 |
|
Introduction Most stretch knit
garments are seamed with overedge and/or coverstitch seam constructions
because these stitches offer both seam elasticity and coverage of the
raw edge of the fabric.
|
However, a common
quality problem is having broken stitches or "stitch cracking" when
the seam is stretch excessively. Generally, the greater
the elasticity of the fabric, for example, high Lycra® content, the
more likely you are to have broken stitches if you do not understand
how to optimize seam elasticity in the stitch.
Most broken stitches
are the result of one of the following:
- Not using the
correct thread type & size.
- Not having enough
stitches per inch.
- Not sewing with the
correct stitch balance or thread tensions set incorrectly.
- Not using the
correct seam margin.
The most common threads
used to sew stretch knit fabrics are textured polyester or nylon threads
like A&E’s Wildcat Plus® or Best Stretch®. Textured threads are ideal
for overedge and coverstitch seams because they offer excellent seam
coverage and seam elasticity. Generally, light weight knits are sewn with
T-18 or T-24 size threads. Medium weight knits are generally sewn with
T-24 and T-35 size threads. Spun polyester threads such as A&E’s
Perma Spun® and Excell® are also used instead of 100% cotton threads because
they have a higher elongation offering greater seam elasticity. In cases
where the garment is subjected to harsh wash programs, many manufacturers
prefer to use Perma Core® threads.
Why Is It Important To
Have the Proper Number of Stitches Per Inch & Stitch Balance?
|
Seam elasticity can be
accomplished by a combination of having the correct number of stitches per
inch and having the proper stitch balance. The more stretch the fabric
has, the more stitches per inch that are required.
Common ranges of stitches per inch are from 12 to 14 spi. |
|
Applying & Releasing
Stress
Across the Seam
 |
|
However, even when the correct number of stitches per inch are being used,
if the proper stitch balance is not used, you will still experience
excessive seam failure.
One way of checking the stitch balance is first check the seam for "seam
grinning" by pulling perpendicular to the seam. |
|
Pulling the Seam Along the Stitch-line
 |
After applying the stress across the seam, remove the stress and check to
make sure the seam goes back to a closed seam.
Next pull the seam along the stitch-line to the maximum stretch level that
the garment will be subjected to and see if the threads fail.
If they do, generally
there is not enough needle thread in the stitch. Therefore loosen the
needle thread tension and check the seam again.
What You Should Know about Seam Margin
|
The seam width on overedge and coverstitch seams also has a major factor
in producing a seam with the proper seam elasticity. Take a zig-zag
stitch for example. The wider the stitch and the more stitches per inch
that are used, the greater the seam elasticity. |
|
504 and 514 Overedge Stitching
|
A zig-zag stitch will
stretch until it becomes a straight stitch at which time all the stress is
applied to the thread.
On overedge machines, the
chaining finger or "stitch tongue" on the needle plate and the adjustment
of the overedge machine’s trimming knives determines the seam width. |
|
1/8" and 1/4" Needle Spacing
|
Seams sewn with a wider
"bite" or seam margin and more stitches per inch will have greater seam
elasticity.
The two most common overedge stitch formations used for seaming knit
garments today include the single needle 504 stitch and the two needle 514
stitch.
Obviously, a two-needle
514 overedge stitch offers greater seam elasticity because this stitch
generally is sewn in a wider seam margin. However, many times this wider
stitch is not desirable on intimate apparel or knit underwear because it
creates a more bulky seam. In this case, a narrow bite 504 stitch is
preferred with more stitches per inch and the correct stitch balance.
The needle spacing on a
406 bottom coverstitch types and similar stitch types have a tremendous
impact on the seam elasticity.
The wider the needle spacing, the more elasticity the stitch will have at
a given stitch length. Therefore, when a narrow needle spacing is desired
on high-stretch fabrics, the sewing machine should be set for more
stitches per inch to minimize thread failure in the seam.
Making Seams
on
Stretch Knit Fabrics with a Flat Seam Appearance
| Most sewing machines
designed for sewing stretch knit fabrics have a differential feed system
that includes two independently driven feed dogs. Tandem differential feed
systems have a front differential feed dog and a back main feed dog.
|
|
Tandem
Differential Feed System
|
If both feeds are feeding at the same stitch length or if a sewing machine
is being used with a single feed, then the resulting seam will appear "wavy"
like the ocean.
|
To compensate for the stretch in the
fabric, the front differential feed is set to feed more fabric in than the
back feed is feeding out resulting in a flat seam appearance. |
|
Setting Differential Feed To
Compensate for Fabric Stretch
|
Most binding or border
machines are also equipped with an "off-set" differential feed
system to optimize seam appearance.
Operator Handling Can
Effect Seam Appearance
When sewing stretch
knit fabrics, it is important to train the sewing operators to guide the
fabric into the sewing machine without stretching the fabric excessively.
Excessive stretch of the fabric when feeding the fabric into the seam can
impact seam appearance, seam elasticity, and garment fit.
Whenever knit fabrics
are being sewn, needle cutting is always a concern. For more information
about how to minimize needle cutting, refer to the bulletin Reducing
Needle Cutting.
|
