What Injection Molding Does

Resin teeth are molded, not cut. Polyoxymethylene or a glass-filled polyamide is melted in a screw barrel, injected into a multi-cavity mold that closes around a woven tape, and allowed to freeze. The cavities form a row of teeth that wrap over the tape edge and interlock with the opposite chain. One shot produces a molded segment, and the tape indexes forward for the next shot, so the chain leaves the machine as a continuous strip of teeth anchored through the tape.

This is the highest-volume tooth-making method in the industry and the one with the tightest process window. The tooth must be dimensionally exact enough to mesh with a die-cast slider, flexible enough to survive bending at a garment seam, and bonded strongly enough that it does not pull off in a laundry drum. All three properties are decided inside a mold in a few seconds.

Equipment and Key Parameters

The workhorse is a horizontal reciprocating-screw machine of 50 to 150 tonnes clamping force, paired with a hardened steel mold of 20 to 80 cavities and a tape indexing mechanism. Mold maintenance matters as much as machine settings: a worn cavity produces a tooth that is 0.03 mm small, which is enough to change how the slider feels.

  • Melt temperature: typically 190 to 215 degrees Celsius for POM, 230 to 260 for glass-filled polyamide. Above the upper limit the material degrades and releases formaldehyde, which pits the mold.
  • Mold temperature: typically 60 to 90 degrees Celsius. A hot mold improves surface and bond strength but lengthens the cycle.
  • Injection pressure: typically 60 to 120 MPa, with holding pressure at 50 to 70 percent of injection pressure.
  • Cooling time: typically 6 to 15 seconds, depending on tooth section thickness.
  • Cycle time: typically 12 to 30 seconds per index, including mold open, tape advance and close.
  • Tape tension and position: held to plus or minus 0.2 mm laterally. A tape that wanders produces teeth of unequal leg length.
  • Material dryness: POM below 0.1 percent moisture, polyamide below 0.15 percent.

Common Defects

  • Short shot. An incomplete tooth, usually at the last cavity or the far end of the runner. Caused by low pressure, cold melt or a blocked gate.
  • Flash. Thin fins at the parting line. Cosmetic at first, but flash on the head profile increases slider drag and can break off inside the channel.
  • Sink marks and voids. Thick sections shrink away from the mold wall. A void inside the tooth reduces bending strength with no external sign.
  • Weak tape bond. Teeth that sit correctly but pull off at 30 to 40 N instead of the specified 60 N or more. Usually a cold mold, low hold pressure or contaminated tape.
  • Gate blush and colour streaks. Local overheating at the gate, visible as a dull patch on a pigmented tooth.
  • Dimensional drift across cavities. The first and last cavities differ because of fill imbalance in the runner.

How the Defect Shows Up Downstream

Runner balance is the parameter nobody records. When a mold is qualified the tool room measures the first and the last cavity; in production nobody repeats the measurement. Gate erosion moves the fill balance slowly over months, so the same mold produces a shipment in which most cavities are perfect and two or three sit at the tolerance limit. Those teeth pass a dimensional check and fail in the field, and the failure is always described as random because the cartons were mixed at packing.

Cooling time is the other silent variable. A cycle shortened by three seconds to raise output leaves the tooth warm at ejection, and a warm tooth deforms slightly as the tape indexes. The deformation is recovered by the next mold close, which hides it, but the internal stress remains, and that tooth is the one that whitens first in the flex test.

Short shots and flash reach the assembly line as slider problems. A tooth that is 0.1 mm short lets the slider ride over the interlock, and the chain opens under modest lateral load. Flash does the opposite: it fills the channel, the slider feels stiff, and the operator forces it, which wears the slider guide surfaces and shortens its life.

Weak bonds are invisible until the garment is worn. A tooth that survives the factory pull test at the edge of the strip may fail after ten domestic washes, when the tape has swollen and shrunk and the mechanical grip has loosened. The complaint arrives as teeth falling off, months after the shipment, and it is almost impossible to trace back to a mold temperature that ran 15 degrees low on a night shift.

Dimensional drift across cavities shows up as inconsistent feel within a single order. One carton of zippers runs smoothly; the next feels tight. The buyer experiences it as unreliable quality even though every measurement is inside the drawing.

How Buyers Can Judge

  1. Measure tooth pitch and head height on ten teeth from each of three positions in the shipment, and compare against the drawing rather than against a sample.
  2. Pull-test five teeth per chain on a tensile stand and record the value. Ask for the supplier distribution, not a single pass figure.
  3. Flex the chain through 180 degrees twenty times and inspect for whitening at the tooth root, which indicates over-stress or a cold weld line.
  4. Look at the parting line under ten times magnification. Flash thicker than 0.05 mm will cause drag in a die-cast slider.
  5. Check colour under a D65 lamp against the approved standard, and check again after a wash at 40 degrees Celsius.
  6. Ask for cavity traceability. A molder who can identify which cavity made a returned tooth is controlling the process; one who cannot is only inspecting it. YCC molds resin teeth with per-cavity records for this reason.