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Basic knowledge about seam quality by AMANN Group, Germany... part II

 

No quality product without quality seams

Seam quality criteria

Quality means performance, conditions, and properties. If you want to assess a seam under these aspects, you need a detailed requirement profile. Yet, different seams make different requirements. Airbag seams, jeans seams, or upholstery seams – they all ask for different properties of the seam connections and thus have their individual requirement profiles.

However, there are indicators that are true for almost all seams. Their evaluation is the basis for assessing their quality. Every seam analysis begins with the checking of these properties. In addition, there are individual quality criteria. They must be worked out depending on the application and function. The image below shows the most important standard indicators and gives examples for individual quality criteria.

 
Criteria of seam quality

Seam stability

For assessing the seam stability, it is important to check the crossresistance (EN ISO 13935) and the seam slippage (EN ISO 13936). Seam breaking strength describes the resistance of seams to tensile stress on the seam, in crosswise direction. If a seam’s breaking strength is insufficient, the seams will tear during the later use. A well-known example from practice here are broken back seams on trousers. The seam slippage relates to the tendency of the warp and weft threads in a fabric to shift in crosswise direction to a seam under tensile forces. This is a fabric-related issue. A low seam shifting resistance leads to open seams, and possibly even broken seams. Many fashion fabrics show this problem.

An unfavourable seam slippage can be improved only marginally by changing the sewing conditions, for example through additional topstitching seams at the relevant seams, or the use of lap seams. Partly, it helps to use an additional non-woven interlining to reinforce the fabric in the area of the seam – an emergency solution, which is very time and cost consuming. The used sewing thread, however, does not have any influence on the seam slippage, despite various statements to the contrary. Basically, it is up to the fabric producer to improve the seam slippage.  

The seam’s breaking strength is determined by the fabric, the sewing thread, and the sewing parameters. The basic level of a seam’s stability is firstly determined by the fabrics tensile strength (in warp, weft, and diagonal direction). Apart from that, the used sewing thread is the primary factor for the possible seam breaking strength of a seam – its raw material, strength, and construction. Cotton or polyester, fibre thread or multifilament construction, coarse or fine counts – the possible stability of the different threads varies extremely. For a first evaluation of the sewing thread requirements the product data sheets can be helpful. Here you will find information on their raw materials, construction, count, and all the other important data. The following figure shows the influence of the thread size. The clear difference in stability, even to the next smaller or bigger size, emphasises the need for a thorough selection of the size.

 
Comparison of seam breaking strength of different thread strengths

When choosing the sewing parameters, the stitch density, stitch type, and the thread balance in the seam have a great influence on the seam’s breaking strength. Increasing the stitch density by only one stitch/cm, for example, leads to a 25 – 30 % increase of seam breaking strength. When comparing the most important stitch types for joining seams, the lockstitch and the double chainstitch, the double chainstitch is to be favoured under the aspect of seam breaking strength.

 

Seam abrasion resistance

The seam abrasion resistance characterises the thread’s resistance to abrasion stress in the seam. It is one of the most important criterions for assessing a threads behaviour in use. Seams are often exposed to great abrasion stress in use: buttonholes, exposed ornamental seams, seams on stone-washed jeans, cover seams on biking pants – just to name a few examples – are under great stress. Quite frequently, they show signs of wear after short use periods only, with a frayed seam pattern or totally worn and torn seams.

The seam’s abrasion resistance is determined by the fabric, the seam construction, and the sewing thread. The characteristic of the fabric is decisive for how flat or prominent a seam is, thus having more and more exposure to the abrasion stress. Fleecy and voluminous fabrics protect the sewing thread from rubbing, hard and glossy fabrics expose the seams more and thus increase the abrasion stress.

Accordingly, the seam construction (seam and stitch type) influence the abrasion resistance. For example, lap seams for joining two material plies cause a more intense abrasion stress for the needle thread than inside positioned safety stitch seams. Thus, the bobbin thread in a double chainstitch is exposed to abrasion stress much stronger than in a lockstitch.  

However, the seam’s abrasion resistance is primarily determined by the sewing thread, and in this regard, mainly by the raw material, which determines its general resistance to abrasion. The following figure shows the big differences in a seam’s abrasion resistance depending on the raw material. The sewing thread construction and its strength do have some influence, too, but it is much lower than that of the raw material. Corespun threads feature a better abrasion resistance than fibre threads, for example. Multifilament threads are the best. Coarser threads tend to be more resistant to abrasion than finer threads. But this is tricky, too, because coarser threads are more prominent, thus increasing the abrasion stress, which can even reverse the expected result. Then, the abrasion resistance of a finer thread is better again.

 

Comparison of seam abrasion resistance of different sewing threads of the same size (Modified Martindale test):

1 Cotton thread
2 Polyester cut staple thread
3 Cotton polyester corespun thread
4 Polyester corespun thread
5 Polyester multifilament thread
6 Polyamide multifilament thread

 

Seam elasticity

Seam elasticity means the elasticity and the behaviour of a seam under tensile stress in lengthwise direction. It is primarily determined by the thread reserve in the seam. Today, elastic seams are important for so many applications that a complete chapter should be dedicated to this topic.

 

© The Fashion Machine News, 2010, August

 

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