Method of maintaining a constant predetermined contact pressure on elements for cutting under water in granulator machines and relevant machine with control and actuation device for said pressure

Abstract: Method of maintaining a constant, predetermined contact pressure on elements for cutting under water in granulator machines, comprising the following phases: bringing the blades (7) of the cutters into bearing against the die (3), applying to the blades a predetermined thrust force of constant value towards the die, setting the drive shaft (5) in rotation, detecting (109), moment by moment, the value of the hydrodynamic thrust force generated by the rotation of the cutters in the water and acting on these cutters towards the die, applying to the blades of the cutters a force (14, 15) in the opposite direction to said hydrodynamic thrust force and of a value equal to the value of same, and granulator machine for cutting under water, adapted for the application and control of a constant pressure according to the above-described method, comprising actuator means with associated feed adapted for applying axial thrusts to the drive shaft for the cutters within a slidable bushing, means for detecting the hydrodynamic thrust force acting on the cutters and regulator means adapted for regulating the force supplied to said actuators.

Description: The subject of the present invention is an apparatus and a method of controlling and keeping constant at a predetermined value the contact pressure of cutting elements against a die in granulator machines for cutting under water, and the relevant control and actuation device for said pressure. Document US-A-4,529,370 discloses an apparatus according to the preamble of claim 6. There are also known in the art machines having granulating heads for cutting under water, composed basically of a chamber closed on one side by a die and traversed axially by a revolving shaft, to which are firmly secured cutting elements composed basically of shaped blades which, brought into contact with the die and set in rotation by the shaft, carry out the cutting of the material. To this chamber, the plastics material to be granulated, which is in the softened state, is fed through the die; at the same time a strong current of water is caused to flow inside said sealed chamber, in a direction transverse to the axis of the shaft and of the die, said flow of water has the dual purpose of cooling the plastics material leaving the die to make it suitable for cutting into granules and, once the cutting has been carried out, of removing the granulated material from this chamber by bringing it into the next processing zone. Because the cutting of the material must be carried out in contact with the die and said contact must be the most uniform and constant possible, it is usual, in the present state of the art, to subject the cutters to a pre-loaded thrust adapted for achieving said contact, said thrust being applied by various means, after the correct positioning of the cutters relative to the die has been achieved by means of suitable mechanical sliding and stopping means. All the efforts of the prior art have, however, been directed towards assuring said thrust against the die even, for example, by introducing a compression spring against the die itself. It has, however, been found that the cutters when set in rotation and immersed in the water generate a hydrodynamic force substantially proportional to the square of the speed of rotation of the cutters, to the shape of the cutters themselves and to their number. Said hydrodynamic force has two components: a first component parallel to the die and responsible for the resistant torque on the cutters, and a second component perpendicular to the die, which induces a reaction force of the water itself, tending to exert a thrust on the blades towards the die, greater by about one order of magnitude than the thrust applied by the aforementioned prestressed spring. Because the quality of cutting, the wear of the blades and, above all, the wear of the die, which is the most expensive component, are substantially dependent upon the contact force between the blades and the die, the technical problem arises of balancing, from one moment to the next, said axial component of the hydrodynamic force for the purpose of avoiding deterioration of the quality of cut and excessive wear of the die, by keeping applied only the constant prestressing force which, for some applications, could even be zero. Said results are achieved by the present invention, which provides a method of maintaining a predetermined constant contact pressure on elements for cutting under water in granulator machines constituted of a water chamber, on the walls of which there are present two axial apertures, respectively for seating a die for feed of the material to be granulated and for seating a drive shaft integral, at its end inside the chamber, with the element carrying the cutters and, at its end outside the chamber, with actuator means adapted for actuating the drive shaft in the axial direction, comprising the following phases: bringing the blades of the cutters into bearing against the die, applying to the blades a predetermined thrust force of constant value towards the die, setting the drive shaft in rotation, detecting, instant by instant, the value of the hydrodynamic thrust force generated by the rotation of the cutters in the water and acting upon said cutters towards the die, applying to the blades of the cutters a force in the opposite direction to said hydrodynamic thrust force and having a value equal to the value of same, in this way obtaining a total force acting on the die equal only to the constant value of the prestress force. Said constant prestress force should possess such a value as to absorb any possible oscillations in the hydrodynamic force around the mean operating value and acting in a direction away from the die. In the method according to this invention, the instantaneous value of the hydrodynamic force may be obtained indirectly on the basis of the variable element constituted of the rotational speed of the cutters or upon the basis of the power consumption of the motor driving the shaft of the cutters. Alternatively, the instantaneous value of the hydrodynamic force may be obtained directly by the interposition of appropriate measuring means between the fixed element and the movable element, opposite each other in the axial direction, of the support and control structure for the cutters unit. The method according to this invention is carried out by a granulator machine for cutting under water, equipped with control and actuation means for a constant pressure on blades of cutters in contact with a die, said cutters being rotationally driven by a drive shaft supported by a first slidable bushing inside a second, fixed bushing and comprising actuator means with relevant feed, these means being adapted for applying axial thrusts to said slidable bushing, means for detecting the hydrodynamic thrust force acting on the cutters, and regulator means adapted for regulating the force supplied to said actuators. Said actuators are preferably composed of a pair of double-acting fluid cylinders, preferably supplied from a motor acting on a pair of pumps or the like. In the machine with a control and actuation device according to this invention, said regulator means are constituted of a pressure regulator which can be adjusted for supplying to a first chamber of said cylinders the pressure corresponding to the prestress force, and of a controllable pressure regulator for supplying, instant by instant, to the second chamber of the cylinders the pressure corresponding to the balancing force for the hydrodynamic thrust. Said detection means are preferably constituted of an electronic circuit adapted in known manner for detecting the rotational speed of the drive shaft and for processing this information by converting it into an electrical signal corresponding to the value of the present hydrodynamic thrust force by carrying out an indirect measurement of the hydrodynamic thrust force or, alternatively, said detection means are constituted of load cells interposed between said fixed shell and said sliding shell which, by directly measuring the hydrodynamic force acting on the cutters, supply at their output an electrical signal adapted to be fed to said control circuit. In the machine having a control and actuation device, it is also provided that said controllable pressure regulator shall receive, at its input, both a constant pressure derived from said feed pump and, instant by instant and by means of a digital/analogue converter, a control signal coming from said detection and comparison means and shall supply, at its output, the second chamber of said cylinders with a pressure corresponding to the force for balancing said present hydrodynamic thrust. Further details can be found from the following description of the method and of an example of application, prepared with reference to the attached drawings, in which there are shown: in Figure 1, a partial section through the granulator machine and, in Figure 2, a diagrammatic example of a control circuit. As Figure 1 shows, a granulator machine 1 is composed basically of a water chamber 2, on which are realized two axial, opposite apertures, respectively closed by die 3 supplied with softened plastics material through an annular pipe 3a disposed outside said chamber and by a lid 4 designed for supporting a drive shaft 5, integrally connected at one end 5a, inside the chamber 2, with a rotor 6, to which are rigidly secured the cutters 7, variable in number as required and, at the other end 5b, outside the chamber, to a hydraulic motor 8 - supplied and driven by an associated circuit shown diagrammatically by the block 108 in Figure 2 - which supplies the rotary movement to said shaft. On said chamber there are also formed apertures 9 for inlet and outlet of the water, disposed in such a way as to cause said water to execute a rotary motion similar to that of the cutters 7, but in a spiral, which causes the water to leave in a direction perpendicular to the cutting direction. In greater detail, the drive shaft 5 is housed in a bushing 11, slidable along a fixed bushing 12, the latter integral with the water chamber 2, on which fixed bushing said sliding bushing 11 can slide, being itself rigidly attached, at its end outside the chamber, to a transverse flange 13 which also supports the motor 8. The method of controlling the thrust pressure of the cutters 7 against the die 3 according to this invention consists, essentially, in the phases of bringing the blades of the cutters 7 into contact with the die 3, of applying an axial thrust force having a predetermined constant value and orientated towards the die, to the shaft carrying the cutters 5, of detecting, also by means of different components associated therewith by relationships of analytical type, the hydrodynamic force produced by the effect of the rotation of the cutters 7 immersed in the water, and of balancing, with an equal and opposite axial thrust, the entire hydrodynamic force in such a manner as to keep only the prestress force, necessary for the maintenance of a correct contact between blades and die, even in the presence of possible turbulence of the water flow which could cause variations in the hydrodynamic force itself even in operation. Said method may be carried out by means of a control and actuation device, an example of embodiment of which will be found in the following description. Said device is composed, essentially, of two double-acting cylinders 14 and 15, rigidly connected at one of their ends to a flange 12a of the fixed shell or bushing 12 and at the other end to the transverse flange 13, in turn rigidly connected to the sliding bushing or shell 11, and of a hydraulic circuit for controlling and supplying the pressure to said cylinders. With reference to the diagram of Figure 2, the hydraulic control and actuation circuit is composed of a motor 100, which drives a first pump 101 and a second pump 102. The pump 101 generates a pressure which, set by means of the pressure regulator 103, fixes the value P₁ of the prestress pressure, programmed for the specific operating conditions, to be fed by means of the valve 104 to the chambers 14a and 15a of the cylinders 14 and 15, causing actuation of the pistons in the direction of thrust against the die 3 and in this way producing the prestress of the cutters 7 against the die 3. The pump 102, in contrast, generates a much higher pressure, which is fed to the controlled regulator 105. As the hydraulic motor 8 starts up, which sets in rotation the shaft 5, the electronic circuit comprising microprocessor 106 is also activated, so as to detect, instant by instant, the rotational speed of the shaft 5 and, on the basis of a preset programme based, for example, on a known analytical function, to calculate the value of the corresponding hydrodynamic force, which acts axially on the cutters. Said instantaneous value of the hydrodynamic force having been detected, a corresponding electrical signal is emitted, which is fed by a digital/analogue circuit 107 to the regulator 105, which supplies a corresponding pressure P₂ to the other chamber 14b and 15b of the cylinders 14 and 15, in such a manner as to exert upon the piston an action in the opposite direction to that of the hydrodynamic thrust, thus producing on the shaft a balancing of the undesired hydrodynamic thrust and the maintaining of the constant prestress pressure P₁. Alternatively, the detection of the hydrodynamic force may be carried out by means of a direct detection by the use of detection devices of the type, for example, of load cells 109 interposed between the fixed shell or bushing 12 and the transverse flange 13. Said load cells emit an electrical signal which is a function of the compression to which they are subjected, which, when fed to the processing circuit 106, enables the desired control of the hydrodynamic force to be carried out in the manner already described. Another possible variant could comprise the use of an electric motor for driving the cutters, and in this case the detection of the value of the hydrodynamic force could be carried out, again indirectly, on the basis of the power consumption of the motor. Numerous variants can be introduced to the device according to the present invention, without thereby departing from the scope of the claims.

Claims: Method of maintaining a constant predetermined contact pressure on elements for cutting under water in granulator machines which are composed of a water chamber, on the walls of which there are present two axial apertures, respectively for seating a feed die for the material to be granulated and for a drive shaft integral, at its end inside the chamber, with the element carrying cutters and, at its external end, with actuator means adapted for actuating the drive shaft in the axial direction, comprising the following phases: bringing the blades of the cutters into bearing against the die, applying, to the blades, a predetermined thrust force of constant value towards the die, setting the drive shaft in rotation, detecting, instant by instant, the value of the hydrodynamic thrust force generated by the rotation of the cutters in the water and acting on said cutters towards the die, applying, to the blades of the cutters, a force in the opposite direction to said hydrodynamic thrust force and of equal value to same, in this way obtaining a total force acting on the die equal only to the constant value of the prestress force. Method according to Claim 1, characterized by the fact that said constant prestress force must absorb also any oscillations in the value of the hydrodynamic force about the mean operating value in a direction away from the die. Method according to Claim 1, characterized by the fact that the instantaneous value of the hydrodynamic force can be obtained indirectly on the basis of the variable element constituted of the rotational speed of the cutters. Method according to Claim 1, characterized by the fact that the instantaneous value of the hydrodynamic force can be obtained indirectly on the basis of the power consumption of the motor driving the shaft for the cutters. Method according to Claim 1, characterized by the fact that the instantaneous value of the hydrodynamic force can be obtained directly by the interposition of appropriate detection means, incorporated between the fixed element and the movable element, opposed to each other in the axial direction, of the support and control structure of the cutters assembly. Granulator machine for cutting under water with means for actuating and controlling a constant pressure on cutter blades in contact with a die according to the method of the preceding Claims, said cutters being rotationally driven by a drive shaft supported by a first slidable bushing inside a second fixed bushing, characterized in that said actuator means are associated with feed adapted for applying axial thrusts to said slidable bushing, with means for detecting the hydrodynamic thrust force acting on the cutters, and with regulator means adapted for regulating the force supplied to said actuators. Machine comprising control and actuation means according to Claim 6, characterized in that said actuators are constituted of a pair of double-acting, fluid-operated cylinders. Machine comprising control and actuating device according to Claim 6, characterized in that said feed means are preferably composed of a motor acting on a pair of pumps or the equivalent. Machine comprising control and actuation device according to Claim 6, characterized by the fact that said regulator means are constituted of a settable pressure regulator for supplying, to a first chamber of said cylinders, the pressure corresponding to the prestress force and of a controllable pressure regulator for supplying, instant by instant, to the second chamber of said cylinders, the pressure corresponding to the force for balancing the hydrodynamic thrust. Machine comprising control and actuation device according to Claim 6, characterized in that said detection means are preferably constituted of an electronic circuit adapted in known manner for detecting the rotational speed of the drive shaft and for processing said information by converting it into an electrical signal corresponding to the value of the present hydrodynamic thrust force. Machine comprising control and actuation device according to Claim 6, characterized in that said detection means are preferably composed of load cells interposed between said fixed bushing and slidable bushing which, by directly measuring the hydrodynamic force acting on the cutters, supplies at the output an electrical signal suitable to be fed to said control circuit. Machine comprising control and actuation device according to Claims 6, 9 and 10 or 11, characterized in that said controllable pressure regulator receives, at its input, both a constant pressure derived from said feed pump and, instant by instant and by means of a digital/analogue converter, a control signal coming from said detection and comparison means, and supplies, at its output, the second chamber of said cylinders with a pressure corresponding to the force for balancing the present hydrodynamic thrust.

Inventors: GUGGIARI ANDREA, GUGGIARI, ANDREA
Applicants: POMINI SPA, POMINI S.P.A.
Application year: 1990
Main classification: B29B9
IPC codes: B01J2, B29B9