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What to do when there's "hard head" in fine yarn?

December 09,2023

"Hard head" in fine  yarn , also known as the inability to stretch, is primarily caused by the stretching force being greater than the holding force. Factors that affect the stretching and holding forces include raw materials, processes, machinery, temperature, and humidity, which can significantly impact the quality and production efficiency of fine yarn.


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Twist coefficient of coarse yarn

When the twist coefficient of coarse yarn is large, the tightness of the yarn increases, the fiber-to-fiber friction resistance increases, and the stretching force response increases. Under the conditions of constant spacing and stretching multiple in the later section of fine yarn, the tassel cannot be effectively untwisted, resulting in a large twist entering the main stretching area and causing the inability to stretch. However, the twist coefficient should not be too small since it may lead to unexpected stretching during feeding into the fine yarn machine, causing defects in the product.


Motion distance of fine yarn guide roller

When the motion direction of the fine yarn guide roller coincides with the twist direction of the tassel, the stretching force is maximum, and the holding force is less than the stretching force, leading to an inability to stretch. When the twist direction of the tassel is opposite to the movement direction of the guide roller, the stretching force is relatively small, which reduces the occurrence of hard head.


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Coarse yarn count

An increase in coarse yarn count means an increase in stretching force when fine yarn is stretched. Under the same roller state, stretching multiple, and twist coefficient of coarse yarn, the holding force needed for heavy coarse yarn is greater than that for light coarse yarn. Thus, coarse yarn count places high demands on the holding force of fine yarn stretching. The count of coarse yarn should be appropriate for the clamp distance to avoid severe fluctuations of the yarn at the clamp point, affecting the normal stretching of fine yarn.


Spacing of fine yarn roller

A smaller spacing is advantageous for controlling the floating fibers in the tassel, which increases the stretching force and leads to hard head. Increasing the spacing weakens the capacity for controlling floating fibers, allowing fibers to be controlled correctly for normal stretching and avoiding hard head.


Stretching of fine yarn in the later section

Increasing the stretching multiple of fine yarn in the later section reduces the number of fibers in the cross-section of the tassel entering the main stretching area, thereby reducing the stretching force, which is beneficial for clamp control of the fiber in the tassel and reducing hard head. However, it generally does not contribute to improving the quality of spun yarn. Many production enterprises adopt larger fine yarn rear roller center distance, smaller stretching multiple in the later section, and larger twist coefficient of coarse yarn.


Clamp distance of fine yarn

A smaller clamp distance enhances the clamp's control ability of fine yarn, particularly at the edge fibers when the fibers' speed changes, improving the spun yarn. Increasing the distance weakens the clamp's control ability, which can reduce hard head but has some effects on the quality of the spun yarn.


Fine yarn roller pressure

Lower fine yarn roller pressure weakens the clamp's control of the fibers, leading to hard head. Increasing the roller pressure increases the friction force of the clamp on the fibers, allowing the fibers to be better controlled for normal stretching. The pressure at the front, middle, and rear positions of the rocking frame should be reasonably allocated to prevent hard head caused by ineffective stretching in the later section.


Fine yarn leather roller

For new rubber rollers with a coated layer, a thicker coating layer weakens the holding force of the rubber roller surface, making it slippery during stretching, which causes hard head. Old rubber rollers have a thinner coating layer with a larger friction coefficient between the rubber roller and the tassel, providing stronger holding force than the stretching force, allowing normal stretching of the tassel. Unprocessed rubber rollers have a large friction coefficient with the tassel, which can overcome a greater stretching force, making it an effective solution for hard head. In winter, due to lower temperatures, untreated leather rollers can become hard, increasing slipping rates and leading to hard head.


Raw material conditions

In daily production, raw materials are constantly changing. For example, if the fiber quality and length in cotton bales improve (e.g., with increased combed sliver content), the friction coefficient between fibers in the tassel increases, and the stretching force response increases. When the clamp cannot effectively control the fibers in the tassel under unchanged fine yarn processing, the tassel cannot be stretched normally, resulting in hard head. Adjustments to other processes are necessary.


Temperature and humidity

When the workshop's humidity increases, the friction coefficient between fibers in the tassel increases, and the friction resistance increases, which increases the stretching force, causing hard head. The workshop should reasonably control temperature and humidity based on seasonal changes, raw materials, and other factors to ensure normal production and avoid hard head.


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