Physical Properties of Seams

The basic function of a seam is to hold pieces of fabrics together. The seam must be carefully chosen after considering the fabric to be sewn. The characteristic of a properly constructed seam will be,

• seam must be strong in the both lateral and longitudinal directions

• seam must be stretch and recover with the fabric (should have good extensibility)

• seam must be durable.• Seam must be secure against fraying apart or

unraveling of stitches• seams must not present and uncomfortable ridge or

roughness to the skin (specially with close fitting garments)

• seams must have good appearance in order to improve quality.

Seam StrengthA seam must be strong, the criteria being maximum strength for minimum thickness and economy of sewing thread. Seam must be strong in two directions.

Lateral-across the seamLongitudinal- along the length of the seam

Lateral - Seam allowances are important, because if these are too narrow seam may pull apart, particularly if the fabric has a tendency to fray. Strength can be improved by increasing seam allowance, by using strong thread and increasing stitch per cm.

• Longitudinal- Closely related to the extensibility along the seam as insufficient strength will result in cracking in wear.

Factors affecting the seam strength• fabric structure• properties of sewing thread• stitch density• seam and the stitch type

Every seam has two components, fabric and the sewing thread. Seam failure must from the breakage of either the fabric or the thread or both simultaneously.

Fabric structure

In seaming fabric may break due to two reasons• seam slippage• fabric damage as a result of the incorrect needles

which weaken the material to be sewn.

Properties of sewing thread

 Seam failure due to sewing thread breakage creates fewer problems than fabric failure because it can be repaired.Factors to be considered when selecting sewing thread to achieve maximum seam strength-sewing thread should be even in thickness and free from knots and faults-frictional properties of thread must be uniform so that the each stitch is pulled equally tight

-thread should have high abrasion resistance-thread should have ability to resist high temperatures-thread should have a considerable twist

Stitch density

Seam strength normally increases with increasing stitch density up to a point where the concentration of the needle holes starts to weaken the material. After this,a stronger thread should be used with lower stitch density.

Seam type

Seam consisting more number of plies under the stitching line is stronger than a normal seam.

eg. French seam is stronger than plain seam.Seam strength can be improved by top stitching the seam. Normally seam strength is directly proportional to the raw of stitches on the seam line.

 

Stitch typeLock stitch is secure because breaking of one thread will not be cause breaking of whole raw of stitch. End and beginning of the stitching line can be secured by back tacking. Stitches in lock stitch beds well in to the material thus improving abrasion resistance.

Chain stitch and over edge stitches are slightly stronger than L/S but their use is limited because of its narrow bight. (distance between needle and fabric edge).

Seam extensibility

This is required in all seam but varies according to the fabric being sewn.

 Factors affecting seam extensibility

• fabric structure (action stretch/ comfort stretch)• stitch type and the seam type• thread type • stitch density 

Fabric structureWhen knitted fabrics are considered, it is difficult to separate seam strength and seam extensibility because the majority of seam problems arise from thread breakage. This happens when a seam is stretch in longitudinal direction and stretches in excess of the amount that the stitch type and the stretch in the sewing thread can accommodate. Fabric structure plays a big role in deciding the seam extensibility because seam should process extensibility as much as the fabric. There are many fabrics, which stretch greatly.• action knit fabrics stretch 100% or more.• comfort knit fabric stretch up to 30%

When seaming in these two fabrics it is required to select a proper seam which stretches and recover with the fabric.

Stitch type and seam type • Both lock stitch (L/S) 301 and chain stitch (C/S) 401 if

carefully adjusted will give adequate stretch for comfort stretch fabric.

• However with L/S there is a limit to the extent to which the stitch tensions can be loosen to increase the stretch, while obtaining satisfactory stitch formation.

• With C/S 401 it is possible to set the stitches looser. To achieve maximum stretch with this stitch needle thread to looper thread length ratio in the seam should be high

needle to looper thread ration 1:3 If increase more than this seam grinning will result

 

• 504( 3 thread o/l) stitch provides the maximum attainable extensibility in high stretch fabrics (action knit wear). In garments such as corsetry and swimwear lock stitch zig-zag stitches are used to provide high stretch as well as high seam security. Class 600(covering chain stitches) stitches can also be used where appropriate because it provide high stretch and low bulk.

Thread type• With the use of synthetic threads seam stretch can

be improved by 10% more than which can be achieved with cotton threads. (Cotton thread typically have extensions at break 6-8%, synthetic thread in medium size have 15-20% or up to 25% in heavier sizes).

• Increase in stretch can be achieved by using spun polyamide threads which have extensions suitable for high stretch fabrics. How ever because of their specially engineered high stretch properties non of these high thread can be sewn efficiently as needle threads but can be used either as bobbin thread or

looper.

Stitch density • Increasing stitch density seam stretch can be

improve but only up to certain point. In L/S seams an increase in stitch density actually reduced the proportion of threads lying on the surface of the fabric and available for direct extension with the seam unless thread tension is adjusted at the same time as the stitch is shortened.

• With overedge stitches increasing stitch density will increase the seam stretch. But a limit is reached when the fabric is jammed between the crowed stitches. The seam extension is then limited and the fabric edge cannot relax completely after stitching.

Seam Durability

A seam must be durable, long lasting and wear easily every day of the garment. The length of life of a seam in a garment should be as long as that of the material. The seam can be considered to have failed in durability terms, if either the thread or the fabric fail at an early stage. Factors affecting seam durability

• stitch type• stitch tightness• fabric construction• seam type • thread type• extent to which the thread beds into the material

stitch type

• The thread in a L/S beds well into the material which improves abrasion resistance.

Stitch tightness (stitch tension) • Seams sewn at higher tension are much less subject

to damage than those which are sewn at less tensions. Also the life of the looper thread in a chain stitch increase if the stitch balance is adjusted so that the needle thread to looper thread of being concentrated at each stitch hole. (1:1)These stitch tightness adjustments are done to improve bedding in of the thread to increase the degree of protection by improving the abrasion resistance.

Fabric construction • Seam sewn in tightly woven and coated material

cannot bed –in and therefore more quickly abraded than those sew in loosely woven and softer materials. Closeness of thread in fabric structure makes it difficult to accommodate the sewing thread between them.

Seam type

• Durability can be improved by the proper selection of seam type.

• Eg: Inseam of a denim jean is area which subjected to high abrasion. For this seam french or a superimposed seam is more suitable because the threads in the seam do not have any contact with the surface.

sewing threads

• For the durability of the seam sewing thread plays an important role. In L/S machines during the process of stitch formation thread travel through the needle and guide points. Sewing threads must withstand these stresses and to retain its initial strength during stitching.

• To reduce friction with machine surface a lubricating finish apply to the sewing thread. In button holding machine spun polyester or polyester covered coarse spun threads used to give both durability and good appearance.

Seam Security  

All the factors discussed under seam strength, extensibility and durability are important in discussing seam security.

Different stitch types have different levels of security. The selection of most suitable stitch type promotes seam security. L/S can be considered as the most secured stitch type mainly due its interlacing of threads during stitch formation and facility of tacking at the beginning and end of stitching .

The least secured stitch type is class 100 that is chain stitches. A loop of thread is passed through the material and secured by intralooping. (each loop of the thread is secured by the succeeding loop of the same thread) This way each stitch is dependent on the succeeding one stitches in this class are insecure and unravel easily.

Level of security can be increased through-careful thread selection (use a hairy thread rather than a smooth thread)

-careful machine adjustment

-by using a machine which can condensed few stitches at beginning and end of a seam

Stitches in class 400,500 and 600 have intermediate security between the class 300 (L/S) and class 100 (C/S).

Seam Comfort

Discomfort in a seam may occur due to • seams tight against the body• seams are presenting ridge or roughness to the skin,

because of an unsuitable seam type, stitch or thread• Thread ends or label corners

If a seam is tight against the body it suggests that the extension in the fabric is greater than that in the thread in the seam thus causing cracking of the seam. To overcome this an alternative stitch type may be used.

• eg. Stitch in class 600 can achieve more flat and high elastic seam.

If seam forms an ridge it is likely to be because of a close overedge with a high stitch density or safety stitches which is more bulkier than an open seam. Any seam which is likely to be rough against the skin can be soften by the use of a crimp textured thread in the looper of the overedge stitches or by using such a thread in covering stitch of class 600. A considerable discomfort is associated with the use of a monofilament polyamide thread due to the long ends which must be left to prevent unravelling along the cut or melted end. Discomfort may also due to the label type and the method of attaching them. Many of the less expensive labels are printed on fabric made from thermoplastic fibres and cut and sealed to narrow widths. For the economy many labels are simply folded and inserted into a row of overedge stitch or with lock stitch close to the sealed edge.

Appearance • Evaluation of the appearance of stitches and

seams has traditionally been carried out using subjective visual procedures where the effect which stitching has on fabric, rather than the stitches themselves, assessed. Developments using instrumental techniques also deal with effects of stitching on the fabric.

Dimensional instability of threads can affect seam appearance by surface undulation (pucker) along the seam line. Fabrics used for shirting, typically polyester/cotton blends. Usually light in weight and with a smooth texture, need to be stitched with threads which have a shrinkage of less than 1% in order to avoid undesirable effects on product appearance.

• Topstitching emphasizes design and construction lines, so thick and /or lustrous thread is used. The smooth, satiny appearance required of embroidery stitching is achieved using fine, mercerized cotton.