Agitating – Rubber or heavy plastic working – Rotating and reciprocating stirrer
Reexamination Certificate
2000-07-11
2002-09-24
Cooley, Charles E. (Department: 1723)
Agitating
Rubber or heavy plastic working
Rotating and reciprocating stirrer
C366S080000, C366S088000, C366S089000, C366S090000
Reexamination Certificate
active
06454454
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a screw type extruder for melting solid polymer materials. More specifically, the invention relates to a screw extruder that transforms solid polymer materials to a melted fluid material by contact of the solid polymer materials with a heated surface. The heated surface is provided with a plurality of circumferentially-spaced, axially extending, radial slots such that the fluid material passes outwardly or inwardly through the radial slots and is removed from the heated surface to ensure that the heated surface remains in close contact with the unmelted solid polymer material at all times for improved heat transfer.
2. Related Art
Devices for converting solid thermoplastic materials to a melted condition are well known in the art. Known devices for use in manufacturing plastic products utilize, in one form or another, an extrusion device to melt, mix, and pump molten polymer for further processing. A commonly used extruder is known as a single screw extruder. A typical extruder mechanism of the single screw extruder type utilizes a heated barrel, a helical auger or plasticating screw within the barrel and a feed throat. The screw is rotated by conventional means. The melting of the plastic is accomplished by heat conduction and shear energy dissipation at the inside wall of the barrel. The shear energy is generated as a result of relative motion between the packed solid polymer particles in the screw channel (solid bed) and the heated stationary barrel surface. As the solid polymer particles are melted, a layer of melt is formed on the barrel's inner circumferential surface, which is continuously wiped off by the screw thread or flight. Barrier type screws are also known for use in the single screw extruder wherein the external thread of the screw includes a rear side portion of full thread radius and a leading side portion of smaller radius, between which a groove is formed. The barrier screw can provide an improved means for separating the melt from the solid bed in order to increase the efficiency and quality of the melt.
In U.S. Pat. No. 3,689,182 to Kovacs, a screw extruder is shown wherein the polymer melt is bled radially inwardly into a central axial bore containing a second coaxial screw. In Kovacs, radial holes 14 of ⅛ inch diameter are distributed along the leading face of the thread of the external screw at uniform angular spacings to dispose six or eight radial openings per turn of the thread. In Kovacs, the assumption is made that the polymer melt forms in a small pool along the leading side of the thread on the external screw. Hence, Kovacs states that satisfactory removal of the melt is obtained with openings of ⅛ inch in diameter distributed serially along the leading face of the thread. In the embodiment shown in Kovacs employing a barrier type screw having a rearward portion of full thread radius and a leading side portion of a smaller radius of thread, Kovacs assumes that a spiral solid thermoplastic body will develop along the full length and volume of the channel defined by the thread. Melted material is only removed from the channel by radial holes positioned serially along the groove defined between the rearward and leading side portions of the thread. Thus, a continuous layer of melted polymer is allowed to form along the entire circumferential extent of the spiral channel defined by the thread, with the melt only being bled off at the leading edge of the thread.
The second embodiment of the Kovacs patent shown in FIGS. 4, 5, 6 and 7 employs a screw 30 having a thread 31 which terminates at its downstream end at a location upstream of, and spaced from, a head 33 as shown in FIG. 5 of the patent. Head 33 has a smooth perimetric area machined to form a desired clearance with the inner surface extruder barrel to restrain the passage of material through said clearance except for that necessary to prevent fouling as discussed in the paragraph beginning in Column 5, Line 55 of Kovacs. The head 33 has grooves 57 which are closed at their front ends and open only at their rear ends with the purpose of the grooves being to trap solid particles. Head 33 also has additional grooves 58 which are closed at both their front and rear ends and which collect melted material from grooves 57. Thus, the second embodiment of Kovacs does not disclose linear threads extending from or adjacent the downstream end of the thread 31 nor is such structure shown elsewhere in the Kovacs patent.
Cheng U.S. Pat. No. 5,098,267 discloses an elongated rotatable screw plunger 12 positioned for rotation and reciprocation in a barrel 10 with the screw plunger 12 extending through an upstream metering section 28 and a mixing section 40 immediately downstream of metering section 28. The screw 12 includes a bore 50 solely located in its downstream end in mixing section 40 as shown in FIG. 5. Lands 38 are provided in metering section 28 and are separated by metering section grooves 36 and a gap space is provided between the downstream end of the spiral thread of the screw and the upstream ends of lands 38.
The mixing structure immediately downstream of the lands 38 consists of mixing section grooves 42 provided between a “land” 44 which would better be described as a barrier plate or flight which has a reduced height relative to the general surface of plunger 12 and a “land” 46 which could better be described as a wiper plate since it is adjacent the inner surface of the barrel 12. Rotation of the plunger 12 causes plasticate to flow across “land” 44 and then downwardly into slot 48 because of the shape and dimension of land 46 as discussed in the paragraph beginning in column 4, line 26 of the Cheng patent.
Soviet Union publication SU 1763207 discloses a screw having spiral threads and an internal bore but not having linear threads spaced from the spiral threads or being closely spaced from the inner surface of the barrel in which the screw is mounted.
In prior art screw-type extruders, such as Kovacs and others, the thickness of the melt film formed along the inside of the heated barrel directly affects the efficiency and rate of melting since the heat developed by dissipation of shear energy between the solid polymer and the barrel is reduced. This reduction in melting rate is especially important on large extruders wherein the circumferential length of the solid bed is very large.
SUMMARY OF THE INVENTION
The present invention provides a device for transforming solid thermoplastic materials into fluid material by contact with a surface, wherein melting of the thermoplastic material is caused by heat conduction and shear energy dissipation. The shear energy is created by relative motion between the solid polymer material and a surface, and the surface is provided with a plurality of circumferentially spaced, axially extending, radial slots there through such that melted polymer material can pass through the surface into an area for storage or removal. The provision of circumferentially spaced, axially extending, radial slots allows the surface to remain in close contact with the solid material at all times by providing means for draining off melted polymer at several locations around the circumference. Hence, the layer of melt interfaced between the solid polymer material and the surface is kept very thin, thus improving heat transfer and shear energy dissipation.
In one embodiment, the present invention is provided with an external barrel that is heated by heating coils, an internal screw, and a floating cylindrical sleeve having a plurality of circumferentially spaced, axially extending, radial slots there through located between the external barrel and the internal screw. The internal screw comprises a central core and an external thread so interrelated in cooperation with the floating cylindrical sleeve as to define a continuous spiral channel between the screw core and the floating cylindrical sleeve. The portion of the spiral channel located radially inwardly from the heating coil
Cooley Charles E.
Jacobson & Holman PLLC
LandOfFree
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