Sewing thread:-

Sewing Threads and their Technical Application:-

Sewing thread:

 According to the definition given by ASTM, sewing thread is a flexible, small diameter yarn or strand usually treated with a surface coating, lubricant or both, intended to be used to stitch one or more pieces of material or an object to a material. It may be defined as smooth, evenly spun, hard-twisted ply yarn, treated by a special finishing process to make it resistant to stresses in its passage through the eye of a needle and through material involved in seaming and stitching operations.

Figure 1: Sewing threads

Sewing threads are used in garments, upholstery, air-supported fabric structures and geotextiles to join different components by forming a seam. The primary function of a seam is to provide uniform stress transfer from one piece of fabric to another, thus preserving the overall integrity of the fabric assembly.

Threads for the high temperature applications are required to withstand and hold the seams secure in their position in extreme temperature conditions between 260ºC-1100ºC. Threads are usually made from glass, carbon, polytetrafluoroethylene, steel and aramid fibers. Polyesters, Polypropylene, Nylon 6, Nylon 66, are widely used for low temperature applications such as car upholstery, leather industry, packing like cement and fertilizer bags. Medical sutures are used for wound closure and are specially design and sterilized to fulfill the end applications.

Seam can be formed by the following techniques:

.Mechanical: 

stapling, sewing.

.Physical: 

welding or heat-setting.

.Chemical:

by means of resins.

The formation of seams by physical and chemical methods is restricted to a few specialized applications, as these processes tend to alter certain properties of the textile material. Among mechanical sewing techniques, sewing maintains its prevailing position by virtue of its simplicity, sophisticated and economical production methods and the controllable elasticity of the seam produced.

Essetial Properties Required for Sewing Threads:

Industrial sewing techniques make specific and often very exacting demands on the threads involved in the sewing process. The sewability of sewing threads is of major importance, having a very profound effect on seam quality and production costs. The sewing and the seam performance of a sewing thread are largely influenced by the material to be sewn, the sewing technique and the end-use for which the sewn material is intended.

These requirements can be defined as:

a) The ability of the sewing thread to meet the functional requirements of producing the desired seam effectively.

b) The ability of the sewing thread to provide the desired aesthetics and serviceability in the seam.

c) The cost of sewing thread and that associated with producing the desired seam.

The different important properties required by a sewing thread are discussed below:

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

2. 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.

This is a function of the dynamic and thermal loading of the thread and is influenced by the thread frictional properties, thread tensioning during sewing, needle size, stitch length and number of fabric layers in the seam. The thread should therefore possess adequate strength and elongation in order to perform satisfactorily during sewing and in seam.

3. 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.

4. 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.

5. 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.

6. 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.

7. 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.

Various studies show that the sewing thread influences the needle temperature significantly. Its movement through the needle reduces the needle temperature by an average of 21- 45%, the amount of reduction depends on the sewing condition and the structure, fineness and composition of sewing thread.

Lubrication of sewing thread with a mixture of wax, emulsions with synthetic resins, and silicon based products may minimize heat generation, and the fibers surface of spun yarns may be an advantage in that a thin layer of the surrounding air will move with the thread and promote needle cooling.

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

9. 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.

10. 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.

11. 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.

The sewing threads should possess better evenness and should contain minimal number of knots, faults and neps, etc. Thread should have very low level of imperfections and classimat faults.

12. Good luster in the thread improves appearance of the seam.

13. Threads must be uniformly dyed in a good match to the materials being sewn and also the dyed thread should have properties like colorfastness to washing, light, perspiration, and sublimation.

14. The ability of the thread to perform efficiently in the sewing machine is defined sewability. It can be assessed by the number of breaks that occur during the sewing of a certain number of stitches. However, owing to the generation of needle heat in high-speed sewing, the threads could be damaged without breaking. The long knot-free evenner yarns in case of rotor and air-jet can give better sewability.

15. The characteristics of properly constructed seam are strength, elasticity, durability, stability and appearance. The relative importance of these qualities is determined by the end-use of the sewn product. The factors that govern these properties are seam and stitch type, thread strength and elasticity, stitches per unit length of seam, thread tension, seam efficiency of the material. The hairiness of sewing thread is important to decide seam appearance. The shrinkage potential of the thread and hence the seam is also major importance for proper seam appearance. The serviceability of a garment depends not only on the quality of the fabric but also on that of the seam. The seam quality is measured by stitching parameters of the threads and seam parameters such as size, slippage and strength.

The failure of seam produced by traverse loading can generally be classified as: Type I: the failure due to thread breakage, Type II: the failure due to fabric breakage, Seam breakage: the failure due to the slippage of cloth yarns at right angle to the seam.

Seam slippage is the most probable cause for seam failure that leads to garment rejection in wear. The durability of a seam depends largely on its strength and its relationship with elasticity of the material. It is measured in terms of seam efficiency, where Seam Efficiency = (Seam tensile strength/fabric tensile strength) x 100, generally ranges between 85 to 90%. The minimum loop strength correlates well with the stitch breaking strength. Further resistance to abrasion and wear of the seam during everyday use, including laundering is also essential for the longer seam.

16. Seam puckering can be defined as a differential shrinkage occurring along the line of a seam and is mainly caused due to seam instability, due to high tension imposed during sewing. Though currently available threads have a certain amount of controlled elasticity and elongation they get over-stretched when the sewing tensions are high. During relaxation the thread recovers its original length, thus gathering up the seam. Threads for use in apparel are also required to have good stability to laundering, ironing and other treatments since differential shrinkage between the sewing thread and the fabric of a garment can cause puckering.

Further, Seam pucker can be determined by measuring the differences in fabric and seam thickness under a constant compressive load. The seam-thickness strain is calculated by using the formula:

Thickness strain (%) = (seam thickness – 2 x fabric thickness) x 100 / 2x fabric thickness

Functions of Various Machines in Sewing Thread Manufacturing Process:

Yarn Singeing:

Sewing thread must be singed to ensure that the projecting fibers do not interfere with downstream processing. Percentage of singeing can be achieved varied by varying the yarn collection speed. Hair removal efficiency at singeing machine is normally 30 – 50%. Flame temperature is around 800°C. Singeing is mainly done in Cotton Sewing Threads.

Features:

The heart of the machine – the burner, serves to singe reliably the projecting fibers of yarn running through at high speed, without inflicting burns on yarn itself. There is a choice between the gas burner and the electric burner. Gas burner is widely used. The gas burner consumes about 55 gm of natural, propane or butane gas an hour, depending on singeing rate and yarn type.

Speed: 300-1200 m/min.

Hank to Cone Winding:

Conversion of hank in to cone of suitable weight

Waxing for reducing co-efficient of friction in sewing thread.

Featured of new machines:

Twin in Rollers:

At the front of the head are the twin-input rollers, set to a fixed speed but proportional to the winding speed. The main functions of this unit are to eliminate unwanted tensions prior to yarn entering waxing unit.

The speed of machine ranging from 400-700 rpm, with possible traverse from150-200 mm.

Polishing:

Some threads for special end uses like leather industries, bag stitching, kite flying are treated with starch, softeners, whitener, etc on this machine. Cooked starch is mixed with other chemicals and different recipes are made for different qualities depending on the end uses.

Main objective of Polishing are:

.Extra ordinary smooth surface

.Thread becomes round

.Stiffness increases

.Increase in tensile strength (7-10%).

Cross Winding and lubricantion:

.Winding in various types of sewing thread packages like cone, cop , tube, ball, vicone and spool.

.Threads are treated with special waxes for achieving best workability during sewing operation.

.Lick roller lubrication is applied on industrial sewing thread where thread has to run on high speed sewing machine; the basic ingredient of the most of the lubricants is parafin wax. .

.Although silicones are also used because of their stability to heat and various additives are also included to give some special properties.

Geometry of Packages:

Threads are wound in many forms. Small length spools are employed in retail store distribution, whereas somewhat larger spools are used to a limited extent industrially. Some of the very fine soft threads are wound on cones, very coarse soft threads are in skein form, but the largest proportion by far is wound on the one headed tube with base or straight tubes.

These packages in some instances are put on a weight basis; however, the larger percentage is marked on a length basis. Ready-wound bobbins in a number of styles to accommodate the various sewing machines employed are also available. Cross winding threads are generally laid in with traverse ratio 1: 6 (one double traverse = six spindle revolution) and 1: 4 for coarser counts.

The following types of packaging are used in sewing thread ( with commonly used dimensions.)

Spool:

Spools are small flanged plastic or wooden bobbins; they are both with tapered (so-called Diabolo spools) and straight flanges. Mainly parallel winding (because side unwinding is easy) is done. Cross-windings are also possible on the spools. They contain relatively short length of 100-500 meter thread. The length of traverse on spool is 2.9-3.8 cm.

Figure 2: Sewing threads in spool.

End uses:

 Upholstery, footwear, leather goods manufacturing, and in hand sewing operations.

Cops:

Cops are small cylindrical flangeless spools, with precision cross winding. They are mostly made of paper and plastic. They are of two types, small Cop (Tube) and medium Cop (Cop). The lack of flanges facilitates regular off winding on industrial sewing machines although their small diameter makes them less well suited to the faster thread take off machines. Smaller Cops are popular make-up in fashion trades, where a variety of shades are used and production runs for any one color or style of garments. The length of thread wound ranges from 100-2000 m on small cop and 400-4000 m on cop. The length of traverse on small cop is 5 – 6.3 cm and on cop is10 cm.

End uses:

Kite flying, Upholstery, ready-made garment, tailoring, hosiery, umbrella, and shoe stitching.

Cone:

This is self-supporting, cross wound conical package. It is easier to withdraw yarn over end from a cone than from a cheese and because of this, cone is more widely used. They contain relatively long lengths 1000-25000m with length of traverse ranging from 10-15 cm. They give trouble free thread unwinding at intermittent or continuous high speeds. Cones are the most economical packages for conventional sewing threads in situations where thread consumption is high and production runs are long.

End uses:

Readymade garment, tailoring, hosiery, leather stitching, Upholstery, shoe stitching, denim, embroidery, and kite flying.

Vicone or King Spool:

Vicones are parallel tubes or low angled cones with an additional base in the form of a raised flange, which may incorporate a small tip. The build of vicone depends on the exact conformity of the taper with the angle of vicone’s base. Coarse yarns require a large traverse for the taper — fine yarn a small one. They contain lengths of 1000-5000 m with length of traverse 6.5-9 cm.

End uses:

Embroidery, core-spun, and filament threads.

Prewound Bobbin:

Prewound bobbins are precision parallel wound thread package designed to replace metal bobbins on a variety of lock stitch machines.

Skein:

A very small hank of soft twisted plied thread (around 8 m) of coarse count is parallel wound with the help rotating flyer, which withdraws the thread from the supply package cone.

End uses:

Embroidery.

Ball:

A typical ball like structure, wound with the help of four types of different winding. They are: Rough base winding, form winding, surface layer winding, and circumference winding. The initial winding provides firmness at the base. Next winding process makes space for placing the identification ticket. The third stage of winding provides firm gripping of the ticket. The last stage of winding makes a band over the ball, which retains its shape. An easy unwinding of thread is possible.

End uses:

Embroidery, fishing net and bag closing.

Cocoon:

Cocoons are self-supporting i.e. center-less thread package specially designed for the insertion in the shuttle of multi needle quilting and some types of embroidery. Cocoons are used in the shuttle of multi needle quilting and some types of embroidery machines.

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