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Introduction |
Measure SPI |
Seam Strength |
Typical Stitch Lengths
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Selecting the Right SPI |
When writing garment
specifications, you should always specify the proper number of
stitches per inch (SPI) that should be used in your sewn products. Why?
Because the number of
stitches per inch can have a direct influence on the following: 1)
the seam strength; 2) the stitch appearance; and, 3) the seam
elasticity on stretch fabrics.
Seam
Strength
| An estimated seam strength formula was developed years ago for woven
seams where one piece of fabric is placed on top of another with a
specific seam margin and stitched with either a 301 lockstitch or
401 chainstitch.
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Superimposed Seam
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Obviously, the seam strength is dependent on a number of
factors including:
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Type & Weight of the Fabric
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Stitch & Seam Construction
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Stitches per Inch
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Thread Type and Size
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Stitch Balance (Thread Tensioning)
Formulas for
Estimating Seam Strength
| From the formulas
to the right, you can see the impact that stitches per inch, thread
strength, and stitch selection have on the strength of the seam. Generally, the more stitches per inch, the greater the seam
strength. There are
rare cases where adding stitches per inch can actually damage the
fabric so that the seam is weakened, however, this only happens on
specific fabrics that can be easily damaged by excessive needle
penetrations. |
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Examples Using
Formulas
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301 Lockstitch
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Estimated Seam Strength |
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= SPI
X Thread Strength (lbs.) X 1.5*
*1.5 is a factor based
on the average loop strength ratio of most sewing threads.
=10
SPI X 4.0
lbs. X 1.5 = 60 lb. strength
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401
Chainstitch -
Estimated Seam Strength |
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= SPI
X Thread Strength (lbs.) X 1.7*
*This
factor is higher than a lockstitch because almost twice as
much thread is consumed per inch of seam uisng a chainstitch.
=10
SPI
X 4.0
lbs. X 1.7 = 68 lb. strength |
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As you can see from the chart
to the right, the stitches per inch
has a tremendous impact on the strength of the seam, as long as the
fabric doesn抰
rupture before the thread.
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SPI |
6 |
8 |
10 |
12 |
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Estimated
Seam Strength |
25
Lbs. |
34
Lbs. |
42
Lbs. |
50
Lbs. |
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Given:
301 Lockstitch, Superimposed Seam; Thread Used, Top &
Bottom - T-24 Perma Core?/a> -
Strength, 2.8 Lbs. |
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If the same seams were sewn with a lower tenacity spun polyester
thread, this also will effect the resulting seam strength.
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SPI |
6 |
8 |
10 |
12 |
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Estimated
Seam Strength |
20
Lbs. |
26
Lbs. |
33
Lbs. |
40
Lbs. |
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Given:
301 Lockstitch, Superimposed Seam; Thread Used, Top &
Bottom - T-27 Spun Poly - Strength, 2.2 Lbs. |
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Some manufacturers
substitute a smaller bobbin thread when sewing lockstitch seams to
minimize the number of times a sewing operator has to change the
bobbin. Remember, however, that the resulting seam strength will be
much less and will be determined by the strength of the bobbin
thread and not the needle thread.
How to Measure the Stitch Length or Stitches Per Inch
The stitch length is measured by measuring the number of
lengths of thread found within one inch. Stitch counters are available from A&E that make this
measurement easier. However, you can place a ruler next to the seam and perform the same task.
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Stitch Counter Measuring SPI
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SPI is measured by counting the number of lengths
of thread found within one inch. As you can see here, there are approximately 9 SPI sewn in
this seam.
Typical Stitch Lengths Recommended for Wovens and Knits
Below is a list of
garments and the recommended typical number of stitches per inch.
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Woven
Garments |
SPI |
Comments |
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Blindstitch
Operations on Slacks, Dresses, Skirts, etc. |
3-5 |
A
long stitch length is desirable to minimize the dimple or
appearance of the needle penetration on the outside of the
garment. |
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Buttonhole
(1/2" purl or whip stitch) |
85-90 |
Generally,
sewn vertically - approximately 85-90 stitches with a lockstitch
buttonhole machine. |
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Buttonsew
(4 hole button) |
16 |
Buttonsew
machines are cycle machines with a predetermined number of
stitches per cycle. |
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Casual
Shirts, Blouses, Tops |
10-14 |
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Childrenswear |
8-10 |
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Denim
Jeans, Jackets, Skirts |
7-8 |
Fewer
stitches per inch generally will give more contrast stitch
appearance. |
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Dress
Shirt or Blouse |
14-20 |
Using
more SPI allows the use of smaller diameter threads that will
minimize seam puckering |
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Dresses,
Skirts |
10-12 |
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Twill
Pants or Shorts |
8-10 |
More
stitches per inch will help minimize seam grinning. |
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Trousers,
Dress Pants, Slacks |
10-12 |
On
some operations, like serge panels, it may be desirable to use a
longer stitch length. |
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Knit
Garments |
SPI |
Knit
Garments |
SPI |
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Dresses,
Shirts |
10-12 |
Jersey
T-Shirts, Tops, Polos |
10-12 |
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Fleece |
10-12 |
Sweaters
(Medium to Heavy) |
8-10 |
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Hosiery,
Socks |
35-50 |
Stretch
Knits (Lycra? Spandex? etc.) |
14-18 |
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Infantwear |
10-12 |
Swimwear |
12-16 |
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Intimates |
12-16 |
Underwear |
12-14 |
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Factors to
Consider
When setting standards for stitches per inch, you should
always keep in mind that the more stitches per inch used in a seam
requires longer sewing cycles to complete the seam. Longer sewing cycles translate into higher labor costs and
lower production levels. A
sewing machine running at 5,000 SPM (stitches per minute) at 8 SPI
will sew 17.4 yards of seam per minute. A
sewing machine running at 5,000 SPM at 14 SPI will sew 9.9 yards of
seam per minute.
More stitches per inch will also consume more thread for
every inch of sewn seam. This
will contribute to higher seam strength and more elastic seams, but
will also increase the consumption of thread required to sew the
garment.
Therefore, the prior recommendations are common stitch levels
that provide adequate seam strength but also take into consideration
the already mentioned production and cost factors.
When sewing knit fabrics, you should always check for
excessive seam grinning; and also check for stitch cracking. Seam
grinning occurs when the thread stitch balance is too loose allowing
the seam to open up too much when stress is applied to it. Stitch Cracking is checked by applying pressure on the seam in
the stitching direction. If
the thread tensions are too tight or unbalanced, or if you are not
using enough stitches per inch, the threads will rupture as stress is
applied on the seam.
Summary
From the comments and information
listed above, you can see how important it is to add stitches per
inch to your garment specification instructions. Using
the correct number of stitches per inch can greatly enhance the
strength, appearance, and performance of the seam for a given fabric
type and application. If
you need assistance in determining what the appropriate stitch level
for your sewn products would be, please contact A&E抯
Technical Service team for assistance.
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Technical Information |
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Need
More Information? |
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Glossaries |
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New
Bulletins |
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Performance Measurement |
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Textile
Resources |
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A&E
Technical Services |
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Internet Sites |
AATCC:
American
Association of Textile Chemist and Colorists.
www.aatcc.org
ATMI:
American Textile Manufacturing Institute.
www.atmi.org
North
Carolina Center for Applied Textile Technology: Part of
the NC Community College System, NCCATT provides instruction in
textile technology and related fields.
www.nccatt.org/home.asp |
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Technology |
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