Sewing Thread

●Needle thread must pass freely through the small eye of the needle; consequently they must be uniform, knot-free, non-torque and fault free.

●Tensile strength/breaking strength is one of the essential properties of the thread. It must be capable of withstanding several kinetic/lateral movements during sewing. The strength of the sewing thread must be higher than that of the fabric so that the thread does not rupture during use. During sewing at high speeds, the needle thread is subjected to repeated tensile stresses at very high rates. The thread also comes under the influence of heat, bending, pressures, torsion and wearing. The value of these stresses depends on the sewing speed, machine settings and the thread used. The stresses created within the thread have a negative effect on the processing and functional characteristics of the thread, and there is significant reduction in the thread strength after sewing.

●For good performance in a sewing machine moderate to low extension-at-break of the thread is usually preferred. Needle thread with different elongation-at-break has been found to behave quite differently during stitch formation. The determinants of success of sewing a thread with certain elongation per cent without any problem the machine setting and special properties of the sewing thread itself.

●The elasticity of the sewing thread must be uniform along its length in order to enable equal length stitches to be formed, and it must closely match the elasticity of the fabric being sewn; otherwise either seam thread fracture, or tearing of the adjacent fabric may arise during garment use. Clearly, the requirements of woven and knitted fabrics will be different.

●The forces that are developed in the sewing thread are mostly due to the friction between the thread and machine parts, the most severe action taking place between:
The thread and the needle.
The thread and the fabric being sewn.
A controlled level of both static and dynamic friction is required; this must not be too high, which could cause lack of thread control. High static friction values are necessary to allow the stitches to lock and prevent “run-back” of seams. Spun threads are particularly good in this respect when compared with filament thread. The worst is the monofilament threads. The frictional properties are affected by lubrication. The factors that influence the frictional properties are:
Uniform application of lubricating agents.
Adhesion of the finishing agent on the thread.
The quantity and quality of finishes are very important. Special finishes like silicone compounds have been found to exhibit clear advantage over standard paraffin wax.

●Good abrasion resistance is essential for good sewing performance. The thread is under tension condition, especially when the stitch is being set. The thread must be resilient enough to return to shape after the distortions, and then must maintain its physical properties to provide good performance in the seam after the sewing process is complete. Nylon and polyester offer the best resistance to abrasion.

●Good resistance to heat is a very important requirement of a sewing thread. The temperature reached by the sewing needle during sewing very much depends on:
The nature of the fabric to be sewn (density, thickness, finish)
The speed of the sewing machine
The type of needle used (size, shape, surface finish)
Size and finish of the sewing thread.
The needle temperature is especially critical for fabrics and sewing threads of thermoplastic fibers, where it may exceed their melting temperature. Needle heating causes sewing thread breakage, cross-thread, skipped stitches, seam damage and physical damage to the needle.

●The hairiness of sewing thread also affects the appearance of the seam. Sewing threads for decorative seams are singed, squeezed and gloss-brushed.

●The final direction of twist insertion may be important to enable the stitch forming mechanism of the sewing machine to perform correctly; most sewing machine require Z twist, but there are a few where performance is better with S twist.

●Color fastness is a general requirement for sewing thread. It is important that the selected shade retain its color throughout the life of the garment. Two aspects of fastness are important:
The thread must not change color.
The thread must not stain any material adjacent to the seam.

●Low shrinkage during washing and ironing is required. Shrinkage due to fiber swelling causes seams to pucker, especially if the fabric exhibits less shrinkage than threads. Synthetic threads suffer less from this problem than cotton threads owing to their much lower moisture absorbency; however they are liable to residual shrinkage problems if unsuitable manufacturing processes are employed. Synthetic threads can suffer from the problem of thermal shrinkage during ironing but this difficulty can be solved by the use of high temperature setting, which stabilizes the thread at temperature above those normally encountered during the ironing process.