Team Apparel Resources (AR) presents a series of five technical articles based on the premium sewing thread range offered by A&E andGütermann – a leading high-end thread provider – and its importance in the garment factories. The series will talk about the five different challenges faced by Sewing Machine Operators in factories. In the second offering, the detailing is shared about sewing of stretch knit fabrics. With an in-house sewing lab facility and Tech Centre based at Bawal Factory , A&E and Gütermann provides technical consulting services to brands and factories as the lab enables the company to perform R&D and random testing on the threads on all sorts of new-age sewing machines such as SNLS, overlock, heavy-duty, flat lock and button-hole offered by premium sewing brands such as Juki, Duerkopp Adler and Yamato.

Most stretch knit garments are seamed with over edge and/or cover stitch seam constructions because these stitches offer the best seam elasticity and coverage of the raw edge of the fabric. A quality problem that is common with stretch knits is excessive ‘broken stitches’ or ‘stitch cracking’ when the seam is stretched excessively. Generally the greater the elasticity of the fabric, i.e., higher Lycra® or Spandex® content, the more likely you are to have broken stitches if you do not understand how to optimise elasticity in the seam.

WHAT CAN BE DONE TO MINIMISE BROKEN STITCHES?

Most broken stitches are the result of one of the following:

– Not using the correct thread type and size, needle

– Not having enough stitches per inch

– Not sewing with the correct stitch balance or machine thread tensions set incorrectly

– Not using the correct seam margin or needle spacing for the application

WHAT THREAD TYPE AND SIZE ARE COMMONLY USED TO SEW STRETCH KNIT FABRICS?

The most common threads used to sew stretch knit fabrics are textured polyester or textured nylon threads like A&E’s Wildcat, Anesoft, Best Stretch, E-Series, Puma, Princess Hs. Textured threads are ideal for overedge and coverstitch seams because they offer excellent seam coverage and seam elasticity. In cases where performance garments are subjected to harsh end-use applications and wash cycles, many manufacturers prefer to use Perma Core, Mara/Ultimate  threads in the needles and textured threads in the looper positions.

Seams should be sewn at 14 to 18 SPI with proper stitch balance.

Since many performance garments are very tight fitting, sometimes there are complaints that the seam is ‘rough’ to the touch and not soft like they should be. This appears to be more of a problem with the thread on the inside than on the outside of the sewn product. When sewing with a coverstitch or bottom coverstitch, the thread that you feel on the inside is actually the needle thread loops rather than the looper thread. To minimise this roughness, use the smallest needle thread possible and make sure the needle loop is pulled up to the bottom side of the fabric. Then you will primarily feel the looper thread which if sewn with a textured thread, will have a nice and soft feel. Another solution is to sew the garments with the looper thread side to the outside of the garment.

WHY IS IT IMPORTANT TO HAVE PROPER NUMBER OF STITCHES PER INCH AND 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 for stretch knit fabrics range from 14 to 18 SPI. However, even when the correct number of stitches per inch are being used, if the proper stitch balance is not set correctly, you will still experience excessive seam failure. One way of checking the stitch balance is to first check the seam for ‘seam grinning’ by pulling perpendicular to the seam. After applying the stress across the seam, remove the stress and check to make sure the seam goes back to a closed seam.

WHAT YOU SHOULD KNOW ABOUT SEAM MARGIN

The seam width of overedge or coverstitch seams also has a major impact on seam elasticity. Take a zigzag stitch, for example. The wider the zig-zag and the more stitches per inch that are used, the greater the seam elasticity. A zig-zag stitch will stretch until it becomes a straight line at which time the stitch loses its elasticity. 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.

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 514 two needle overedge 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. If a narrow bite 504 stitch is preferred, the seam should be sewn with more stitches per inch and the correct stitch balance.

The needle spacing on 406 bottom cover stitch types shown below and similar cover stitch types have a tremendous impact on the seam elasticity.

The sample sewn with the wider needle spacing (1’4”) has more elasticity than seam sewn with the narrower needle spacing (1/8”) because it has more thread in the stitch. Therefore, when a narrow needle spacing is desired on high-stretch fabrics, the sewing machine should be set for more stitches per inch to minimise 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. 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.

Most binding or border machines are also equipped with an ‘off-set’ differential feed system to optimise seam appearance.

OPERATOR HANDLING CAN AFFECT 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.

REDUCING NEEDLE CUTTING ON KNITS:

Needle cutting is the breaking of yarns in the fabric as the needle enters the seam. On knits that are made with inter looping yarn constructions, a cut yarn will result in a ‘run’ along the seam line. Therefore, when the needle enters the seam, the needle point should not cut the yarns but push them aside as it penetrates the fabric. Below is a checklist that will aid in reducing needle cutting:

NEEDLES AND THREAD

Use a ball pointed needle as small as possible. A ball pointed needle should not cut the yarns in the fabrics as the needle penetrates the seam but shift the yarns to one side. This also requires a small diameter thread. On some classes of needles, there are different types of ball points available. Make sure the type of ball point matches the size of the yarn in the fabric. Too large of a ball point will burst the knit fabric made from very fine yarns. Too small of a ball point will pierce and possibly cut or weaken larger yarn sizes used in sweaters.

A small diameter thread with a high tenacity is generally required to give the required seam strength.

Typical thread types and sizes used for seaming knits would include:

• Use a tapered blade needle with a ball point if it is available. A tapered blade needle allows the use of the smallest possible needle with minimum needle breakage.
• Inspect the needles at regular intervals and replace them if they show signs of having a sharp or burred point. Sometimes the sewing machine can be out of adjustment allowing metal to metal contact that will damage the point. Readjust the sewing machine if necessary to minimise this condition.

• Check for signs of needle heat that may be melting the fibres. If needle heat is identified as the problem by observing melted fibres around the needle hole, reduce the needle heat by one of the following methods: a) use a double grooved needle if available; b) make sure the thread has a good lubricant on it; and c) use some type of needle cooler.

OTHER MACHINE PARTS

• Check for any sharp edges on the needle plate, feed dog and presser foot. Slightly bevel the top of the needle plate if necessary. Remove all sharp edges that might snag the fabric as it is being sewn. Care should be taken not to remove too much to allow excessive flagging or the material.
• Use minimum presser foot, so that the fibres in the fabric are allowed to shift as the needle penetrates the seam. Sometimes a presser foot with a special plunger or yielding section is used to reduce the pressure on the seam at the needle.

OTHER CAUSES OF FABRIC DAMAGE

Not all fabric damage attributed to needle cutting is caused by the needle during sewing. Many times, the ‘cuts’ are created before the sewing process. A way of determining if the problem is being caused by the needle or something else is by evaluating: Are the holes occurring at the seam where the needle enters the seam or at other locations away from the seam? If the ‘cuts’ are found away from the stitch line, the holes could be caused by: a) staple holes caused by attaching the marker on the lay before cutting; and b) other rough surfaces that the fabric might come in contact with during handling.

FABRIC STORAGE

Fabric storage is very important when sewing 100 per cent cotton piece goods. Some fabrics can dry out and become brittle if they are not stored properly. If you suspect that the fabric is particularly susceptible to needle cutting, try the following:

– Try conditioning the fabric in a humidity chamber prior to the sewing process.

– If this helps, store your piece goods in a controlled environment.

– Try not to pre-heat the garment pieces prior to sewing which might dry out the fabric. Cotton yarns are actually stronger when they are wet.

– Ask your fabric supplier or an independent test company to test the strength of the fabric and evaluate if the fabric has been finished properly. Even the best fabric suppliers sometime make inferior piece goods.

TROUBLE-SHOOTING LOGIC

Below is a basic logic used to determine the cause of needle cutting:

1. Is the needle cutting occurring on most sewing operations on the garments being sewn? If so, the problem is probably related to the fabric. Make sure the fabric has been finished and conditioned properly. 100 per cent cotton fabrics usually loose strength when they dry out. One solution is to have a humidity controlled conditional area where the cut parts are moved prior to going to the sewing floor.
2. Is the needle cutting only occurring on a few sewing operations? If yes, can you determine if the needle cuts are on the top or bottom ply being sewn? If the needle cuts are always on either the top or bottom ply, then most likely the problem is NOT needle cutting but some other part of the machine that is damaging the fabric. Check for burrs or sharp surfaces on the needle plate, presser foot, folder, etc.

3. Is the needle cutting on a few sewing operations and intermittent on both the top and bottom plies of fabric being sewn? If yes, then the problem most likely is needle cutting. Check the needle points for damage. Check to make sure the correct size and needle point are being used for the fabric. Using a smaller needle is always the first step to needle cutting unless the needle points are being damaged during sewing.
4. If the needle points are being damaged, what should you look for? Check to make sure that the

needle point never makes contact with any metal surface in the sewing machine like a needle guard, needle plate, etc. If so, readjust the sewing machine to eliminate the metal to metal contact.

Reach us for any kind of query and Technical Assistance at: amefird@vardhmanthreads.in, sumesh. minhas@vardhmanthreads.in and uday.bammanwadi@guetermann.com