Kinematics of wet forming
Velocity distribution on spinning line during wet forming
1. Kinematic Analysis of Spinning Line in Wet Laying Process
(1) Without stretching: The spinning solution sent out from the metering pump is pressed into the spinneret channel (The diameter of the spinneret hole is Do; The channel length is Lo), and then extruded from the spinneret orifice. The thin stream of spinning solution expands at the orifice. At this moment, the speed of the thin stream is free flow velocity (Vf) and the diameter of the expanded thin stream is Df which keeps constant size along the axis of the thin stream.
(2) Under stretching: The thin stream is drawn out under the stretching of the first guide roller. The outflow velocity is Vo. The thin stream become thinner gradually after passing the maximum diameter (Dm). This process of getting thinner continues until the freezing point.
The spinning process can be divided into four sections:
Area I: Pore flow area
Area II: Swelling area
Area III: Thinning Area
Area IV: constant velocity travel region of coagulated fibers.
2. Stretched state determined by stretching speed
(1) Positive stretch: VL>Vo, Nozzle stretch ratio of melt spinning and partial wet spinning: (VL-Vo)/ Vo >0
(2) Zero stretch: VL≈Vo, Nozzle stretch ratio of wet spinning: (VL-Vo)/ Vo =0
(3) Negative stretch: VL<Vo, Nozzle stretch ratio of wet spinning: (VL-Vo)/ Vo <0
Different from melt spinning, when the spinning stock solution is extruded from the spinneret hole, the stock solution has not yet solidified, the tensile strength of the spinning line is very low, and it cannot withstand excessive stretching of the spinneret, so wet forming usually adopts spinneret negative stretch, zero stretch, or not much positive stretch.
3. Characteristics of the four zones along the spinning process
Area I: Pore flow area: After the spinning solution enters the spinneret hole, it flows forward along the axial direction. The flow velocity Vx of the stock solution remains constant along the X-axis: dvX/dx=0.
In this case, the orientation of the macromolecular chains can occur and store elastic energy, which will affect the size of the pore opening area.
Area II: Swelling area: Vx decreases along the spinning process line, Vx↓. The axial velocity gradient is negative, that is, dvX/dx<0. In the case of changing the draw ratio of the spinneret, the expansion ratio B decreases with the increase of the draw ratio VL/Vo. When the stretch ratio increases by a certain value, the swelling zone (II) can completely disappear
Area III: Thinning Area: The filament stretches and flows, elongates and becomes thinner.
IIa: dvx/dx>0, d2vx/dx2>0
IIb: dvx/dx>0, d2vx/dx2<0
Area III is the critical area for fiber formation and the main area where the initial structure of the fiber is formed. The stretching flow orientation increases, and the stretching deformation orientation of the jelly skin layer increases.
Area IV: constant velocity travel region of coagulated fibers: Curing of filaments.
Vx=K, d=K,, =0
The primary structure of the fiber continues to complete.