PP Flat Yarn Extrusion Line: Common Faults and Practical Solutions

Introduction

In PP woven bag production, the flat yarn extrusion line serves as the critical starting point. When this system operates efficiently, the entire downstream process — weaving, coating, printing, and bag making — runs smoothly. However, even well-maintained equipment encounters issues that can disrupt production and compromise quality.

At Haosen Leituo Group, we've accumulated extensive field experience in diagnosing and resolving extrusion line problems across hundreds of customer installations worldwide. This guide addresses the most frequently observed faults — from extrusion system irregularities to drawing section failures — providing practical solutions that help you reduce downtime and maintain consistent yarn quality.

Whether you're troubleshooting a specific issue or establishing a preventive maintenance routine, these insights will help keep your production line running at peak performance.

1. Extrusion System Faults

The extrusion system is where raw material transforms into molten film — and where many production problems originate.

No output or insufficient output
This is often caused by hopper blockage, over-high feeding temperature, screen clogging, or damp and caked raw materials. Solutions include clearing the hopper, reducing feeding temperature, replacing screens regularly, and ensuring materials are fully dried before processing.

Unstable extrusion current
Common causes are moisture or impurities in the resin, uneven temperature control, worn screw components, or an unstable inverter. To resolve this, dry and purify materials, repair the temperature control system, replace worn screws, and recalibrate the inverter settings.

Uneven die output with thickness deviation
This typically results from uneven die temperature, inconsistent lip gap, partial screen blockage, or unstable cooling water flow. Solutions involve checking die heaters, fine-tuning the lip gap, replacing screens, and stabilizing water temperature and traction speed.

Bubbles, pores, or fish eyes on film
These surface defects are usually caused by excessive moisture in raw materials, thermal degradation from high temperature, impurities in the melt, or carbonized residue buildup in the die. Fully drying materials, reducing processing temperature, cleaning the die thoroughly, and running at low speed for exhaust can effectively eliminate these issues.

Die lip buildup and plastic beads
Abnormal drying processes, excessive die temperature, or poor melt fluidity often lead to material hanging on the die lip and forming plastic beads. Adjust drying parameters, reduce die temperature, and clean and polish the die lip regularly to prevent this problem.

2. Flat Film Forming Faults

Once the melt exits the die, proper film formation is essential for downstream drawing and slitting.

Stripes and scratches on film surface
Foreign particles stuck on the die lip, worn die lip sections, or a dirty cooling roller can cause surface defects. Clean the die lip carefully, repair or replace worn sections, and clean and polish the cooling roller surface to eliminate scratches.

Film wrinkling with uneven tension
Low temperature at both die ends, uneven cooling water distribution, or unbalanced traction tension often lead to wrinkling. Raise the temperature at die sides, balance water flow across the cooling system, and calibrate roller parallelism and tension settings.

3. Drawing & Slitting Core Faults

The drawing and slitting stage determines the final dimensional stability of the flat yarn. Problems here directly affect weaving performance and fabric quality.

Frequent yarn breakage
Excessive drawing ratio, abnormal oven temperature, poor material toughness, or roller slipping are the primary causes. Reduce the drawing ratio appropriately, calibrate oven temperature settings, check material quality, and clean roller surfaces to increase friction.

Bamboo-joint effect and uneven thickness
High water temperature, speed fluctuation, undried raw materials, or unstable heat profile can cause these issues. Lower water temperature, stabilize roller speed, ensure complete drying before processing, and maintain consistent production temperature throughout.

Slitting jumping and uneven cutting
Dull blades, wrong blade angle, high die temperature, or unstable film tension lead to poor slitting quality. Replace blades regularly, correct blade angle settings, reduce die temperature appropriately, and stabilize film tension before the slitting section.

4. Daily Maintenance Standards

Preventive maintenance is far more effective than reactive repairs. We recommend the following daily practices to keep your extrusion line in optimal condition:

Raw material control — Both virgin and recycled materials must be dried and purified before processing. Wet materials are strictly prohibited as they cause bubbles, degradation, and unstable extrusion.

Screen replacement cycle — Replace screens every 72 hours for virgin materials and within 40 hours for recycled materials. Regular screen changes prevent pressure buildup and ensure consistent melt quality.

Temperature control — Strictly control extrusion, die, and oven temperatures to prevent material carbonization. Monitor thermocouples and heaters regularly for accuracy.

Lubrication — Regularly lubricate bearings and reducers. Use high-temperature-resistant grease for components exposed to heat zones to prevent premature wear.

Daily cleaning — Clean die lips, cooling rollers, and traction rollers after each shutdown to prevent residue buildup that can scratch film and yarn surfaces.

5. Emergency Shutdown Conditions

To protect equipment and ensure operator safety, immediate shutdown is required when any of the following conditions occur:

• Continuous large-area yarn breakage with no possibility of recovery

• Severe die material buildup accompanied by smoking

• Severe equipment vibration, abnormal noise, or belt slipping with smoking

• Electrical alarm activation, temperature control failure, or motor overheating

When any of these conditions are observed, stop the line immediately and conduct a thorough inspection before restarting.

Final Thoughts

A well-maintained PP flat yarn extrusion line is the foundation of profitable woven bag production. Understanding common fault patterns and their solutions enables your team to respond quickly when issues arise — minimizing downtime and protecting product quality.

At Haosen Leituo Group, we go beyond manufacturing. We partner with our customers to optimize their entire production process through technical support, operator training, and tailored machine configurations. Our engineering team is always available to provide remote diagnostics and on-site assistance.