What sewing thread should underwear use?

Short answer: Underwear seams need to stretch with the fabric, stay strong and feel soft against the skin. Many factories use spun or core-spun polyester in the needles for strength and textured, or bulked, polyester or nylon in the loopers of overlock and coverstitch seams for stretch and softness. Textured threads fill the seam and feel smoother, while a hard, untextured thread can feel scratchy and break when the seam stretches. Seam popping usually means the combination of thread, stitch type and stitch length cannot stretch as far as the fabric, not just that the thread is weak. Choose thread type and size with the stitch type, match stitch density to the fabric, and test seam stretch and strength after washing.

Common thread types

• Spun polyester: strong, economical, common in needles.
• Core-spun polyester: polyester filament core with a fibre wrap, strong and smooth.
• Textured polyester: soft, bulky, stretchy, often used in loopers.
• Textured nylon: very soft and stretchy, but less resistant to heat and light than polyester.

Why seams pop

• Stitch length too long or too short for the fabric.
• Thread too fine or too stiff for the stitch.
• Tension too high in production.
• Stitch type not suited to stretch, such as a plain lockstitch on a stretch seam.

Why seams feel scratchy

• Hard thread on the inside of the seam.
• Thread ends and loose loops not trimmed.
• Excess overlock width at sensitive areas.

Testing

• Stretch seams to the fabric's working elongation; fabric stretch data from ASTM D4964 helps set the target.
• Wash and dry with an ISO 6330 procedure that matches the care label and check seams.
• Test seam strength at a lab if failures persist.

A worked example

If seat seams pop, compare stitch density and thread with an approved sample and with fabric stretch. A common fix is to use textured thread in the loopers, adjust stitch density and reduce tension, then retest seam stretch before bulk.

Colour

Match thread colour to the fabric. Textured threads take dye well, but check colour fastness on dark colours.

Heat and finishing

Textured nylon thread is soft but can be damaged by high heat in finishing or ironing. If garments go through heat transfer, heat setting or tunnel finishing after sewing, check that the thread tolerates it, or use textured polyester instead.

Limits

Thread choice works together with stitch type, machine settings and operator skill. Change one at a time and test.

Facts, recommendations and uncertainty

Established: underwear seams need thread and stitch combinations that stretch with the fabric; textured threads add softness and stretch. Recommendation: specify thread by position and test seam stretch after washing. Uncertain: which factor causes a specific failure until tested.

Sources and further reading

• ASTM D4964 — Tension and elongation of elastic fabrics
• ISO 6330:2021 — Domestic washing and drying procedures for textile testing

Standards and regulations are referenced by their published scope. Test parameters, cycles and pass levels should be agreed with your laboratory for the specific product; legal requirements should be confirmed for each destination market.