Seam Engineering (part 1/2)

Introduction 

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…
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 "unqualities," such as, re-stitched seams or seam failure due to the thread rupturing, we would say the garment is "under-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. 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 "rules of thumb" for thread size selection on seams sewn with 301 lockstitch, 401 chainstitch, and 504 overedge.

301 Lockstitch SeamsFor 301 lockstitch seams, we generally recommend using the same needle thread size as the bobbin thread size in the seam. Why? Because "a chain is only as strong as its weakest link."

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. 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 ChainstitchNotice 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. 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 Seams504 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.
Factors that Affect Seam Strength
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.




continue in part 2
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