Screw element having shearing and scraping flights

A machine for processing solid and viscous plastic materials includes a screw element having a shearing flight and a scraping flight. Upon rotation of the screw, the shearing flight shears the plastic material and the scraping flight guides the material to the shearing flight. Preferably, the screw includes a plurality of threads which extend from one end of the screw to the other, and which include a plurality of shearing and scraping flights.

Some of the most difficult mixing or compounding jobs are those that 
involve dispersion of solids in a viscous matrix such as a fluid polymeric 
material. For example, in compounding rubber for tires and other 
applications, carbon black is dispersed into a rubber matrix. Similarly, 
the dispersion of inorganic and organic pigments as well as many other 
solid additives into plastics is another common practice in the 
compounding field. 
Successful compounding generally requires both dispersive mixing and 
extensive mixing. One manner of accomplishing successful dispersive mixing 
is to expose the fluid elements containing the solids to high shear 
stresses in order to rupture the solids into fragments. Presently 
available mixers generate such stresses by passing material to be 
compounded through narrow gap regions in the mixer, thus creating high 
shear rates. Multiple passes of the same material through the narrow gap 
regions is desired in order to reduce the size of the agglomerates to 
acceptable levels. 
Extensive mixing involves mixing various portions of the matrix with one 
another. Thus, after the ruptured solid particles and surrounding matrix 
pass through the narrow gap region, this part of the matrix must be mixed 
back into the rest of the matrix for composition uniformity. 
Many commercial mixers include narrow gap regions for dispersive mixing. 
For example, the narrow gap region is present in the nip of a roll mill 
mixer, in the gap between the tip of a Banbury type rotor and wall, in the 
gap between the tip of the kneading element and the barrel in an 
intermeshing twin screw extruder, and in the clearance between the mixing 
pin and the disk in a corotating disk processor. While these commercial 
mixers are effective in dispersive mixing, they are quite expensive and 
complex compared to single screw extruders. 
Screw type processing equipment such as single screw extruders are the most 
popular continuous processors of plastics and rubber. Such extruders have 
numerous desirable features including simplicity, relatively low cost, 
high throughput and versatility. However, application of single screw 
extruders and other similar processing equipment in the field of 
compounding has been rather limited. Although single screw extruders can 
be designed to provide good extensive mixing, they generally provide poor 
dispersive mixing. Only a small fraction of the processed material ever 
experiences high stresses in narrow gap regions or flight clearances 
between the flights of the screw and the barrel. Only a negligible amount 
of material passes repeatedly through these clearances. Even if the size 
of the flight clearance or radial distance between the flights and the 
barrel were increased to increase the number of passes through the flight 
clearances, the same material would stay close to the barrel as the flight 
clearance passes by, with little mixing or exchange of material. In other 
words, the same material would pass repeatedly through the flight 
clearance or gap with the bulk of material not having a chance to pass 
through the flight clearance at all. As a result, there would be a loss of 
output, poor heat removal, poor melting, and other detrimental effects. 
Accordingly, prior to the present invention there have been acute, unmet 
needs for screw-type processing equipment such as single screw extruders 
and other similar equipment which could combine the generally desirable 
characteristics of such equipment with improved dispersive mixing ability. 
SUMMARY OF THE INVENTION 
The present invention addresses these needs. 
One aspect of the present invention provides a machine for processing solid 
and viscous plastic and polymeric materials comprising a hollow barrel 
defining a barrel chamber. The barrel has an inner surface, an outer 
surface, a first end, a second end, and a longitudinal axis extending 
between the ends. The barrel defines axial directions along the axis, 
radial directions traverse to the axis and circumferential directions 
around the axis. The present invention also includes inlet means for 
introducing a material into the barrel chamber, and a screw extending 
along and rotatable about the longitudinal axis in a circumferential 
rotation direction. The screw has a first end adjacent the first end of 
the barrel and a a second end adjacent the second end of the barrel. 
The screw includes a shearing flight extending to a spaced distance from 
the inner surface of the barrel, and the flight shears material between 
the shearing flight and the inner surface of the barrel. The screw also 
includes a scraping flight radially extending to adjacent the inner 
surface of the barrel, and the scraping flight removes material from the 
inner surface of the barrel. The flights are constructed and arranged such 
that upon rotation of the screw in the rotation direction, the scraping 
flight collects and guides the material to the shearing flight. 
Desirably, the shearing flight and scraping flight are helically disposed 
on the screw. The screw may include a helical thread extending about the 
longitudinal axis which has a shearing flight at one portion of the thread 
and a scraping flight at another portion of the thread. A plurality of 
such threads may be included in the apparatus. Further, each thread may 
include a plurality of shearing and scraping flights such that the 
shearing flights alternately follow the scraping flights in series along 
each thread. 
Preferably, the shearing flight of one of the threads is followed in the 
rotation direction by the shearing flight of another thread. 
Alternatively, the shearing flight of one of the threads may be followed 
in the rotation direction by the scraping flight of another thread. Yet 
further, the shearing flight of one of the threads may be followed in the 
rotation direction by both a scraping flight and a shearing flight of 
another thread. 
One thread may continuously extend from a first spaced distance from the 
first end of the shaft to a second spaced distance from the second end of 
the shaft and another thread may continuously extend from a different 
first spaced distance from the first end of the shaft and a different 
second spaced distance from the second end of the shaft. 
Preferably, at each axial location along the screw, the spaced distance 
between the shearing flight and the inner surface of the barrel gradually 
decreases in the direction opposite the rotation direction until the 
shearing distance reaches a minimum spaced distance. This minimum spaced 
distance is a shearing flight tip, which may be rounded. Also, the portion 
of the scraping flight closest to the inner surface of the barrel, the 
scraping flight tip, may have a scraping corner adjacent the inner surface 
of the barrel. The may be substantially rectangular. 
The apparatus may also comprise a second barrel chamber having a second 
longitudinal axis and a second shaft extending along the second 
longitudinal axis, whereby the barrel chamber communicates with the second 
barrel chamber via an opening. 
Desirably, the scraping flight radially extends to the inner surface of the 
barrel. Alternatively, the scraping flight may radially extend to a spaced 
tolerance distance from the inner surface of the barrel, and the spaced 
distance between the shearing flight and the inner surface of the barrel, 
the shearing distance, may be greater than the tolerance distance. The 
shearing distance also may be greater than ten times the tolerance 
distance. Preferably, the tolerance distance is about 0.2% of the radial 
distance from the longitudinal axis to the inner surface of the barrel. 
Where the total shearing length is defined as the totalled sum of the 
periods of all of the shearing flights, and the total scraping length is 
defined as the totalled sum of the periods of all of the scraping flights, 
the total scraping flight may be greater than the total shearing length. 
Alternatively, the total scraping flight may be less than the total 
shearing length. Yet further, the total scraping flight may be about equal 
to the total shearing length. 
Preferably, the number of threads is equal to four, and the helix angle of 
the threads is about equal to about 17-18 degrees. 
Desirably, the screw defines a plurality of channel sections between the 
threads where channel section has a channel surface which is radially 
spaced from the barrel wall by a channel depth. At least one of the 
channel sections is bounded in the rearward direction by a scraping flight 
and has a channel depth which is at a maximum adjacent the bounding 
scraping flight and which decreases in the forward direction away from the 
scraping flight. Further, the channel section may be bounded in the 
forward direction by one of the shearing flights, and the channel depth 
may be at a minimum adjacent the bounding shearing flight and increase in 
the rearward direction away from the scraping flight. Yet further, the 
channel section is further bounded by a second shearing flight which is 
connected to the bounding scraping flight at a connecting point and has a 
channel depth which increases immediately adjacent the bounding scraping 
flight and the bounding second shearing flight in the directions away from 
the connecting point. 
In another aspect of the claimed invention, a machine for processing solid 
and viscous plastic and polymeric materials comprises a hollow barrel 
defining a barrel chamber, the barrel having an inner surface, an outer 
surface, a first end, a second end, and a longitudinal axis extending 
between the ends. The barrel defines axial directions along the axis, 
radial directions traverse to the axis and rotational directions around 
the axis. The machine also comprises an inlet means for introducing a 
material into the barrel chamber, and a shaft extending along the 
longitudinal axis and having a first end adjacent the first end of the 
barrel and a second end adjacent the second end of the barrel. 
A plurality of screw segments are disposed on the shaft, and each screw 
segment is capable of rotating in a circumferential direction. Each screw 
segment includes either a scraping flight for removing the material from 
the inner surface of the barrel, or a shearing flight radially extending 
to a spaced distance from the inner surface of the barrel. Upon rotation 
of the screw segments, the scraping flight collects and guides the 
material to at least one of the shearing flights of at least one of the 
screw segments, and the material is sheared between the shearing flight 
and the inner surface of the barrel. 
Preferably, each screw segment includes at least one shearing flight and at 
least one scraping flight. 
In yet another aspect of the present invention, a screw element for 
processing and mixing materials comprises a body defining upstream and 
downstream ends, a longitudinal axis extending between the ends and 
forward and rearward circumferential directions about the longitudinal 
axis. The screw element also includes a plurality of flights spaced apart 
from one another and a plurality of channels between the flights. The 
flights include at least one scraping flight having a crest at a scraping 
radius from the axis and at least one shearing flight having a crest at a 
shearing radius from the axis, the shearing radius being less than the 
scraping radius. The flights are constructed and arranged so that upon 
rotation of the screw in the forward circumferential direction within a 
barrel, the scraping flight collects material disposed within the barrel 
and guides the material to the shearing flight. 
Preferably, the flights are generally helical. 
Desirably, the shearing flight has an inlet surface extending from the 
channel rearward of the crest of the shearing flight. The inlet surface 
slopes radially outwardly in the rearward circumferential direction toward 
the crest of the shearing flight so as to define a smooth transition from 
the channel to the crest of the shearing flight. Further, the shearing 
flight may have an outlet surface extending rearwardly from the crest of 
the shearing flight, the outlet surface sloping radially inwardly in the 
rearward circumferential direction away from the crest so as to define a 
smooth transition from the crest of the shearing flight to a channel. 
The screw element may also have a shearing flight whose leading surface 
faces in the circumferentially forward direction and extends substantially 
radially outwardly. 
The screw element may further include a plurality of shearing and scraping 
flights. The plurality of scraping and shearing flights may be arranged in 
alternating sequence around the circumference of the body at a first axial 
location. The plurality of scraping and shearing flights may also include 
a plurality of scraping flights at a first axial location and a plurality 
of shearing flights at a second axial location. 
Yet further, the flights of the screw element preferably include first and 
second sets of flights extending in axially opposite directions from a 
common meeting plane, the flights of the first and second sets being 
offset from one another in circumferential directions at the meeting plane 
so as to define discontinuities between flights at the meeting plane. 
Preferably, the body includes a plurality of generally cylindrical body 
sections, each body section carrying at least one of the flights, and 
means for securing the body sections to one another in coaxial, end-to-end 
relationship. The securing means may be operative to secure the sections 
to one another in a plurality of different relative positions in the 
circumferential directions, whereby the relative circumferential positions 
of the flights on the sections may be adjusted. 
The flights may also constitute a plurality of substantially continuous, 
generally helical threads extending from adjacent the upstream end to 
adjacent the downstream end, each thread including a plurality of flights. 
Preferably, the shearing radius is less than the scraping radius by a 
difference equal to between about 0.2% and about 0.4% of the scraping 
radius. 
The advantages and flexibility of the present invention apply in almost any 
context where dispersive mixing, extensive mixing, composition or 
temperature homogenization, melting and mixing and devolatilization is 
desirable.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
FIG. 1 depicts the general layout of a compounding extruder in accordance 
with one embodiment of the invention. A tubular, generally cylindrical 
barrel 10 provides an inlet port 12 at a first or upstream end 30 of the 
barrel and an outlet port 14 at the other or downstream end 20 of the 
barrel. A shaft 34 extends from upstream end 30 to downstream end 20 of 
the barrel along the longitudinal axis 24 of barrel 10. The shaft is 
supported for rotation within the barrel about axis 24. At upstream end 
30, shaft 34 extends beyond the end of barrel 10 and is connected to 
rotary drive device 40. Rotary drive device 40 may include a conventional 
motor and gear train (not shown) arranged to rotate the shaft in a 
preselected direction of rotation 46 about axis 24. 
For ease of reference, directions are stated in this disclosure with 
reference to the longitudinal axis 24. Thus the terms "axial" and 
"axially" should be understood as referring to the directions parallel to 
the longitudinal axis, whereas the terms "radial" and "radially" should be 
understood as referring to the directions transverse to this axis. The 
term "radially inward" refers to the directions towards the axis, whereas 
"radially outward" refers to the directions away from the axis. 
"Circumferential" or "rotational" directions refer to the directions 
around axis 24 such as that indicated by arrow 46. 
A generally cylindrical screw element 32 is mounted on shaft 34 coaxially 
therewith, so that the axis of screw element 34 is coincident with the 
axis 24 of the barrel. The screw element has an upstream end 32a disposed 
at the upstream end 30 of the barrel and a downstream end 32b disposed at 
the downstream end 20 of the barrel. Screw 32 has a body 33 and a mixing 
section 39 with four helical threads 71-74. Helical threads 71-74 are 
formed integrally with body 33, and wrap around the body from over the 
upstream to downstream length of mixing section 39. Screw 32 further 
includes a feed section 38 upstream of mixing section 39 and a metering 
section 31 downstream of the mixing section. The feed section and metering 
section are of generally conventional construction as commonly employed in 
extruder screws. For example, the feed section may include one or more 
threads and channels of constant or progressively decreasing 
cross-sectional area, whereas the metering section typically includes one 
or more threads and channels having relatively small cross-sectional area 
so as to restrict or meter flow of material through the apparatus. 
As used herein, the term "helical" does not necessarily imply that a 
structure completes a full rotation about the longitudinal axis. "Helical" 
also refers to those structures which both form an arc of a circle in the 
rotational direction, and which also extend in the axial direction. The 
terms "forward" and "rearward" are used herein with respect to 
circumferential or rotational directions around axis 24. As so used, the 
"forward" direction is the direction corresponding to the direction of 
rotation which would appear to move the threads of the screw towards the 
downstream end of the barrel. Thus, for a right-hand screw as illustrated 
in FIGS. 1 and 2, the clockwise direction of rotation as seen from the 
downstream end 20 of the screw is the forward direction, whereas the 
counterclockwise direction as seen from the downstream end is the rearward 
direction. For a left-handed screw, these directions would be reversed. A 
first point described as being "before" a second point in the rotational 
direction shall mean that the first point is disposed in the forward 
rotational direction from the second point. A first point being described 
as "following" a second point in the rotational direction shall mean that 
the first point is disposed rearwardly of said second point. The four 
threads 71-74, inner barrel surface 11, and screw body 33 cooperatively 
define helical spaces or "channels" 26. The distance between screw body 
surface 33 and inner barrel surface 11 is referred to as the "channel 
depth". 
As shown in cross-sectional view in FIG. 2, screw element 32 is disposed 
concentrically about shaft 34. Screw 32 maintains a fixed position with 
respect to shaft 34 via key 35. Key 35, which is integral with shaft 34, 
extends radially outward from shaft 34 and fits into accepting slot 37 of 
screw 32. As slot 37 is substantially the same shape as key 35, rotation 
of shaft 34 will cause likewise rotation of screw 32. However, while screw 
32 has a fixed rotational relationship with shaft 34, key 35 does not 
prevent axial movement of screw 32 along shaft 34. Therefore, screw 32 may 
be removed and replaced on shaft 34 by axially sliding the screw element 
on and off the shaft. During operation, axial movement of screw 32 is 
prevented by a stop (not shown). 
FIG. 2 also illustrates the configuration of the four threads 71-74 at a 
particular axial location along the screw, i.e., at a particular distance 
from the upstream end 32a of the screw, and hence at a particular axial 
distance from the upstream end 30 of the barrel 10. Specifically, a 
portion 75 of thread 71 is shown in cross-section, as is a portion 76 of 
thread 72, a portion 77 of thread 73, and a portion 78 of thread 74. 
The portion 78 of thread 74 shown in FIG. 2 is configured as a shearing 
flight. The portion of the shearing flight 78 closest to the inner barrel 
surface 11 is shearing tip or crest 50. An inlet surface or shearing feed 
area 48 begins forwardly of shearing crest 50, in a channel 26, and slopes 
radially outwardly in the rearward circumferential direction so as to 
provide a smooth transition between the channel and the shearing tip or 
crest. An outlet surface 49, also referred to as a scraping feed area, 
extends rearwardly from the shearing tip or crest 50 towards the next 
following channel 26. Outlet surface 49 slopes radially inwardly in the 
rearward circumferential direction. The shearing tip or crest 50 is 
rounded to provide a smooth transition from the region immediately before 
the shearing tip (inlet or shearing feed surface 48) to the area 
immediately after the shearing tip in the rotation direction 46 (outlet or 
scraping feed surface 49). 
The tip or crest 50 of the shearing flight lies at a shearing radius 
R.sub.shear from axis 24. The shearing radius is less than the radius 
R.sub.barrel of barrel 10 by a preselected shearing clearance C.sub.shear 
so that there is a gap 54 between shearing tip or crest 50 and the inner 
surface 11 of the barrel. This narrow gap between shearing tip 50 and 
inner barrel surface 11 is small enough to provide high shearing forces to 
any material passing through the gap. However, narrow gap 54 is not so 
small that it prevents compounding material from passing from the shearing 
feed area 48 to the other side of the shearing tip. The distance between 
screw surface 33 and inner barrel surface 11 increases with increasing 
circumferential distance from shearing tip 50. In other words, in shearing 
feed area 48, the distance between the surface of screw body 33 and inner 
barrel surface 11 tapers to the narrow gap 54. In the outlet or scraping 
feed region 49, the surface of screw body 33 is similarly tapered. Portion 
76 of thread 72 is likewise configured as a shearing flight, with the 
identical cross-sectional structure of shearing flight 78. 
In contrast to shearing flights 78 and 76, portions 75 and 77 of threads 71 
and 73, respectively, are configured as scraping flights. Scraping flight 
75 has a leading wall 58 facing forwardly towards shearing flight 78 and 
scraping feed region 49 and a trailing wall 59 facing rearwardly towards 
the other shearing flight 76. The leading and trailing walls of the 
scraping flight extend substantially radially outwardly from the surface 
of screw body 33 to the outermost tip or crest 60 of the scraping flight. 
Almost directly abutting inner barrel surface 11 is scraping tip 60 of 
scraping flight 75. Scraping tip 60 forms a sharp or almost sharp corner 
with leading wall 58, preferably at a right angle. Thus the scraping tip 
60 is disposed at a scraping radius R.sub.scrape greater than the shearing 
radius R.sub.shear of the shearing flights, so that the scraping tip or 
crest 60 is disposed at only a very small tolerance distance from the 
inside surface 11 barrel. This tolerance distance D.sub.t (the difference 
between the scraping radius R.sub.scrape and the interior radius of the 
barrel R.sub.barrel) should be as small as practicable and should be small 
enough to substantially prevent compounding material from passing between 
the scraping flight and the barrel. Preferably, the tolerance is less than 
about 0.4%, and more preferably about 0.2% of the radial distance from the 
longitudinal axis 24 to inner barrel surface 11. The diameter of screw 
element 32 is defined as twice the radial distance from longitudinal axis 
24 to the tip 60 of scraping flight 75. 
In essence, the scraping flight provides an area of very tight flight 
clearance, i.e., very little distance between the tip of the scraping 
flight and the inner surface of the barrel, and the shearing flight 
provides an area of loose flight clearance, i.e., the tip of the shearing 
flight is a relatively larger spaced distance from the inner surface of 
the barrel than the scraping flight. Stated another way, the shearing gap 
clearance C.sub.shear is substantially larger than the tolerance distance. 
Although the optimum value of the shearing gap clearance will vary 
depending on the application, a shearing gap clearance equal to about 2% 
to about 7% of the scraping radius or barrel radius, and more desirably 
about 3% to about 6% of the scraping radius or barrel radius, can be used. 
A shearing gap clearance of about 4.5% of the scraping radius is 
particularly desirable. The preferred length of the clearance may be 
effected by numerous variables, including the operating screw speed, the 
nature of the materials, and the barrel size. 
Barrel 10 includes axially-extending holes 64, through which a cooling 
medium, such as water, is provided to cool the barrel and materials in 
contact with the barrel. 
FIG. 3 illustrates how the shearing and scraping flights alternate along 
the threads. FIG. 3 is a projection of a schematic of the threads, as if 
screw element 32 were rolled through one complete rotation along a flat 
surface and threads 71-74 left the markings shown in the figure. As shown 
by dimension 42, the distance covered by one full rotation of screw 
element 32 is equal the circumference, or diameter D of screw 32 
multiplied by Pi. Only a portion of the entire axial length of the screw 
is shown; screw element 32 continues in the direction of the ends 20 and 
30 as indicated. 
Within the mixing section 39, each thread alternates in the axial direction 
from a scraping flight to a shearing flight along its entire length. For 
example, thread 74 includes shearing flight 78, followed in the axial 
direction 43 by scraping flight 80, followed by shearing flight 81, 
followed by scraping flight 82, followed by shearing flight 83, etc. 
Scraping flights are shown as solid lines, while shearing flights are 
shown as broken lines. 
As shown in the preferred embodiment of FIG. 2 and as shown in the 
projection of the preferred embodiment in FIG. 3, the shearing and 
scraping flights also alternate in the rotational direction. Specifically, 
shearing flight 78 of thread 74 is followed in the rotation direction 46 
by scraping flight 75 of thread 71. Likewise, scraping flight 75 is in 
turn followed in the rotation direction 46 by shearing flight 76 of thread 
72, which is in turn followed by scraping flight 77 of thread 73, and 
which is in turn followed by shearing flight 78 of thread 71, thus 
completing the cycle. The period of individual shearing and scraping 
flights is measured with respect to the length of the individual flight in 
the rotational direction 46, and if the flights are all of equal length, 
the period is equal to: 
##EQU1## 
Accordingly, because there are four shearing flights and four scraping 
flights of equal length, the period of the individual shearing flights and 
scraping flights of the preferred embodiment will be D*Pi/8. The channel 
width 80 between any two threads is equal to the distance between the 
threads in the axial direction 43. 
In operation, material such as a polymer and an additive to be compounded 
is deposited through inlet port 12 into the feed section 38. In the feed 
section, the polymer is brought to a fluid or partially fluid condition 
(as by melting where the polymer is a thermoplastic) and advanced to the 
mixing section 39 where it enters channels 26 between the screw threads 
71-74, screw 32, and inner barrel surface 11. Rotary drive device 40 
rotates shaft 34, and hence screw element 32, in the forward rotational 
direction 46 about longitudinal axis 24. 
As screw 32 rotates, any material present in channels 26 will come in 
contact with either a scraping flight or shearing flight of the threads. 
For example, as shown in FIG. 2, as material 90 accumulates in the 
shearing feed area 48, some of the material will eventually be pushed 
through narrow gap 54 between shearing tip 50 and barrel inner surface 11 
due to the rotation between the screw and barrel. 
As the material 90 passes through clearance 54, the material will 
experience high shear stresses between shearing tip 50 and inner barrel 
surface 11. These high shear stresses will rupture the solid particles 
into fragments, thus compounding the solid fragments into the surrounding 
fluid polymer matrix. After the material 90 passes through the high stress 
clearance 54, it will accumulate in front of scraping flight 75 in 
scraping feed area 49. 
Because of the extremely tight flight clearance between scraping tip 60 and 
inner barrel surface 11, the material will not be able to pass over 
scraping flight 75, but will instead collect along wall 58 and in scraping 
feed area 49. 
From there, a typical flow pattern for the material is shown as path 92a in 
FIG. 3. Because the material cannot pass over scraping flight 75, it will 
be moved in the barrel rotation direction 47 along scraping flight 75 of 
thread 71, until it encounters shearing flight 90 of the same thread. The 
material will pass over shearing flight 90, where it will again experience 
high stress and a rupturing of the solids. After the material passes over 
shearing flight 90, it will be gathered by scraping flight 91 of the 
following thread 72. Scraping flight 91 will guide the material to 
shearing flight 89 of the same thread, where it will again experience high 
shearing forces. As shown by path 92b, after the material passes over 
shearing flight 89, the material will be collected by scraping flight 93 
of the following thread 73. 
This process of shearing, scraping, guiding to shearing a flight, and then 
shearing again continues as the material travels down the length of the 
screw. The repeated shearing will result in a highly effective compounding 
of the material. The helical nature of the screw, the continuous of influx 
of material into the screw, and the rotation of the screw all combine to 
force the material from upstream end 30 towards downstream end 20. 
Eventually, the material will pass through the metering section 31 and out 
of the extruder 1 via outlets 14. 
Although paths 92a and 92b show one possible route a given particle of 
material may take through the compounder, the probable path for most of 
the material is contemplated to be different. For example, the size of 
narrow gap 54 for the shearing portions is preferably not so large that 
all the material in shearing feed area 48 will pass through the gap. 
Rather, most of the material should take a path similar to path 93, where 
a portion of the material 93a in channel 94 passes over shearing flight 95 
of thread 72, and the rest of the material 93b continues down channel 94, 
to be eventually sheared by another shearing flight. Further, the material 
93b traveling down channel 94 will mix with the material 96 entering 
channel 94 via shearing flight 97. Consequently, the nature of the 
alternating shearing and scraping flights allows for not only a great deal 
of high stress areas, but also a great deal of random mixing. A particular 
stream of material will be split apart and recombined with other streams 
of materials many times before finally exiting compounder 1 via outlets 
14. 
Extruder 1 is extremely effective in accomplishing four important aspects 
of a good compounder. First, it provides high shear stress regions by 
passing the material through narrow gap regions. Second, it has a flow 
pattern which brings about repeated passages through the high shear 
regions. 
Third, it provides good extensive mixing. With typical flow paths such as 
that shown by paths 93 and 96 in FIG. 3, streams of material will be split 
and recombined as the screw rotates, thus providing extensive mixing. 
Fourth, it provides good temperature control in the shearing regions. 
Generally, shear stress is a product of viscosity and shear rate, and as 
the temperature increases, viscosity drops and stress levels become 
insufficient for rupturing agglomerates. Extruder 1 prevents the 
temperature from exceeding unacceptable levels by its cooling tubes 64, 
the scraping flights 75, and the shearing flights 58. The inner surface 11 
of barrel 10 is cooled by passing a cooling medium such as water through 
tubes 64. Scraping flight 52 removes material from the inner surface 11 of 
barrel 10, thus preventing material from accumulating as a stationary 
layer on the interior surface of the barrel and insulating the rest of the 
material from heat transfer. Thus, the scraping flights aid in maintaining 
a relatively high heat transfer coefficient on the interior surface of the 
barrel. Therefore, when material passes through the narrow gap between 
shearing tip 50 and inner barrel surface 11, the material will remain at a 
relatively low temperature, thus securing the high stress levels required 
for effective dispersion. 
The screw and extruder as described above can be used in achieving 
dispersion, phase contacting and/or heat transfer with many fluid or 
semifluid materials. As mentioned above, a particularly preferred 
application is in forming dispersions where one or both phases are 
polymeric materials as, for example, in incorporation of additives such as 
pigments, stabilizers, fire retardants, antimicrobial agents and fillers 
into polymers. However, the screw and extruder may also be used with 
systems where both phases are polymers. The present compounding extruder 
is especially relevant to the dispersion of noncompatible polymers for the 
preparation of blends and alloys in addition to routine compounding. The 
repeated stress regions and mixing will prevent non-compatible plastics 
from separating after the material leaves the screw. 
Due to the leakage of material from one channel to another, extruders and 
screws as described above provide for extensive mixing independent of the 
high stress rates. They are also useful in causing reactions between two 
separate materials, because good reactions require good mixing. 
Screws as described above are also applicable to melting and 
devolatilization (i.e., removal of low molecular weight volatile 
components such as residual solvents monomer, etc.). These are important 
unit operations carried out in processing machines. In the melting 
process, at the point the material is partially molten, repeated shearing 
and scraping of the partially molten material effectively completes the 
melting process and creates a homogeneous molten mass of material. The 
source of the melting energy is the conversion of mechanical energy into 
heat in the high shear regions. This is termed as 
"dissipative-mix-melting". In devolatilization, the molten material in a 
partially filled machine is exposed to high vacuum. The volatile material 
diffuses through the exposed surfaces to the vapor space. Repeated 
shearing and scraping of the material will effectively devolitize the 
material such that all the material is repeatedly exposed to the high 
vacuum vapor space. 
The invention allows for a variety of options, many of which directly 
affect the typical stream paths of the material through the screw, and, 
thus, the dispersive behavior. 
Many configurations of the shearing and scraping flights are possible. For 
example, in FIG. 3, at any particular point along the screw, the shearing 
and scraping flights alternate in the rotation direction 46. In another 
preferred embodiment, FIG. 4, the shearing and scraping flights do not 
alternate in the rotation direction. Rather, in rotation direction 146, 
each shearing flight is followed by another shearing flight, and each 
scraping flight is followed by another scraping flight. For example, 
shearing flight 121 of thread 111 is followed in the rotation direction 
146 by shearing flight 122 of thread 112. Likewise, shearing flight 122 is 
followed in the rotation direction by shearing flight 123 of thread 113, 
shearing flight 123 is followed in the rotation direction by shearing 
flight 124 of thread 114, and shearing flight 124 is followed in the 
rotation direction by shearing flight 121 of thread 111, thus completing 
the cycle. 
FIG. 4 also shows a variation in the period of the scraping and shearing 
flights. As there are four scraping and shearing flights along a single 
rotation of one thread, as shown by shearing and scraping flights 121, 
132, 133, and 134, the period of the individual shearing and scraping 
flights is D*Pi/4. 
Although the present invention is not limited by any theory of operation, 
the preferred embodiment of FIG. 4 is contemplated to provide a different 
flow path than the preferred embodiment of FIG. 3. Because the shearing 
flights are larger (in period) and immediately follow one another in the 
rotation direction, the material to be compounded will experience a higher 
number of passes than the embodiment of FIG. 3. Path 40 illustrates that a 
particular portion of the material might pass through quite a few shearing 
flights before it is diverted along paths 40a, 40b, 40c, or 40d, and 
before flowing into channels 41, 42, 43, or 44, respectively. Because it 
is contemplated that more material will pass over the shearing flights 
than travel down the channels as compared to the preferred embodiment of 
FIG. 3, the velocity of the flow of material through the individual 
channels in FIG. 4 will decrease, and, therefore, the flow velocity 
throughout the entire screw will also decrease. 
FIG. 5 shows yet another preferred embodiment, where the periods of the 
scraping and shearing flights are variable, and where a a portion of a 
scraping flight 210 is followed in the rotation direction by a shearing 
flight 211, and another portion of scraping flight 210 is followed in the 
rotation direction by another scraping flight 212. By varying the periods 
of the individual flights and their axial position with respect to one 
another, it is contemplated that a large variety of parameters can be 
emphasized or deemphasized, such as the average number of shearing passes 
for a given particle of material, the percentage of material experiencing 
shearing passes, the flow rate of the material, and the extent of mixing 
of different streams. For example, if the period and number of scraping 
flights are substantially greater than the period and number of shearing 
flights, then more of the material will tend to travel down the same 
channel, without passing through high stress regions or mixing with other 
streams. Likewise, if the period and number of shearing flights are 
substantially greater than the period and number of scraping flights, then 
while the material will experience repeated stress and extensive mixing 
with other streams, it will probably not flow as quickly down the 
channels, and the overall flow rate for the compounder will decrease. 
While FIGS. 4-5 are all directed to a four-flighted screw, i.e., a screw 
with four threads, it is also possible to increase or decrease the total 
number of threads. 
It is also possible to change the angle of the threads to affect the flow 
material through the compounder. For example, FIGS. 3-5 all show a 
four-flighted square-pitched screw with a 17-18 degree helix angle. The 
helix angle could be increased or decreased depending on the needs of the 
material to be compounded. Because large helix angles provide better 
pumping performance, it contemplated that large helix angles are 
particularly advantageous in a compounder of the present invention because 
the increased pumping performance will compensate for the leakage of 
material over the shearing flights. It is also possible to vary the helix 
angle with increasing axial distance from one of the screw ends. 
Yet another parameter which may be varied is the channel depth. FIG. 7 
illustrates a manner of varying the channel depth in order to effect the 
flow path. The length of hash marks 410-427 indicate the depth of the 
channel at that point, such that the longer hash marks indicate greater 
depth (hereinafter "deep") and the shorter hash marks indicate lesser 
depth (hereinafter "shallow"). Specifically, channel 400 is "deep" at hash 
marks 411, 413, and channel 410 is deep at hash marks 424 and 426. The 
channel 400 is "shallow" at hash marks 415-422, and "medium" at hash marks 
410, 412, and 414. Channel 410 is also medium at hash marks 423, 425, and 
427. 
Path 490 indicates a typical path of material through channel 400. As the 
material passes through shearing flight 450, it is contemplated that the 
material will accumulate in the deeper sections of the channel, namely 
near hash mark 411 and adjacent to scraping flight 451, and away from the 
more shallow portions of the channel (hash marks 420-422). As the screw 
rotates and forces the material down channel 400, the channel depth 
adjacent scraping flight 451 decreases towards medium hash mark 412, 
forcing more of the material towards the middle of the channel. 
Accordingly, despite the fact that channel 400 is shallow near shearing 
flight 455 (as indicated by hash marks 417-420), some of the material 490b 
will pass over the shearing clearance and into channel 410. However, most 
of the material 490a will be forced away from shearing flight 455 because 
of the shallow depth, and will enter the deeper portion of the channel 
near hash mark 413. There, the material 490a will mix with the material 
491 passing over shearing flight 452. The combined mix of material will 
then continue down channel 400. Accordingly, it is contemplated that the 
amount of material passing through the shearing flights can be regulated 
not only by varying the period of the shearing flights and the position of 
the shearing flights with respect to the scraping flights, but also by 
varying the channel depths between the threads. 
In order to allow users the greatest amount of flexibility in configuring 
their systems to their particular needs, it is preferable to break the 
screw element into different segments for placement on the shaft. As shown 
in FIG. 6, three such screw segments 331-333 are coaxially disposed on 
shaft 334 so that the screw segments lie in end-to-end relation with one 
another. Each screw segment includes slot 337 for receiving key 335, which 
is integral with shaft 334. Thus, all the segments maintain a fixed 
rotational relationship with shaft 334. 
Segment 331 includes scraping flights 337 and 375, and also includes 
scraping flight 378 and another scraping flight (not shown). Similarly, 
screw segment 332 includes scraping flight 380 (and another scraping 
flight which is not shown) and shearing flights 381 and 382, and screw 
element 333 includes scraping flights 390 and 393 and shearing flight 392 
(and another shearing flight which is not shown). The flights, either 
scraping or shearing, are not continuous at the common meeting planes 
between the segments. Instead, at the common meeting planes, the flights 
are offset from one another in circumferential directions. Thus one end of 
scraping flight 380 terminates at the edge of segment 332, adjacent to 
channel 348 between scraping flight 377 and shearing flight 378. Likewise, 
the other end of scraping flight 380 terminates in the channel between 
scraping flight 393 and shearing flight 392. It is contemplated that such 
circumferential offsetting and lack of continuous threads will introduce 
more random and extensive mixing of the material. 
In essence, by using separate segments, the user can set up the screw to 
fit his needs to suit the material to be compounded. For example, if it is 
preferable for the segments to form continuous threads across the entire 
length of shaft 334, the segments could be rotated with respect to the 
shaft to form the continuous threads. As shown in FIG. 6, key 335 could be 
placed in slot 339 instead of slot 337 in order to form a continuous 
thread from segment 331 to 332, the thread being made up of scraping 
flight 377 and scraping flight 380. 
If key 335 were placed in yet a different slot (not shown), a continuous 
thread could include a shearing flight 378 along segment 331, and a 
scraping flight 380 along segment 332. In this manner, the preferred 
embodiment of FIG. 3 could be created with the segments of FIG. 6, where 
the shearing and scraping flights alternate in both the rotational 
direction and axial directions. 
The keys and slots mentioned above thus are arranged to secure the segments 
of the screw element to one another, and to the shaft, in a plurality of 
different relative positions in the circumferential directions. Any other 
mechanical arrangement which securely fastens the segments to one another 
can be used to accomplish the same function. Merely by way of example, the 
shaft and segments may be provided with corresponding splines. 
Alternatively, the shaft may be omitted and the segments may be connected 
to one another as by bolts, pins or other suitable fastening devices. If 
desired, the screw can be provided with internal heating and/or cooling 
devices. 
In yet another preferred embodiment, each individual screw segment might 
have all shearing flights or all scraping flights. Such segments would 
allow the creation of the preferred embodiment in FIG. 4 if all the 
flights of the different segments were lined up to form continuous threads 
across the length of the shaft. The segments may also be used to create a 
non-cylindrical surface of revolution for the outer screw surface. For 
example, the diameter of the outer surface of the screw, whether measured 
from the crests of the threads or the depths of the channels, may increase 
or decrease with changing axial distance from the ends. Clearly, the use 
of segments permits an infinite number of combinations and a great amount 
of flexibility. 
Taking into account all of the above considerations and possibilities, and 
although the present invention is not limited to particular dimensions or 
parameters, one preferred embodiment includes a six-flighted screw with a 
25 degree helix angle to pump 1000 lb/hr of general purpose polystyrene 
(453 kg/hr) when operating at 90 RPM. The scraping flights and shearing 
flights are arranged similar to FIG. 3, where the scraping flights and 
shearing flights alternate in the rotation direction at any point along 
the length of the screw. The period of each individual shearing flight and 
each individual scraping flight is equal to (D*Pi)/12, such that on one 
revolution of one thread, there are six scraping flights and six shearing 
flights. The diameter of the screw is about 6 inches (15.24 cm), and the 
axial lead is 8.79 inches (22.3 cm). The mean channel depth is 0.2 to 0.3 
inches (0.50-0.75 cm). A narrow gap distance of 56 milles provides a 
shearing rate of 500 s.sup.-1. According to fluid dynamics theory and 
under the above constraints, the flow rate over the shearing portions 
should be equivalent to one-half the net flow rate through the channels. 
Accordingly, the flow rate in any individual channel will tend to vary 
between 75% to 125% of the mean flow rate of all the channels. The average 
number of passes for a typical particle of material will be three. The 
approximate flow path for all the material will be such that 1.5% of the 
material in the screw experiences 0 passes or 6 passes, 9.4% will 
experience 1 pass or 5 passes, 23.5% will experience 2 passes or 4 passes, 
and 31.3% will experience 3 passes. 
A screw according to the present invention may also be used in more than a 
single screw configuration. A screw element including shearing and 
scraping flights could be used in almost any processing device, compounder 
or mixer which uses screws, including, but not limited to, twin screw 
extruders (intermeshing or non-intermeshing, corotating or 
counterrotating), and multiple screw machines. Screws according to the 
present invention can also be used in screw injection molding machines, 
and screw blow molding machines. A screw of the present invention would 
replace the screw in the above machines. In fact, the advantages and 
flexibility of the present invention applies in almost any context where 
dispersive mixing is desirable. 
FIG. 8 is a cross-sectional view of non-intermeshing twin screw 500 
containing two screw elements 532 and 533 disposed on shafts 534 and 535. 
Screw elements 532 and 533 have shearing and scraping flights as described 
extensively above. Screw 532 and shaft 534 are disposed within barrel 
chamber 510, and screw 533 and shaft 535 are disposed within barrel 
chamber 511. Barrel chamber 510 communicates with barrel chamber 511 via 
opening 550. The operation of the twin screw compounder 500 is similar to 
the operation discussed above, but with the additional exchange of 
materials from barrel chamber to the other. 
As these and other variations and combinations of the features described 
above can be utilized without departing from the present invention as 
defined in the appended claims, the foregoing description of the preferred 
embodiments should be understood as being illustrative rather than as 
limiting the invention as defined in the claims.