Agitating – Rubber or heavy plastic working – Stirrer is through-pass screw conveyor
Reexamination Certificate
1997-08-12
2001-05-08
Pyon, Harold (Department: 1722)
Agitating
Rubber or heavy plastic working
Stirrer is through-pass screw conveyor
C366S088000, C366S089000, C366S090000, C425S204000, C425S208000
Reexamination Certificate
active
06227692
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a screw for plasticating plastic material and a method for mixing plastic material that has been at least partially plasticized and contains inhomogeneities.
Extrusion and injection molding processes form shapes from thermoplastic resin by softening or plasticating solid material to a molten state, shaping the product in an extrusion die or in a closed mold, and solidifying the shaped product by cooling. The properties of the final product depend to a major extent on the processing history of the material, and particularly on the thermal and physical homogeneity of the molten material prior to forming it, as well as the length of time the material is heated. For example, acid aldehyde formation in polyethylene teraphthalate (PET) type resins is directly related to the temperature, shear history and total cycle time. Accordingly, processing of a homogeneous melt at the lowest possible temperature, with minimized shear and time is a major concern in the injection molding and extrusion industries.
Over the years, a great deal of effort has been spent on designing the screws used to plasticate the material to be molded. As a result, the prior art is replete with a wide variety of differently configured plasticating screws. U.S. Pat. Nos. 2,753,595 to Dulmage and 3,006,029 to Saxton describe plasticating screws that employ distributive mixing elements to break down agglomerations or inhomogeneities in the melt. U.S. Pat. Nos. 3,687,423 to Koch et al.; 3,701,512 to Schippers et al.; 3,858,856 to Hsu; 3,870,284 to Kruder; 3,941,535 to Street; 4,000,884 to Chung; 4,079,463 to Miller; 4,128,341 to Hsu; 4,215,978 to Takayama et al.; 4,639,143 to Frankland, Jr.; and 4,925,313 to Nunn illustrate some other types of plasticating screws that employ dispersive mixing. Still another type of plasticating screw is shown in abandoned U.S. patent application Ser. No. 757,042, filed Jan. 5, 1977. There is disclosed in this abandoned application a screw provided with a helical flight and a valley wherein the bottom surface of the valley is divided into a plurality of parallel strip shaped portions having wavy contours with a predetermined phase difference therebetween which form a plurality of cross points between the wavy contours along the dividing plane.
New developments in plastic resin compositions now allow for PET type products to be used under relatively high temperatures, in so called “hot-fill” applications. In order to maintain physical properties at these higher temperatures, a polymer blend of PET and polyethylene napthalate (PEN) is normally used. However, PEN and PET are immiscible, so distributive mixing is required. Conventional PET screws cannot successfully process these new copolymer blends.
SUMMARY OF THE INVENTION
Accordingly, it is an object of the present invention to provide a screw for plasticating plastic material which ensures homogeneous mixing and melting without increasing overall shear.
It is a further object of the present invention to provide a screw as above which can successfully process both PET and PET blends.
It is yet a further object of the present invention to provide a screw as above which can successfully process non-PET resins and blends.
It is yet a further object of the present invention to provide a screw as above which can be used in extruders and injection molding machines.
It is yet another object of the present invention to provide a screw as above in which shear heat is minimized during resin melting, in order to minimize acid aldehyde.
It is still another object of the present invention to provide a method for mixing plastic material that has been at least partially plasticized and contains inhomogeneities.
The foregoing objects are attained by the screw and the method of the present invention.
In accordance with the present invention, a screw for plasticating plastic material broadly comprises: first means for receiving feedstock, usually in pellet form, and for compacting the feedstock into a solid material; second means for compressing the solid material and for forcing the compressed solid material against a heated wall to initiate melting of the plastic pellets; third means for breaking up the compressed solid material and for creating a melt with the solid particles being substantially equally distributed in said melt; fourth means for trapping solid particles having a size greater than a particular size and for allowing solid particles having a size less than the particular size to pass; and fifth means for completely melting the solid particles and for creating a substantially homogeneous melt. In a preferred embodiment of the present invention, the third means comprises a first melting zone having two helical channels. Each channel preferably has a plurality of peaks and valleys and with the peaks and valleys in the different channels being offset from each other so as to break up solid agglomerations and distribute them equally in the liquid melt. The fourth means preferably comprises a spiral mixer section having a plurality of helical channels. The fifth means preferably comprises a second mixing and melting zone having two helical channels. Each channel preferably has a plurality of peaks and valleys, with the peaks and valleys being offset from each other so as to completely melt the plastic material and generate a substantially homogeneous melt at the discharge end of the screw.
The method of the present invention broadly comprises the steps of: providing plastic feedstock to a rotatable screw; rotating the screw at a desired rate of rotation; melting a portion of the plastic feedstock in a first portion of the screw while the screw is rotating; trapping solid particles above a certain dimension in the molten plastic material in a mixer section of the screw adjacent the first portion while allowing molten plastic material with solid particles having a dimension less than the certain dimension to pass through the mixer section; and further melting the molten plastic material with the solid particles therein in a second portion of the screw adjacent the mixer section.
Other details of the screw and the method of the present invention, as well as other objects and advantages attendant thereto, are set forth in the following detailed description and the accompanying drawings in which like reference numerals depict like elements.
REFERENCES:
patent: 2753595 (1956-07-01), Dulmage
patent: 3006029 (1961-10-01), Saxton
patent: 3184790 (1965-05-01), Araki
patent: 3584340 (1971-06-01), Koch
patent: 3687423 (1972-08-01), Koch et al.
patent: 3701512 (1972-10-01), Schippers et al.
patent: 3788612 (1974-01-01), Dray
patent: 3788614 (1974-01-01), Gregory
patent: 3858856 (1975-01-01), Hsu
patent: 3870284 (1975-03-01), Kruder
patent: 3941535 (1976-03-01), Street
patent: 4000884 (1977-01-01), Chung
patent: 4053143 (1977-10-01), Hosokawa et al.
patent: 4079463 (1978-03-01), Miller
patent: 4107788 (1978-08-01), Anders
patent: 4128341 (1978-12-01), Hsu
patent: 4135870 (1979-01-01), Wallace et al.
patent: 4140400 (1979-02-01), Lovegrove
patent: 4173417 (1979-11-01), Kruder
patent: 4215978 (1980-08-01), Takayama et al.
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patent: 4511319 (1985-04-01), Takayama
patent: 4639143 (1987-01-01), Frankland, Jr.
patent: 4708623 (1987-11-01), Aoki et al.
patent: 4729666 (1988-03-01), Takubo
patent: 4840492 (1989-06-01), Nakamura
patent: 4925313 (1990-05-01), Nunn
patent: 4944906 (1990-07-01), Colby et al.
patent: 5035509 (1991-07-01), Kruder
patent: 5698235 (1997-12-01), Satoh et al.
patent: 5735599 (1998-04-01), Ishibashi et al.
Bachman & LaPointe P.C.
Husky Injection Molding Systems Ltd.
Leyson Joseph
Pyon Harold
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