The General Characteristics Of Melt Spinning Process Are Analyzed In Depth.

Melt spinning is a one component system, which mainly involves heat pfer between polymer melt wire and cooling medium, and there is no change in the spinning system.

The main technological processes include spinning melt preparation, melt extrusion from spinneret, melt wire drawing, cooling and solidification, and filament winding and oiling.

The spinning process of melt spinning is shown in Figure 9-1.

The melt is extruded from the silk hole by a certain amount of pump supply, and the shear deformation, extrusion swell, tensile deformation, cooling and solidification of the fine flow in the spinneret of the melt occur on the spinning process, thus forming a certain supermolecular structure of the primary fiber.

1. Diameter variation and velocity distribution of wire rod on spinning process

When the melt is extruded from the spinneret, it expands due to the normal stress of the melt, and then the fine flow is accelerated and stretched. After cooling and solidification, the velocity and fiber diameter no longer change.

According to the strain rate (i.e. the axial velocity gradient) of the fine flow, the whole spinning process can be divided into three regions, the extrusion swell area, the deformation zone and the solidified wire movement area, as shown in Figure 9-2.

The junction between the first and second areas corresponds to the largest diameter swell, usually less than 1omm from the spinneret.

The second zone is the main area for the development of elongational flow. The total length varies with spinning conditions, usually between 50 and 150cm.

In this region, the maximum value exists in the middle ages. Due to the cooling effect of the air, the temperature of the fine stream decreases and the viscosity increases, and the fine current is elongated, leading to the preorientation of the melt macromolecules.

The second zone is also the key area for controlling cooling conditions.

The third zone is a solidified wire movement area. At this time, the velocity and diameter of the fiber basically remain unchanged at =o. The melt flow is pformed into a completely cooled solid primary fiber, in essence, the fiber continues to cool from the solidification temperature to near room temperature, so it is the cooling zone.

2. Kinetics of melt spinning process

In melt spinning process, polyester melt is extruded from the spinneret hole and subjected to axial tension of winding force, and the wire bar will be elongated and refined in the course of operation.

The force on the thread of the spinning process is shown in Figure 9-3.

The tension Fext (x) from the spinneret (x=0) to the X at the tip of the spinneret can be expressed as: Fext=Fi (x) +Fs (x) +Ff (x) +Fr (0) (()).

The Fext (x) is the winding tension of the X wire; Fi (x) is the inertia force to overcome the axial acceleration motion for the wire rod; Fs (x) is the surface tension that the customer needs to take on the spinning process; Ff (x) is the frictional resistance produced by the air on the wire surface; Fr (0) is the rheological resistance that is overcome by the axial flow of the melt flow at the outlet of the spinneret hole; Fg (x) is the force acting on the wire by the gravity field.

In the winding area (x=L), the upper form can be written as: Fext (L) =Fr (L) =Fi (o) +Fs (0) +Ff (0) +Fr (0) -Fg (0).

The upper surface tension Fs (0) exists in the small section of the filament in liquid. The surface tension Fs (0) and gravity Fg (0) account for less than 1% of the total spinning tension in conventional spinning or high speed spinning, so it can be neglected.

The inertia force Fi (0) exists only in the range of acceleration movement of the wire in the spinning process. Therefore, after the wire is cured, the speed will no longer change, and the inertia force is zero. The friction resistance Fi (0) is near the spinneret, because the velocity of the melt wire is very small, and the air resistance is very small.

In fact, most of the air friction resistance contributes to the wire after reaching the winding speed, and increases with the square of spinning speed.

Air friction resistance contributes little to total tension at low speed spinning, but air friction resistance is a factor that can not be ignored when spinning at high speed.

At the exit of the spinneret hole (x0=0), the rheologic resistance Fr (0) is:

Formula: AXX (0) is the average tensile stress at the outlet of the melt flow at the spinneret hole; R0 is the radius of the spinneret hole.

The winding tension is very sensitive to spinning conditions (winding speed and mercury content, cooling conditions or rheological properties of extruded melt), so it can be used to characterize the stability of industrial spinning process.