Underwater pelletizer with separator

Details Underwater pelletizer with separator Underwater pelletizer with separator

2000-03-22

2003-07-15

Davis, Robert (Department: 1722)

Plastic article or earthenware shaping or treating: apparatus

Immersed shaping orifice discharging directly into liquid...

C425S313000, C425S311000, C425SDIG002

Reexamination Certificate

active

06592350

ABSTRACT:

CROSS-REFERENCES TO RELATED APPLICATIONS
This application claims the priority of German Patent Application Serial No. 199 14 116.9, filed Mar. 23, 1999, the subject matter of which is incorporated herein by reference.
BACKGROUND OF THE INVENTION
The present invention relates to an underwater pelletizer for granulating thermoplastics. The present invention further relates to a method for granulating thermoplastics.
Thermoplastics are transformed through injection molding or blow molding into finished products which are typically fed in the form of particles or granules to respective plastics processing machines. This granulated material is made from raw plastics which is oftentimes mixed with additives such as pigments or reinforcing agents, and processed in extruders and pelletizers for cutting the raw material extruded from the extruding dies.
In water-ring pelletizers, thermoplastic material is plasticized and pressed through a plurality of die holes in a die plate, which is located at the exit of the extruder, for producing comparably thin single strands when leaving the die holes. Immediately after leaving the die, the strands are cut to relative fine pellets or particles of relatively short length by blades of a rotating cutting tool which moves in close proximity past the exit plane of the die plate. Surrounding the cutting tool and the die plate is a ring-shaped cooling chamber which has a substantially cylindrical inner wall for conducting a thin stream of cooling water that has been injected tangentially. Water-ring pelletizers of this type are known, for example, from German Pat. Nos. DE-AS 1 221 783 and DE-AS 1 454 888.
Other types of pelletizers include so-called underwater pelletizers which differ from the water-ring pelletizers by the fact that cooling water circulates through and fills the entire cooling chamber. Normally, cooling water is injected from below or conducted through the driveshaft of the cutting tool into the chamber to ensure a complete filling of the chamber with cooling water throughout. Cooling water leaves the chamber through an outlet, which is normally located at an upper area of the chamber, and carries off the granulated material, produced in the cooling chamber in the form of a solid/water mixture, when leaving the chamber. Directly connected to the outlet is a conduit for transport of the granulated material to a separator in which the solidified particles are separated from the cooling water and subsequently dried.
An underwater pelletizer of this type is described, for example, in British Pat. No. GB 2 010 288 A. The pelletizer includes an extruder with a die plate for directly extruding plastic material into a cooling chamber filled with cooling water, whereby cooling water is injected from below and exits from above. Arranged in the cooling chamber is a cutting tool which rotates about a horizontal pivot axis and is operated by an electric motor. This conventional pelletizer is not intended for making particles of plastic material but is provided to realize a rapid production of an aqueous solution of a water-soluble polymer which is extruded in the form of a gel through the die plate. For that reason, a further chamber is arranged between the cooling water outlet and the cooling chamber, for accommodating a rotating rotor therein in order to accelerate the solution of the gel in water through applying an intense shear action.
Underwater pelletizers are suitable for use with thermoplastics, such as PP, PE or PVC, whereas their application for other materials, which have the undesired tendency to absorb water at a relatively high degree, is accompanied by deteriorating quality of the finished product. Pelletizing of such plastics, which are sensitive to cooling water, is normally carried out by a process, called strand cutting, in which relatively long plastics strands are continuously extruded through a die plate, and subsequently transported, freely suspended, along a short path through air and then conducted through a water bath which is located downstream of the extruder. As a result of the significantly reduced specific (as relating to volume) surface of a plastics strand compared to granulated material, the water absorption is maintained within narrow limits. After cooling down the individual strands in the water bath, the individual strands are dried and transferred, pre-dried and cooled, to a pelletizer. Thus, granulation does not take place in a thermoplastic state, but in a solid state. Subsequently, drying is typically executed again.
While the strand cutting process ensures a short cool-down period and good drying of the particles, this process requires relatively complex maintenance works, especially when plants of high output are involved, because individual extruded strands oftentimes tear off. Moreover, deposits are formed outside on the die plate and adversely affect the quality of the product. This is true, in particular, for polyamide compounds. Moreover, especially when plants with high throughput are involved, appropriate drying of the particles may be difficult. In the event of an excessive temperature rise of the material, degradation may be experienced, causing damage to the plastic material being processed.
SUMMARY OF THE INVENTION
It is thus an object of the present invention to provide an improved underwater pelletizer, obviating the afore-stated drawbacks.
In particular, it is an object of the present invention to provide an improved underwater pelletizer which is suitable for plastics that have the undesired tendency to absorb water when coming into contact with cooling water.
These objects, and others which will become apparent hereinafter, are attained in accordance with the present invention by providing at least one extruder having at least one die plate formed with die holes for extruding thermoplastics into single strands, at least one motor-driven cutting tool rotating about a horizontal pivot axis for cutting the strands in short pieces in a cutting plane in parallel relation to an exit plane of the die plate, thereby forming a granulated product, whereby the exit plane of the die plate and the cutting tool are positioned within the chamber which is filled with cooling water streaming through the chamber, with the chamber having a cooling water inlet, which is arranged above the pivot axis of the cutting tool, and an outlet, arranged beneath the pivot axis of the cutting tool, for a mixture of water and granulated product, with the outlet being configured as free discharge opening, and a separator connected to the outlet and positioned immediately underneath the outlet for separating cooling water from the granulated product.
Compared to conventional underwater pelletizers, an underwater pelletizer according to the present invention ensures at operation an extremely short retention time of produced granulated material in cooling water, because the water inlet is disposed above and the outlet for cooling water is disposed below the pivot axis of the rotating cutting tool used for pelletizing, and because the outlet is configured as a free discharge opening which is immediately followed by the separator positioned underneath the outlet. Thus, the provision of a conduit for transport of cooling water and particles to the separator is eliminated, as, in accordance with the present invention, the particles are transferred by cooling water, falling from top to bottom through the pelletizer, directly to the separator after an extremely brief retention time in cooling water of about 1 to 2 seconds. This short retention time in cooling water is not long enough to attain a sufficient cooling of individual particles of the granulated product; rather, a granulated product is realized which has solidified but still retains sufficient residual heat.
As stated above, plastics, such as polyamide or polyester, have a tendency to absorb water. These plastics have a relatively distinct melting point, and, after solidification, do not show a tendency for adhesion. The present invention exploits these properties becau

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