The invention relates to a strand shaping part for connecting an extruder to a granulating system, preferably for thermoplastic material, having at least one supply channel that can be flow-connected to the extruder for the melt, which leads to a gear pump with which the flow of melt to a channel that leads to a strand die can be blocked during start up.
Furthermore, the invention relates to a method for starting a granulating system provided with a strand shaping part of this type.
Granulating systems offer the advantage of being able to granulate very low viscous materials, in particular thermoplastic materials, preferably for recycling purposes. In an underwater granulating system, the melt delivered by the extruder directly reaches the granulator housing through which water flows. The cutting tools which pass the perforated plate and thereby divide the strands coming out of the holes of the perforated plate which form the dies into granulated particles also run underwater. On the other hand, in a conventional strand granulating system, the strands coming out of the strand dies are left in a water bath in which the melt solidifies, after which the strands are removed from the water bath and conveyed to the granulating blades.
To be able to start systems of this type, the melt must be promptly supplied to the dies to prevent individual dies from freezing as, in particular in underwater granulating systems in which the perforated plate is in contact with water, the melt cools off quickly. To ensure the aforementioned prompt supply in a strand shaping part having the known construction described above, the part carrying out the blocking is formed by a valve which is opened as soon as the extruder is brought to a full output. Until then, the melt material conveyed by the extruder is, discharged via a lateral outlet, primarily onto the ground. Aside from the noted loss in material, the disadvantage is the fact that irregular throughputs occur at the dies or the holes of the perforated plate, which results in irregular properties of the granulated material produced.
A granulating system is known from U.S. Pat. No. 5,723,082 in which a slide gate is used to block the flow of melt to the dies. This slide gate can also be omitted, in which case the gear pump has to assume the function of the slide gate. The flow of melt to the dies can also be blocked by switching off the gear pump in a construction according to EP 0 894 594 A2, also in a construction according to DE 101 17 913 A1. However, in all of these known constructions, the disadvantage of irregular throughputs on the dies or holes of the perforated plate can also not be satisfactorily overcome.
The object of the invention is to avoid these disadvantages and to improve a construction of the aforementioned type in such a way that at least an almost uniform throughput is ensured at all dies. The invention solves this object in that several gear pumps which each have the same delivery behaviour are driven by a common shaft, wherein a channel from each gear pump leads to a strand die. Therefore, each gear pump is connected by a single channel with the strand die allocated to it. In this way, it is ensured that each gear pump doses the flow of melt to the die allocated to it. Since the gear pumps each have the same delivery behaviour and are driven by the common shaft, this means that each strand die supplied by the respective gear pump is uniformly supplied with a volume of melt per unit of time. Thus, uniform conditions set in for all holes or strand dies, so that a uniform quality of the granulated material produced is ensured. The dimensions of the granulated material are then in a very narrow tolerance zone.
Furthermore, the advantage is offered that, in addition, every gear pump mixes the melt supplied to it by the extruder, so that a further uniformity of the quality of the melt is obtained.
Finally, the advantage is given that, when the gear pump is at a standstill, a preliminary pressure can be built up at the entrance of the gear pump by means of the extruder. If the gear pump is not rotationally driven until said preliminary pressure has been reached, the full flow of mass sets in immediately in the respective channel leading to the die, as a result of which it is prevented that these channels freeze.
To safeguard the advantage that no varying conditions occur at the die outlets, it is advantageous if, within the scope of an embodiment of the invention, each gear pump of the strand die in question is arranged so as to be adjacent, so that the length of the channels leading from the gear pump outlets to the dies is short. In contrast thereto, for the most part, there are large channel lengths between the shutoff valve or the gear pump and the strand dies or the holes of the perforated plate in the known constructions, which results in the noted difficulties. This channel length, which is kept short in accordance with the invention, can be easily realized structurally in that the strand dies are provided on a die carrier which forms a housing for the gear pump. In this case, it is not necessary to form the strand dies from separate components, of course, the die carrier can also be a perforated plate head, whereby the strand dies are formed by holes of a perforated plate fastened to it. Moreover, the use of gear pumps offers the advantage that it is possible to be able to set, optionally, the volume flow by changing the drive of the gear pumps, perhaps differently for individual pumps of several gear pumps, so that melt is discharged uniformly everywhere from the holes.
The method according to the invention for starting a granulating system which is provided with a strand shaping part according to the invention and supplied with melt by an extruder, is characterized in that the extruder is first driven until a sufficient pressure is built up at the entrance of each gear pump, and that each gear pump is not driven from its thusfar stillstand state until this pressure has been attained, whereby, in an underwater granulating system, the granulating blades are rotationally driven last. This method ensures the aforementioned avoidance of the melt freezing in the channels leading to the strand dies or holes of the perforated plate and ensures the uniform quality of the granulated material cut from the individual strands.
Examples of embodiments of a strand shaping part according to the invention are schematically illustrated in the drawings.
The strand granulating system shown in
To improve the starting conditions and to ensure uniform quality properties of the granulated particles cut off, each supply channel 8 leads to a gear pump 21 situated in the die carrier 30, said gear pump having a constant delivery behaviour for the material conveyed to it for each specific number of revolutions. The embodiment shown has four gear pumps 21 and, thus, four supply channels 8 and four channels 11. The two gears 22 of each gear pump 21 are rotationally driven in direction of arrow 23. For this purpose, each gear 22 of each gear pump 21 is mounted so as to be secured against rotation on a shaft 24 (
A single channel 11 is connected to the outlet 26 of each gear pump 21, as
The outlet of each strand die 9 is directed downward, so that the extruded melt strand 31 coming out of it falls into a strand cooling bath 32 which is filled with water 33. The solidified strands 31 are again removed from this strand cooling bath 32 and conveyed to a granulator 34 (only shown schematically) from which the cut granulated particles fall downward in direction of arrow 35.
As
As
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Each melt channel 8 leads to several gear pumps 21 which are arranged in the perforated plate head 7, each gear pump having a constant delivery behaviour for the material supplied to it for each specific number of rotations. The embodiment shown has four gear pumps 21 which are driven in a similar manner and mounted on drive shafts 24, as was described in association with the embodiment according to
A single channel 11 leads from the outlet 26 of each gear pump 21 to a strand die 9 formed by a hole on the perforated plate 10 (
When starting a system of this type, whether it be an underwater granulating system or a strand granulating system, one proceeds in such a way that the gear pumps 21 are first stopped and, consequently, the material flow between the extruder 1 and the dies 9 is completely blocked. The extruder 1 is then started and conveys the plastic material to be processed into the inlet 6 of the die carrier 30 which simultaneously forms a housing for the gear pumps 21. As a result, a pressure in the plastic material supplied builds up at the inlet 27 of each gear pump 21. As soon as this pressure has reached a preset value, which can be monitored by pressure sensors (not shown), all gear pumps 21 are promptly started and thus convey the plastic material supplied to them with constant delivery behaviour into the channels 11 to the dies/dies 9.
Throughput capacities of 20 to 60 kg per hour and per hole of the perforated plate can be obtained with a granulating system designed according to the invention, which signifies an increase of the maximum throughput capacity in comparison to known constructions.
Number | Date | Country | Kind |
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A 78/2005 | Jan 2005 | AT | national |
Filing Document | Filing Date | Country | Kind | 371c Date |
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PCT/AT06/00013 | 1/12/2006 | WO | 00 | 3/17/2008 |