Agitating – Rubber or heavy plastic working – Stirrer is through-pass screw conveyor
Reexamination Certificate
2002-11-06
2004-03-30
Cooley, Charles E. (Department: 1723)
Agitating
Rubber or heavy plastic working
Stirrer is through-pass screw conveyor
Reexamination Certificate
active
06712495
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to an apparatus for mixing thermoplastic materials, and more particularly to screws for mixing multiple polymers, which screws can be used in extruders and injection molding machines.
2. Description of the Related Art
The use of screws to extrude, compound, and injection mold polymer is well known. Turning to
FIG. 1
, there is shown a conventional screw
11
for use in melting and mixing polymers. The screw includes three zones: a feeding zone
13
, a compression or transition zone
15
and a metering zone
17
. Screw
11
is housed in a hollow barrel
19
having a constant inner cylindrical diameter and a smooth inner surface. Polymer resin, which may be in any form such as pellets, granules, flakes or powder, is fed through an opening
21
in barrel
19
into feeding zone
13
where screw
11
turns to pack and then push the polymer into compression zone
15
. The polymer is melted in compression zone
15
and then conveyed to metering zone
17
where the molten material is homogenized. Afterwards, the homogenized melt is extruded, injection molded or processed further.
Screw
11
includes a screw shaft
23
having a thread spirally positioned about shaft
23
to form flights
25
. Flights
25
are characterized by their depth, which is the height of flight
25
above shaft
23
and by their pitch, which is the distance P between two adjacent flights
25
plus one flight width. The outside diameter OD of screw
11
includes the depth of a flight
25
above and below shaft
23
, whereas the root diameter RD of screw
11
is the diameter of shaft
23
only, without including the depth of flights
25
.
U.S. Pat. No. 3,486,192 discloses a mixing element for incorporation into an extrusion screw, which mixing element is shown in FIG.
2
. The mixing element
30
is an elongated cylinder with a surface that is machined with surface grooves
32
and
34
which are separated by raised lands
36
. The input grooves
32
are open at the end of the mixing element facing the end of the element
30
from which polymer is fed by the screw to the mixing element and the input grooves
32
are closed at the opposite end of the mixing element. The output grooves
34
are open at the end of the mixing element facing the end of the element
30
toward which polymer is discharged and the output grooves
34
are closed at the opposite end of the mixing element. The lands
36
form a barrier between the input grooves
32
and the output grooves
34
, but there is a clearance between the top of the lands and the inside surface of the barrel
19
. The mixing element
30
rotates with and is driven by the turning of the screw shaft
23
. The rotation of the screw forces molten polymer into the input grooves
32
of the mixing element
30
, over the top of the lands
36
through the clearance between the lands
36
and the inside surface of the barrel
19
and out through the output grooves
34
. The polymer melt is subjected to a high sheer stress as it is squeezed between the top of the lands
36
and the inside surface of the barrel
19
. Additional screw mixing element configurations are disclosed in U.S. Pat. No. 6,136,246.
With mixing elements like that described above, it has been difficult to obtain a very high degree of polymer mixing without at the same time generating excessive heating of the polymer which frequently causes degradation of the polymer. Accordingly, there is a need for a mixing screw for use in extruders and injection molding machines that achieves a very high degree of mixing and homogenization but that does not generate undue heating or degradation of the polymer. There is also a need for a screw with a mixing element that generates a high degree of polymer mixing but does not constrain flow of the polymer through the screw so as to make reductions in screw rotation speeds and overall polymer resin throughput rates necessary. What is needed, therefore, is an apparatus that will produce a homogeneous melt from multiple polymers without causing substantial degradation of the polymer resins.
SUMMARY OF THE INVENTION
The present invention relates to an apparatus for mixing thermoplastic materials, and more particularly to screws for mixing multiple polymers which screws can be used in extruders and injection molding machines. The apparatus includes a barrel having a hollow space therein with a substantially round cross section and a longitudinal axis, with the barrel hollow space having an inside surface and a screw rotatably mounted in the barrel hollow space. The screw is mounted coaxially with the longitudinal axis of the barrel hollow space so as to transmit a polymer material through the barrel hollow space from an inlet end of the barrel hollow space to an outlet end of the barrel hollow space when the screw is rotated. The screw has a screw shaft with a thread spirally extending around the screw shaft so as to form a plurality of flights, and the screw has a zone through which melted polymer is conveyed. The screw has a flow cross-sectional area between adjacent flights of the screw that is equal to the radial height that the screw thread of the adjacent flights extends above the screw shaft multiplied by the width between adjacent flights. The screw has a portion within the zone through which melted polymer is conveyed which screw portion is without a screw thread and where the screw shaft forms a mixing element having an inlet end directed toward the inlet end of the barrel hollow space and an outlet end directed toward the outlet end of the barrel hollow space.
The mixing element of the screw comprises a drum-shaped surface extending above the screw shaft that is coaxial with the screw shaft. This drum-shaped surface has a plurality of input grooves on the drum-shaped surface extending in a generally axial direction with groove openings at the inlet end of the mixing element, wherein the input grooves terminate before reaching the outlet end of the mixing element, and a plurality of output grooves on the drum-shaped surface extending in a generally axial direction with groove openings at the outlet end of the mixing element, wherein the output grooves terminate before reaching the inlet end of the mixing element. The plurality of output grooves alternate with the plurality of input grooves such that the input and output grooves are contiguous with each other over a portion of the surface of the mixing element. Lands extending between the input grooves and output grooves separate the input grooves from the output grooves, with the length of each land being substantially equal to the length of the contiguous portion of the adjacent input and output grooves. Each of the lands has a top surface extending the length of the land that has a thickness between the input and output grooves of less than 2 mm. The top surface of the lands extends toward but does not touch the inside surface of the hollow barrel. The clearance between the top surface of said lands and the inside surface of the barrel hollow space is preferably less than 0.5 mm.
According to the invention, there is a land cross-sectional shear area corresponding to each land that is equal to length of the land multiplied by the clearance between the land and the inside surface of the barrel hollow space, and the summation of the land cross-sectional shear areas for all of the lands of the mixing element is greater than or equal to 95% of the flow cross-sectional area between the adjacent flights of the screw through which polymer passes immediately prior to entering the mixing element.
REFERENCES:
patent: 2607077 (1952-08-01), Dulmage
patent: 3486192 (1969-12-01), Le Roy
patent: 3555616 (1971-01-01), Parks
patent: 3564651 (1971-02-01), Covington, Jr. et al.
patent: 3730492 (1973-05-01), Maddock
patent: 3756574 (1973-09-01), Maddock
patent: 3762692 (1973-10-01), Schippers
patent: 3771774 (1973-11-01), Hook
patent: 3788612 (1974-01-01), Dray
patent: 3870284 (1975-03-01), Kruder
patent: 3942774 (1976-03-01), Sokolow
patent: 3957256 (1976-
Cooley Charles E.
E. I. du Pont de Nemours and Company
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