Stitch Selection Guide for FIBC (Bulk Bag) Components.

Each part of the bag experiences different stresses, so the stitching method must be tailored accordingly. This guide covers the three basic stitch types used in FIBC manufacturing and provides specific recommendations for each major component.

1. Basic Stitch Types Used in FIBCs

2. Component‑by‑Component Stitch Recommendations

• 2.1 Bag Body (Side Walls)

Recommended Stitch: Lockstitch

Rationale: The body experiences uniform static pressure from the contents. Lockstitch offers reliable, consistent seam strength with a flat, neat appearance, meeting standard load requirements.

Key Process Parameters:

Stitch must be straight, with no skipped stitches, broken threads, or loose loops.

Stitch pitch: typically 7–9 mm per stitch (0.7–0.9 cm).

• 2.2 Bag Bottom

Recommended Stitch: Multiple rows of high‑strength lockstitch (3 rows)

Rationale: The bottom bears the entire weight of the contents – it is the most critical stress point. Industry standards require three parallel stitch rows on the bottom seam to create a redundancy that ensures absolute safety.

Key Process Parameters:

The seam is often folded double (double hem) before stitching.

Total seam width: about 20 mm, with evenly spaced rows (no overlapping).

Minimum seam strength: side seam ≥ 67% of base fabric strength; bottom seam ≥ 42% of base fabric strength.

• 2.3 Lifting Loops (Straps / Belts)

Recommended Stitch: Combination of lockstitch and chainstitch with reinforcement patterns

Rationale: Lifting loops carry the entire dynamic lifting load – they are the "lifeline" of the bag.

Lockstitch is used for the main attachment to the bag body, with stitch lines placed 5 mm from the edge and 10 mm from the strap end.

Chainstitch is often added for dynamic load areas, as it can increase tensile strength by 30‑50% under impact.

Reinforcement stitches (e.g., "X‑box", "square", "zig‑zag" or multiple passes) are applied at the attachment points to distribute stress.

Key Process Parameters:

At least 3 back‑stitches (reverse stitches) must be made at the start and end of every seam to prevent unraveling.

• 2.4 Inlet and Outlet Spouts

Recommended Stitch: Lockstitch for attachment + Overedge for edge finishing

Rationale: Spouts are frequently handled and subjected to abrasion and pulling.

Lockstitch secures the spout firmly to the bag body.

Overedge protects the raw edge of the spout from fraying, extending its service life.

Key Process Parameters:

The innermost stitch line on the spout‑to‑body seam must align with the inner marking line.

The second stitch line should be spaced about 5 mm from the first, with no overlap.

• 2.5 Reinforcing Patches (e.g., corner reinforcements, baffle attachments)

Recommended Stitch: Lockstitch or 4‑needle 6‑thread safety stitch (a high‑strength compound stitch)

Rationale: Patches reinforce high‑stress zones (e.g., around lifting loops or spouts).

Standard lockstitch suffices for moderate reinforcement.

For extreme duty applications, 4‑needle 6‑thread stitching provides 2‑3 times the tensile strength of ordinary plain seams.

Key Process Parameters:

Stitches must be straight and perpendicular; starting point is usually 10 mm from the upper left corner of the patch.

• 2.6 Raw Edges (cut edges of fabric, top openings, etc.)

Recommended Stitch: Overedge (overlock)

Rationale: PP woven fabric tends to unravel at cut edges. Overedge stitching encloses the edge, effectively preventing yarn slippage and improving overall durability and appearance.

3. General Stitching Quality Requirements (All Components)

All stitches must be straight, without skipped stitches, broken threads, or loose loops.

Back‑stitching (reverse stitching): at least 3 stitches at the beginning and end of every seam.

For lifting loops: stitch line must be 5 mm from the edge and 10 mm from the loop end.

Bottom seam: use 3 rows of stitches with even spacing (approximately 2 cm total width), no overlapping.

Needle and thread must be compatible with polypropylene fabric to avoid damage or weakened seams.

4. Choosing the Right Stitch for the Right Place

The optimal stitch selection for FIBCs balances strength, elasticity, and cost.Lockstitch is the workhorse for most seams.

Chainstitch is preferred where shock‑loading and elongation are concerns (especially lifting loops).

Overedge is essential for edge finishing to prevent premature failure from fraying.

By applying these recommendations and strictly controlling process parameters (stitch pitch, back‑stitching, and seam alignment), manufacturers can ensure that every FIBC meets the required safety and performance standards for its intended use.