Friday, March 3, 2017

Fabric Seam Slippage Mechanism, Process and Way to Improve

Seam slippage is one of the most important defects in case of woven garments that affect appearance and performance of the garment. And after all it decreases the garment quality and hampers the brand image of the manufacturer very badly. There are various factors of seam slippage in garments. In this article we will discuss about what is seam slippage? Mechanism of seam slippage. How to occur seam slippage in fabric? And how to improve seam performance in garments?

What is seam slippage?
When the seam is under some transverse strain, then displacement of the stitch relative to one or more of the fabrics can occur. This is called seam slippage. The stitch displacement produces some displacement of one yarn system in the fabric against the other, which causes opening in the fabric. This phenomenon is an adverse feature of some woven fabrics, and it decreases the range of possible end-uses and causes problems in the garment.
Seam slippage in fabric
Fig: Seam slippage in fabric
The amount of seam slippage or the fabric resistance to seam slippage depends on:
  1. Yarn-to-yarn friction
  2. Contact angle between threads
  3. Stitch density
  4. Yarn flexural rigidity
An increase in the values of these factors increases the fabric resistance to seam slippage. Seam slippage depends on weave, fabric raw material, type of seam, stitch density, and sewing thread tension.

Mechanism of fabric seam slippage:
When two fabrics are sewn together parallel to a yarn system and the seam is open by applying some tension transverse to the seam axis, this tension imposes strain on the seam. The tension in the sewing thread in the seam depends on thread tension during sewing, fabric tension, and stitch density. The tension applied to the fabric through the seam creates a pushing force on the threads transverse to the seam axis. This causes deformation of stitch geometry and displacement of threads along the seam.
Seam slippage of rayon fabric
Fig: Seam slippage of rayon fabric
Seam slippage is basically a consequence of tensile elongation. Seams in the garments are constantly being subjected to stresses in various directions, among which those perpendicular to the seams are most common. When two pieces of woven fabric are joined by a seam and an increasing force is applied to the assembly at right angles to the seam line, rupture ultimately occurs at or near the seam line and at a load usually less than that required to break the unsewn fabric.
Seam slippage test
Fig: Seam slippage test
When fabrics are stretched or load is applied, some changes are expected in the structure of the fabric, which are dependent not only upon the tensile properties of the fabric but also upon other mechanical properties like bending, shear (along with frictional hysteresis), and the surface coefficient of friction and roughness.

As a load is applied to stretch the fabric, at first the crimp is straightened, and then the yarn extends and overcomes the inter-yarn friction. The higher cover factor in the cross-direction gives a greater number of cross-over points. Therefore, yarn slippage of such a fabric will be more difficult. Another important consideration is that decrease in crimp in the direction of the pull increase the crimp in the opposite direction. Therefore, fabric elongation becomes more difficult due to an increase in the frictional resistance resulted by an increase in the contact area between yarns.

When force is applied perpendicular to a seam joining two pieces of fabric, some of the above movements would take place but with a difference. The stitching line acts like a clamp, which is flexible. Its flexibility would therefore determine the load elongation behavior of a seamed fabric.

Fabric seam slippage process:
When a tensile load is applied to a fabric seam, it has to overcome two types of frictional forces. One is the inter-yarn frictional forces within a fabric, and the other is the frictional force of the stitch assembly. The former is dependent upon the crimp, yarn diameter, fiber content, and number of cross-over points. The latter, however, is dependent upon the fabric properties like fiber content, type of yarn (spun or filament), thickness, lateral compression, cover factor (threads per cm), bending, shear, tensile and surface roughness, and coefficient of friction. It is also dependent upon the properties of the sewing thread like fiber content, diameter, coefficient of friction, initial modulus, and extensibility. All of these properties, together with the machine variables like needle and bobbin thread tension and the stitch length, make up the frictional force of the stitch assembly. Thus different combinations of these would be expected to provide different frictional resistance and hence different loads at which seam slippage may take place.

How to improve seam slippage?
Seam slippage is usually caused by poor fabric design such as too loose of a weave or too narrow of a seam margin. Poor stitch balance or not using enough stitches per inch can also contribute to seam slippage.
  1. To minimize seam slippage problem below techniques should follow:
  2. Seam slippage depends upon the stitch rate, so higher stitch rate will increase the grip, hence minimize seam slippage.
  3. Increasing seam allowance will reduce seam slippage.
  4. Seam slippage frequently occur by single needle stitch, then can try with double needle stitch.
  5. Sewing with interlining fabric or fusible tape along the seam-line on seams that will help to reduce seam slippage.
  6. Tight stitch will reduce seam slippage because a tighter stitch will more grip of fabric.
  7. Make sure the stitch is properly balanced with the minimum thread tensions.