A METHOD OF PRODUCING FIBRES FROM MINERAL MELT WITH THE AID OF CENTRIFUGAL FORCE

Abstract: The invention relates to a method of controlling fibre generation in a process where mineral melt is thrown into fibres with the aid of a centrifugal force. With this method, controlled vibrations are caused in the spinning wheel.

Description: A method of producing fibres from mineral melt with the aid of centrifugal force This invention relates to a method of producing fibres from mineral melt with the aid of centrifugal force. The method is especially directed to controlling vibration conditions on the fibre producing surface/s of a spinning wheel unit employed for producing fibres from mineral melt. The invention is described below in connection with a so-called cascade spinner, but it can also be used with all prior art defibration methods where the centrifugal force plays a determining role at the initial stage of the fibre generation. In a cascade spinner, the fibre generation takes place on the outer periphery of a spinning wheel. A cascade spinn¬ er normally consists of two to four spinning wheels, each forming a unit with a driving axle, devices for the supp¬ ly of air, cooling agent, binding agents and possible other additives. The spinning wheels are normally installed onto a movable carriage. Fibre generation takes place as follows: the mineral melt is supplied onto the peripheral surface of the first spinning wheel unit from which it is thrown in a drop cascade form onto the next spinning wheel located at a slight angle below and from there onto the peripheral surface of the next and subse¬ quent spinning wheels. The number of spinning wheels is usually four, but it may be more or less. The mineral melt fixes onto the peripheral surface of each spinning wheel in a ribbon form. Fibre embryos are thrown out of this melt ribbon and are drawn into fibres with the aid of jets of air. The fibres are collected onto a receiving conveyor and are formed into a mat. During the defibration of mineral melt, a balance pre¬ vails between the spinning wheel defibration surface, the layer of melt on the spinning wheel and the ambient gas. This state of equilibrium is determined by the conditions between the surface forces, the hydrodynamic fluid forces and the prevailing acceleration forces within the system. If the hydrodynamic, chiefly aerodynamic forces and ac¬ celeration forces exceed a certain value, the melt layer will break forming drops and fibres. An uncertainty factor at the generation of the said equi¬ librium is caused by the vibrations of the defibration element; i.e. the spinning wheel. These vibrations are difficult to control. The vibrations result in loss of control of the sequence of events turning melt into fibres. These vibrations are at least partly due to the rigid suspension arrangement of the spinning wheel giving rise to sideways forces which adversely affect the con- trol of the fibre generation process. The sideways forces affect the fibre embryo generation on the spinning wheel's peripheral surface as well as the circumstances prevailing just before the onset of favourable conditions for fibre generation. The said uncertainty factor caused by the spinning wheel suspension can be essentially eliminated by providing the spinning wheel with non-contact magnetic bearings. This enables the control of the spinning wheel's rotation movement and allows the rotation of the spinning wheel or wheels with the centre of mass as the rotation centre. This is due to the fact that the centre of mass is shift¬ ed from the centre of the wheel as a result of the un¬ steady distribution of material around the spinning wheel - one of the results of fibre generation process. This .imbalance cannot be eliminated when using mechanical spinning wheel bearings; instead, the vibration caused by the difference between the centre of mass and the rota¬ tion centre will continuously disturb the fibre gener- ation process. Uncontrolled vibration of a rotating spinning wheel gives rise to great acceleration forces directed at the melt fibre embryos. In extreme conditions, these forces either strengthen or repel the effect of the centrifugal force. The nominal value of these forces is almost as large as the centrifugal force. The centrifugal force is the main external force during the initial fibre generation stage. On the other hand, the vibration directed at the melt surface causes a wave phenomenon with the peaks serving as embryos for the fibres being generated. The main pur¬ pose of this invention has been to conceive a method for controlling this wave phenomenon. A primary precondition for the functioning is, however, that uncontrolled vibra¬ tions are entirely eliminated from the spinning wheel rotation movement. In this respect, the essential step is to control the spinning wheel's magnetic bearings into a vibrationless basic rotation state where the rotation centre is located in the center of mass. Once this is accomplished, it becomes possible to control the spinning wheel vibrations and use them for regulating the distri¬ bution of the fibre being generated. The aim in fibre generation is to accomplish a distribution of fibres with a dominating thickness of 2 - 6 μm. The intention is to avoid the generation of thinner and thicker fibres re- spectively. In accordance with the main principle of this invention, the fibre generation is controlled by accomplishing con¬ trolled vibrations in the spinning wheel. These vibra- tions produced in the spinning wheel may be directed radially or axially and can be caused alternatively or simultaneously.§ Once radial vibrations are accomplished, it becomes poss- ible to make use of the spinning wheel's said basic pro¬ perty; i.e. the difference between its constructive rota¬ tion centre and the center of mass. The rotation center can be intentionally displaced from the centre of mass, which naturally causes a spinning wheel vibration depend¬ ing on the rotation speed. The direction of the vibration amplitude maximum can be controlled around the wheel by regulating the direction of the change in the rotation centre. It is thus possible to direct the vibration am¬ plitude maximum towards an advantageous point on the spinning wheel's periphery; for instance by taking into account the confrontation zone of the melt being suppli- ed. With the help of the magnetic bearings, it becomes poss¬ ible to direct radial vibrations of essentially higher amplitudes onto the spinning wheel, in stead of or together with the said mass center - rotation center con¬ trol. This vibration has two different effects, depending on the vibration frequency. At lower frequencies of below 100 Hz, the melt route through the group of wheels is mainly affected and thereby also the amount of fibres and their properties. At higher frequencies of up to 500 Hz, the said melt surface wave phenomenon is affected in such a manner that the vibration is in resonance with an ap¬ propriate wave dimension or wave form causing the fibre producing process to start from the selected wave dimen- sion or wave form. The vibration amplitude is restricted to the magnetic bearing's degree of freedom; in practice this restriction is to approx. 1 mm. With the said con¬ trol method it is possible to influence the dimension and form distribution of the fibre being generated. It is possible/expedient also to direct this high frequency vibration purposefully in a certain direction in relation to the melt stream supplied. In order to accomplish the control, it is necessary to have information concerning the process variables. This is obtained from the detectors used for reading various process variables. The values obtained are compared with the pre-programmed default values, and the existing dif¬ ferences feed impulses to the bearings of the spinning axles for correcting the spinning wheel's rotation points making the values measured comply with the default values. The following is a description of an advantageous form of implementation for a spinning wheel unit in accordance with this invention, with reference to appended drawings in which Fig. 1 shows a front view of a spinning wheel unit with the melt ribbon on the peripheral surface of the spin¬ ning wheel, and the spinning wheel's geometric centre and the centre of mass marked Fig. 2 a) - c) shows the invention's control alterna¬ tive where high frequency radial vibrations are direct¬ ed onto the spinning wheel, Fig. 3 shows a schematic control system for a spinning machine in accordance with the invention including four spinning wheels (the control circuit for a possible fifth spinning wheel is drawn using dashed lines in the lower part of the figure) , and Fig. 4) shows a schematic control circuit for a spin¬ ning axle, Fig. 5 shows a schematic end view of a spinning wheel with a control system for the magnetic bearings pro¬ vided with the corresponding descriptions of the con¬ trol circuit components in Fig. 4. Figure 1 shows a cascade spinner with each of the spinn¬ ing axles suspended using at least one magnetic bearing. The thickness of the melt ribbon on the spinning wheel's peripheral surface has, however, been exaggerated. It mainly shows the correct proportion of the ribbon thick¬ ness at different points of the peripheral surfaces and on different spinning wheels. The spinning wheels' geo- metric centre has been indicated with rp. The spinning wheel's centre of mass has been indicated with mp. It is .easy to see that the varying thickness of the melt ribbon on the peripheral surfaces and the pulsing melt stream into the fibre generation device cause a displacement of the mass centre. By using magnetic bearings for axle sus¬ pension, the wheel's rotation axles are shifted from the geometric centre to the wheel's centre of mass. With this arrangement, steady and vibration-free rotation is achieved, which is the primary precondition for the method in accordance with this invention. Figure 2 a) shows an example of the bearing's radial con¬ trol value in the direction of an opposite pair of coils (e.g. 61, 61 in figure 5) where the bearing during an axle revolv actively prevents load changes from occurring and keeps the rotation centre within the system's centre of mass (by changing the bearing forces on opposite sides of the axle) . If the intention is to programmed change the fibre gener¬ ation conditions, it is necessary to consciously displace the axle of rotation from the mass centre. Figure 2 b) shows an example of a high frequency vibration intended to be directed to the axle in the direction of an oppo- site pair of bearings. Here, it is necessary to generate the control signal for one of the above axle's coil pairs in accordance with the figure 2 c) during the same revolv as shown in figure 2 a). In figure 2 the vibration ampli¬ tude on the y-axle is shown in mm. It is, of course, possible to cause corresponding (or other) displacements anywhere within the radial x-y level or in the direction of the axial z-axle, also at the same time. An example of the control system includes two main cat- egories of control; a control circuit for the spinning machine and separate control circuits for the spinning machine's spinning wheels. Fig. 3 shows a schematic control circuit for the spinning machine. The spinning machine's control device 31 is provided with continuous fibre generation variable data supplied to the system from the fibre process detectors. The fibre gener¬ ation variables indicate, among other things, the various properties of the melt, such as temperature, viscosity, surface tension, and other concrete fibre generation values from the end-product, etc. The control circuit supplies control information 33 with regard to tempera- ture, vibrations, positions, powers, etc. The spinning axle control circuits 32 also supply signals 35 to the spinning machine control circuit. These signals may indicate the load imposed by the melt cascade on the magnetic suspension, the power consumption of spindle drive motors, position data, etc. Figure 4 shows in schematic form the part of the spinning axle control circuits used for controlling the magnetic bearings which is thus of importance for this invention. The control circuit is drawn for an axle with two pairs of magnetic coils in each bearing. The magnetic bearing's control circuits are of prior art type and control the current fed to the magnetic coils 61 based on the signals received from position detectors 7 and 9. δ In the radial bearing the control circuit 21 is combined with a balancing circuit 22 used for relaying the rotor rotation onto the retardation axle which is major import¬ ance for the generation of fibre from mineral melt on the peripheral surface of a spinning wheel. The axle cooperates with the spinning machine's other axles via the spinning machine control circuit (fig.3) . Control circuit 32' constituting a part of the control circuit 32 in figure 3 has been inserted between the regulating circuit 21 and balancing circuit 22. Control circuit 32' prepares signals for the spinning axles con- tained in the magnetic suspensions. The circuit is in continuous contact with the spinning machine control sys¬ tem 31, figure 3, receiving parameter value data from it. The regulating circuit 32' can accept weakened output signals for certain magnetic coils, but can also supply the magnetic coil with control signals for active vibra¬ tion of the fibre generating element. This active vibra¬ tion can be executed, for example, by amplifying the vol¬ untary vibrations or by modifying the counter-acting fre¬ quency. The signals handled by the balancing circuit 22 must not, however, be totally ignored, as the material distribution on the spinning wheel would cause an imbal¬ ance jeopardizing the whole fibre generation process. Variations of rotor stiffness are actively used for vi¬ bration control via the amplifiers' frequency control, but rotor speed variations are not used to a very large extent, as the speed variations are primarily used for fibre generation instead of for seeking critical fre¬ quencies.

Claims: Patent claims: 1. A method of controlling fibre generation in the manu¬ facture of fibres from mineral melt with the aid of cen- trifugal force, in which method the mineral melt is sup¬ plied onto a spinning wheel provided with at least one controllable magnetic bearing, or a corresponding spinn¬ ing wheel group, from which it is thrown off in the form of fibres or fibre embryos which are drawn into fibres using a gas jet, characterized in that the fibre gener¬ ation is controlled by causing vibrations in the spinning wheel in a controllable manner. 2. A method as claimed in patent claim 1, characterized in that radial vibrations are caused in the spinning wheel by controlling the magnetic bearings of the spin¬ ning wheel. 3. A method as claimed in patent claim 2, characterized in that radial vibrations are caused in the spinning wheel, the vibrations being directed in relation to flow direction of the melt. 4. A method as claimed in patent claim 2 - 3, character- ized in that radial vibrations are caused in the spinning wheel by adjusting the distance between the centre of mass and the rotation centre of the spinning wheel. 5. A method as claimed in patent claim 2 - 4, character- ized in that in the spinning wheel is caused through the magnetic bearing a radial vibration whose frequency is decisively higher than the rotation vibration frequency. 6. A method as claimed in any of the patent claims 1 - 5 characterized in that an axial vibration is caused in the spinning wheel.

Inventors: AASTRAND ERIK AXEL, HOIKKA TIMO OLAVI, AASTRAND, ERIK, AXEL, HOIKKA, TIMO, OLAVI
Applicants: PAROC OY AB
Application year: 1991
Main classification: C03B37
IPC codes: C03B37