3.1 Problem Analysis of MES
TRIZ theory is an effective method for resolving conflicts and eliminating contradictions. Among them, the material-field model analysis method is a method used to establish a functional model linked to the problem of an existing system or a new technological system, and in the process of problem solving, the corresponding general and standard solutions can be found according to the problem described by the material-field model [22-24]. To address the problems of MES, a melt spinning design solution strategy based on the material-field model and conflict resolution theory is proposed to eliminate the whip instability in electrospinning to obtain regular patterned fibers with fine particle size. The MES system shown in Figure 1 can be decomposed and represented by multiple material-field triangles modeled, and the obtained material-field model is shown in Figure 2.
The process of solving this problem by the material-field model analysis is as follows.
(1) Specify the substances: S1-forming nanofibers, S1'-formed nanofibers, S2-molten polymer, S3-heating temperature control device, S4-high voltage electrode.
(2) Determine the fields: F1-thermal field, F2-high voltage electric field, F2'-force field between nanofibers.
The analysis of the material-field model shows that: (1) the heating temperature control device S3 keeps the molten polymer S2 always in the molten state under the action of the thermal field F1 (effective effect); (2) the high-voltage electrode is connected to the melt nozzle or needle part, and the electric heating or other direct heating methods are used, which will easily lead to the breakdown of the high-voltage electrode S4 (harmful effect). The melt spinning distance (the distance between the collector and the nozzle) is continuously adjustable, when the spinning distance is far: (3) the jet stream of molten polymer S2 is continuously stretched under the action of high-voltage electric field F2 until it is pulled into nanofiber S1 (effective effect); (4) the nanofiber S1 formed in the high-voltage electrostatic field is homogeneously charged, and the nanofibers repel each other under the action of inter-nanofiber force field F2', and the final formed nanofiber S1' is difficult to have a good morphology (harmful effect). In addition, the fibers can cure prior to deposition, which affects the positional deposition performance, and requires higher spinning voltages, increasing the risk of partial discharge and breakdown. When the spinning distance is shortened, the fibers have not reached the whip instability stage has been stacked on the collector, so that the fibers have not had time to pull the fine stacked shape, which eliminates the harmful effects of F2', can get a regular arrangement of fibers, but at the same time lead to F2 does not play a corresponding stretching effect, the obtained fiber particle size is coarse. That is, MES exists between the contradiction of fiber size and alignment rules, it is difficult to get both fine particle size and regular arrangement of the fiber pattern.
This system is a complete material-field model of harmful effects. The main problems are the poor safety of the spinning system and the irregular shape of the collected nanofibers or the coarseness of the collected fibers that do not reach the nanoscale.
3.2 Problem Solving of MES
The analysis of the forming process and the material field model of MES revealed that: (1) In order to ensure the state of the polymer melt, the temperature of the melt must be ensured, and the high voltage electrostatic is usually loaded to the syringe needle and other jet generating end, however, the jet generating end in the MES device is also often the plasticizing end of the melt . If electric heating or other direct heating is used to ensure the temperature of the melt, it is easy to load the voltage too high and lead to the breakdown of the heating circuit or the metal original with the high voltage electrode, thus terminating the spinning process. (2) The newly formed nanofibers are homogeneously charged, and the collected nanofibers are irregular in shape due to the repulsion of the charges; the high voltage can make the nanofibers being formed have a greater force in the axial direction, which makes the repulsion effect between the nanofibers less obvious, so that the nanofibers with better shape can be collected, but the power consumed is too large and the safety risk is greater.
For the above-mentioned harmful material-field models, the effects of harmful effects can be eliminated by introducing new physical fields or by inventive principles [22, 24]. Using the TRIZ theory of technical conflict resolution to analyze the above problems: (1) The temperature of the melt must be guaranteed, and "temperature" is the parameter to be improved, which corresponds to the general technical parameter "22-temperature"; the safety is affected by the breakdown of the circuit or high-voltage electrode, and "safety" is the parameter to be deteriorated, which corresponds to the general technical parameter "37-safety". "Safety" is the parameter to be deteriorated, corresponding to the general technical parameter "37-safety". A query of the TRIZ theory conflict matrix yielded 5 recommended inventive principles: 24 (with the help of intermediaries), 1 (segmentation), 3 (local mass), 35 (change of physical or chemical parameters), 28 (substitution of mechanical systems). (2) Well shaped nanofibers need to be obtained, "well shaped nanofibers" is the characteristic to be improved, corresponding to the general technical parameter "9 - shape"; loading higher voltage causes excessive energy consumption, "increased energy consumption" is the characteristic to be deteriorated, corresponding to the general technical parameter "16 - energy consumed by moving objects". A query of the TRIZ theory conflict matrix yielded 8 recommended inventive principles: 3 (local mass), 14 (surfacing), 28 (substitution of mechanical systems), 2 (extraction), 24 (with the help of intermediaries), 35 (change of physical or chemical parameters), 6 (diversification), 34 (discard or regeneration). The inventive principle 28 (substitution of mechanical systems) was selected after comparative analysis. Using a strong magnet instead of high-voltage electricity and adding ferromagnetic particles to the polymer melt to make it a magnetic fluid, the magnetic field generated by the strong magnet is used to spin the magnetic fluid, which retains the advantages of MES and solves two pairs of technical conflicts: (i) the high-voltage circuit is eliminated, the temperature of the melt will not be affected, and the safety of the melt temperature and preparation process can be ensured at the same time; (ii) the high-voltage electric field is eliminated and the magnetic field is used, which ensures the morphology of the patterned nanofibers obtained and does not increase the energy consumption.
A schematic diagram of the modified MMS device using the conflict matrix is shown in Figure 3, and a model of the MMS material-field is shown in Figure 4.problem by the material-field model analysis is as follows
The MMS process is analyzed using the material-field model as follows.
(1) Specify the substances: S1-nanofiber, S2-magnetic fluid, S3-heating temperature control device, S4-strong magnet.
(2) Determine the fields: F1- thermal field, F2-magnetic field.
The analysis of the material-field model shows that: (1) the magnetic fluid S2 is always in the molten state under the action of the thermal field F1 through the heating temperature control device S3 (effective effect); (2) the jet stream of the magnetic fluid S2 is continuously pulled thin under the action of the magnetic field F2 until it is formed into nanofiber S1 (effective effect).
This system is an effective and complete material-field model, using the inventive principle, a strong magnet is substituted for high voltage electricity, and the MES device is improved, thus solving the problems of poor safety and irregular shape of the collected patterned fibers in the MES device.
3.3 Device Improvement of MMS
In the problem solving session, the problems of poor safety and irregularity of the collected patterned fiber morphology in the MES device were solved, and the melt magnetic spinning device was obtained. However, the working platform of this device and the position of the magnetic fluid injection device are fixed, resulting in the prepared nanofiber morphology is almost constant, limiting the versatility and operational flexibility of the prepared nanospinning, and the prepared nanofibers are coarse and difficult to reach the nanometer scale. Some researchers have solved the problems of coarse nanofibers and low spinning efficiency by changing the fixed working platform into a rotating working platform , but there is still a lack of research on how to form patterned fibers, which cannot meet the demand of neatly arranged nanofibers. Therefore, the MMS device shown in Figure 3 is optimized to solve this problem by the three-dimensional movement between the magnetic fluid injection device and the working platform. Figure 5 shows a schematic diagram of the three-dimensional mobile MMS device after the optimization of the MMS device.
The three-dimensional mobile MMS device spinning process is analyzed using the material-field model (shown in Figure 6) as follows.
(1) Specify the substances: S1-coarser nanofibers, S2-finer nanofibers, S3-magnetic fluid, S4-heating temperature control device, S5-strong magnet.
(2) Determine the fields: F1-thermal field, F2-magnetic field, F3-mechanical field.
The analysis of the material-field model shows that: (1) the magnetic fluid S2 is always in the molten state under the action of the thermal field F1 through the heating temperature control device S4 (effective effect); (2) the jet stream of the magnetic fluid S3 is continuously pulled thin under the action of the magnetic field F2 until it is formed into the nanofiber S1 (effective effect); (3) the coarser nanofiber continues to be pulled thin under the action of the mechanical field F3 until it becomes the nanofiber S2 in a more ideal state (effective effect).
This system is an effective and complete material-field model. In this system, the original fixed magnetic spinning device is changed into a three-dimensional magnetic spinning device that can be moved up and down, left and right, and back and forth, by optimizing the melt magnetic spinning device for an effective complete object field model. This device can obtain finer nanofibers and can generate regular patterned fibers with uniform nanofiber particle size by constantly changing the relative positions of the magnetic fluid preparation device and the mobile table.
3.4 Device Details Optimization
In-depth analysis of the three-dimensional mobile MMS device shown in Figure 5 found that the device also has some details of the problem. For example, when the magnetic fluid is extruded out of the capillary syringe little by little through the piston, it is difficult to control the extrusion force and the speed of the magnetic fluid outflow due to the manual extrusion operation, which makes the extruded magnetic fluid not uniform, thus leading to the inconsistent diameter of the prepared nanofibers. To address this problem, a stepper motor magnetic fluid extrusion device is designed in this paper, as shown in Figure 7.
The stepper motor is connected to the screw nut mechanism, and the screw nut mechanism is connected to the pushing member so as to control the piston movement, and the piston movement is used to regulate the pressure acting on the magnetic fluid inside the chamber, thus making the magnetic fluid extruded more stably and uniformly, so as to prepare the fiber with uniform particle size, which solves the problems of the traditional feeding device that the feeding is not uniform and stable enough and the spinning effect is poor.
3.5 Three-Dimensional Mobile MMS Principle
After further optimization of the magnetic fluid spinning device, a three-dimensional mobile MMS device was finally developed, the structure of which is shown in Figure 8. The MMS device consists of four main parts: the magnetic fluid preparation device for producing magnetic fluid, the three-dimensional console for fixing the magnetic fluid preparation device, the collector located below the magnetic fluid preparation device, and the strong magnet located below the collector.
The spinning material is put into the sealed chamber 4, and then the magnetic fluid preparation device is fixed on the support of the three-dimensional console, and the relative position of the nozzle 6 of the magnetic fluid preparation device and the strong magnet 7 of the three-dimensional console is adjusted to the set initial position. By controlling the stepping motor 1, the propulsion device 3 is moved downward to the sealing chamber 4 under the action of the filament nut mechanism 2, and the pre-prepared magnetic fluid in the molten state is added inside the sealing chamber, which is always in the molten state under the control of the heating temperature control device 5. Under the further action of the propulsion device, the magnetic fluid inside the chamber is extruded from the nozzle under pressure to form a liquid droplet. The synchronous motor 11 is connected to the screw, which is connected to the guide rail to control the front and back relative position of the working platform and the nozzle, the stepping motor 14 controls the left and right relative position of the working platform and the nozzle, and the synchronous motor 16 controls the up and down relative position of the working platform and the nozzle. The relative position of the nozzle of the magnetic fluid preparation device and the strong magnet of the three-dimensional console can be freely controlled, and the droplet-shaped magnetic fluid formed according to the designed trajectory forms a liquid bridge under the action of the magnetic field of the strong magnet, and the liquid bridge is cured to form nanofiber filaments.