Seam Engineering (part 2/2)

Fabric Type and WeightFabric type and weight can affect seam performance depending on the following:
• Fiber content (100% cotton, cotton/polyester blend, nylon).
• Fabric construction:
o Woven or knit.
o Type of weave used (plain, twill, jersey, tricot).
o Fill count.
o Yarn type and size.
• Pattern placement and seam direction.
• Propensity of the yarns in the seam to shift or pull out of the seam.
When engineering seams, we recommend doing tensile tests on the fabric to determine its strength. You cannot specify seam strength requirements that are stronger than the fabric itself.

Thread Fiber Type, Construction, and Size

These all have a definite effect on seam strength including the following factors:
• Fiber Type:
o Some fibers are stronger than others and have greater loop strength contributing to greater seam strength. For example, a 100% spun polyester thread will give greater seam strength than a 100%
cotton thread of the same size.
o Synthetic fibers like polyester and nylon are much more resistant to abrasion and chemical degradation (such as bleach) than cellulosic fibers. Cellulosic fibers on the other hand have superior heat resistance.
o Kevlar® and Nomex® threads were designed to resist high temperatures in protective clothing.
• Thread Construction (Spun, Core, Textured, Multifilament, etc.):o Core threads, made with continuous filament polyester core, generally will provide higher seam strength than spun and textured threads.
o Continuous filament polyester or nylon thread construction will provide greater resistance to abrasion and seam degredation.
o Some thread constructions are less subject to shearing or cutting each other when interlooped together in the seam. Air entangled, textured, and monocord thread constructions exhibit the best loop strength characteristics.
• Thread Finish (Soft, Mercerized, Glaced, Bonded, Etc.):
o Glace or bond finished threads generally have superior abrasion resistance to soft finished threads.
o Mercerized threads are stronger than soft cotton threads of the same fiber type and size.


• Thread Size (Tex, Metric, Yarn Size):
o Given a specific fiber type and thread construction, the larger the thread size, the greater the seam strength. As previously mentioned, different fiber types and thread constructions have different loop-strength characteristics. In many cases, a smaller thread size will imbed itself in the seam making it less prone to surface abrasion.
Stitch and Seam Construction
• Stitch Types:o Generally, the more thread consumed in a stitch, the greater the seam strength. This holds true when comparing 301 lockstitch seams to 401 chainstitch seams.
o Threads used in 301 lockstitch seams are more susceptible to shearing each other than 401 chainstitch and 504 overedge seams because of the way the threads are interlocked together rather than interlooped together.
• Seam Types:o Many seam constructions are more resistant to both stress and abrasion than other constructions. For example, a Fed. Spec 751a `LSc' or ISO 4916 2.04.06 felled seam is the strongest of all seams because the stress is shared by the fabric and the thread.
Stitches per Inch
• Generally, the greater the number of stitches per inch in a seam, the greater the seam strength. This refers back to the point that the more thread you put in the seam, the stronger the seam. However, on some fabrics, too many stitches can cause damage to the fabric by cutting the yarns enough to weaken it.
• Excessive stitches per inch can also contribute to seam puckering and reduce the speed through the machine resulting in loss of production.
Stitch Balance• As a rule, the more needle thread that can be put into a seam, the greater the seam strength. This can be accomplished by adjusting the sewing machine thread tensions, thread control guides, and eyelets, etc.
• Care should be taken not to put too much needle thread in the seam to cause the seam to "grin" or open up when stress is applied to it. Excessive sewing machine thread tension will cause reduced seam strength as well as create other sewing problems. Estimating Seam Strength on Wovens. To the right are two formulas that were developed for estimating the seam strength on woven fabrics. To do the calculations, you need to
know the following:
• Stitch type (301 Lockstitch or 401 Chainstitch)
• Thread Strength (Single-end breaking strength of the thread, measured in pounds)
• Stitches per inch

301 Lockstitch Formula
SPI X THD. Strength X 1.5* = Estimated Seam Strength
For example: 10 X 4.0 lbs. X 1.5 = 60 lbs.
* Factor based on normal loop strength of threads used for apparel.

401 Chainstitch Formula

SPI X THD. Strength X 1.7* = Estimated Seam Strength
For example: 10 X 4.0 lbs. X 1.7 = 68 lbs.
*Factor based on normal loop strength of threads using double loop on underside.
Abrasion Resistance

Abrasion resistance has always been an important factor to upholstery, footwear, and carpet manufacturers. Recently, abrasion resistance has also become important in apparel due to the stringent laundering cycles and pre-wash processes used in garment preparation. Factors that affect the abrasion resistance of a sewing thread in a seam include:
• Fiber Type (nylon, polyester, cotton, etc.).
• Fiber size and shape (denier per fil, round, trilobal, etc.).
• Thread construction (monocord, corespun, spun, air entangled, etc.).
• Thread size.
• Stitch and seam construction.