An elastic tape is a core plus a cover. The core stores energy. The cover is what you sew, dye and feel. Buyers who specify “elastic, 3 cm, black” are specifying the photograph. The mill is left to choose latex versus elastane, woven versus knitted, and a modulus that either holds a waist or cuts into it.
TrimGem treats elastic webbing as a catalogue family next to rigid tapes and cords: same colour story, different mechanics.
What is inside the tape
The core is usually cut rubber (latex), extruded elastomer, or elastane / spandex filament (including branded fibres such as Lycra). Apparel volume has moved toward elastane and synthetic rubber because ageing, odour and chlorine stories are easier to control than natural latex — though latex still appears in cheap waistbands and some medical wraps.
A covered elastic yarn (rubber or elastane wrapped in polyester or polyamide) is the building block of many knitted and braided tapes. Double covering hides the core, improves abrasion and lets the mill dye the cover to a shade. Bonded multi-filament cores (several elastomer ends glued along their length, then braided) reduce the chance that one broken filament empties the tape.
Modulus is not “stretchy / not stretchy.” It is force at a stated elongation. A sports waist and a lingerie picot can share a width and still be different articles.
Three constructions
Woven elastic. Cotton or synthetic warp and weft, plus a set of rubber or elastane ends bound in on a regular pattern. Plain, rib, twill, double-cloth and jacquard grounds all exist. The tape is dense, stable in the width, and the default for waistbands, cuffs, bra bands, sock tops, shoe openings and medical wraps. Buttonhole (slot) elastics for maternity and kids’ adjusters are usually woven.
Knitted elastic. Warp-knit with weft insertion, or a crochet construction. Needle loops form chains; rubber is trapped in the chain or sandwiched by wefts. The tape is softer, more open, and easy to run with picot edges, stripes and small motifs. Nylon textured yarn is common. Lingerie, briefs and light apparel use it because it sits on skin. It is a poor choice where a woven would have been specified for recovery after industrial laundry.
Braided (spindle) elastic. Warps braid in a figure-eight around the rubber. The face is a herringbone. Widths are typically 3–20 mm. Hand sits between woven and knitted. Patterning is limited. It is a garment tape, not a bra-engineering tape.
Fold-over elastic (a centre crease woven or knitted in) is a construction for binding a knit edge. Grip (silicone-print or rubber-dot) elastics are a finish on top of one of the three structures. Printed and heat-transfer elastics are a decoration, not a new core.
Where it belongs on the garment
Underwear, trousers, infant goods, knitwear, sports, bridal, T-shirts, hats, bras and masks all eat elastic. That list is not a spec. Match construction to job:
- Waist and cuff that must survive wash: woven, named elongation and recovery.
- Lingerie and next-to-skin picot: knitted nylon, soft modulus, colourfast cover.
- Narrow garment tape and cheap bindings: braid.
- Adjustable openings: buttonhole woven elastic, latex or elastane core named.
- Fold-over on a knit hem: FOE with a stated width and a stitch that does not cut the rubber.
TPU and other thermoplastic elastomers appear in some “latex-free” programs. They are a material claim, not a weave. If the brief is medical or infant, write the polymer and the biocompatibility pack; do not assume a fashion elastic is a device elastic.
What fails first
Cores oxidise, hydrolyse and take a set. Covers pill, crock and melt under a too-hot folder. A tape that measured 150% elongation on day one and 80% after twenty washes was never “the wrong colour.” It was the wrong core, the wrong cover, or a mill that skipped ageing.
Put width, construction (woven / knitted / braided), core (latex / elastane / TPU), cover fibre, elongation at a stated load, recovery after a stated cycle, and wash temperature on the PO. Picot, fold-over, buttonhole and grip are extras. They do not replace the core line.
The recovery-and-ageing note in this series is where those numbers are tested. This article is the article you sew.