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Seam
Engineering |
One of the forgotten
elements in sewing manufacturing is seam engineering. For
example, a typical pair of branded five pocket jeans retailing for $30
to $75 is sewn with approximately 250 yards (200 meters) of thread.
The cost of a quality thread for these jeans is approximately $0.25
U.S. per garment.
Many jean manufacturers
will spend anywhere from $1 to $6 per jean to have them stone washed,
acid washed, or sandblasted. Incredibly, the
thread is expected to hold the seams together for the life of the
garment regardless of what abuse it has been exposed to! In such cases,
what some forget is that?/font>
Though thread makes up a small percent of
the cost of the garment, it shares 50% of the responsibility of seam
quality.
If a manufacturer has frequently returned garments for excessive seam
搖nqualities,?/i>
such as, re-stitched seams or seam failure due to the thread rupturing, we
would say the garment is 搖nder-threaded,?meaning that either the wrong
type or size of thread is being used. Quality seam engineering relates to
many areas of concern including seam strength and seam durability.
Obviously thread plays an important role in all of these areas that are
controlled by the factors described below.
Note: For additional information, refer to A&E 抯
Thread Selection Guides by
Application to determine the correct thread type and size generally used
for quality seams. Also, see Technical Information.
Seam Engineering - Thread
Size
We are frequently asked by sewing factories if they should use the same size
thread throughout the sewn product or if they could save money by using a
smaller thread size on the underside of the seam or in overedge seams.
Before these questions can be answered, the manufacturer should first take
into consideration:
- The number of colors that will be sewn in a season.
- If it is even feasible to use more than one thread size.
If you are sewing childrenswear or womenswear where there are always many
colors being sewn at any one time, then we would recommend staying with the
same type and size of thread throughout the garment. On the other hand, if
you are sewing products that consist of several basic colors, the best
option is to use smaller threads on the inside of the garment to
significantly reduce the thread cost.
Another factor that should be considered is the affect that this downsizing
of thread will have on your seam strength. Following are some common-sense
搑ules of thumb?for thread size selection on seams sewn with 301
lockstitch, 401 chainstitch, and 504 overedge.
301 Lockstitch Seams
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For 301 lockstitch seams, we generally recommend using the same
needle thread size as the bobbin thread size in the seam. Why?
Because
揳
chain is only as strong as its weakest
link.?/i> |
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If a smaller, weaker thread is
used in the bobbin, then the seam will only be as strong as the bobbin
thread. This is particularly true with lockstitch seams because of the
way the needle and bobbin threads are interlocked together.
In some cases though, this does have exceptions. Sometimes a smaller size thread with a
different fiber type and/or thread construction can be used and still maintain seam performance. An example would
be to use a higher tenacity Corespun in place of a Spun
thread. To take this a step further, you can use an
Anecord Nylon?i>,
monocord bobbin instead of a spun or corespun thread in the seam. Also,
exceptions can be made when a larger topstitching thread is desired to give
a bold appearance on the outside of the garment when it is not necessarily
being used to add strength or durability to the seam.
401 Chainstitch
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Notice that in the diagram of the 401 chainstitch, a loop of looper thread is holding the needle thread through the
seam. Also, notice that the threads are interlooped rather than
interlocked as we saw on the lockstitch seam. |
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This allows a looper
thread to be downsized to at least 60% of the needle thread size
without adversely affecting the seam strength (for example: T-60
needle thread ?T-40 looper thread). This is one way to reduce
thread cost without adversely affecting the seam quality. However, you
need to determine if carrying another SKU (stock keeping unit) will
significantly increase the cost of inventory and supervision on the sewing
floor to make sure the right size thread is being used in the correct
position.
504 Overedge Seams
504 Overedge seams have basically the same needle thread formation as
the needle thread on a 401 chainstitch seam. Therefore, the same rule
applies. You can use a looper thread approximately 60%
of the needle thread size without adversely affecting the seam strength
(for example: T-24 needle thread ?T-18 looper threads).
Just remember that smaller looper threads will not give the seam
coverage of larger thread sizes.
Five factors that determine the strength of a seam include:
- Fabric type and weight.
- Thread fiber type, construction, and size.
- Stitch and seam construction.
- Stitches per inch.
- Stitch balance.
Any one of these factors can adversely affect the performance of a sewn
product depending on the end-use of the sewn product. Following are a few
general comments related to the factors listed above.
Note:
Technical bulletins focused on optimizing seam performance when sewing denim
or when sewing stretch knits are available through A&E. Refer to the
Technical Information section.
Fabric Type and Weight
Fabric type and weight can affect seam performance depending on the
following:
- Fiber content (100% cotton, cotton/polyester blend, nylon).
- Fabric construction:
- Woven or knit.
- Type of weave used
(plain, twill, jersey, tricot).
- Fill count.
- 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:
- 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.
- 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.
- Kevlar?and Nomex?
threads were designed to resist high temperatures in protective
clothing.
- Thread Construction
(Spun, Core, Textured, Multifilament, etc.):
- Core threads, made with continuous filament polyester core,
generally will provide higher seam strength than spun and textured
threads.
- Continuous filament polyester or nylon thread construction will
provide greater resistance to abrasion and seam degredation.
- 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.):
- Glace or bond finished
threads generally have superior abrasion resistance to soft finished
threads.
- Mercerized threads are
stronger than soft cotton threads of the same fiber type and size.
- Thread Size (Tex,
Metric, Yarn Size):
- 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
- 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.
- 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:
- Many seam
constructions are more resistant to both stress and abrasion than other
constructions. For example, a Fed. Spec 751a
慙Sc?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
揼rin?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
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301 Lockstitch Formula |
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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. |
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401 Chainstitch Formula |
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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. |
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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.
Currently, no ASTM, AATCC,
or ISO abrasion test exists for evaluating the toughness of sewing
threads. However, A&E conducted comprehensive tests using a modified
crockometer to achieve the following results.
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