A practical guide to choosing bag-closing sewing thread
Match the thread to the bag weight, the machine, and the environment. This guide walks through the core principle, the specs, and the details that keep seams reliable.
Why it matters
Polyester bag sewing thread is a critical consumable for industrial package sealing. Its thickness and tensile strength directly determine sealing reliability and transportation safety. The core principle is simple: match thread strength to bag weight, while also considering bag material, sewing equipment, and service environment. An undersized thread risks seam breakage; an oversized thread adds cost and may damage the fabric through incompatible needle size.
Ne
Yarn count (English Cotton Count)
A lower Ne number means a thicker single yarn. The count is combined with a ply number (e.g. NE12/4).
Tex
Linear density
A higher Tex value means a thicker, stronger thread. Tex groups threads by density for direct comparison.
Three yarn-count families
Ne10 · Ne12 · Ne20 — three ways to build a thread
The Ne number tells you how thick each single yarn is. Lower Ne = thicker single yarn, fewer plies, stiffer feel. Higher Ne = finer single yarn, more plies, softer seam. All eight models fall into three families:
Thick single yarn · 3–4 plies
Stiff, rigid, built for high-speed closing lines
Coarse 10-count yarn twisted into just 3 or 4 plies. The result is a stiff, dimensionally stable thread that feeds reliably through high-speed bag closers and skips rarely. Ideal for thick PP woven fabric and heavy-duty lines where rigidity matters more than a flat seam.
Medium single yarn · 3–6 plies
The universal workhorse — four models cover Tex 150 to 300
The broadest family in the range. Ne12 single yarn twisted into 3, 4, 5, or 6 plies gives a smooth strength ramp from light sacks all the way to cement. Balanced feel, compatible with most domestic and imported bag closers — the first choice for most production lines.
Fine single yarn · 6–7 plies
Soft, smooth, leaves a flat and neat seam
Finer 20-count yarn twisted into 6 or 7 plies. The multi-ply build gives a soft, supple surface that glides through the needle and lays flat on the bag. Preferred for food, feed, and consumer-goods sacks where seam appearance matters, and for thin or soft bag fabric that a stiff thread would cut.
Selection details
Six rules that protect the seam
Beyond the weight tier, these details prevent the most common seam failures.
Rule 01
Keep a strength safety margin
Reserve a safety factor of at least 30% over actual working strength. For example, if the maximum seam load is about 120 N, choose a thread with breaking strength ≥ 156 N to prevent seam burst from drop impact or stacking pressure.
Rule 02
Match the needle size
Thread diameter must match the needle. Too thick with a small needle causes needle breakage and skipped stitches; too thin with a large needle leaves oversized holes that can tear the fabric.
Tex 150 / 180 / 200 / 210 — generally compatible with 18#–20# bag closing needles
Tex 240 / 250 / 300 — generally compatible with 21#–23# bag closing needles
Rule 03
Adjust for bag material
PP / PE woven sacks: prioritise polyester thread with good abrasion resistance; double-thread chain stitch is recommended for heavy-duty use.
Multi-wall paper bags: choose a smooth, moderately soft thread (such as NE20/6) to avoid cutting the paper layers with a stiff thread.
Rule 04
Consider the service environment
Polyester thread is naturally moisture-, acid/alkali-, and UV-resistant. But for long-term outdoor exposure, low temperatures, or contact with highly corrosive materials, choose high-tenacity polyester or UV-stabilised products to avoid ageing and brittle fracture.
Rule 05
Cooperate with the seaming process
Single-thread sewing is fine for light-duty sacks; double-thread sewing is recommended for medium and heavy-duty sacks — it can raise seam strength by over 50%.
Keep stitch density at 7–9 stitches per inch. Too dense damages the fabric; too sparse leaves the seam weak.
Rule 06
Check compliance & quality
For food, feed, and pharmaceutical packaging, select polyester thread that meets food-contact safety standards. For products exported to the EU and North America, confirm the thread complies with REACH, TPCH and other environmental regulations.
Comments