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| Issue date:01/08/2007 |
| ATA Journal for Asia on Textile & Apparel - Aug 2007 Issue |
| Source:Journal for Asia on Textile & Apparel |
| by Sumit Mandal, Dr Frency Ng and Dr Patrick Hui |
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This article investigates the different mechanical properties of sewing threads that affect the seam quality, and to identify the interrelation between each property with seam serviceability and seam appearance. It is observed that the mechanical properties of a sewing thread are highly correlated with seam serviceability and seam appearance.
The apparel engineers are always concerned about properties of fabric used in apparel manufacturing because the fabric is the prime raw material in apparel industry. Fabric quality is one of the primary requirements for production of high seam quality in apparel. However, fabric quality alone does not fulfill all the criteria for the production of high quality garment. The conversion of a two-dimensional fabric into a three-dimensional garment involves many other interactions, such as the selection of a suitable sewing thread, optimization of sewing parameters, level of easeness to converse fabric into garment and actual performance of the sewn fabric when wearing the garment.
Sewing thread is also a key variable contributing to satisfactory seam quality. A sewing thread is selected based on its properties in a completed garment to improve seam quality. Good seams are essential factors in garment quality. Defective seams may spoil the appearance of a garment and be the cause of ultimate failure. Seam quality depends mainly on the following sewing thread properties: tensile, bending properties, dimensional stability, and friction on the fibers used.
Figure 1: Tenacity-elongation curve for different fibers sewing thread |
Tensile properties
If an external force is applied to a material, it is balanced by internal forces developed in the molecular structure of the material. In a high-speed sewing machine, the external force applied on the needle thread is as high as 200gf. In order to withstand this force during sewing, a thread must possess adequate strength and elongation. Since different materials have different molecular structures, their behavior in response to this force varies (Figure 1). Some examples of different fiber tenacity levels are given below.
Comparison of fiber tenacity, breaking extension, work of rupture and initial modulas [Meridith (1945) and Farrow (1956)] |
The straight strength and elongation of a sewing thread should be adequate to achieve good sewing performance as well as good seam strength. Highly extensible thread is generally required only for extensible knitwear garments. It is also essential to measure loop strength and loop elongation of a sewing thread. The loop strength of a thread contributes directly to the stitch strength and hence to the seam strength, while loop elongation is an indicator of the degree to which a seam, under stress, can be stretched without a thread breaking.
In terms of quality, the initial modulas are one of the key properties in case of the sewing thread. A high initial modulas is always essential to prevent slip stitching and seam puckering. However, initial modulas are not only a measurement of sewing seam quality. A moderate-to-high level of toughness (area under the tenacity elongation curve) helps improve the amount of damage during sewing hence seam quality. Thus, adequate yarn strength, tenacity-elongation characteristics and their recovery behavior are important to the proper performance of a sewing thread.
Comparison of kinetic and static friction in fibers [Pai, Munsi, Ukidve (1982)] |
Frictional properties
Frictional forces are often created between fabrics and machine parts. The most severe friction occurs between the thread and the needle, as well as where the thread and the fabric is being sewn. In general, static friction and kinetic friction can be found. Static friction is the force necessary to initiate motion from the rest, while kinetic friction is related to the magnitude of the velocity.
Although fibers do not have a genuine, constant coefficient of friction, it is useful to quote values of μ= F/N to express the magnitude of the friction under particular conditions. The coefficient of friction varies with different yarn materials and on different surfaces (see tables below).
Comparison of the coefficient of friction between fibers [Morton, Hearle (1975)] |
Comparison of the coefficient of friction between fibers and different guides [Buckle and Pollitt (1948)] |
In a fast sewing machine, a needle thread rubs against the needle or thread eyelet placed at several points in the machine. However, the coefficient of friction between a needle-thread and a stainless steel or other guide should be less than 0.2. As a result, all synthetic fiber needle threads require a lubricant finish to reduce friction to an acceptably low level.
In terms of quality, a moderate-to-high level of static friction values is necessary between the fibers to allow the stitches to lock and prevent "run back" of the seam. Spun threads are particularly good in this respect when compared with the continuous filament threads. The worst offenders are threads made from monofilaments. However, friction must be in optimum. Friction in excess causes the thread to break, while the thread control will be lost if there is insufficient friction. Earlier studies show that coarse and medium fine yarns demonstrate a more hairy appearance after friction, but poor seam appearance. In contrast, damage is not discernible to fine yarn fiber and good seam appearance is resulted.
Dimensional stability
Dimensional stability refers to the ability of a sewing thread to retain its original condition when subject to varying degrees of temperature, moisture, or other kinds of stress.
Heat is generated during the sewing process as a result of the friction between fabric and needle. The most severe heat generation often takes place in two areas, namely the portion of the yarn that is pushed through the fabric, and the part of the emerging loop that is wedged between the needle and the fabric.
Research data show that the needle temperature can rise up to 350℃. As a result, all the thread must be protected to ensure it passes through the sewing machine and into the seam as smoothly as possible.
In addition, a sewing thread made from silk and cotton can be operated effectively at a needle temperature of up to 400℃. Meanwhile, synthetic fiber, staple fiber sewing thread and core spun thread exhibit a higher melting resistance at high sewing speeds than continuous filament threads, owing to the existence of higher temperature gradient in the yarn cross section.
Washing usually follows after a garment is manufactured. When a sewing thread absorbs water, it changes in dimension, and swelling also occurs transversely and axially. All the moisture-absorbing fibers show a large transverse swelling (see table 6), but in some, the axial swelling is very small, so that the swelling anisotropy is very high.
Swelling is generally less severe in synthetic fiber. A sewing thread made from nylon or polyester often exhibits higher dimensional stability during wetting, whilst sewing threads made from natural fiber have higher transverse swelling, which is the result of less dimensional stability.
Both dry and wet dimensional stability is key to seam quality. Heat often causes burn marks on natural fibers such as cotton or wool, and it causes synthetic fiber to soften or melt, leaving a weakened seam or a melted residue on the fabric surface. Sewing thread may also break because of this, and may damage the surface of the fabric. In the wet condition, a cotton sewing thread increases in diameter, and its shrinks in length leading to seam puckering in sensitive fabric and a change in the seam appearance of fabric.
Bending properties
The flexibility of a fiber depends on its shape, modulas and density and fiber. The most severe bending often takes place when a needle thread is bent at the needle hole, or at the thread eyelets.
Bending property has severe impacts on seam quality. Thus, all the thread must be flexible to ensure it passes through the sewing machine and is being imparted into the seam as smoothly as possible.
Sewing thread made from spun yarns has good sewing performance, good dimensional stability, and good stitch locking properties. In terms of quality, pucker is not favored as it refers to the wrinkling appearance along a seam in an otherwise smooth fabric. Among other factors, high sewing thread bending rigidity is a reasons explaining the happening of seam puckering that leads to poor seam appearance.
The apparel engineers are constantly working to avoid the two major groups of problems when textile materials are sewn, namely the problems of stitch formation give rise to poor seam appearance and performance, and the problems of fabric distortion known as "pucker" also lead to poor seam appearance.
All in all, it is critical to understand the interaction between sewing thread mechanical properties and seam quality, so that apparel engineers can work to attain the optimal sewing thread selection in apparel manufacturing.
Sumit Mandal is conducting researching in the in The Institute of Textiles & Clothing, The Hong Kong Polytechnic University (PolyU), Kowloon, Hong Kong. Dr Frency Ng and Dr Patrick Hui are both Assistant Professor of the Institute of Textiles & Clothing at PolyU.
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| Copyright © Adsale Publishing Limited. Any party needs to reprint any part of the content should get the written approval from Adsale Publishing Ltd and quote the source "ATA Journal for Asia on Textile & Apparel", Adsale Textile English Website - www.AdsaleATA.com. We reserve the right to take legal action against any party who reprints any part of this article without acknowledgement. For enquiry, please contact Editorial Department. |
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