Most manufacturers of denim and twill garments that pre-wash garments after they are assembled have experienced problems with excessive “cut” or “broken” stitches. In fact, many manufacturers have found this problem to be significant reaching in excess of 20 to 30% of the products being sewn.
Many times this problem occurs when a previously sewn stitch line is crossed during a subsequent operation and the needle damages the thread in the seam. Broken stitches can also occur when there is excessive abrasion or chemical degradation of the thread during the wash process. Let’s now discuss what are some of the major causes and solutions to these problems.
1) Many manufacturers have significantly reduced the number of “cut” and “broken” stitches by using high-performance sewing threads. Make sure the correct thread type and size are being used in both the needle and bottom (looper) positions. Core threads that have a continuous filament polyester core in each singles yarn are more resistant to cutting and degradation than 100% spun polyester threads and other thread constructions.
2) Usually the larger the thread size, the more resistant the thread is to being cut by the needle or feed or to failure due to chemical degradation or heat. Because of this many manufacturers have increased the thread size on critical operations like waistbanding, seat seaming, etc. Typical thread sizes used on heavy denim run from T-150 down to T-60 depending on the desired look. Typical thread sizes used on twills used in the manufacturing of chinos run from T-40 to T-60.
3) Inspect the needle point at regular intervals and check for sharp or burred points. If the needle point is damaged, replace the needle. Many companies have found that it is best just to replace the needle on critical operations once or twice a day.
4) Use proper thread tensions.Â Make sure the stitch on the seam line is loose and be able to move if the needle hits it during a subsequent sewing operation.Â This will also allow the stitch to flex during washing minimizing broken stitches.Â Tight machine thread tensions will not allow proper flexibility in the stitch and will increase “cut-stitch” damage.Â Generally, on chainstitch seams, the ideal stitch balance is when the needle loop on the underside of the seam lays over half way to the next needle penetration.Â This can be checked by unraveling the looper thread and observing the needle thread on the underside of the seam.
5) Check the edges of the needle plate and presser foot needle holes to make sure they do not have any sharp edges or burrs that can damage the thread during sewing.Â Properly remove all burred or sharp surfaces making sure not to oversize the needle holes which can lead to excessive “flagging”.
6) Inspect the feed dogteeth directly behind the needle holes and make sure they are not sharp. If required, buff the feed dog teeth with a wire wheel or with a stone if they appear to be sharp. Be careful not to remove too much of the feed dog teeth that could hinder the feeding or interfere with chaining.
7) Use the minimum amount of presser foot pressure to get a uniform stitch length. Excessive presser foot pressure can cause the thread to be damaged when it is compressed against a relatively sharp surface. On some machines it is sometimes necessary to use a presser spring with fewer coils per inch to give more consistent pressure even when crossing heavy seams.
8) The proper type and capacity folder should be used to prevent stalling when crossing heavy seams. Feed stalling will increase the chances of “cut” stitches.
9) Check for signs of needle heat or excessive heat exposure during laundering that may be melting the thread.Â Usually, if the thread has been damaged by heat, the thread will have a hard melted surface that can be felt or seen using a magnifying glass.Â If you suspect that needle heat is a problem, try using a special coated needle or needle coolers to reduce needle heat.Â Make sure the thread has the proper type and amount of lube.Â Most major thread suppliers have developed high-performance lubricants to minimize heat damage on polyester threads.Â A cotton wrapped core thread may be more resistant than a 100% polyester thread.