Coats does not recommend adding any additional lubricant / silicone on thread. Thread colour may change, it can create large variation of frictions and cause oil staining. Atomised silicone may also become a health issue if breathed by operators / machinists.

Coats does not recommend thread rewinding since this can have detrimental effect on thread itself. The lubricant on thread will be lost during the process. The rewinding will add extra twist on thread causing snarling/lively. Rewinding may also cause damage to bonding on bonded threads.

Coats offers a full package service called Coats Sewing Solutions / CSS where Coats technical personnel can be involved from development to final stages of production.

1. Use better and stronger threads.
2. Maintain proper SPI and stitch types throughout the whole garment.
3. Change stitch type.
4. Change seam type.
5. Increase stitches per SPI.
6. Use corespun threads.

Check thread-needle size compatibility and needle condition. Check components in contact with thread (thread path, tension discs, needle eye, bobbin, rotary hook) and also machine timing.

1. Check the ply adhesion of bonded thread.
2. Poor thread lubrication - increase lubrication.
3. Machine setting adjustment.

Check working condition and parts of sewing machines (i.e. hook, bobbin case, throat plate, needle tip, sharp thread guide) plus thread quality and material thickness.

1. This is a machine-related issue. The throat plate aperture enlarges due to wear and tear over a period of time causing the needle to push the fabric through the aperture before penetrating the fabric.
2. This can also happen if the needle size (thickness) is changed and the throat plate is not changed accordingly.
3. Solution: Throat plate must be changed at regular intervals after checking for wear and tear.
4. Throat plates must be changed in accordance with the needle size even if there are no signs of wear and tear.

We recommend the use of Super Lubricated products in conjunction with titanium nitride needle to minimise needle overheat.

1. This is the transfer of colour from shoe materials into threads which have been sewn into the seam. The result is unsightly seam appearance. Possible causes are poor leather tanning, poor fastness of dye-stuff materials, solvent or adhesive reacting with dye chemicals or excessive needle heat.
2. Fibre migration is the breakdown of material fibre which attaches to the threads causing contamination. Possible causes are fibre migration from poor quality material, or material damage caused by excessive needle heat.
3. Possible Solutions: Ask factory to test the colour fastness of the material; use GEBEDUR needles to reduce needle heat; apply air blower cooling (6 mm from needle eye).
4. Recommend speed for normal MC is 1,400 rpm or less, recommend speed for computerised stitching is 2,500 rpm or less.
5. Use of Super Lubrication threads (SL) such as bonded nylon threads or GRAL polyester threads is recommended.

1. Check your presser foot has enough pressure applied to the fabric to give positive feed.
2. Check and clear your feed dogs of any lint that may have built up.
3. Check and adjust the height of the feed dogs.
4. Check your stitch length regulator as it may be set to zero stitch length.
5. Check the thread tension as it may be loose and the thread may be clogging under the presser foot.
6. Check your needle plate to ensure it is fixed securely.

To improve seam elongation it is possible to increase the stitch rating per cm or inch, increase the bite, reduce the needle / looper tensions on affected seams or change thread types used.

1. Seam slippage may occur when the fabric at either side of the stitching distorts and the fabric yarns slide away from the seam resulting in a permanent gap within the fabric. Seam slippage most often occurs on fabrics made from shiny, slippery continuous filament based yarns.
2. To help prevent seam slippage the following points could be used, Increase seam allowance. Change seam type. (Example: fell seam).Increase or decrease SPI depending on fabric, (minor results).Usage of fusible tape or cut fabric on a bias.

1. Skipped stitching at the start on lockstitch machines could be as a result of the thread take-up spring too strong or the thread take-up spring operating range too large.
2. Is the needle bar in the up position at the start?
3. Is the length of thread trailing out from the bobbin too short?
4. Are the threads being trimmed cleanly?
5. Has the bobbin been wound correctly, does the bobbin case have a check spring or is the bobbin spinning after the machine has stopped and tangling the thread.

If the needle cuts are always on the top or bottom fabric ply, then most likely the problem is NOT needle cutting but some other part of the machine that is damaging the fabric. Checking machine surfaces is important in such cases.

More details are available on our website. Please navigate to Solutions/On-line Training/Common Sewing Problems and Solution/Chapter 2 - Needle Point Style.

1. Is the thread tension too tight causing a small loop to form? The tension or the take ups on the sewing machine may need adjusting.
2. Has the needle become bent or blunt and needs to be replaced.
3. Is the needle the wrong size and needs to be changed to the correct size or does the sewing machine need adjustment to accommodate a different needle size.
4. Has a different needle or thread been issued to the one previously used? Check and change as required.
5. Has the sewing machine been threaded correctly? This needs to be checked and then rethreaded as required.
6. Is the pressure foot pressure sufficient to allow positive feed or does it need increasing to ensure positive feed motion.
7. Has the needle been set in the machine correctly? Does it need to be re set into the correct position.

Stitch type, seam type, thread type and size etc. play vital roles in seam strength. Choosing all these properly can enhance seam strength.

A generic formula is available to determine seam strength:

For lockstitch = Stitch per inch (SPI) x Thread breaking strength (STS) x 1.5

For chainstitch = Stitch per inch (SPI) x Thread breaking strength (STS) x 1.7

1. Has the stitch length has been set to zero? If so adjust to suit.
2. Is the pressure on the presser foot too low? If so adjust to suit.
3. Have the feed dogs moved and are set either too high or low? If so adjust to suit.
4. Is the thread tension to low and the thread knotted under the fabric, if so check threading and tensions and adjust as required?

1. Lockstitch machines which use tight tensions have reduced seam elongation and may display premature seam failure because of the seam's reduced ability to absorb load. The needle thread tension should be set as light as possible whilst still achieving a balanced stitch that does not gape.
2. The bobbin thread tensions which is set first to ensure a neat seam appearance uses a standard measure for thread tension, this is equal to that of the bobbin case including a full bobbin.
3. Using better and stronger threads will increase seam strength as will increasing the stitch rating, changing the stitch type or changing the seam used.

1. Has the sewing machine been running at a slower speed for a long period resulting in a lack of oil to circulate around the machine? Turn the power of for 20 minutes for the machine to cool and restart.
2. Has the power supply to the sewing machine been affected if so wait for the power supply to return.
3. Has a large amount of lint accumulated under the needle plate if so turn off the power to the machine, clean the machine and restart?
4. Has thread got caught around moving parts in the machine or caught in the shuttle. If so turn of power to the machine (give time for the motor to stop) remove thread blockage and restart.
5. Has a safety device activated, if so turn off the machine correct the error and restart the machine.

1. Are the correct size, type and point of needle being used?
2. Is the machine threaded correctly check and rethread as required.
3. Is this a result of a loose upper or lower thread tension? Adjust as required.
4. Is the fabric getting caught on a damaged piece of the sewing machine or is the fabric being pulled out of the sewing machine.
5. Does the pressure foot have sufficient pressure to ensure the correct feeding of the fabric is more pressure required?
6. Is the presser foot set correctly? Is it loose and needs tightening?
7. Is the bobbin wound evenly? Unwind if unsure and then rewind.
8. Is the stitch length set correctly for the material being sewn?
9. Is the point of the needle blunt or damaged and needs replacing.
10. Is the thread tension set correctly.
11. Is the fabric too sheer or soft? Are different foot, feed dogs and plate required for this lighter fabric?
12. Are two different sizes or types of thread being used.
13. Are the needle plate sat correctly and the screws holding it in place tight?
14. Is there a build-up of lint between your feed dogs? If so this needs to be removed.
15. Is there needle vibration or deflection which is causing the stitch to look as if it’s not straight with sewn in a straight line, if so Increasing the needle size or changing to a reinforced or tapered needle may help.
16. Is there movement in the feed dog, if so tighten or remove play.

1. Is the threading of the sewing machine correct? Check and change as required.
2. Is the thread getting caught in worn or defective thread guides and is causing uneven stitch balance, check, adjust or exchange as needed.
3. Has the bobbin case been threaded correctly? Check and adjust as needed.
4. Is the looper or hook timing set correctly? This may need checking and adjustment made.
5. Is the looper or hook damaged and needs replacing or polishing.

Most broken stitches are the result of not using the correct thread type and size for the process it has to go through changing the stitch rating, stitch balance and setting machine thread tensions correctly will help to reduce issues as will thicker or stronger thread types. Use Coats corespun threads to provide better abrasion resistance. Also consider using a looser stitch so the stitch rolls rather than pops or cracks during contact with stones or enzyme.

This is the transfer of colour from materials into threads which have been sewn into the seam. The result is an unsightly seam appearance and can be caused by poor tanning of the leather, poor fastness of dye-stuff materials, solvent or adhesive reacting with dye chemicals, excessive needle heat, fibre migration is the break down of material fibre which attaches to the threads causing contamination.

1. Has the bobbin been fully inserted or seated correctly in the bobbin case? If unsure remove, check and insert again.
2. Is the bobbin threaded correctly? Is it tangled up and needs to be unthreaded and wound again.
3. Does the bobbin turn smoothly in bobbin case? If not the bobbin needs to be checked to ensure the thread on the bobbin has been wound evenly, at the correct tension and that too much thread has not been wound onto the bobbin. A pre wound bobbin would help to eliminate this problem.
4. There is Lint in the bobbin case or shuttle which needs to be removed by cleaning.
5. Is the tension too tight or does the bobbin over spin. The bobbin case may need adjusting and a washer or spring adding to the case to help prevent over-running.
6. Are there sharp edges on the bobbin case/spring or looper eyelet which need to smoothed and polished.
7. Does the bobbin fit in the bobbin case correctly? The size and type of bobbin needs to be checked for suitability, checked for flange distortion and replaced as needed.

1. Is the thread being wound off correctly, if not check to ensure the sewing thread is being pulled off vertically by setting the eyelet from the thread stand directly above the centre of the cone? Also check to ensure the sewing thread is being pulled of the cone smoothly and not being trapped under the cone (spillage), if it is a foam pad could be used under the cone to help prevent trapping if spillage does occur. Check the height of the eyelet above the cone of thread is set at the correct height, this should be around 2 ½ times height of the thread package.
2. Is the sewing thread getting trapped at the thread guides, check and rethread if required.
3. Is the sewing thread snarling up before going into the tension discs? If it is increase the wraps on the pre tension thread guides to help reduce this problem or reduce the thread tension and ensure that the tension discs are smooth.
4. Is the thread quality being used for the operation? Is the thread weak or of an inferior quality or is it a very old thread cone if it is try replacing the thread to see if this resolves the issue.
5. Are there broken or misaligned parts fitted to the sewing machine? Check to ensure all parts are fitted and working correctly paying particular attention to the check springs on lockstitch machines. Check to ensure check springs are fitted and working correctly, replacing or adjusting as required.
6. Is the thread fraying at the needle? If so the use of a finer thread or coarser needle as appropriate could help.
7. Is the needle eye is getting blocked, if it is then excessive needle heat may be causing this problem, try using an alternative finish to the fabric where possible. Change to an alternative needle with better coating, finish or point. Apply a needle lubricant via the thread or you could try to use one or more types of needle cooling devices.
8. The hook on the sewing machine is very hot. Check the supply of oil getting to the hook and check the needle to hook clearance.
9. Is the sewing machine threaded correctly, check or double check and rethread as required.
10. Did the sewing thread have a knot in it and the machine just needs to be rethreaded.
11. Is the sewing tension (force applied) on the sewing machine to high and breaks the thread. If so the sewing tension needs to be adjusted or the sewing thread used needs to be changed to a stronger or a thicker thread.
12. Is the needle in the sewing machine damaged in some way e.g. blunt, bent or has a sharp eye and needs to be replaced.
13. Is the needle used the wrong size and needs to be replaced with a needle of the correct size.
14. Has the needle been inserted correctly? If not it needs to be removed and inserted correctly.
15. Is the size of the needle correct for the size of thread used? This may cause the thread to break. Check and change either the needle size or the thread size to ensure a suitable match.
16. Is the thread take-up lever/spring threaded incorrectly or missing? Is so rethread or replace.

1. Needle heat is caused by friction. That may be friction between the needle and the fabrics being sewn or from the machine or its settings. Blunt or damaged needles will increase temperatures quicker which can melt or fuse the sewing thread.
2. Good quality sewing threads have specially developed lubricants applied to them which help the sewing thread to deliver excellent sewability and to dissipate this needle heat.
3. This excessive needle heat may be caused by the needle eye being too small for the size of sewing thread being used. Other reasons for needle heat include The hole in throat plate is too large for the needle size being used. The needle is off centre to the plate hole causing it to rub the edge of the throat plate. The needle guard in the machine is rubbing against the needle, or the hook or stitch forming device, striking the needle at the time of taking the loop of needle thread to create the stitch.

Staggered stitches have a negative impact on seam appearance as deviation from the stitching line leads to a perception of poor quality. The needle is vibrating or deflecting.

1. Is the correct needle being used, check and change as required.
2. Is the needle bent or does it have a blunt needle point, check and change as required.
3. Is the needle to thread size correct, check and change as required.
4. Is there movement in the feed dog, if so remove play in feeds.
5. Is this due to poor fabric control and presser foot bounce, if so, increase pressure on presser foot.

1. The thread used may be poorly finished or unsuitable for the fabric or sewing operation. Change to a better quality thread or change to a corespun thread.
2. The fabric may have been poorly, harshly finished or densely woven. The fabric finish may need to be improved. A change of needle to a more suitable type/finish may help as would the use of needle coolants.
3. The needle may be damaged or the needle overheated after thread breaks and a new needle is just required.

1. Has the machine stopped in a position other than Top dead centre making it difficult to thread the needle.
2. Has the needle hole clogged up and needs either cleaning out or replacing.
3. Is the size of thread used to large or the needle size used too small?

1. Ensure adequate oil supply. Check needle to hook clearance.
2. Polish edges and contact surfaces.

1. Check and remove any lint that has built up between your tension disks.
2. Check and rethread your machine
3. Check your feed dog height and raise your feed dogs as required
4. Check and adjust the sewing tension settings
5. Check and adjust the sewing machine foot pressure.

1. Check the needle type size and brand given matches the needle used previously if not change.
2. Check the threading of the machine to ensure it has been threaded correctly after changing needle.
3. Check to ensure the needle has been put in correctly (both up far enough and it is the correct way round.
4. Try reducing the thread tensions on your machine.

1. Check your feed dog height and raise your feed dogs as required.
2. Check and adjust the sewing tension settings.
3. Check and adjust the sewing machine foot pressure.
4. Was a thin needle used for sewing a heavy weight material and the size of the needle needs to be increased.
5. Has the needle been fully inserted into needle bar?
6. Is the needle clamp screw loose or worn and the needle has dropped down while sewing.
7. The presser foot is not correct one and is catching the needle.
8. Has the presser foot become loose?
9. If there is a puller device fitted to the machine is this pulling to fast bending the needle.
10. Has the machine settings (loopers or hook) moved and need resetting.
11. Check that the presser foot has not moved and is rubbing against the needle.
12. Is the needle being sewn over pins/rivets etc. if so this will be a contributing factor for needle damage?
13. Is the fabric being pulled through while you sew and is bending then breaking the needle?
14. Was there a knot in th thread which has caused the needle to break while sewing?
15. Is the needle plate in the correct position and the screws holding it in place tight?

1. Try to improve feed mechanism.
2. Try to feed through the fabric evenly along the seam and try not to pull through or hold back on the fabric while sewing.
3. Try to ensure the machine speed is constant, running at the same speed over the whole of the seam length.

1. Is the thread too thick for the trimmers?
2. Is the knife damaged or worn?
3. Is the thread trimming timing correct?
4. Is the knife blunt?
5. Are skipped stitches causing the issue?
6. Badly wound thread on bobbin. Adjust bobbin winder alignment. Use prewound bobbins.
7. Tension too tight or bobbin over-running. Adjust bobbin case tension. Insert washer or spring to prevent over-running.

Coats does not recommend adding any additional lubricant / silicone on thread. Thread colour may change, it can create large variation of frictions and cause oil staining. Atomised silicone may also become a health issue if breathed by operators / machinists.

There are spiral / coil zips, plastic moulded and metal zips. See the Product Information on our website.

Sewn Coil Zips are made from sewing a coil of monofilament, normally polyester wrapped around a polyester cord , onto 2 tapes , normally also made of polyester. See the Product Information on our website.

Previously these zips may have been made from a range of manmade materials like nylon, polyester, polypropylene, poly butyl phthalate (PBT). Nylon was a common material used for zips due to its smoother feel and so this remained as a name for this category of zips.

Spiral zips are made from integrally weaving monofilament into the centre of the zip as one continuous process. Because this process only involves two materials this is a more quality efficient process. See the Product Information on our website.

Plastic moulded zips are normally made from Polyester Tapes and Poly Oxy Methylene (POM) teeth. Alternative materials can be used for teeth such as Polypropylene for clear teeth materials. Other more highly technical plastic materials can be used for more specialist end use. Zip tape is generally polyester.

Metal zips are normally made from Polyester Tape and Brass Teeth. Other materials can be used for the tape such as Cotton, a mix of Poly/Cotton. See "Choosing the right zip" Garment Dye. The teeth can also be made from Aluminum to reduce weight and Steel / Copper for some specialist industrial applications.

Metal zips are made by attaching the teeth to the beaded edge of the tape. The teeth can be made from flat brass wire or shaped wire and attached directly to the tape. Or the teeth can be stamped first and then polished / plated before attaching making a very high quality zip. Usually, these are for high end bags / garments where very smooth teeth are required. In some cases the teeth can be die cast directly onto to the tape, but this process is now very seldom used due to advances in the other technologies.

Closed end zips (i.e. pockets/skirts/trouser), open end zips (i.e. centre front of jackets), two way zips (allow the zip to be opened from the bottom and also fully separated (i.e. long coats), X type zips (two sliders facing away from each other, i.e. large art folders that can be accessed from right or left side), O type zips (two sliders facing one another, i.e. generally used in bags/suitcases). For more information see the Product Information.

At the top of the zip there are the Top Stops that stop the Slider (moving part) from detaching from the zip. At the bottom of a closed end zip (pocket/skirt zip) there is a Bottom Stop that prevents the bottom of the zip from opening. On an Open End Zip (Seperating Zip as used in the Centre front of a jacket) the bottom components are generally referred to a Free Pin and Box Pin. The box pin being the larger female part of the two components that allows the free pin to be inserted to close the zip. For detailed informaiton of Top Stops, Bottom Stops, Slider and Pullers see Product Information on our website.

The top end stops on metal zips are normally made from brass wire. This is also the case for Spiral zips when metal stops are used. Aluminum wire stops are sometimes used. Polyester filaments can also be used to form top and bottom stops on spiral zips, as can Poly Oxy Methylene(POM) which is also generally used on Plastic Moulded zips. Some spiral zips can also have Brass Pin Bottom Stops, these are especially suited for footwear due to their high strength. Some Spiral/Sewn Coil zips have simple welded bottom stops sealing together the teeth. These are not usually very strong and normally stitched into the seam. Details can be found in the Product Data Sheets. See Opti Website for Product Data Sheets.

The slider are generally made from a Zinc Alloy, thought working components inside the slider maybe made from Steel or Brass? Plastic sliders are also available, though these are not as strong. Sliders made of stamped brass are also used generally for jeans applications.

The pullers can also be made from zinc alloy and a variety of materials such as Polyurathane, Wood, Silicone, PVC, Fabric, Textile Tapes, Brass - the material is usually requested by the designer though advice must be sought when using materials unusual for the application.

There are many ways to measure the length of a zip, Coats Opti follow the British Standard Method, where all measurements are based on measuring from the end of the components or head of the slider where fitted. More clear details and images can be found in the Product Information on our website.

The top and bottom tape end extensions (see Zip Component Descriptions) can be important for positioning the zip in the application. This may vary from one zip company to another so it is important to check the Product Data Sheet for this property when making a change. The Chain width also denotes the size of the zip. It is important to use the right size of zip in the right application. Please see choosing the right zip from our FAQ and website for advice on which zip to use.

Typical zip sizes for Opti Spiral / Metal / Plastic Moulded and Invisible zip and local market terms can be found in the detailed information on our website. See also Opti Website for Product Data Sheets.

Invisible zips, sometimes refered to as concealed zips, have a fold in the tape behind the sewn coil area so they can be sewn with a special sewing foot to give a hidden / invisible appearance to the teeth. The teeth are hidden from view. See Choosing the right zip and Do’s and Don’ts for further advice on invisible zips.

Bottom stop should be protected with a bar tack to prevent direct strain on the zip. If bar tack is fitted but breakage still happens, check effective opening of the garment. Try garment on model or dummy to see possible causes of the breakage.

Change the zip for reversed coil type zip and slider.

Spiral/coil zips have an S twist and Z twist in the teeth creating a "smile" or "frown" appearance when the teeth are joined. The left and right teeth should be both in the same direction/condition. If this is not the problem check the sewing tension is equal on both sides of the zip.

Spiral/coil zips have an S twist and Z twist in the teeth creating a "smile" or "frown" appearance when the teeth are joined. The left and right teeth should be both in the same direction/condition. If this is not the problem check the sewing tension is equal on both sides of the zip. Also check for any high levels of shrinkage in the fabric and any interlining.

Check the sewing tension is equal on both sides of the zip. If this is only noticed after pressing heck for any high levels of shrinkage in the fabric material and any interlining.

In long centre front applications the end of the zip can be placed under high strain. Check that the end consumer does not create too much pressure to the bottom end of the zip when wearing the garment. If the application is a long coat a two way zip maybe required.

Check that there is sufficient clearance for the slider to pass along the length of the zip. Check that no side seams interrupt the travel of the slider along the chain. Check that the edge fo the coil is not stitched to the base tape of the zip.

Ensure an M46 or M60 zip is being used. Ensure zip slider is closed during washing and ensure the button is fastened where ever possible. If button is not present protect the zip from the stresses of the washing process. Ensure no overloading of the washing is occurring.

Ensure an M46 or M60 zip is being used. Ensure zip slider is closed during washing and ensure the button is fastened where ever possible. If button is not present protect the zip from the stresses of the washing process. Ensure no overloading of the washing is occurring. Check if washing machine hole sizes in the drum are compatible with the slider puller. Check there are no gaps / slots where the puller can be trapped in washing or drying equipment.

Ensure an M46 or M60 zip is being used. Ensure zip slider is closed during washing and ensure the button is fastened where ever possible. If button is not present protect the zip from the stresses of the washing process. Ensure no overloading of the washing is occurring. Check if washing machine hole sizes in the drum are compatible with the slider puller. Check there are no gaps / slots where the puller can be trapped in washing or drying equipment. Check that a bar tack is present protecting the bottom of the zip.

Ensure an M46 or M60 zip is being used. Ensure zip slider is closed during washing and ensure the button is fastened where ever possible to protect the teeth with the fly piece. If button is not present protect the zip from the the chemicals of the washing process using a piece of cotton fabric. Ensure no direct dosing of the chemicals onto the zip teeth or slider. Ensure the temperature and timing controls are correctly calibrated and set to the required process conditions.

Check for any solution / sprays / polishes being used on the zip teeth. Check for any tanning chemical residues on the surface of the leather. Check that the zip has been ordered with Protecto Treatment to prevent any reaction of the pigments In the leather with the copper of the zip.

Check for any solution / sprays / polishes being used on the zip teeth. Check for any excess oil being used on sewing threads if sewn application or check temperature of heat seal if sealed zip application.

Spiral / coil zips have an S twist and Z twist in the teeth creating a 'smile' or 'frown' appearance when the teeth are joined. The left and right teeth should be both in the same direction/condition. If this is not the problem, check the sewing tension is equal on both sides of the zip.

Check that the slider and chain are from the same zip supplier and are the correct sizes to match one another.

Check that there is no special process such as turning out the bag that creates high pressure on the slider pushing the slider against the bottom end of the opening. Check that there is sufficient clearance to allow the slider to run smoothly without rubbing against the fabric / leather of the item or the lining of the item. Check that no side seams interrupt the smooth closure of the slider and that both slider and chain are from the same supplier. If the zip is metal, check the direction of the chain is correct. Metal chain has a correct / wrong way to mount the slider. This is called preferred / non preferred direction. If the slider is to be used in both directions, than special symmetrical teeth or two way teeth are needed.

Please inform our Customer Service department when placing orders for goods for outdoor use. For such goods, we recommend standard puller with silver plated slider, or special plastic sliders.

Please inform our Customer Service department when placing orders for goods for outdoor use. For such goods, we recommend standard puller with silver plated slider, or special plastic sliders. Outdoor goods should be dried before storage. They should not be packed away wet.

Please inform our Customer Service department when placing orders for boat covers / marine upholstery. For boat covers / marine upholstery, we recommend standard puller with silver plated slider, or special plastic sliders.

Check the needle size used to insert the zip is not too large. If a large needle is used for back tacking the bottom of the zip, this can split the reinforcement. Check that the load to assemble / close when under any pressure is not too high. The end of the zip and force on the zip during closure should not exceed 5 kg for normal applications.

Sewn coil zips are more loose than S type zips. Check that an S type zip is being used for best results.

Check that no excess levels of sewing machine oil are being used in stitching the zip to the seat. Check the colour fastness of the seat leather, thread and zip and check for any residual chemicals left in the leather from the leather manufacturing process.

Check for any excess oil / lubrication used on the sewing machine when inserting the zip. If sewn coil product is being used in raw condition then this may have more lubrication content of the sewing thread used to construct the zip. Use a Coats Opti zip that is made by carefully controlled levels of lubrication in the thread, or preferably use an S type zip that is made from only yarns and so has less oil content.