An Investigation Into Some Aspects Relating To The Design Of Melt Blowing Nonwoven Die

The flowing of polymer fluid from the inlet of coat-hanger die to the outlet of spinneret plate in melt blowing nonwoven process was simulated in this dissertation. Based on the simulation, a study of designing melt-blown die in several aspects was made. The following five main parts were included.1) The fluid flowing in the coat-hanger die with linearly tapered manifolds was simulated by three dimensional finite element method, and the efficiency of the numerical simulation was verified by experiments.2) Two coat-hanger die with different linearly tapered manifolds were optimized by evolution strategy (ES). The objective function is the uniformity of flow rate distribution at the die outlet, and the object variables are the main design parameters of the die.3) Based on the numerical simulation, the causes and impact factors of stagnation phenomenon of the fluid in the coat-hanger die was analyzed, meanwhile, the improving method was proposed.4) Taking the outlet velocity distribution of the coat-hanger die as the inlet condition of the spinneret plate, the fluid flowing from the outlet of the coat-hanger die to the outlet of the spinneret orifices was simulated by three dimensional finite element method. The influence of the combination of various orifice parameters on the flow rate distribution at the spinneret orifice outlet was discussed. And it was concluded that, with a proper combination, the uniformity of flow rate distribution at the outlet of spinneret orifices could be better than that at the coat-hanger die outlet.5) Broadening design of the coat-hanger die in the melt blowing nonwoven process was studied preliminarily. The feasibility to achieve broad combined coat-hanger die through jointing small width dies in tandem was discussed. In order to ensure the uniformity of cross flow rate distribution at the combined die outlet, the jointing conditions of each die were also discussed.This dissertation included 8 chapters.In chapter 1, the references relevant to this research field at home and abroad were reviewed.In chapter 2, three dimensional finite element numerical simulation of the polymer fluid flowing in the coat-hanger die was mainly introduced. In order to verify the simulation results easily, the coat-hanger die with simple linearly tapered manifolds and 1% CMC aqueous solution were selected. The solution's rheological parameters were measured by advanced rotating rheometer. Flowing vectogram and velocity distribution of the fluid inside the die were obtained by simulation. Through the observation of the flowing vectogram in the central plane of the die, it was found that not all of the fluid in the manifolds flow along the manifold's axial direction and the fluid which flow into the slot section from the manifolds do not flow along the die longitudinal direction entirely, which was not consistent with the assumed conditions in the one dimensional analysis. The velocity magnitude in the land of the die showed that the fluid velocity was lower in the middle part and higher in the two ends at the whole width of the die, and this velocity distribution was kept until to the outlet of the die. Moreover, velocity distribution at the outlet of the die reflected the influence of non-slip condition of the wall. Although the fluid velocity was higher at the two ends of the die, the velocity of the fluid closed to the wall are still very low. The flowing of polymer fluid in the coat-hanger die designed by one dimensional analysis method was also simulated in this chapter. The simulation results showed that the uniformity of velocity distribution at the die outlet is better than that at the simple linearly tapered coat-hanger die outlet. In chapter 3, the experimental work of using particle image velocimetry to verify the simulation results presented in Chapter 2 were introduced. Comparing the flowing vectogram in the central plane of the coat-hanger die, it was found that the simulation results were fundamentally consistent with the experimental results. Comparing the velocity distribution at the die outlet, it was found that they were also very close to each other. The value of CV% was 21.3228% and 23.8308% respectively, which indicated the efficiency of the numerical simulation method used in this dissertation.Two different coat-hanger dies were optimized in chapter 4. One is a simple linearly tapered manifolds coat-hanger die, and the other is a linearly tapered manifolds coat-hanger die based on the design of one dimensional analysis. The CV% value of the flow rate distribution at the die outlet was taken as the objective function; the slot width and the manifolds angle of the die were taken as the objective variables. ES was used to search the optimum objective variables. When a pair of objective variables was found, the corresponding objective function would be obtained by numerical simulation. The searching and simulation would continue until the prescribed terminal condition was reached. Finally, the minimum value of objective function and its corresponding objective variables were selected as the optimal results. For the simple linearly tapered manifolds die, CV% value of the flow rate at the die outlet decreased from 22.0586% to 1.772%, the decreasing amplitude was large; for the linearly tapered manifolds die based on the design of one dimensional analysis, CV% value decreased from 2.9501% to 0.9465, the optimal results was better but the decreasing amplitude was much smaller than the former one. It was because its velocity distribution at the die outlet was better than the former one before the optimization. The optimized results indicated that the optimization design method proposed in this dissertation was feasible and effective. Based on the optimal results, the optimized linearly tapered manifolds coat-hanger die based on the design of one dimensional analysis was used for the subsequent study of the dissertation.Despite the unevenness of velocity distribution at the coat-hanger die outlet, stagnation of the fluid caused by the difference of fluid velocity in the die will also influence the quality of the product. Therefore, the stagnation phenomenon in the die, which was designed by one dimensional analysis method and optimized in chapter 4, was analyzed through numerical simulation. Through observing the fluid experience time at the die outlet, it was found that there are 67% of the fluids at the die outlet under the average experience time 0.47s, most of which are neared to central plane and symmetry plan at die outlet. Fluids nearby the die end has the longest experience time of 3.60s～7.19s, but they are only 0.28% of the fluids at the die outlet. The fluids which have relatively long experience time of 0.47s～3.60s are 32.72% of the fluids at the die outlet and are near to the wall and the symmetry plane at the die end. Through the further analysis of the three dimensional velocity distributions in the die, it was found that the stagnation phenomenon was caused by the low velocity zone near the wall of the manifolds. In order to discussing the impact factors to the stagnation, stagnation zone was defined in this chapter. Through comparing the areas of the stagnation zone, it was found that design parameters of the die, such as slot width and manifolds angle, and non-newtonianism of the polymer fluids impacted the size of stagnation zone in a certain extent. In addition, it was also found that the stagnation in coat-hanger die with tear drop manifolds is less than that with circular manifolds.In melt blowing process, fluid is distributed by the coat-hanger die first, and then it will flow through the spinneret plate and be transformed into many fluid strands through the spinneret orifices on the plate. Therefore, the distribution of the flow rate at the outlet of spinneret orifices influences the cross-section uniformity of the melt-blown products directly. For this reason, the fluid flowing in the spinneret plate was simulated in chapter 6 under the situation of taking the velocity distribution at the outlet of the optimized coat-hanger die designed by the one dimensional analysis as the inlet condition. It was shown by the simulation results that small waves appeared in the velocity distribution at the outlet of the spinneret orifices and the CV% value increased from 0.9465% to 1.8140%. Therefore, velocity distribution at the outlet of spinneret orifices was then studied. The fluid flowing in nine different spinneret plates was simulated respectively, which were obtained by pair wise combining three different spinneret orifice diameters and three different spinneret orifice densities. It was found that the flow rate distribution at the outlet of spinneret orifices was influenced by different combination of orifice diameters and densities. In the 9 schemes, the combination of 0.3 mm diameter and 12 orifices/cm density of spinneret orifice was the best, the velocity distribution of which was the most uniform, and CV% value was 0.694% which was lower than the CV% (0.9465%) value at the coat-hanger die outlet.Technological problems of the coat-hanger die broadening in melt blowing equipment were discussed in Chapter 7. The height of the die, the die slot's deformation and the unevenness of flow rate distribution at the die outlet will become larger with the increase of the die width under the same other conditions. Therefore, it isn't feasible to achieve broad die through merely increasing the width of a single die. However, jointing many single small coat-hanger dies in tandem to constitute a combined broad die may not cause those problems. Two coat-hanger dies were jointed in tandem in this dissertation. There were two inlets and one outlet in the combined die and no wall obstructer in the jointing position. Through numerical simulation of the fluid flowing in the combined die; it was found that combined die inherited fluid distribution characteristics of the single die. However, the fluid velocity was lower in the jointing position, which influenced the uniformity of flow rate distribution at the combined die outlet. Through adjusting jointing position of the two dies, it was found that velocity distribution was different at the jointing position when they were jointed at different positions. With the increase of the distance between the jointing position and the die end, velocity distribution at the jointing position changed from concave state to convex state gradually. Method of bisection was used to adjust the jointing position. It was found that the unevenness of velocity distribution can be eliminated fundamentally when the distance between the jointing position and the die end is the 0.625% of the single die width. On this condition, CV% value of the flow rate distribution at the combined die outlet was 0.782%, which was lower than 0.9465% of the single die. It was, therefore, a feasible technological approach to achieve broad melt blown coat-hanger die through jointing many small single dies in tandem at the right position.Conclusions and outlooks were presented in Chapter 8. Main research findings and insufficiencies of this dissertation, and further research points involved in this field were described one by one.