Flow guide for underwater pelletizer

Details Flow guide for underwater pelletizer Flow guide for underwater pelletizer

1999-09-21

2003-04-22

Silbaugh, Jan H. (Department: 1722)

Plastic article or earthenware shaping or treating: apparatus

Immersed shaping orifice discharging directly into liquid...

C425S311000, C425S313000, C425S315000, C425SDIG002

Reexamination Certificate

active

06551087

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention generally relates to improvements in underwater pelletizers and more particularly to providing a water flow guide for the water box of an underwater pelletizer to more efficiently utilize pressurized water flowing through the water box. The water flow guide of this invention produces an accurately guided pressurized water flow which improves pelletization of polymers of various diameters, especially micropellets, and polymers having a wide range of specific gravity and rheological characteristics.
2. Description of the Prior Art
Underwater pelletizers are well known and include a die plate with multiple orifices through which molten polymer strands are extruded with the orifices terminating at a die face on the die plate. A powered rotary cutter including a cutter hub mounted on a shaft with a plurality of blades supported from the cutter hub is associated with the die face for shearing the extruded polymer strands into pellets as the polymer is extruded beyond the die face. A water box encloses the die face, cutter hub and blades to form a cutting chamber through which water flows to quench and rigidify the extruded polymer strands, thus enabling the cutting blades to better shear the extruded strands into pellets. The flow of water through the cutting chamber defined by the water box also conveys a slurry of the water and sheared pellets through the outlet of the water box to a pellet dewaterer and/or a dryer. The above described underwater pelletizers are disclosed in related U.S. patents including U.S. Pat. Nos. 4,123,207, 4,251,198, 4,621,996, 4,728,276 and 5,059,103, all owned by the assignee of this invention. Additional U.S. patents which relate to pelletizers are as follows:
3,207,818
4,245,972
4,978,288
3,341,892
4,300,877
5,215,763
3,353,213
4,846,644
3,862,285
4,943,218
U.S. Pat. No. 3,341,892 illustrates cutter hub assemblies directing water flow to the cutting area of the die plate. However, the water enters through the pelletizer shaft and is routed through the cutter hub with the water serving as an aid to pelletization. The water is not pressurized in the cutting chamber and gravity is relied upon to actually get the pellets into a solid stream of water to transport the pellets to the lower portion of the cutting chamber where the pellets exit. This patent also illustrates a flute style spray nozzle assembly that introduces a spray of water directed at the cutter blades where its purpose is to assist in so-called “hot face” die plate pelletization.
U.S. Pat. No. 3,862,285 illustrates a cooling system for the production of plastic sheets incorporating a flow guide design. U.S. Pat. No. 4,245,972 illustrates the use of flow control that is actually designed to keep water away from the cut pellets until centrifugal force and gravity transport the pellets to a water ring formed within the bowl (formed at 14 and 10 in the patent).
U.S. Pat. No. 4,300,877 uses the cutter hub as an integral component of the flow control which could result in plastic contamination and cause tedious cleanup issues for operating personnel. U.S. Pat. No. 4,846,644 illustrates what is called a high speed “cooler” and is essentially a water ring pelletizer somewhat similar to the structure disclosed in U.S. Pat. No. 4,300,877 and which does not use water to encapsulate/transport the pellets as they are cut.
U.S. Pat. No. 3,207,818 illustrates a cutting process where the specific gravity of the polymer plays an important role in operation. The “chamber” has no specific water (or process medium) flow guiding devices. The flow guide of this invention operates independently of any specific gravity considerations. U.S. Pat. No. 4,978,288 illustrates a water ring pelletizer where water is again isolated from the pelletizing process until well after cutting has taken place and U.S. Pat. No. 5,215,763 also illustrates a water ring pelletizer.
While the above listed patents disclose various structures associated with underwater pelletizers, the invention disclosed in the instant application utilizes the controlled flow of water not only to assist in cutting or shearing the pellets but also to quench and isolate pellets immediately after cutting has occurred. The type of pelletization known as “hot face”, as shown in U.S. Pat. No. 3,341,892, uses an assist spray and is not underwater pelletization as utilized in the present invention. Underwater pelletization as in the present invention utilizes the pressurized water to encapsulate and isolate the pellets and also uses the pressurized water to transport the pellets away from the cutter hub.
The water flow guide of the present invention produces a pressurized water flow, accurately guided, to allow the underwater pelletization of polymers of nearly any diameter, even micropellets, i.e. pellets formed from polymer strands extruded through orifices having a diameter less than or equal to 0.050 inches, as well as polymers of a wide range of specific gravity/rheological characteristics. The flow guide within the underwater pelletizer relies on the water pressure/flow as opposed to gravity, such as is used in water ring pelletizers, and the pressurized water actually transports the pellets from the moment they are cut until they are introduced into a drying structure.
SUMMARY OF THE INVENTION
The water flow guide of the present invention is positioned within the cutting chamber defined by the water box of an underwater pelletizer in order to selectively guide the pressurized water passing through the water box. By selectively guiding the pressurized water in the cutting chamber, it has been found that the water can more efficiently quench the extruded polymer strands to cool and rigidify the strands, thus enabling the cutter blades to more effectively and cleanly cut or shear the extruded strands into pellets. Also by selectively guiding the pressurized water with the water flow guide of this invention it is possible to increase the efficiency of the pressurized water flowing through the water box to convey the sheared pellets away from the cutting chamber to the water box outlet and then to a pellet dewaterer and/or a pellet dryer.
In addition, the flow guide of this invention permits flow management of the pressurized water flow into and around the cutter within the cutting chamber. This leads to a reduction or even elimination, of cavitation which occurs when water cannot get into the central area of the cutter hub and die plate, thus reducing or eliminating pellet clusters or agglomerates forming in the cutting chamber, especially when producing micropellets. By reducing or eliminating pellet clusters or agglomerates, expensive shutdowns of the equipment and operator requirements and time to clean up and restart are significantly reduced. In addition, the wasting of polymer materials is reduced, all increasing productivity of the pelletizer. Moreover, if agglomerations and cutting chamber clog ups occur, the agglomerate or clog within the cutting chamber is easier to clean off and remove by the operator because there is no longer a large open cutting chamber to fill up with molten, but solidifying polymer. Furthermore, the reduction of cavitation allows the pelletizer to produce good quality, well sheared pellets at higher speeds and/or with more cutter blades, thus making it possible to yield higher rates of production, in pounds/hour or kilograms/hour, which results in a higher capacity for the same size pelletizer.
Furthermore, it is believed that the flow guide of the present invention allows the pelletizer to operate with less load, and thus pulls less amperes by the motor which, in turn, allows the pelletizer to operate more efficiently with less power requirements. Also, in utilizing the water more efficiently with the flow guide of the present invention, less water flow is required for the pelletizer. Less water flow translates into smaller pumps and/or less pump energy consumption.
The water flow guide of this invention is oriented in the cutting chamber between the

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