Studies on the mechanism of draw resonance in melt spinning

The physical mechanism of draw resonance of polymer fluids in melt spinning has been studied. It is proposed, according to the cross-sectional area dependence of the local draw ratio of the filament along the spinline, that filament drawing can be divided into three modes which are given in the main text. In particular, it is found that the nonuniformizing draw mode is the necessary condition of draw resonance. The mechanism is certified through the analytical solution of isothermal melt spinning under uniform spinning stress and the critical draw ratio of isothermal and uniform tension melt spinning. The mechanism was employed to analyze the promotive and suppressive factors of draw resonance in a real spinning system and the development of filament unevenness along the spinline.     

Studies on melt spinning. III. Flow instabilities in melt spinning: Melt fracture and draw resonance

An experimental study has been carried out to investigate threadline instabilities in melt spinning. Two types of melt threadline instabilities, draw resonance and melt fracture, were observed under both isothermal and nonisothermal spinning conditions. Polymers investigated were polypropylene and polystyrene. Draw resonance was observed as an increase of the take-up speed above a critical value. It was also observed that an increase in take-up speed reduces the severity of melt fracture, whereas once draw resonance occurs the amplitude and frequency of the pulsing thread diameter increases with the take-up speed. The phenomenon of draw resonance was investigated by taking motion pictures of the pulsing molten threadline spun vertically downward. Furthermore, a stability analysis was carried out to explain the experimentally observed draw resonance.     

Studies on melt spinning. VI. Simulation of draw resonance using Newtonian and power law viscosities

Draw resonance, a periodic variation of spin line diameter in unstable melt spinning, was measured for its wave form under 34 different spinning conditions for PET and PP. In and attempt to simulate the measured wave form, the equations of continuity and momentum for the isothermal melt spinning of power law fluids were solved for their limit cycle solutions expressed in the time variations in the cross-sectional area at the take-up. Power law exponent p and draw down ratio ψ_w uniquely define the solution. Theoretical curves were superposed on the experimental amplitude-versus-ψ_w diagram and oscillation period-versus-ψ_w diagram to assign p value to each experimental point. Excellent agreement between theory and experiment was obtained with PET in that p values were nearly independent of ψ_w and of the diagram used in the determination of the p value, amplitude diagram, or oscillation period diagram. Motion pictures (16mm) of the side profiles of the pulsing spinline showed good agreement with the theoretical side profiles constructed from the corresponding limit cycle solution. It was proposed that the stability of melt spinning has no direct equivalence to the spinnability of fluids.     

Finite-amplitude stability and draw resonance in isothermal melt spinning

The spinning of fibers from polymer melts is sometimes limited at high takeup rates by the onset of a flow instability known as “draw resonance,” which is characterized by large amplitude fluctuations in the takeup area and the force. In this work, the onset and growth of the spinning instability is analyzed for an isothermal Newtonian liquid by examining the nonlinear dynamics of the most unstable spatial mode. At draw ratios below the critical value D_R = 20·21 computed from linear theory the system is stable to finite-amplitude perturbations. At higher draw ratios all disturbances approach a stable finite-amplitude oscillation which agrees in period and magnitude with experimental observations of draw resonance.     

Studies on melt spinning. VIII. Transfer function approach

Transfer function analyses were carried out on a linearized perturbation model of melt spinning previously developed. Results are as follows. (1) Formal proof was obtained of the criterion of spinline stability stated in terms of transfer function. (2) A new concise statement of the stability criterion was obtained: the spinline is stable when the transfer function G₂(s) connecting the spinline tension to the spinline velocity does not have a zero in the right-hand half of the complex s plane. (3) Effects of various external disturbances on filament unevenness were predicted theoretically by expressing the transfer function between each disturbance input and cross-sectional area at take-up in the form of frequency response Bode diagrams.     

Numerical simulations of draw resonance in melt spinning of polymer fluids

This article deals with numerical simulations of draw resonance of polymer fluids by employing direct difference methods to solving the governing equations in melt spinning. The stability of each difference method was studied by a comparison of the results obtained from simulations with the theoretical solutions or values. The numerical simulation confirms that the critical draw ratio of draw resonance in an isothermal and uniform tension spinning of a Newtonian fluid is between 20 and 21. The cross-sectional area of a spinline in draw resonance was found to decrease monotonically from a spinneret toward a take-up bobbin, although the taken-up filament shows periodical variation. This study has also illustrated the mechanism of draw resonance previously proposed by the author. © 1993 John Wiley & Sons, Inc.     

Studies on melt spinning. V. Draw resonance as a limit cycle

The exact wave form of draw resonance in isothermal spinning of Newtonian liquids was sought by solving numerically the simultaneous partial differential equations¹ of melt spinning in their original nonlinear form without recourse to perturbation. When the draw-down ration of spinning exceeded 20, solution of the equations became a limit cycle, a sustained oscillation having amplitude and period independent of initial conditions. As the draw down ratio was further increased, the amplitude of the limit cycle grew very rapidly, and the wave form became close to a pulse train predicting an extreme thinning of the thread at regular intervals along the thread. The above solution for Newtonian liquids agreed well with experiment with respect to oscillation period. Agreement, however, was poor in amplitude, indicating that possibly the amplitude of draw resonance is affected by deviations of polymer viscosity from Newtonian.     

熔体静电纺丝的工艺条件对纺丝纤维直径的影响

主要研究纺丝温度、纺丝电压、接收距离等参数对聚丙烯(PP)熔体静电纺丝纤维直径的影响。采用了只变一个参数,其它参数固定的常规实验方法。在实验条件范围内,随着纺丝温度的升高,纤维的平均直径逐渐减小,得到PP的最佳纺丝温度240℃。在固定电压的情况下,得到最佳接收距离7cm。在固定接收距离的情况下,随着电压的增加,电场中的喷射流熔体受到的电场力逐渐增大,得出最佳纺丝电压35kV。     

Studies on melt spinning. III. Velocity field within the thread

The velocity field within a molten spinning thread was analyzed quantitatively by solving the equations of continuity and momentum for Newtonian liquids. In solving the equations, the viscosity was assumed known and was given by the expression where x and r are distances in cylindrical coordinates. A series solution in velocity v having the expression was obtained when several simplifying assumptions were made on the equations. The series solution was found to converge when cr² < 1 is satisfied. μ₀e^βx and ν₀e^αx above are tangents on semilog paper at x = x to the macroscopic viscosity and velocity profiles μ(x) and ν(x) which were computed separately by means of a technique developed previously by the author.^1,2 The value c was derived from the temperature profile across the thread at x = x computed separately using another technique developed by the author.³ The above series solution showed numerically that under most conceivable spinning conditions the velocity field within the thread is for practical purposes flat across the thread and, in addition, purely extensional.     

Studies on melt spinning. VII. Temperature profile within the filament

An attempt was made to numerically compute the temperature profile within the melt spinning filament without assuming axisymmetry and using the data on the variation of the coefficient of heat transfer from a cylinder cooled by cross flow of air as given by Eckert.⁴ The computed constant-temperature contours were approximately concentric circles with their center shifted from the filament center in the downstream direction of the cooling air flow. A filament yarn melt spun under spinning conditions corresponding to the computation was dyed, and its cross sections were observed under the microscope. The border between the dyed and undyed portion of the cross sections approximately coincided with one of the computed temperature contours, suggesting indirectly the general validity of the computed temperature profile.     

The effects of heat transfer in melt spinning

Several heat transfer models are examined in order to assess the influence of convection and radiation upon the melt spinning of a Newtonian fluid. In the spinning of glass fibers, radiation is shown to be equally as important a mode of heat transfer as convection. Small changes in the spectral emissivity are shown to have a large effect on the shape of the jet and the drawing force required to attenuate the fiber.     

Studies on melt spinning. II. Analysis of the deformation and heat transfer processes

A generalized, empirical equation is proposed which takes into account the dependence of elongational viscosity on both elongation rate and temperature. From this, a mathematical model for simulating the melt spinning process has been developed. The model has been tested against experimentally observed velocity profiles in fibers of polystyrene and high-density polyethylene spun into an isothermal chamber. It has been found that predicted velocity profiles agree well with experimentally observed ones. The mathematical model has been used to predict velocity and temperature profiles in fibers spun into a cooling medium. The simultaneous solution of momentum and energy balance equations by means of a numerical integration scheme has generated important information such as distributions of force components involved in spinning and distributions of the total rate of heat transfer along the spinning way.     

Studies on melt spinning. IV. Spinning through a ribbon die

An experimental study has been carried out to investigate effects of stretch ratio on molecular orientation in polypropylene monofilaments which are melt spun from a ribbon die into a water bath with an adjustable air gap distance between the two. By varying the air gap distance and the rate of stretching, a variety of filaments of different molecular orientations were obtained. Measurements were taken of fiber birefringence of finished filaments under a polarizing microscope with camera attachment and mercury lamp. It has been found, according to the already established relationship between the molecular orientation and birefringence, that the molecular orientation in polypropylene filaments is increased with the rate of stretching. Two other interesting observations were made. One was that the filaments form crimps whose frequency increases with the rate of stretching. The other was that the phenomenon of draw resonance was observed when the rate of stretching was increased beyond a certain critical value.     

Studies on melt spinning. V. Elongational viscosity and spinnability of two-phase systems

Isothermal melt spinning experiments were carried out to investigate the elongational flow behavior and spinnability of polymer blends and a calcium carbonate-filled polypropylene. Blends chosen for study were mixtures of polystyrene (Dow Chemical, Styron 678) with high-density polyethylene (Union Carbide, DMDJ 4309), and mixtures of polystyrene (Dow Chemical, Styron 686) with high-density polyethylene (Union Carbide, DMDJ 4309). For the study, measurements were taken of the thread diameter by photographic techniques and of the thread tension by means of a Saxl tensiometer. These measurements were later used to determine the elongational viscosity of the material investigated. It was found that, in all the blends and filled systems investigated, the elongational viscosity decreases with elongation rate and that the relationship between the elongational viscosity and blending ratio is very complex. An attempt is made to offer explanations of the observed complicated relationship with the aid of microphotographs of fiber samples, which display the complexity of fiber morphology in two-phase systems. It was also found that there exists some correlation between the elongational viscosity and the maximum stretch ratio which may be considered as representing fiber spinnability.     

Studies on melt spinning process of hollow polyethylene terephthalate fibers

Dimensional change and profile development in the melt spinning process of polyethylene terephthalate hollow fibers were studied through the numerical simulations and experimental results. The simulation predicts the final dimensions and profiles development of the hollow fibers at various positions from the die. Experimental results show that the melt extruded from the spinneret coalesces initially to form a hollow inner core and the cross‐sectional shape holds for over the whole spinline with only variation in the hollow portion. Analysis of the effect of spinning parameters on hollow portion shows that the spinning temperature, mass throughput rate, and take‐up speed are the most critical variables in controlling the hollow portion followed by quench air velocity. The quench air temperature has relatively less effect than the other variables. As the mass throughput rate and quench air velocity increase and the take‐up speed and spinning temperature decrease, the hollow portion increases. To investigate the effect of die geometry, die having a different ratio of inner to outer diameter was used. The effect of change of process variables decreases as the die gap becomes narrow. POLYM. ENG. SCI. 46:609–616, 2006. © 2006 Society of Plastics Engineers     

Effect of molecular weight on high-speed melt spinning of nylon 6

An extensive experimental study of the structure and properties developed in as-spun nylon 6 filaments is reported. Five polymers representing different molecular weights in the range 25,000–73,000 g/mol (viscosity average) were studied. These polymers were melt spun over a range of spinning speeds using an air drag type of drawdown device. Maximum take-up velocities achieved were in the neighborhood of 4000 m/min. The structure and properties of the as-spun filaments were characterized using density, DSC, WAXS, SAXS, birefringence, and tensile tests. The structural characteristics and properties of the filaments are strongly dependent on molecular weight. Generally, higher molecular weight leads to higher modulus and filament tenacity and lower elongation to break in the as-spun filaments. The structural changes with molecular weight are rather complicated; the complications are explained in terms of changes of crystallization rate and attainable crystallinity.