually measured by digital encoders.

Presser- foot displacement and compressing force: Presser- foot is the part of the sewing machine that holds the fabric in place as it is being sewn and fed through by the feed dogs. The feeding system is one of the most important constituting systems of the sewing machine. If this system is not efficient then it results into irregular seams and many other defects, especially when running on high speed. To evaluate feeding efficiency, the force on the presser foot is measured with the help of electric or other sensors.

Needle penetration force measurement: The interaction of needle with fabric is very crucial. Fault-free needle penetration depends chiefly on the properties of fabric and needle choice. Needle penetration force is one of the variables whose measurement is important for the analysis of quality problems or "sewability testing" and also quality monitoring. It takes various factors into consideration, such as the needle geometry (including the point angle and point length of the needle), the friction between the needle and the fabric, the friction between the needle eye and the thread along with fabrics' property, and the sewing conditions.