It can be acceptable, but treat it as a specification change and check the seams first. Spun polyester thread is made from short polyester fibres twisted together. Core-spun thread has a continuous polyester filament core wrapped in polyester or cotton fibres, which makes it stronger for the same thickness while keeping a soft, spun surface. That extra strength matters most on seams that take strain.

Where the difference matters

On hoodies, the side seams, armholes, pocket corners and cuff and hem joins take the most stress. Thread strength, thickness and stitch density together decide how well a seam holds. If a weaker thread is used in the same size, seams may crack sooner when stretched or after washing. If a thicker spun thread is used to compensate, it may need a larger needle, which can damage knit fabric.

Overlock and cover stitch

Many knit garments use textured, bulked polyester or nylon thread in the overlock loopers, because it is soft and covers the seam edge well. That is a different choice from the needle thread. Make sure the proposal states which thread changes in which position: needle, looper or cover thread.

What to ask for

Ask the factory to name the current and proposed threads with supplier, type, size or ticket number, and colour reference, and to state which seams and positions are affected. Ask for the saving per garment, so you can weigh it.

How to check

Make garments with the proposed thread and compare them with the approved sample. Stretch key seams by hand to see whether stitches crack, wash samples with a named procedure such as ISO 6330 at your label settings, and check for seam cracking, puckering and colour change. If you have a lab, seam strength can also be compared on fabric seams. Then decide.

Put it in writing

If you agree, update the thread specification and re-approve a pre-production sample. In final inspection under your ISO 2859-1 plan, include seam checks on the stressed areas. If you do not agree, say so in writing, since thread changes are often made without notice when the line is busy.

Thread colour also needs matching across lots, especially on visible topstitching.