After yarn twisting of polyester or nylon 66 sewing thread, heat set is one important process. Heat set, also known as steam stabilization, is a process that uses hot steam to stabilize the twisted yarn. During the production of high-strength polyester and nylon 66 filament yarns, repeated mechanical stress and friction cause internal stress imbalance, deformation, fatigue, and static electricity. Additionally, after twisting, the yarn retains significant untwisting torque, which can adversely affect subsequent production processes. Increasing temperature or moisture regain can weaken the intermolecular forces within the fibers, allowing them to relax and eliminate internal stress resisting the twist, thereby stabilizing the yarn twist.
1. Heat Set Steps
Step 1: Preheating
During steaming, water spots on yarn are a common defect that can lead to dyeing issues and other problems in downstream processes. Therefore, preheating is essential to avoid condensation-induced water spots.
Step 2: Vacuum Extraction
Reduces the boiling point of water to generate low-temperature saturated steam required for the process.
Step 3: Heating
Allows low-temperature saturated steam to penetrate cone or bobbin yarns. Each production step (spinning, twisting, weaving, etc.) induces tension and entanglement in the yarn and fibers. Stress relaxation in the molecular structure of synthetic fibers causes fiber shrinkage, while tension and entanglement create challenges for downstream processes.
Step 4: Heat Preservation
Transitioning from vacuum to low-temperature saturated steam ensures uniform treatment of the entire yarn package, including packaging materials. Moisture from the steam is transferred to the yarn, achieving consistent moisture distribution, uniform yarn strength, and friction values. Heat treatment also relaxes the twist. During production, fiber moisture continuously evaporates, resulting in a cumulative moisture loss of over 4% from raw material to bobbin yarn. Higher machine speeds exacerbate weight (moisture) loss, while high temperatures and friction within machines degrade yarn quality, adversely affecting downstream processes.
Step 5: Secondary Vacuum Extraction
A unique secondary vacuum design in the steaming machine ensures optimal yarn treatment.
2、Suggested Heat Set Temperature ℃
Heavy Twisted Yarns | 90～95 |
Filament Polyester Yarns | 112～120 |
Core Spun Yarns | 70～75 |
Filament Polypropylene Yarns | 130～140 |
Filament Nylon Yarns | 90～110 |
3. Effects of Heat Set
3.1 Impact on Yarn Mechanical Properties
Yarn breaking strength and elongation are closely related to moisture regain. Properly increasing moisture regain enhances mechanical properties, reducing yarn breakage and improving efficiency in subsequent knitting or weaving.
3.2 Impact on Yarn Friction Coefficient
Moisture regain directly affects friction coefficient uniformity, hairiness, and static electricity. Steam stabilization balances friction coefficients across yarn layers, reduces static, improves unwinding efficiency, and enhances fabric softness.
3.3 Impact on Twist Stabilization
Torque generated during yarn processing causes uneven unwinding tension, reduced knitting efficiency, and quality defects. Two methods address torque:
- Natural torque decay over time (slow and ineffective).
- Steam stabilization to relax internal stress and fix twists (fast and highly effective).
3.4 Other Benefits
High temperature, humidity, and pressure improve yarn structural stability, bulkiness, shrinkage, and subsequent dyeing uniformity and vibrancy. Overall, steam stabilization enhances fabric dimensional stability, softness, and quality while boosting production efficiency and shortening lead times, giving manufacturers a competitive edge.
# Heat Set of Sewing Thread # High Tenacity Filament Polyester Sewing Thread
# Bonded Nylon 66 Thread # Raw White on Dye tube