Analysis of Frequent Thread Breakage in Bag Sewing Machines

Analysis of Frequent Thread Breakage in Bag Sewing Machines

Frequent thread breakage in bag sewing machines (often used for closing sacks of rice, flour, chemicals, etc.) is a common issue that disrupts production. The causes generally fall into four categories: Needle Issues, Thread Quality, Machine Settings, and Material Handling.

Here is a detailed analysis of the causes and solutions.

1. Needle-Related Issues

The needle is the most critical component. If it is damaged or incorrect for the material, the thread will snap.

• Blunt or Bent Needle: A needle tip that is not sharp enough cannot pierce the woven bag material cleanly, causing excessive friction and heat. A bent needle will hit the throat plate or hook, snapping the thread instantly.

• Wrong Needle Size: Using a needle that is too fine for thick bag material forces the thread to squeeze through a small eye, causing breakage. Conversely, a needle that is too large leaves a hole that is too big, affecting the loop formation.

• Needle Installation: If the needle is not inserted fully or is rotated in the wrong direction, the timing between the needle and the hook (looper) will be off.

Solution: Replace the needle regularly. For woven polypropylene bags, use a heavy-duty needle (e.g., size 18-22) with a sharp point. Ensure the long groove of the needle faces the direction of the thread feed.

2. Thread Quality and Condition

The thread itself is often the culprit, especially in industrial bag closing.

• Poor Thread Quality: Old, brittle, or low-quality thread lacks tensile strength and snaps under tension.

• Thread Path Obstruction: If the thread snags on a rough spot on the spool pin, a burr on the tension discs, or a dirty eyelet, it will fray and break.

• Improper Threading: If the thread misses a guide or the tension discs, the tension balance is lost.

Solution: Use high-quality, waxed nylon or polyester thread designed for bag closing. Check the entire thread path for rough edges or burrs and polish them if necessary.

3. Tension Settings

Incorrect tension is a leading cause of breakage.

• Excessive Upper Tension: If the tension dial is set too tight, the thread is stretched beyond its limit as it passes through the fabric.

• Unbalanced Tension: If the top tension and bobbin (or looper) tension are not synchronized, the knot forms incorrectly, putting stress on the thread.

Solution: Loosen the tension dial gradually until the thread flows freely. The stitch should lock in the middle of the fabric layers, not pulling too tight on either side.

4. Timing and Hook/Looper Issues

In bag sewing machines (often chainstitch machines), the timing between the needle and the looper is precise.

• Incorrect Timing: If the looper catches the thread too late or too early, it pulls on the thread loop violently, causing it to snap.

• Burred Looper: The tip of the looper (hook) can develop a rough edge or burr over time, which acts like a knife cutting the thread.

Solution: This requires a skilled technician. Check the timing marks on the handwheel and inspect the looper tip. If there is a burr, it must be polished with a fine stone or replaced.

5. Material and Operator Handling

How the bag is fed into the machine matters.

• Pulling the Bag: If the operator pulls the bag through the machine faster than the feed dogs move it, the needle is forced to bend, and the thread is yanked, causing breakage.

• Thick Seams: Sewing over the folded edge of the bag or multiple layers of thick material without adjusting the presser foot height can jam the mechanism.

Solution: Train operators to guide the bag gently, letting the machine pull the material. Do not force the fabric.