Agitating – Rubber or heavy plastic working – Rotating and reciprocating stirrer
Reexamination Certificate
1998-03-27
2001-03-27
Drodge, Joseph W. (Department: 1723)
Agitating
Rubber or heavy plastic working
Rotating and reciprocating stirrer
C264S050000, C366S087000, C425S00400R, C425S199000
Reexamination Certificate
active
06206558
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates generally to single extruder systems for processing, screening and extruding foamable plastic material. More particularly, the invention relates to a system for flowing plastic material through a screen changer at an upstream point where the material is at relatively high temperature and pressure.
To manufacture foam plastic products, it is known to flow molten foamable material through a screw extruder, then through a screen changer, and then through a die. The molten material is cooled as it flows through the extruder. The cooled material should have sufficient viscosity to prevent premature foaming.
High viscosity material, however, may require a larger or more complicated screen changer. If the screen changer is not large enough, the pressure drop across the screen changer may cause premature foaming. Also, the increased pressure demand at the end of the extruder results in more shear heat in the extrudate, lessening cooling efficiency.
A prior art screen changer is shown in U.S. Pat. No. 3,856,277 (Tiramani).
SUMMARY OF THE INVENTION
The problems and disadvantages of the prior art are overcome to a great extent by providing a foam extrusion system with a screen operatively located between first and second in-line screw sections.
In one aspect of the invention, a seal structure directs the molten material from the first screw section and toward the screen. In a preferred embodiment, the seal structure has reverse flights to provide a visco-dynamic seal. Preferably, the reverse flights cause a small amount of molten material to be recycled upstream, eliminating stagnation and eventual degradation of the extrudate.
In another aspect of the invention, the seal structure is rotated by the first screw section, and the rotation of the second screw section is driven by the seal structure. In a preferred embodiment, the screw sections and the seal structure are integrally connected together and located within a common barrel-shaped housing.
The screen may be located within a screen changer. In a preferred embodiment of the invention, a system of bypass conduits provide fluid communication between the barrel housing and the screen changer.
In another aspect of the invention, foamable plastic material is extruded by rotating a primary screw section, a seal section, and a secondary screw section, and by causing the material to flow through the primary screw section, then through the screen changer, and then through the secondary screw section. In a preferred method, the plastic material has a gaseous foaming agent entrained therein. As an alternative, the screen changer can be positioned at a location along the barrel before the foaming agent is injected into the process.
An object of the invention is to provide an economical, uncomplicated and easy to use foam extrusion system.
Another object of the invention is to provide an extrusion system that does not require a large screen and screen changer.
Another object of the invention is to locate a screen changer midway within a single extrusion line. Preferably, the plastic material is relatively cool and viscous at the downstream end of the line, near the die. The screen changer may be located at a point where the plastic material is at relatively high temperature (and hence lower viscosity) and where a pressure drop is more acceptable to the process.
Another object of the invention is to avoid stagnation and degradation of plastic material within the extrusion system. In a preferred embodiment of the invention, stagnation is avoided by providing the seal with reverse flights. The seal is rotated integrally with the screw sections. Consequently, the reverse flights cooperate with the interior of the barrel housing to cause a small amount of plastic material to recycle through the screen changer.
The present invention provides an uncomplicated drive system for an extrusion system. This object may be achieved by constructing the seal as an integral driven part of the screw sections. With this arrangement, the second screw section may be driven directly from the rotation of the first screw section. The invention avoids the need for multiple, flighted extruders.
The present invention may be used with a wide variety of plastic materials and foaming agents, including but not limited to, high and low density polyethylene, polystyrene, polypropylene, PET and the like.
These and other objects, features and advantages of the invention will become apparent from the following detailed description of preferred embodiments of the invention.
REFERENCES:
patent: 3193877 (1965-07-01), Edwards
patent: 3360824 (1968-01-01), Schippers
patent: 3856277 (1974-12-01), Tiramani
patent: 4419309 (1983-12-01), Krutchen
patent: 4436679 (1984-03-01), Winstead
patent: 4476077 (1984-10-01), Bullard et al.
patent: 4615664 (1986-10-01), Kolossow
patent: 5045255 (1991-09-01), Kurz
patent: 5129728 (1992-07-01), Winstead
patent: 5419634 (1995-05-01), Bacher et al.
patent: 5651944 (1997-07-01), Schulz et al.
Dickstein , Shapiro, Morin & Oshinsky, LLP
Drodge Joseph W.
Sencorp Systems Inc.
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