What Coil Forming Does

A nylon coil zipper begins as a single filament of polyamide. Polymer chips are melted, extruded through a spinneret, drawn to align the molecular chains and wound onto spools. The forming line then reheats that filament, winds it into a helix around a rotating mandrel and cools it so the spiral keeps its shape after release. What leaves the machine is a continuous coil with a fixed pitch, a fixed leg height and a fixed head profile, ready to be woven onto tape.

The operation looks mechanical and forgiving, but it fixes almost every dimension the finished zipper will have. Pitch decides how many elements sit in a given length, and therefore whether a size 5 slider will mesh correctly. Leg height decides how much of the coil the tape can grip. Head geometry decides how the two halves interlock and how much drag the slider feels. A coil formed outside tolerance is not recoverable downstream: dyeing shrinks it slightly, setting relaxes it slightly, but no later step adds or removes material.

Equipment and Key Parameters

Coil forming runs on a line that combines a filament feed, a forming head with a heated mandrel, a cooling zone and a winder. On integrated machines the coil is formed and woven in one pass, which removes a spooling step and the stretching that comes with it. Standalone forming is still common where weaving widths change often.

  • Filament diameter: typically 0.50 to 0.90 mm. Size 3 and size 5 garment chains usually sit at 0.60 to 0.70 mm.
  • Mandrel temperature: typically 180 to 230 degrees Celsius. Too low and the filament springs back; too high and the surface oxidises and yellows.
  • Winding speed: typically 300 to 800 revolutions per minute. Output rises with speed, but the cooling window shortens.
  • Filament tension: typically 20 to 60 cN, held within a few centinewtons. Tension drift is the usual cause of pitch drift.
  • Cooling: air or water quench directly after the forming point, bringing the coil below its glass transition before it leaves the mandrel.
  • Pitch: counted over 100 elements or measured as elements per inch; a size 5 coil typically runs 12 to 14 elements per inch depending on the standard used.
  • Winder tension: light, typically 10 to 30 cN, so that the spool does not compress the coil on the inner layers.

Two parameters are routinely under-recorded and routinely cause trouble: filament moisture before forming, and mandrel concentricity. Polyamide absorbs water, and a damp filament foams at the die, producing micro-voids that appear later as weak points. A mandrel that runs 0.02 mm out of true produces a coil that is alternately tight and loose along its length, which no inspection of a short sample will reveal.

Common Defects

  • Pitch drift. Element spacing creeps along the spool because feed tension or mandrel temperature wandered. Short samples look perfect; a metre-long check does not.
  • Oval or flattened coil. Insufficient cooling before release lets the helix sag under its own weight. The coil measures correct on height but wrong on width.
  • Diameter variation. Moisture in the chips or extruder surging produces thick and thin sections that dye differently and bend differently.
  • Surface damage and fuzz. Worn guides or an overheated mandrel scratch the filament, creating a rough flank that later catches in the slider channel.
  • Micro-voids and bubbles. Trapped moisture or gas at the die. They are invisible at forming and become fracture origins in the pull test.
  • Poor splices. Where spools are joined, an oversized or misaligned splice jams the weaving needle and creates a weak element.

How the Defect Shows Up Downstream

Pitch drift is the most expensive because it hides. A chain with two percent pitch variation sews and dyes normally, then fails at the slider: the slider rides up, the elements no longer align, and the chain opens behind it. In a finished garment this reads as a self-opening zipper, and the customer blames the slider rather than the coil.

Oval coil shows up at the sewing machine first. The needle that should pass between elements clips the flattened flank, breaking filament or pushing the coil off the tape line. The operator compensates by slowing down, and the cost appears as lost output rather than as a defect.

Diameter variation shows up in the dye house. Thick and thin sections take up dye at different rates, so a chain dyed in one batch comes out with a visible barre that no amount of levelling agent will fully remove. Rough flanks appear as slider drag in the wear test, and micro-voids appear as a low pull strength on a chain that looked sound.

How Buyers Can Judge

  1. Check pitch over a full metre, not over 100 mm. Lay the chain flat, measure the distance across 200 elements, and repeat at three points on the spool.
  2. Measure coil width and height with a profile gauge or a micrometer at ten points. A coil that varies more than 0.05 mm across a metre is suspect.
  3. Run a slider over two metres by hand and feel for tight spots. Repeat after the chain has been washed once at 40 degrees Celsius.
  4. Ask for forming records: mandrel temperature, filament tension, winding speed and the moisture reading of the resin lot.
  5. Ask how many splices per spool the supplier allows and how they are marked. A splice is acceptable only if the weaver can see it coming.
  6. Inspect under ten times magnification for scratches, bubbles and fuzz on the element flank. At YCC, coil is checked this way at the forming line rather than at final inspection, because the defect is easier to see before weaving.