A zipper is a closure and a design line. The engineering choice is three stacked decisions: how much force the opening will see, which chain (coil, moulded POM, metal), and which geometry (closed-end, separating, two-way). Colour and puller come after those three, or the factory will sew a dress invisible into a ski placket.
TrimGem University treats zipper placement as garment construction, in the same file as snap set height and cord exit. The Guangzhou one-stop desk already samples those parts together; the catalogue will keep adding chain and puller options around that engineering.
Strength first, then material
Gauge tracks strength. Same gauge, different materials, different loads: metal #5 is a jeans and work-jacket chain; coil #5 is a sports shell and pocket chain; moulded POM #5 is a down-coat and bag chain. If the opening will be yanked — packed bag, ski placket, kids’ snowsuit — move gauge up or change family. Do not “make it look like metal” with a coated coil and hope.
Moulded plastic reads chunky, takes tooth colour, wears well, and hates a hot press. Coil flexes, dyes with the tape, sits in curves and detachable linings, and is the volume apparel zipper. Metal is the denim and hardware look; brass for better programmes, aluminium when weight and cost lead. Invisible and two-bone coil are skirt and trouser tools; woven coil without a heavy filler cord is the fly that must not ridge a tailored pant.
Geometry follows the pattern
- Jacket placket: separating (open-end) zipper; long coats that must open from the hem for sitting use a two-way separating chain.
- Pockets, pant flies, dress backs: closed-end.
- Reversible garments: double-pull or dual-tape sliders so both faces have a puller.
- Thin dresses and skirts: invisible (reverse coil) or lace-tape invisible — not a #5 metal.
- Trouser fly that should look clean: reverse-coil closed zipper with a standard slider can stand in for invisible when hip tension is modest.
- Detachable linings and hoods: coil, often a one-side separator, because it flexes around a curve.
Pocket zippers should be cut to the lower end of the allowed length, not the upper: a long chain in a short welt arches. Across a waistband or a side seam, leave slider clearance through the ply stack — the same rule as invisible insertion.
Colour is a design spec with a fastness tax
Match tape to shell when the zipper should disappear. Contrast tape and tooth when the zipper is the design. Mills work from zipper shade cards, not from a fabric mill’s cotton card. Tape fastness — wash, rub, and migration onto PVC, PE and coated shells — is a zipper lab row. A perfect visual match that crocks onto a light lining is a return.
Decorative uses (contrast teeth around a pocket, detachable sleeves, shaped panel zippers, exposed metal on a hoodie) do not suspend the strength table. A panel zipper that becomes a load path is a structural zipper. Specify it as one.
Length, shrink and the sewing line
Tape is a textile. It has a shrinkage. Knit, corduroy and enzyme-washed denim shrink more than a heat-set polyester zipper tape; the chain then arches (“smiles”) after laundry. Do a paired shrink test. Keep the stitch ~3 mm from the elements; leave 2–3 mm of free tape above the top stop so the slider does not jam on the first stitch. Clear thread tails. On separating zippers, do not sew through the reinforcement film so close to the box that the peel angle rips it off.
Engineering is the spec: opening type + chain family + gauge + cut length + shrink + stitch clearance. The puller is jewellery on top of that spec.