In a polyester chip spinning plant—or in a direct melt spinning line equipped with an in-line additive system—the screw extruder works like the “heart” of the process, continuously delivering polyester melt to the spinning packs.
Among the key parameters that determine the efficiency and output quality of this “heart,” the screw length-to-diameter ratio (L/D) is particularly important. It affects not only how effectively the polymer is melted, but also the strength and uniformity of the finished fibers, as well as overall production stability.
This article explains how the screw L/D ratio influences polyester spinning operations.
1. What Is the Screw L/D Ratio?
Simply put, the screw L/D ratio is the ratio between the screw’s effective working length (L) and its diameter (D).
For example, if a screw has a diameter of 60 mm and an effective working length of 1,800 mm, its L/D ratio is 30:1:
(1,800 ÷ 60 = 30)
In polyester spinning, you cannot select this ratio arbitrarily. It determines how much time and space the polyester chips have inside the extruder to convey, melt, mix, and vent.
A higher L/D ratio gives the chips a longer processing path, allowing more shear and more thorough mixing. A lower L/D ratio shortens the path and speeds up processing, but the material may not be treated as completely.
2. Three Main Effects of the L/D Ratio
Polyester chips used for spinning are generally made from PET, which has a melting point of approximately 255–265°C. Once molten, PET is temperature-sensitive and may degrade above 290°C.
This limits the practical selection of the screw L/D ratio. Different ratios can produce noticeable differences throughout the spinning process.
2.1 Melting Quality: Does the Fiber Contain Unmelted Material?
Before spinning, polyester chips must be converted from solid particles into a homogeneous melt inside the screw extruder.
☆ L/D ratio too low, such as below 24:1
The chips spend less time inside the extruder and may not melt completely. Small unmelted particles, sometimes referred to as “unmelted chips” or “hard particles,” may remain in the melt.
If these particles enter the spinneret holes, they may block the spinneret, cause filament breaks, or create thick spots in the finished fiber, affecting the fabric’s hand feel.
☆ Moderate L/D ratio, such as 28:1–32:1
The chips have enough time to pass through the complete feeding, compression, and metering sections. This allows the material to melt more uniformly, with almost no unmelted particles remaining.
The resulting melt is clearer, and the spun filaments are more consistent in diameter.
☆ L/D ratio too high, such as above 34:1
Although melting may be more complete, the melt stays at a high temperature longer. This may cause thermal-oxidative degradation and generate acetaldehyde and other low-molecular-weight compounds.
As a result, melt viscosity may decrease, and bubbles may form in the fiber, causing a sharp increase in filament breaks during spinning.
2.2 Spinning Stability: Its Effect on Production Uptime
For a spinning plant, stable operation means higher efficiency. Fewer filament breaks and fewer package changes allow the production line to maintain a higher output. The screw L/D ratio is one of the less visible factors affecting this stability.
☆ When the L/D ratio is too low, insufficient melt mixing may cause large viscosity fluctuations. The melt may alternate between being too thick and too thin.
When these fluctuations reach the spinning pack, filament tension becomes unstable. Minor fluctuations may cause broken filaments or fuzz, while more serious fluctuations can lead to filament breaks. Operators may then need to stop the line frequently to clean the spinneret or replace the spinning pack, potentially reducing the operating rate from 95% to below 80%.
☆ With a suitable L/D ratio, the metering section provides better pressure stabilization. Melt viscosity fluctuation can be controlled within ±0.05, filament tension remains stable, and the filament break rate can be kept below 0.5%.
This reduces unnecessary shutdowns and keeps production running more smoothly.
2.3 Fiber Properties: Does the Finished Product Meet Market Requirements?
The screw L/D ratio also influences the performance of the finished polyester fiber.
☆ If the L/D ratio is insufficient, poor melt homogeneity may lead to variations in crystallinity and molecular orientation within the fiber.
As a result, fiber strength may vary from one section to another. Fabrics made from such fibers may be more likely to snag or pill, increasing the risk of customer complaints.
☆ If the L/D ratio is too high, melt degradation may reduce viscosity and lower the breaking strength of the fiber—for example, from 4.5 cN/dtex to 3.8 cN/dtex.
In curtains, workwear, and other applications that require good abrasion resistance, this strength loss can significantly shorten the product’s service life.
3. Practical L/D Ratio Selection for Polyester Spinning
No single L/D ratio suits every spinning process. The correct choice depends on the fiber type, equipment specifications, and production speed.
The following three common applications serve as a practical reference.
Application 1: Conventional POY and FDY Spinning
POY, or partially oriented yarn, and FDY, or fully drawn yarn, are among the main products in polyester filament production.
Their spinning speeds are typically around 3,000–5,000 m/min, and their melt-uniformity requirements are moderate. A screw L/D ratio of 28:1–30:1 is recommended.
This range allows the chips to melt completely without keeping the melt inside the extruder long enough to cause excessive degradation. It balances production stability and fiber strength and is widely used for conventional fibers in apparel and home textiles.
Application 2: Fine-Denier and Microfiber Spinning
Fine-denier fibers, with an individual filament linear density below 1.0 dtex, and microfibers, with an individual filament linear density below 0.5 dtex, require an extremely clean and homogeneous melt.
Even a small degree of non-uniformity may cause filament breaks or uneven filament diameter.
These fibers are generally spun at speeds of 5,000–8,000 m/min and require stronger mixing and more stable melt pressure. A screw L/D ratio of 30:1–32:1 is recommended.
The slightly higher ratio helps produce a finer and more homogeneous melt, reduces spinneret blockage, and supports continuous production of fine-denier fibers. These fibers are commonly used in premium underwear, facial mask fabrics, and similar applications.
Application 3: Industrial Yarn Spinning
Polyester industrial yarns, including yarns for tire cords and conveyor belts, require a high-viscosity melt—with an intrinsic viscosity above 0.9 dL/g—and high fiber strength.
Their spinning speed is relatively low, normally around 1,000–2,000 m/min, but the process still requires effective shear and melt mixing.
A screw L/D ratio of 26:1–28:1 is recommended.
Because industrial-yarn-grade polyester chips have a high viscosity, an excessively high L/D ratio may create too much shear and cause melt degradation. A slightly lower ratio can provide adequate mixing while reducing residence time and viscosity loss, helping preserve the high strength required for industrial yarns.
4. Two Additional Factors That Should Not Be Overlooked
4.1 The Screw Design Must Be Suitable
The L/D ratio should not be considered on its own. The screw compression ratio—the ratio between the channel depth in the feeding section and that in the metering section—also matters.
For example, even if a screw has an L/D ratio of 30:1, its melting performance may still be unsatisfactory if the compression ratio is too low, such as below 2.5.
For polyester spinning, a screw compression ratio of 2.8–3.5 is generally recommended.
4.2 Operating Parameters Must Match the Equipment
Screw diameter varies between equipment manufacturers, so the L/D value alone does not provide the full picture.
For example:
- A screw with a diameter of 50 mm and an L/D ratio of 30:1 has a length of 1,500 mm.
- A screw with a diameter of 65 mm and an L/D ratio of 30:1 has a length of 1,950 mm.
The machine barrel must be long enough to accommodate the selected screw. Otherwise, installation problems may occur.
In practice, selecting the screw L/D ratio involves balancing melting quality, production efficiency, and fiber performance.
For polyester spinning companies, choosing the right ratio can reduce filament breaks and material losses while improving the consistency and competitiveness of the finished fibers.
Whether you require high-quality polyester chips or wish to explore innovative applications of polyester resin in chemical fiber production, you are welcome to contact Decken Polyester at any time!
🌐 Website: https://www.polyestermfg.com/category/products/
✉ Email: info@polyestermfg.com
We look forward to welcoming experts from the chemical fiber industry for visits and exchanges, and to joining hands with you to drive the industry toward greater refinement, high-end development, and innovation!
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