Agitating – Rubber or heavy plastic working – Stirrer is through-pass screw conveyor
Reexamination Certificate
1999-09-03
2001-01-30
Soohoo, Tony G. (Department: 1723)
Agitating
Rubber or heavy plastic working
Stirrer is through-pass screw conveyor
C366S089000
Reexamination Certificate
active
06179461
ABSTRACT:
FIELD OF THE INVENTION
The present invention is generally related to machinery for processing solid resinous material, and is more specifically directed to extruder machines for mixing and melting said resinous material.
BACKGROUND OF THE INVENTION
Extruder screws employed in the melting, mixing, and compounding of polymeric resinous material typically employ three zones, namely a feed zone, a metering zone, and a melting zone located between the feed and metering zones. Typically the extruder screw is positioned for rotation into an extruder barrel that includes a hopper section adjacent to the feed section of the screw, and a discharge end opposite the hopper section and proximate to the metering section of the screw. During operation, solid resinous material is introduced through the hopper section and presented to the feed zone of the screw. The solid resinous material is then conveyed to the melting zone where it is transformed from a solid, to a molten state. From the melting zone, the molten material is transferred to the metering zone for conveyance to a discharge end of the extruder.
Historically, conventional extruder screws comprised a single helical flight disposed about and cooperating with a root or body section of the screw to form a channel along which the resinous material introduced into the extruder is conveyed. As the material enters the melting section it begins to melt due to the heat created by friction within the material itself, and heat from an external source conducted through the barrel. The melted material forms a melt film that adheres to the inner surface of the extruder. When the film thickness exceeds the clearance between the extruder barrel and the flight, the leading edge of the flight scrapes the melt film off the inner surface of the barrel causing the molten material to form a pool along an advancing edge of the flight. As the material continues to melt, the solid mass normally referred to as the solids bed breaks into agglomerations of solid material which then intermix with the pool of molten material.
When this occurs, the amount of solid material exposed to the heated barrel is severely diminished since the solid material is in the form of agglomerations entrained in the pool of molten material. Therefore, in order to melt the entrained solid material, sufficient heat must transfer through the molten pool to the solids. Since most polymers have good insulating properties, the melting efficiency of the extruder declines once the solids bed has broken up.
In an effort to improve melting efficiency, extruder screws were developed that incorporated a second flight in the melting section that extended about the body portion of the screw and defined a solids channel between an advancing surface of the second flight and a retreating surface of the primary flight. In addition, a melt channel for conveying molten material was also formed between a retreating surface of the second flight, and an advancing surface of the primary flight. The diameter of the root or body section of the screw progressively increased in the solids channel, thereby reducing the channel's depth along the melt section, and decreased along the melt channel, thereby increasing the melt channel's depth. During operation, the melt film formed at the interface between the solid bed and the heated barrel surface would migrate over the second flight into the melt channel thereby minimizing the break-up of the solid bed.
In screws of this type the rate at which the solid material melted was determined by the surface area of the solid bed in contact with the heated barrel wall and the thickness of the melt film formed between the barrel wall and the solid bed. An increase in the surface area of the solid material in contact with the barrel wall caused an increase in the melting rate due to improved heat transfer from the barrel to the exposed surface of the solid bed. However, an increase in the thickness of the melt film between the solids bed and the barrel, acted as a thermal insulator, thereby reducing the heat transfer from the barrel to the solid material and slowing the rate of melting. Accordingly, to transform the solid resinous material to a molten state, the melt section of these extruder screws was quite long, which in turn resulted in increased cost both to manufacture and operate an extruder utilizing such a screw.
Based on the foregoing, it is a general object of the present invention to provide an extruder screw that overcomes the problems and drawbacks of prior art screws.
It is a more specific object of the present invention to provide an extruder screw wherein the solid material introduced into the screw is melted and mixed in an efficient manner.
SUMMARY OF THE INVENTION
The present invention resides in an axially elongated extruder screw that includes a screw body and an axially extending extruder portion. The extruder portion is defined by three zones or sections, namely, a feed section at an inlet end of the extruder screw, a metering section at an outlet end of the screw, and a barrier section between the feed and metering sections. At least one helical primary flight extends about and is coaxial with the screw body. These two portions of the extruder screw, e.g., the primary flight and the screw body, cooperate in the feed section to form a first solids channel for conveying solid resinous material from the feed to the barrier sections.
The barrier section of the extruder screw of the present invention includes at least one helical secondary flight extending from the primary flight at the start of the barrier section, and about the screw body along the length of the barrier section. A helical first surface of revolution is defined by the screw body between the primary and secondary flights. At least one helical tertiary flight extends from the screw body and is positioned between the primary and secondary flights along the length of the barrier section. A second helical surface of revolution is defined between the secondary and tertiary flights, and a third helical surface of revolution is defined between the primary and tertiary flights. Each surface of revolution extends axially along the barrier section of the extruder screw.
A series of circumaxially contiguous cam-like forms are defined by the second and third surfaces of revolution, each spanning a segment of the screw. Each cam-like form includes a root, a crest, a first surface portion extending radially outward from the root to the crest in the direction of screw rotation, and a second surface portion extending radially inwardly from the crest to the root.
In the preferred embodiment of the present invention, the first surface of revolution cooperates with the primary and secondary flights to form a melt channel for conveying the resinous material in a molten state, along the barrier section of the extruder screw. Since the amount of molten material to be conveyed increases in a downstream direction along the barrier section, the depth of the melt channel progressively increases to adequately accommodate the increasing volume of molten material.
In addition to the melt channel, two solids channels are formed in the barrier section of the extruder screw. A second solids channel is defined by the cooperation of the secondary and tertiary flights with the second helical surface of revolution. Since, as will be explained in detail below, the volume of solids in the second solids channel decreases during operation of the extruder screw in the downstream direction along the barrier zone, the depth of the second solids channel progressively decreases in the downstream direction. A third solids channel is defined by the tertiary and primary flights cooperating with the third helical surface of revolution. As with the second solids channel, the depth of the third solids channel also decreases in the downstream direction along the barrier zone.
During operation of the extruder screw, the solids in the third solids channel form an upper molten layer as a result of shear generated in the solids
Christiano John P.
Thompson Michael R.
Davis-Standard Corporation
Michaud Richard R.
Soohoo Tony G.
Thompson Raymond D.
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