Before entering a comprehensive analysis, I believe everyone is an industry insider with a certain understanding of casing and its applications. Therefore, we will delve deeper into the control of casing excess length.
Introduce
Firstly, introduce the sleeve material PBT (Polybutylene terephthalate), which is a semi crystalline thermoplastic with high heat resistance, toughness, and fatigue resistance. The advantage of rapidly crystallizing up to 40% crystallinity at low temperatures is the main reason why we chose it as a casing material.
Sleeves are colored optical fibers obtained through the coating process in the production process of layer twisted optical cables. The most critical process parameter for layer twisted optical cables is the excess length of the casing, which is closely related to whether the fiber is subjected to stress inside the casing, whether the mechanical properties of the fiber such as torsion, tension, bending, flattening, and whether the high and low temperature cycling performance tests meet the requirements, and can directly affect the attenuation performance of the fiber.
So in order to have good control over it, the first step is to understand the source of surplus.
First of all, the conclusion is that the excess length of the casing comes from the cooling crystallization shrinkage of PBT material, causing the length of the optical fiber inside the casing to be greater than the length of the external casing. Therefore, in order to make the remaining length of the casing controllable, we must hope that the more complete the crystallization shrinkage of PBT casing, the more it tends to remain unchanged, the more controllable it is. So the key to process control lies in how to achieve higher crystallinity in PBT sleeves.
We all know that extrusion molding is the process of melting PBT material through an extruder, causing it to change from a particle to a viscous flow state. By adjusting the head mold and extruder speed to control its extrusion outer diameter, it is stretched and covered outside the optical fiber through fiber and filled with fiber paste. The entire process is distributed as a wire rack, electrostatic removal, extrusion machine, hot water tank, main traction, first cooling, second cooling, caliper, diameter die, printing machine, wire collection rack, etc.
Principle
Before the main traction of the sleeve molding, there is a hot water tank and the fiber optic cable before it is laid out. During this process, the sleeve and fiber optic cable both have a certain tension to move in a straight line without generating any extra length of the sleeve. In the hot water tank, the PBT sleeve is stretched, and the internal molecules will generate lattice tendency to produce inward shrinkage stress. When the cold and hot alternation of the main traction wheel is reached (at this time T>Tg), it meets the conditions for crystal nucleus and grain growth. The PBT sleeve will crystallize to eliminate internal stress. However, due to incomplete cooling in actual production, its crystallization is not complete, and its shrinkage internal stress cannot be completely eliminated. In the cold water tank (T: 14-20 ℃, far less than Tg), the chain segments are almost frozen. Unable to crystallize, there may be a situation where the remaining length of the casing gradually increases after production is completed. Due to the winding of the guide wheel around the sleeve, the optical fiber will deviate from the direction of the guide wheel at the center of the sleeve due to tension, resulting in the actual radius of the optical fiber being smaller than the radius of the sleeve, and the optical fiber will be shorter than the sleeve, resulting in negative excess length. So when the crystal eliminates the internal stress caused by shrinkage, it will first eliminate the negative excess length and produce positive excess length, which gradually increases. So the key to controlling the excess length process is to make the crystallinity of PBT casing as high as possible, so that the internal stress it can eliminate is more complete and the excess length generated by shrinkage is smaller.
Influence factor
After understanding the above principles, we can roughly consider the factors that may affect the remaining length of the casing, such as the tension of the wire, the tension of the remaining length, the temperature difference between the cold and hot water tanks, and the viscosity of the fiber paste.
To summarize, there are four points:
- Paying off tension, the greater the tension, the smaller the excess length
- Excess tension, the greater the tension, the smaller the excess length
- The larger the temperature difference, the greater the residual length
- For the viscosity of the fiber paste, firstly, the fiber paste has thixotropy, and the viscosity will recover to ensure that it will not flow out of the casing. Then, its viscosity has an impact on the residual length. When the viscosity is low, the fiber can move freely, making it easier to control the residual length; When the viscosity is high, the optical fiber cannot move freely and it is difficult to effectively control the excess length. When the fiber paste is extruded stably, the impact on the remaining length will be minimized.