Thread Tension Puckering

If a thread is sewn into the seam with heavy machine thread tension so that it has been elongated or stretched as the stitch is being set, the thread will try to recover or return to its original length. This can cause the seam to pucker immediately as the seam is coming out from under the presser foot; however, sometimes the sizing or resins in the fabric will initially maintain a flat seam but later in the day, the seam will appear puckered. This phenomenon also occurs after the garment is laundered and the sizing materials are removed causing the seam to pucker and the inspector to think that there was too much shrinkage in the thread. Excessive thread tension during sewing will not only cause puckered seams but also cause other sewing problems including thread breakage and skipped stitches.

Identification

Carefully clip the thread between adjacent needle penetrations along the seam and observe if the puckering is reduced in the fabric. If it is, then excessive thread tension is the probable cause of the seam puckering.

Solutions to Tension Puckering

To minimize seam puckering caused by thread tension, do the following:

• Use very light machine thread tensions. Begin by setting the bottom thread tension (bobbin or looper) as light as possible but still maintain the proper thread control. Next adjust the needle thread tension to a minimum level necessary to maintain a closed seam and a balanced stitch. This not only reduces the elongation of the thread in the seam, but also improves loop formation and sewability.
• Use a thread with a low elongation or high initial modulus to minimize stretching during sewing. Use a thread with good lubricity characteristics that will allow it to be sewn with minimum thread tension.
• Reducing the thread size will not only help to minimize structural jamming but can usually be sewn with lighter machine thread tension because a smaller thread requires less tension to pull it up into the seam and set the stitch.
• If structural jamming does not appear to be a problem, increase the needle size or use a needle with a ball eye needle to open up a larger hole in the fabric so the stitch can be set with the lightest tension possible
• Properly set the take-up spring so that the stitch can be set with minimum thread tension. Polish or replace all eyelets and thread guides that may be grooved to make sure they do not affect the frictional properties of the thread. On some machines a rotary tension system is available that meters the thread more uniformly that conventional tension disc systems.
• On some machines the thread control guides and eyelets can be adjusted to control the thread more efficiently so less tension is required. This is advisable when possible.

Lockstitch Machines

• On lockstitch machines, make sure the positioning finger is correctly set to allow the thread to pass through the hook with minimum resistance. If the machine has a mechanical opener, set the opener to shift the bobbin case holder so it will allow the thread to pass by the positioning finger with as little resistance as possible.
• On lockstitch machines, it is sometimes necessary to refine the gib or rail on the bobbin case holder so the thread can be released by the hook sooner in the stitch cycle. This sometimes allows the thread to be set with lighter tension.
• On most sewing machines, four elements help in setting the stitch in the seam. They are the take-up system, the feed system, the needle thread tension, and the bottom thread tension. Proper feed timing in reference to the take-up system is critical to sewing with light tensions.
• In extreme cases, it may be necessary to use an oil free hook which uses at Teflon coated rail. It has been found that machine oil migrates on other surfaces of the hook and can increase the surface friction as the thread passes around the hook.

Chainstitch Machines

• On 401 Chainstitch machines, adjust the stitch balance so the needle loops on the bottom side of the seam lay over at least half way to the next needle penetration when the looper thread is unraveled out of the seam. The looper thread must also be as loose as possible.
• Adjust the thread controls to allow the stitch to be set with minimum thread tension. On many chainstitch machines it is best to draw most of the needle thread through the tension discs when the needle is rising, and a small amount when the needle is descending (75% on upstroke & 25% on the downstroke). Advancing the looper thread take-up timing will sometimes allow the needle thread to be released sooner so that it can set with lighter machine thread tension.

Feed Puckering (Uneven Ply Feeding)

Feed puckering occurs when one of the fabric plies is fed into the seam at a different rate than the other ply or plies. This causes a gathering effect in the over-fed ply. Ply mismatching as shown in:

• Usually occurs when the presser foot holds back on the upper ply as the bottom ply is being fed into the seam at a faster rate by the feed dog.
• Usually occurs when the operator holds back on the bottom ply and pushes the top ply into the machine so the fabric edges will come out evenly.

Many seams observed display both of these conditions, with the first usually contributing to the latter because the sewing operator will attempt to correct for the uneven feeding of the sewing machine.

Identification

Make two perpendicular cuts across a sewn seam where the puckered condition is the greatest. Remove the thread in the seam and see if the two plies are of equal length. If one is longer than the other then the puckering is being caused by uneven ply feeding.

Solutions to Feed Puckering

To minimize seam puckering caused by uneven ply feeding, do the following:

• Use the minimum presser foot pressure that will maintain uniform feeding. Make sure the presser foot is clamping the fabric properly both in front and back of the needle. When the feed is up and moving the fabric, the seam should be clamped by the entire bottom surface of the presser foot. This can be checked by inserting a piece of paper under the foot from different angles and observing if the foot is clamping the fabric properly.
• Set the feed dogs at their proper height and check for back-feeding. The feed dog should have the optimum teeth per inch and number of rows of teeth for the operation and fabric being sewn. Puckering can sometimes occur if the material is not held down flat as it is being fed through the machine creating a rippled appearance as the plies conform to the feed dog teeth. Usually lightweight wrinkle resistant fabrics should be sewn with feed dogs with 20 - 24 teeth per inch. Medium weight fabrics like men's trousers should be sewn with feed dogs with 14 - 18 teeth per inch. Heavy weight fabrics are usually sewn with feed dogs with 8 - 12 teeth per inch.
• Use the correct presser foot and needle plate for the material and operation being sewn. The needle plate and presser foot should have relatively small needle holes in relation to the needle size being used. As a general rule, the needle hole should be approximately twice the size of the needle. Check to make sure that the needle plate is not bent down at the needle hole.
• Use a low friction presser foot: Teflon coated, roller bearing, "feeding foot", etc. Use an "anti-puckering" needle plate with a retaining spring that holds back on the bottom ply to match the top ply.
• Use machines equipped with a needle feed or compound feed mechanism where the needle moves with the feed as the fabric is being sewn. This "pinning" of the plies as they are being fed helps reduce feed puckering.
• Whenever possible, use machines equipped with auxiliary top feeding mechanisms such as: walking foot, puller, top driven roller feed, upper belt feed, etc.
• On machines equipped with differential feed systems, set the differential action to slightly stretch the bottom ply to match the top ply so they are fed evenly into the seam.
• Use automatic machines equipped with material clamping systems that prevent the fabric from moving as it is being sewn.
• Make sure you are using the correct capacity of folders and guides for the fabric being sewn.
• Observe operator handling for proper fabric movement to and through the machine.
• Make sure the pieces are cut properly in the cutting room and the proper seam tolerances have been maintained so the pieces are of equal length before seaming.
• If the plies have different stretch characteristics, position the ply with the greatest amount of stretch are against the feed if possible.