Patent Application
20240123645
The invention relates to a plastic strand granulator for pelletizing plastic strands and in this context in particular an adjustment mechanism for adjusting the width of a cutting gap between a cutting rotor and a knife strip arranged at a small distance therefrom.
Plastic strand granulators are employed in plastic strand granulating installations, in which molten plastic strands are extruded from nozzles and fed to a water bath, in which they cool down and acquire the necessary strength in order to subsequently be pelletized into granules in the plastic strand granulator. The water bath can be formed as a trough or, as in EP 0452 376 B1, as a guiding device in the form of a—frequently inclined—water-flushed channel. There are many different variants in which the plastic strands are cooled and brought to the desired granulation temperature from the point at which they exit the extruder nozzle to the point at which they enter the granulator. The granulator has a housing with a feed opening for feeding the plastic strands to be granulated and an outlet opening for removing the plastic granules. Inside the housing there is located a cylindrical cutting rotor as a rotating knife arrangement with cutting knives evenly distributed over the circumference and a fixed knife strip running parallel to the rotational axis of the cutting rotor, on which the plastic strands are granulated into granules by means of the cutting rotor. Due to the force of gravity, the granules fall out of the granulator housing through the outlet opening at the bottom.
There is a cutting gap between the cutting rotor and the knife strip, which in order to achieve a desired granulate quality must be set as precisely as possible and the width of which must be readjusted, in particular during operation. It is known, for example, from U.S. Pat. No. 2,782,853 to mount the knife strip on a pivotable knife holder and to set the cutting gap width via the pivoting position of the knife holder. A control circuit is used in which a button acting on a surface of the rotating cutting rotor records a temperature-related change in the cutting rotor diameter, for example, and sends a corresponding signal to a controller, by means of which the pivoting position of the knife holder is changed in such a manner that the distance between the cutting rotor and the knife strip is restored by turning a gauging screw forwards or backwards accordingly. The gauging screw acts on one end of the knife holder, which is disposed opposite to the knife strip with reference to the pivot axis of the knife holder. Overall, two mutually spaced-apart set screws act on the knife holder in the manner described, so that the parallelism between the knife strip and the rotational axis of the cutting rotor can also be set by turning one or the other set screw forwards or backwards accordingly.
The object of the present invention is to optimize the adjustment mechanism for setting the cutting gap width.
According to a first aspect, the adjustment mechanism comprises at least one actuator with differential thread to adjust the position of the knife strip accordingly. A differential thread usually has a reducing transmission ratio.
A differential thread is characterized by two intertwined threads with different pitches. Differential threads are known from differential thread screws, which consist of three coaxial parts, of which the inner part has a first-pitch external thread, the outer part has a second-pitch external thread, and the central part has a first-pitch internal thread and a second-pitch external thread. With a suitable formation, the inner part can be displaced axially relative to the outer part (or alternatively the outer part relative to the inner part) by turning the middle part, without the axially displaced part rotating in the process. The axial displacement of the aforesaid central part within the aforesaid outer (or inner) part by turning the central part by a relatively large angle of rotation only leads to a small axial displacement of the aforesaid inner (or outer) part, wherein the transmission ratio is all the lower, the smaller the difference between the two slopes is. By means of the differential thread with reducing transmission ratio therefore an even better fine gauging can be achieved than with, for example, conventional fine-thread screws. On the contrary, normal threads with correspondingly higher strength can be employed in the differential thread. In particular, due to the transmission ratio, in addition only small driving forces are required, so that even manual driving without tools is possible.
The differential thread can have a transmission ratio between 1:2 and 1:20, preferably about 1:5, which corresponds to a ratio between the first and the second pitch of 1:1.2.
Advantageously, the adjustment mechanism can have a pivot axis around which the knife strip is pivotably mounted, wherein a pivoting position of the knife strip can be set by means of the actuator, as is fundamentally the case in U.S. Pat. No. 2,782,853 mentioned at the outset. For example, the knife strip can be mounted accordingly on a pivotably mounted knife holder. Insofar as the knife strip on the one hand and a point of action of the actuator on the knife holder holding the knife strip on the other hand are arranged at different distances from the pivot axis, an additional leverage effect must be taken into account with reference to the displacement of the knife strip relative to the axial adjustment of the actuator. Advantageously, the actuator acts on the pivotable knife holder at a suitable point between the pivot axis and the knife strip, specifically as close as possible to the knife strip. If the point of action of the actuator is in the middle between the knife strip and the pivot axis (and the actuator acts on the knife holder in a direction roughly tangential to the pivot axis), the transmission ratio deteriorates by a factor of 2. Of course, it is most expedient if the actuator point of action is even further spaced apart from the pivot axis than the knife strip, wherein this can be difficult to implement for reasons of space.
The actuator can preferably be actuated from outside of the housing in which the cutting rotor is arranged. In particular, due to the particular reducing transmission ratio, as mentioned, the actuator is easy to actuate manually.
The gauging accuracy of the adjustment mechanism can be ensured by executing the differential thread of the actuator in play-free manner, in particular by means of a spring bias, e.g. by means of plate springs.
Similar to the aforementioned U.S. Pat. No. 2,782,853, the adjustment mechanism can have two such actuators with differential threads, which are arranged parallel to one another at the largest mutual distance next to one another. In order to save space, the actuators can be arranged within the granulator housing outer walls such that they move forwards and backwards therein parallel to the outer sides of the walls.
Alternatively, the adjustment mechanism can have only one actuator with differential thread and parallel thereto a sliding guide, wherein the knife strip is aligned transversely to the sliding guide and the adjustment mechanism is formed so as to slide along the sliding guide. The position setting of the knife strip then takes place via only one actuator. If the knife holder on which the knife strip is mounted is formed accordingly to be torsion-resistant and the sliding guide is executed very precisely without significant play, then one actuator is sufficient for setting the cutting gap width. However, this also means that this advantage occurs regardless of whether the actuator is formed with a differential thread with a reducing transmission ratio or is a different type of actuator, for example an actuator with a fine thread. This aspect of the invention is therefore considered to be inventive in its own right. In particular, the parallelism relative to the cutting rotor axis does not have to be set via two mutually spaced-apart actuators, but rather the parallelism to the cutting rotor can be gauged, for example, by accordingly adjusting the knife strip on the knife holder.
Nevertheless, it is advantageous to provide a locking mechanism by means of which the position of the knife strip can be fixed relative to the sliding guide, so that the torsion resistance and/or a play-free sliding guide is not of such decisive importance.
The locking mechanism can comprise a clamping sleeve to fix the knife strip. In particular, the clamping sleeve can be a slotted clamping sleeve, in which a slide pin is shiftably guided. By compressing the slotted clamping sleeve, the slide pin is fixed in the clamping sleeve and thus the knife strip is fixed relative to the clamping sleeve.
According to a second aspect, the adjustment mechanism is integrated into a control circuit, by means of which the cutting gap can be automatically gauged to a predetermined width. Such a control circuit can advantageously have a vibration sensor to record vibrations on the plastic strand granulator, as well as an evaluation device for extracting one or several parameters characteristic of the gap width from the vibrations recorded by means of the vibration sensor, wherein the control circuit furthermore comprises a control device, by means of which the adjustment mechanism is gauged in dependence on the extracted vibration parameters so that the predetermined cutting gap width is set or maintained.
For example, reference parameters obtained in field tests can be stored in a memory and compared by the evaluation device with the vibration parameters extracted from the recorded vibrations in order to produce, based on this comparison, an input value for the control device for changing the cutting gap width. The extracted vibration parameters can comprise at least one vibration amplitude and/or at least one vibration frequency.
The vibrations recorded by the vibration sensor are preferably structure-borne sound waves and/or surface waves that occur on a component of the plastic strand granulator. It is particularly preferred in this context if the structure-borne sound waves or surface waves are introduced to or into the relevant component of the plastic strand granulator by means of a transmitter provided for this purpose, and are correspondingly detected by means of a receiver spaced apart from the transmitter. More than one transmitter and/or more than one receiver can also be provided. In particular, the transmitter can be an ultrasonic transmitter, and the receiver receives the signal from the ultrasonic transmitter. Such sensors are offered by Best Sense AG, for example. Alternatively, the transmitter and receiver can be provided at the same location, wherein the receiver then detects reflected waves from the transmitter.
According to a first variant, at least one transmitter and at least one receiver are arranged in or on a bearing of the cutting rotor. According to a second variant, alternatively or additionally, at least one transmitter and at least one receiver can be arranged in or on the knife strip or a bearing of the knife strip, thus in particular in or on the knife holder. With reference to the cutting gap width, vibration parameters that are particularly meaningful could be achieved by means of an ultrasonic transmitter and a receiver spaced apart therefrom, which are arranged in a bearing of the cutting rotor.
In the following the invention is described by way of example with reference to the accompanying figures. The figures are described as follows:
The rotational axis 6 of the cutting rotor 5 and the pivot axis 4 of the knife holder 2 are aligned parallel to one another, and the knife strip 3 is gauged on the knife holder 2 in such a manner that it is likewise aligned parallel thereto, so that the cutting gap is constant over the entire length of the knife strip 3. In order to now set the cutting gap width between the knife strip 3 and the cutting rotor 5, according to a first embodiment example, two actuators 7 act on the pivotable knife holder 2. These are linear actuators. The actuator or actuators 7 are flanged onto the granulator housing 1 from the outside so that they can be actuated manually. For reasons of space and to implement a granulator housing that is as narrow as possible, the setting pistons 8 of the actuators 7 are accommodated in the side wall parts 1A, 1B, wherein only one of the actuators 7 is recognizable in
When the setting pistons 8 of the two linear actuators 7 are now moved forwards or backwards, the knife holder 2 with the knife strip 3 mounted thereon pivots about the pivot axis 4 away from the cutting rotor 5 or towards the cutting rotor 5, as a result of which the width of the cutting gap between the cutting rotor 5 and the knife strip 3 can be set accordingly. Due to a special configuration of the actuator 7, as will be explained in more detail later, this can take place manually in a simple manner without any tools, but preferably takes place automatically at least during ongoing operation via a corresponding control device that keeps the cutting gap width constant at a predetermined value.
Since it is not easy to ensure simultaneous, precise gauging of the two actuators 7, a second embodiment example provides that only one of the two actuators 7 is present, whereas the setting piston 8 on the opposite side is not part of an actuator, but is only guided in a sliding guide in parallel to the first actuator. Due to the torsion resistance of the knife holder 2 and with a correspondingly play-free mounting of the pivot axis 4 of the knife holder 2 in the granulator housing 1, the knife strip 3 can be moved relative to the rotational axis 6 of the cutting rotor 5 without the parallelism of the knife strip 3 relative to the rotational axis 6 being lost. The setting piston 8 on the side opposite the actuator 7 is only released before the displacement of the knife strip 3 and subsequently fixed again, so that the knife strip 3 is firmly clamped on both sides for ongoing operation. On the other hand, the embodiment variant with two actuators 7 is better suited for an adjustment during ongoing operation.
A rotation of the movable component 13 accordingly results in the construction element 17 moving in the stationary component 14, for example by an amount X in one direction. If the internal thread of construction element 17 and the external thread of internally disposed component 12 had the same pitch, the construction element 17 would only move forwards and backwards between the stationary component 14 and the internally disposed component 12. However, since the two pitches differ slightly from one another, for example by a factor of 1.2, the displacement of the construction element 17 by the amount X within the stationary component 14 results in the movable component 12 also being displaced axially, but only by a correspondingly lesser amount of 0.2×. In other words, the differential thread then has a reducing transmission ratio of 1:5. Preferably, the reducing transmission ratio is between 1:2 and 1:20.
According to a preferred embodiment, the pitch of the external thread of the construction element 17 is 1 mm per revolution, and the pitch of the internal thread of the construction element 17 is 1.2 mm per revolution, which, due to the effect described above, has the result that the setting piston 8 is not shifted axially by 1 mm, but by only 0.2 mm by one revolution of the movable component 13. In order to axially displace the setting piston 8 by 0.05 mm, all that is required is thus a rotation of the movable component 13 by 90°.
However, an axial displacement of the setting piston 8 by 0.05 mm does not result in the cutting gap width also changing by 0.05 mm. Rather, the extent of the displacement of the knife strip 3 depends on the point at which the setting piston 8 acts on the knife holder 2. If the point of action is roughly in the middle between the knife strip 3 and of the pivot axis 4 of the knife holder 3 and the setting piston 8 acts at this point roughly tangentially relative to the pivot axis 4, this results in a leverage effect with a transmission ratio of 2:1. In other words, a rotation of the movable component 13 by 90° does not lead to a displacement of the knife strip 3 of 0.05 mm, corresponding to the axial displacement of the setting piston 8, but to a displacement of 0.1 mm, which is still a very acceptable result for an accurately gauged positioning of the knife strip 3. The closer the shaft 9, on which the setting piston 8 acts, is to the knife strip 3, the more favorable this affects the overall transmission ratio of the adjustment mechanism.
It is of course even more favorable if the knife strip 3 is located radially between the pivot axis 4 of the knife holder 2 and the shaft 9 on which the setting piston 8 acts. In the embodiment example represented, however, this is prevented by the mounting of the cutting rotor 5 in the granulator housing 1. Alternatively, the actuator 7 can therefore, for example, be arranged with its setting piston 8 from the correspondingly opposite side in the side wall parts 1A, 1B, so that a collision with the cutting rotor mount is avoided.
On the outside of the movable component 13 a scaling is applied which translates the rotation of the component 13 into an axial displacement of the knife strip 3 relative to the cutting rotor 5. Furthermore, inside the actuator 7 several axially stacked plate springs 18 are provided which ensure that there is always axial play in the differential thread on a predetermined side.
As previously mentioned, instead of the second actuator 7, a sliding guide can be provided for the setting piston 8, particularly if the actuator 7 is only to be adjusted manually. The sliding guide should of course be fixable for the operating state. Such a fixable sliding guide 19 is explained in more detail below with reference to
For the automatic adjustment of the knife strip 3 relative to the cutting rotor 5 during ongoing operation, a special control device is provided which detects and evaluates characteristic operating states of the granulator via vibration sensors and, based thereon, effects an automatic adjustment of the knife strip 3. In the concrete embodiment example according to
By storing different reference tables for the most diverse granulator statuses (depending on the granulated material, number of granulated plastic strands, strand thickness, rotor rotational speed, cutting gap width, rotor sharpness (sharp/blunt)) conclusions can be drawn accordingly about the respective granulator status, such as the cutting gap width.
A corresponding method for operating the plastic strand granulator thus provides for the adjustment of the cutting gap width by displacing the position of the knife strip relative to the cutting rotor, wherein vibrations, for example in the bearing shell of the cutting rotor 5 or alternatively (or additionally) on the knife strip 3 or on the knife holder 2, are recorded, from which one or several vibration parameters are extracted, such as, for example, the height of the lower amplitudes according to
Specifically, the method can have the course as represented in
| Number | Date | Country | Kind |
|---|---|---|---|
| 10 2019 127 762.0 | Oct 2019 | DE | national |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/EP2020/078482 | 10/9/2020 | WO |
