Extruder screw with multiple flighting

A plastic materials screw has a metering section defined by a series of axially extending discontinuous and partially overlapping flightings. The flightings overlap each other by about a half to one turn to define overlapping portions whereby the flow in the primary channel is divided into a pair of secondary flow channels which have velocity profiles normal to the channel root greater than that in the primary channel. At the end of the overlapped portion, the first flighting is discontinued and the second flighting continues with a result that the flows are recombined in a distributive manner to provide improved mixing with a minimum of additional shear stress.

BACKGROUND OF THE INVENTION 
This invention pertains to an extruder screw for use in the barrel of a 
plastics material extruder, and is more particularly directed to the 
construction and design of the metering section of such a screw. 
Typically, the metering section of the screw immediately follows a section 
in which the plastic material has been fully melted. Often, this section 
is called the "pressure section" since it is the region in which the 
molten material is stabilized in temperature, is further mixed, and is 
delivered under pressure, usually to a final mixing or kneading stage 
before exiting the barrel. 
Mixing is an important task of a single screw extruder, the other tasks 
being conveying and melting. However,, mixing in such single extruders has 
always been a problem due to lack of uniformity and homogeneity in the 
melt. 
The typical single flight single screw extruder inherently produces a 
non-uniform mix. This is because there is a large difference in the shear 
rate and the residence time in the outer and inner regions of the channel 
between the flighting. These result in a variation in the shear strain 
within the channel. The circular flow, both parallel to and normal to the 
screw, results in a shearing action in the outer portion which is 
counteracted by a shearing action in the inner portion, with locations in 
which the shear is zero. Such models lack elongational or extentional 
mixing and distributing mixing mechanisms, and therefore, complete mixing 
must depend upon the extent and efficiency of the customary final mixing 
stage. 
Also, screw designs in the metering section have, commonly not adequately 
provided for a reduction or elimination of pressure fluctuations within 
this section. Flighting channels which vary in depth such as by tapering 
the core diameter, can, in some instances, produce a fluctuation in the 
rate of flow which, in turn, can be seen as a fluctuation at the output 
end of the screw. 
SUMMARY OF THE INVENTION 
This invention is direct to a screw design in which a plurality of 
flightings interact to divide the flow, and then recombine and then 
redivide and recombine the flow through a plurality of divisions. 
Simultaneously, at the regions of division, the flow is divided into two 
smaller channels which provide a higher shear stress within or along the 
channels as compared to that of the single channel. 
Individual lengths or sections of flighting are partially overlapped to 
produce overlapping channels. The overlapped lengths extend for 
approximately one to one and a half flighting turns, at which point the 
next succeeding or secondary flighting becomes a primary flighting and 
delivers the melt to a further successive flighting in overlapping 
relation to the primary flighting. This may be repeated throughout the 
metering section, i.e., a division of flow from single channel to dual 
channel to single channel, in which the succeeding flighting then becomes 
the primary flighting which delivers the melt to second and subsequent 
divisions through regions of mutually overlapping flighting. In this 
manner, the flow is split and divided, and smaller overlapping channels 
are formed between the flights at the overlapping sections in which the 
fluid flow is accelerated both along the turns of the dual flighting and, 
rotationally within the respective channels, to be recombined into a 
single channel with accompanying deceleration. The polymer which was in 
the low shear center portion of the single channel is redistributed into 
the outer high shear areas of the dual channels. At each successive dual 
channel section the polymer on the trailing flank of the primary flighting 
is separated and exchanged to become immediately rearranged and positioned 
on the pushing flank of the new primary flighting. The modification of the 
shear mixing, accompanied by simultaneous distributive mixing during the 
acceleration and deceleration phases results in substantially improved 
overall mixing throughout the length of the metering section. 
Throughout the metering section, it is preferred that the core diameter 
remain constant or relatively constant, i.e., the core is not tapered. 
This provides channels of substantially uniform depth along the metering 
section. 
Accordingly, it is an important object of this invention to increase 
distributive mixing by providing a screw metering section with a plurality 
of partially inter-fitted or overlapping flightings which divide and 
recombine the flow defining between partially overlapping portions and 
conventional portions. The flow stream is divided from a single channel to 
parallel twin channels and then back to a single channel, in which the 
relative positions of the polymer in divided flows is reversed. 
The resulting more complete mixing reduces temperature stratification in 
the melt thereby making temperature throughout the melt more uniform, with 
minimum additional shear energy input. 
These and other objects and advantages of the invention will be apparent 
from the following description, the accompanying drawings and the appended 
claims.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
Referring to the figures of the drawing, which illustrate a preferred 
embodiment of the invention, a metering section of a screw, for delivering 
molten plastic material is illustrated generally at 10 in FIG. 1, 
beginning with a primary flighting 12 on a core 15. The inlet or beginning 
of the metering section is illustrated generally at 13, while the outlet 
end or forward of the metered section is illustrated at 14. A concept of 
this invention may be presented over a shorter or longer metering section 
within the scope of the invention. The inlet end 13 may be fed 
conventionally from a mixing or melt section of the screw or may be made 
according to copending applications (BKD 162 P2) filed concurrently 
herewith while the forward end 14 may feed a high shear mixer in 
accordance with the requirements of the material which may, as an example, 
be a final mixer of the type shown in U.S. Pat. No. 4,321,229. 
The primary flighting as illustrated in FIG. 1 has a generally "square" 
pitch in that the lead spacing between adjacent flights is approximately 
equal to the diameter of the core 15. However, it is within the scope of 
this invention to have flighting of a varying angles which may be greater 
or lesser than the angles shown herein. Further, the radial height of the 
flighting, or alternatively the depth of the channels defined by the 
flighting, remains, in the preferred embodiment, constant throughout the 
length of the metering section. 
The metering section of this invention is made up of a plurality of 
individual flighting segments which are arranged in a series of partially 
overlapping portions as represented in FIG. 1 by L.sub.o. Thus, the 
flighting 12 may be considered as a first or primary flighting segment 
which extends axially along the length of the screw 10 for about 3 to 4 
turns as represented by L.sub.s. A forward end of flighting 12 is 
overlapped by about one half to one turn by a second flighting 18. In the 
overlapping zone or portion L.sub.o a double flight section is formed. The 
second flightings 18 also extends axially along the core about the same 
length as that of flighting 12, that is about three or four turns, and is 
then terminated. The forward end of the flighting 18 is in overlapping 
relation which a third flighting 20 and similarly, the third flight 20 has 
a forward end in overlapping relation with a fourth flighting 22, etc. 
There may be more or less than four flightings, as illustrated, and the 
overlapping extent between the first and the second, the second and the 
third, etc., may be varied one from the other, staying in the approximate 
range of one-half to one. 
A primary channel 25, as illustrated in FIG. 2, is defined by the adjacent 
flanks of the flighting 12, the channel 25 is divided into two secondary 
channels 28 and 30, FIG. 3, fondled by the lead end portion of flighting 
18, dividing the channel 25 into the two smaller channels, 28 and 30, 
which, together, have an effective volume less than the channel 25. In 
FIGS. 2 and 3 and in FIGS. 4 and 5, the screw barrel is represented at 32. 
The decrease in volume corresponds to the displacement of the flighting 
18. This relationship holds true for the interaction between flightings 18 
and 20, 20 and 22, etc. Throughout the metering section, the screw root or 
core 15 is of one-half to one turn. 
Since the flow from the channel 25 is divided into two smaller channels, 28 
and 30, and since the combined volumes of the channels 28 and 30 are less 
than the volume of channel 25, this transition is accompanied by a linear 
acceleration of the material which inherently causes additional 
distributive mixing. Additionally, it can be seen by comparing the cross 
channel velocity vectors profiles represented in FIGS. 4 and 5, that the 
velocity parallel to the screw root as well as the normal velocity vectors 
are increased substantially within the channels 28 and 30 over the 
corresponding velocity victors of the channel 25. This changes in velocity 
vectors also increases distributive mixing. 
Thus, the flow from channel 25 is distributed into channels 28 and 30 and 
then recombine into channels defined by the non-overlapped portion of the 
respective flightings 18, 20, and 22 into wide channels as requested by 
channel 25. At each overlap section, that is the overlap between 
flightings 18 and 20, between flightings 20 and 22, etc., the flow in the 
primary channel is divided into secondary flows for approximately one turn 
and then recombined into a primary flighting. 
The recombining of the separate flows results in further shear mixing and 
distributive mixing due to the interaction between the circulatory flows 
represented in FIG. 3 which combine to form a single flow in a combined 
channel, and the attendant decrease in the velocity vector along the 
channel due to the increase in channel volume (FIG. 4). Further, as 
previously explained, the relative positions of any two flows which are 
split in the secondary channels are reversed when the channels are 
recombined. That is to say, an element of polymer which was in the low 
shear center portion of the single channel becomes redistributed to the 
outer high shear areas of the dual channels. Following each successive 
dual channel section, an element of polymer on the trailing flank of one 
of the primary flights is separated and exchanged to become positioned on 
the pushing or forward facing flank of the next primary flight. 
This interchange of polymer streams is, in itself, a division and mixing 
function which is accomplished with minimum additional shear strain and 
the mixing is increased at approximately the same shear strain as occurs 
in a conventional metering section. However, improved mixing is achieved 
by the periodic splitting and recombining of the flow, the disruption of 
conventional cross channel flow profiles, the change in overall velocity 
profiles and the resulting distributive mixing when these flows are 
periodically returned to a single flow status. Since the mixing is more 
complete, temperature stratifications in the melt tend to become reduced 
or eliminated. 
While the form of apparatus herein described constitutes a preferred 
embodiment of this invention, it is to be understood that the invention is 
not limited to this precise form of apparatus, and that changes may be 
made therein without departing from the scope of the invention which is 
defined in the appended claims.