Manufacturer · Fashion & Home Accessories
When a zip does not close and the coil separates behind the slider, the cause in most cases is a worn or spread slider, not the coil itself. Other common causes are a size too fine for the load, a tight seam around the zip and a damaged coil. With YKK coil zips, the problem almost always comes from use or from the specification, not from a defect in the zip.
Why a zip does not close – how the coil works
The slider is a small “press” with a wedge-shaped interior. When it moves up, it presses the two coils together at a precise angle and they interlock. If the inner walls of the slider spread even by fractions of a millimetre, the pressure is not enough and the coils stay apart. That is why a spread slider is the number one cause.
Quick diagnosis
| Symptom | Probable cause | Solution |
|---|---|---|
| The zip separates right behind the slider | Worn or spread slider | Replace the slider |
| It separates in one place only | Deformed or melted coil | Replace the zip |
| It opens under load, e.g. when sitting | Size too fine or tight seam | Larger size, adjust the cut |
| The slider moves stiffly and “jumps” | Fabric is caught or the seam is too close to the coil | Correct the sewing |
| The open-end zip does not engage at the bottom | The pin does not go fully into the box | Check the pin; replace if damaged |
| The zip opens by itself during wear | Slider without a lock | Auto-lock slider |
Causes related to the specification
Size too fine
If a No. 3 is chosen for a fitted skirt in dense fabric or for a bag with heavy contents, the coil is constantly under tension and sooner or later separates. The rule is simple: the greater the tension across the zip, the larger the size you choose.
Wrong type of slider
A non-locking slider on a fly or a jacket will slip down by itself. This is not a defect, but a wrong choice. In high-stress positions, insist on an auto-lock.
Incompatible slider
When assembling zips from continuous chain, the slider must be exactly for that size and type of coil. A slider for an ordinary coil does not close a reverse zip reliably.
Causes related to sewing
- The seam is too close to the coil. The fabric gets into the slider, friction increases and the slider spreads faster.
- The zip is shorter than the opening. The seam is stretched, the tape is under constant tension and the coil works under stress.
- No reinforcement at the bottom end. On closed-end zips, the load is concentrated below the bottom stop; without a cross bartack the tape tears.
- Ironing directly on the coil. High temperature can deform the turns and they stop interlocking.
Causes related to use
- pulling the slider sideways instead of along the axis of the zip;
- closing the zip on an overfilled bag or a tight case;
- sand, salt and dirt in the coil, especially on outdoor products;
- washing with the zip open, so that the coil bends in the drum.
Prevention in series production
- Test samples with the actual fabric, not just on paper. Open and close the zip repeatedly, sit down in the trousers, fill the bag.
- Determine the size by the load, not just by looks.
- Include the slider type in the tech pack so that it is not substituted on a repeat order.
- Check machine settings when the fabric changes – the distance from the coil, the feed, the needle.
- Train finishing pressers not to press on the coil.
- Add care instructions – zip closed when washing, moderate temperature.
Repair or replacement
If the coil is intact and the problem is in the slider, replacing the slider is a quick and inexpensive repair. If the coil is melted, torn or has missing turns, the whole zip must be replaced. In that case it is worth reviewing the specification – perhaps a larger size or a different type of slider is needed. Standard nylon coil zips and sliders for them are in Polkom’s range, and for products that require the brand, see YKK zips.
Particularities of different types of products
Trousers and skirts
Here the zip is under tension when sitting and bending. The most common mistake is a non-locking slider or a cut that is too tight around the hips. Check the sample seated, not just standing.
Bags and backpacks
The zip bends around the corners and bears the weight of the contents. Choose a larger size, round off the corners and provide an inner tape that takes part of the load.
Jackets and sweatshirts
With open-end zips, problems are often at the bottom end – the pin does not go fully in or the box is damaged. Stitch the bottom part firmly and check that the tape is not gathered around the pin.
When the problem is in the batch
If the same fault appears widely in a new batch of products, with no change in cut or fabric, compare the zip with the approved sample: size, slider type, marking. Sometimes a different item is delivered by mistake on a repeat order. A recorded specification with the exact model is the best protection.
Advice and ordering
Polkom is a Bulgarian manufacturer of accessories for fashion, furniture, footwear and bags that also offers original YKK zips. If you have a recurring zip problem in production, browse YKK nylon coil zips and write to us with a description of the product and photos – we will help you choose a more suitable size and slider and offer a sample.
