Screw refiner

An extruder for processing wood particles, shavings or sawdust is particularly useful for the production of a refined product exhibiting a large number of separated, individual fibers each of a relatively long length as is desirable in the production of a fiber core for a composite particle board. The extrusion apparatus comprises an elongated barrel and a flighted screw rotatable within the barrel, with the final downstream section of the screw being of a configuration to sequentially present three processing zones each having a compression region, a restricted region and a decompression region. The three processing zones of the final screw section cooperate with a compression region and restricted region formed in the final head section to choke the advancement of wood materials through the extruder sufficiently to enable satisfactory processing of the same without the use of a final die or restricted orifice disposed on the outlet of the extruder barrel. In particularly preferred forms, restriction elements in the form of rectangular bars are disposed across the grooves of the screw and grooves of the head in the restricted region, and the bars are oriented in acute angular relationship relative to each other in scissors-like fashion to promote rolling and twisting apart of the fibers while maintaining the longitudinal integrity of the same.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention broadly relates to a method and apparatus for the extrusion 
of wood chips, shavings or sawdust to produce a refined product having a 
multitude of separated fibers of relatively long lengths and narrow 
cross-section. More particularly, the invention is concerned with an 
extruder head and screw each having bar-like restriction elements 
extending transversely across the grooves between flighting, and the bars 
interact in a scissors-like fashion to roll and twist the fibers apart 
without excessive transverse severing of the same. A final, conical 
section of the extruder screw has two juxtaposed grooves each presenting 
three processing zones which each include a compression region, a 
restricted region and a decompression region sequentially arranged along 
the length of the grooves and upstream of an annular outlet, in order to 
choke the flow of wood through the extruder and roll the wood particles 
against each other without the use of a die at the outlet of the extruder. 
The extrudate is of a flowable nature which can be easily storaged, 
conveyed, or handled. Additionally, the extrudate is highly desirable for 
use in the production of a special core for particle board, since the 
relatively thin fibers may be readily and efficiently coated to a smooth 
finish after the board is cut, while the long lengths of the fibers 
interconnect to form a stronger composite board product than would 
otherwise be possible. 
2. Description of the Prior Art 
The use of particle board for furniture, cabinets and other types of 
finished construction projects has significantly increased in recent 
years. Particle board has in many instances replaced plywood due to the 
improved dimensional stability of particle board and the lower cost of the 
same. In most cases, particle board is comprised of relatively flat, small 
chips that are bonded together by an adhesive such as epoxy resin. 
However, particle board tends to have relatively large voids and pores 
between the chips because the latter, during manufacture of the board, 
resist compression to a dense, tightly packed condition. As a result, sawn 
or cut edges of the board present a number of voids and cavities which, 
for the most part, cannot be readily coated to a smooth finish. 
Consequently, the cut edges of the particle board when used in furniture, 
cabinets and the like are normally trimmed with a piece of natural wood of 
a type selected to match the desired finish to be applied to the faces of 
the board and other components of the finished structure. As can be 
appreciated, the usual practice of selecting, cutting and affixing a trim 
strip to the sawn edges of particle board significantly increases the 
overall cost of the finished product as well as the time expended in 
construction of the same. 
Other types of composition board, and particularly those products known as 
fiber board, have been manufactured by processing shavings, wood chips or 
sawdust in a disc refiner or pressurized refiner and then applying an 
adhesive and pressing the refined fibers together in order to form the 
resultant board product. The core of fiber board, being comprised of 
tightly compacted, small fibers, presents a relatively smooth edge when 
sawn due to the small size of the voids between adjacent fibers, in 
contrast to the larger voids presented along the cut edge of particle 
board. Unfortunately, fiber board is relatively expensive, due in part to 
high energy costs which are associated with operation of the disc refiner 
or pressurized refiner, the latter of which requires significant amounts 
of steam energy. 
Hence, it would be a desirable advance in the art if means were provided to 
refine wood chips, shavings or sawdust into a product that is 
characterized as having thin, at least partially separated fibers each of 
a relatively long length, but with significantly reduced costs for energy 
in comparison to the operating expenses associated with the use of disc 
refiners and pressurized refiners. Advantageously, such a refined product 
could be utilized in the manufacture of a special core for particle board, 
so that cut edges of the same can be easily coated to present a smooth, 
finished appearance that matches the face of the boards or other 
components of the finished article. 
SUMMARY OF THE INVENTION 
Our present invention represents an especially effective means for 
processing wood particles of various sizes into a highly refined, 
uniformly-sized product that is comprised of a large number of individual 
as well as partially separated fibers each having a relatively long length 
and a small transverse cross sectional area. In accordance with the 
invention, an extruder of novel construction is provided for producing the 
refined product with an operating cost far less than the costs normally 
associated with disc refiners, hammermills and pressurized disc refiners. 
The extrusion apparatus of the present invention includes an extruder 
barrel having an elongated bore, and a screw rotatable about its 
longitudinal axis within the bore of the barrel. A downstream section of 
the barrel and the screw have generally conical configurations with 
flighting means defining respective elongated, helically shaped grooves 
that cooperate to advance the wood particles toward an annular extruder 
outlet which takes the form of an open, unrestricted area between the 
downstream end of the screw and the surrounding regions of the bore. 
Importantly, the flighting defining the grooves in the final barrel head 
section and the final screw section have a certain compression ratio 
gradient that functions to restrict or choke the flow of particles through 
the extruder upstream of the outlet so that the wood particles are 
sufficiently retained and refined before being discharged through the 
non-plugging, annular outlet. 
In certain forms of the invention, each of two side-by-side grooves formed 
in the final screw section presents three processing zones which are 
sequentially arranged along the length of the groove. Each of the 
processing zones includes a compression region wherein the root diameter 
between flight portions of the groove is increased to compress the 
particles, and a restricted region having at least one bar which extends 
transversely across the groove to further compress the particles and force 
the same through a relatively narrow restricted gap between the outer 
surfaces of the bar and adjacent, stationary portions of the head. 
Finally, each of the three processing zones also includes a decompression 
region located immediately downstream of the corresponding restricted 
region, and preferably the decompression region is established by 
significantly reducing the root diameter of the groove between the 
flighting portions to create an enlarged free volume that enables the 
particles to expand somewhat and rearrange before reaching the compression 
region of the next adjacent, downstream processing zone. 
Thus, the advancing wood particles encounter a series of compression 
regions wherein the velocity of the particles along with the amount of 
pressure applied to the particles is steadily increased, as well as a 
series of decompression regions wherein the velocity of the particles is 
decreased while pressure applied to the particles is relieved to enable 
the same to mix and expand. The grooves of the screw are of relatively 
wide width so that the fibers are separated by the grinding action of wood 
particles against other wood particles, in contrast to disc refiners where 
grinding occurs by shearing wood against metal. In preferred embodiments 
of the invention, the inclined bottom wall of the screw within the 
compression regions is roughened to retard the advancement of wood 
particles and further promote the grinding action between adjacent 
particles. 
The compression regions and the corresponding, adjacent restricted regions 
are each formed, along with adjacent regions of the mating head, to 
present a series of increasingly smaller restricted areas through which 
the particles must pass as the same advance through the refiner. The 
conical configuration of the final screw section and the adjacent head 
section also function to cause the particles to be increasingly compressed 
during advancement along the barrel. In addition, the decompression 
regions following each corresponding restricted region have increasingly 
smaller free incremental volumes available for passage of the particles, 
so that each of the processing zones compresses the product with 
sufficient flow retardation that the use of dies at the outlet of the 
extruder is rendered unnecessary. This latter factor is particularly 
advantageous in that refined wood cannot readily flow through 90.degree. 
turns of a configuration similar to the passages that are normally found 
immediately upstream of extruder dies. 
In preferred forms of the invention, the grooves of the final downstream 
head section are also of particular configuration which establishes a 
compression region and a restricted region located adjacent the third 
processing zone of the screw. In addition, bars extending transversely 
across the groove in the restricted region of the head are oriented at a 
slight acute angle relative to the bars in the third restricted region of 
the screw. The stationary bars of the head cooperate with the bars of the 
rotating screw in a scissors-like fashion so that the fibers are twisted 
apart during passage through the relatively narrow area between the bars 
without excessive severing of the length of the same. As a consequence, 
the longitudinal integrity of the fibers is, in large part, retained which 
renders the refined product extremely desirable for use in the manufacture 
of fiber board cores for particle board as well as in other applications. 
These and other objects of the invention will be made more clear in the 
course of the following description of a preferred embodiment of the 
invention.

DETAILED DESCRIPTION OF THE DRAWINGS 
An extruder constructed in accordance with our present invention is broadly 
designated by the numeral 20 in FIGS. 1-10 and includes a barrel 22 having 
four interconnected head sections 24, 26, 28 and 30 as is illustrated in 
FIG. 1. Each of the sections 24-30 includes an outer casing 32 and an 
inner insert 34 and the inserts 34 represent structure defining an 
elongated bore 36 that extends along the length of extruder 20. The bore 
36 includes an upright inlet 38 located at the upstream end of the first 
head section 24, and also has an outlet 40 at the final or downstream head 
section 30 which will be described in more detail hereinafter. 
Viewing FIG. 1, the insert 34 of first head section 24 downstream of inlet 
38 presents a series of straight, converging, rectangular ribs 42 that are 
spaced around the perimeter of adjacent regions of the bore 36 and extend 
in planes parallel to the longitudinal axis of the latter. Similarly, the 
insert 34 of the third head section 28 also has a spaced series of 
rectangular, essentially straight ribs 44 that extend in planes generally 
parallel to the longitudinal axis of bore 36. The inserts of head sections 
24, 28 are both tapered to present a generally conical configuration. 
The insert 34 of the second head section 26 has double flighting 46 which 
presents two separate, side-by-side grooves 48, 50. Each of the grooves 
48, 50 is elongated and has a generally helical configuration; 
furthermore, the region of bore 36 within the flighting 46 is of an 
untapered, generally cylindrical shape, in contrast to the conical regions 
of the bore 36 that are formed by the tapered configuration of inserts 34 
for the head sections 24, 28. 
The final or downstream head section 30 has "quad" flighting 52 that 
presents four juxtaposed, separate grooves 54-60. Referring to FIG. 2, and 
also to FIG. 3 (which depicts only groove 58), each of the grooves 54-60 
presents in sequential order a passage region 62 that is followed by a 
processing zone 64 having a compression region 66, a restricted region 68 
and a decompression region 70. Viewing FIG. 3, it can be seen that the 
root diameter of the groove 58 decreases in such fashion that a bottom 
wall 72 of the groove 58 is inclined in the form of a ramp 74 between the 
passage region 62 and the compression region 66. Thereafter, the bottom 
wall 72 continues along the length of the groove 58 in a steadily 
decreasing, spiral fashion corresponding to the tapered profile of the 
bore 36. The compression region 66 extends through an arc of approximately 
135.degree. between ramp 74 and the beginning of the restricted region 68. 
Two elongated, rectangular element or cross-bars 76 are disposed within the 
restricted region 68 of each of the grooves 54-60, and extend in 
transverse relationship to adjacent portions of the flighting 52 as can be 
appreciated by reference to FIG. 2. The bottom wall 72 of grooves 54-60 
between the bars 76, 76 is spaced from the innermost surface of bars 76 in 
general alignment with the bottom wall 72 within the compression region 
66, although the free incremental volume (i.e., the groove volume per 
incremental or unit length of the groove along its helical path) of the 
grooves 54-60 between the bars 76 of the restricted region 68 is somewhat 
less than the incremental volume of the grooves 54-60 within the 
compression region 66 due to the conical configuration of the bore 36 
within head section 30. The decompression region 70 begins immediately 
downstream of the second bar 76, and is formed by removing material within 
the insert 34 until the diameter of the bottom wall 72 within the 
decompression region 70 is greater than the diameter of the bottom wall 72 
within the compression region 66 or between the bars 76 of the restricted 
region 68. 
Preferably, the top surface of the bars 76 are flush with adjacent portions 
of the flighting 52. As such, the grooves 54-60 present incremental 
volumes in locations 75, 77 (FIG. 3) in both the compression region 66 and 
the decompression region 70 respectively and directly adjacent the 
restricted region 68 which is greater than any incremental volume of the 
groove 58 in a location 79 that is between the bar 76 and the longitudinal 
axis of bore 36, since the free incremental volume of the grooves 54-60 
inwardly from the bars 76 is equal to zero. Moreover, the restricted 
region 68 is disposed upstream from the outlet 40 of the bore 36 to 
restrict or choke the passage of wood chips advancing through the extruder 
20 before the same reach outlet 40. 
The configuration of grooves 54, 56 and 60 is similar in essential respects 
to the configuration of groove 58 (shown in FIG. 3). However, as can be 
appreciated by a reference to FIG. 2, the processing zones 64, including 
the bars 76, are disposed approximately 90.degree. around the longitudinal 
axis of bore 36 from each other so that the respective compression regions 
66 and bars 76 of grooves 54, 58 are spaced across each other within the 
bore 36 while the compression regions 66 and bars 76 of grooves 56, 60 are 
located across from each other in transverse relationship to the 
compression regions 66 and restricted regions 68 of bars 54, 58. 
Referring again to FIG. 1, an elongated screw, broadly designated 78, is 
rotatable about its longitudinal axis and extends along the length of bore 
36. The screw 78 is comprised of four screw sections 80, 82, 84 and 86 
which are releasably secured together for simultaneous rotation. Each of 
the sections 80-86 presents at least one helically-shaped groove for 
receiving material to be processed and advancing the same in downstream 
direction toward the outlet 40 of bore 36 as the screw 78 is rotated. 
The first or inlet screw section 80 which is depicted in FIG. 1 is 
preferably a single flight, tapered screw that presents a decreasing 
volume in directions away from the inlet 38 of bore 36. The second screw 
section 82 is a modified, double flighted construction which can be 
described as a "11/2" flighted screw. That is, the upstream portion of one 
of the flights is tapered to begin at the shank of the screw section 82 
adjacent the beginning of the same, and then gradually increase in 
diameter until equal in dimension to the constant outer diameter of the 
other flight. 
The screw section 84 has a conical configuration complemental to the 
tapered, overall profile of the bore 36 within head section 28. The screw 
section 84 is in this instance a double flighted screw that is aligned 
with the screw sections 82, 86 such that the ends of the flighting of 
screw section 84 are closely adjacent the respective ends of the flighting 
of sections 82, 86 to present a smooth transition for material passing 
along each of the two grooves. 
The screw section 86 is depicted in greater detail in FIG. 4 and has double 
flighting 88 defining two juxtaposed, elongated grooves 90, 92 having a 
generally helical configuration. The flighting 88 is fixed to a shank 
portion 94 and presents an overall tapered profile or conical 
configuration. 
A relatively short passage region 96 is located at the upstream end section 
of each groove 90, 92 and presents an available or free incremental volume 
that is approximately equal to the free incremental volume of the adjacent 
reaches of the grooves at the downstream end section of third screw 
section 84. Each groove 90, 92 of the screw section 86 also presents, in 
sequential order, three processing zones, 98, 100, 102 which correspond to 
Zones One, Two and Three that are designated in FIG. 4. 
The three processing zones 98, 100, 102 of groove 90 can be better 
understood by comparison of FIGS. 4 to FIGS. 5, 7 and 9. As shown in FIG. 
5, the bottom wall of the groove 90 within the first processing zone 98 
steadily increases to present a compression region 104 that extends in 
this instance in an arc approximately 235.degree. about the longitudinal 
axis of screw 78. A restricted region 106 is formed immediately downstream 
of compression region 104 and includes three spaced, parallel elements or 
rectangular bars 108 that extend across the groove 90 between adjacent, 
continuous portions of the flighting 88 and lie in planes parallel to the 
longitudinal axis of bore 36. Also, as shown, a decompression region 110 
is disposed downstream of the third bar 108 of the restricted region 106. 
The second processing zone 100 of groove 90 is shown in FIG. 7, and 
includes a compression region 112 that extends approximately 90.degree. 
about the longitudinal axis of screw section 86 until reaching a 
restricted region 114 having a single bar 116 which extends between 
adjacent portions of flighting 88 in a plane parallel to the longitudinal 
axis of bore 36. Downstream of bar 116, a bottom 118 of groove 90 is 
inclined in the nature of a ramp and leads to a relatively short 
decompression region 120. 
The nature of the third processing zone 102 of groove 90 can be understood 
by reference to FIG. 9. Zone 102 includes a compression region 122 that 
extends along an arc of approximately 45.degree. about the central axis of 
screw section 86. A restricted region 124 is located immediately 
downstream of compression region 122, and includes four spaced, elongated 
elements or elongated bars 126 that extend between adjacent, continuous 
portions of the flighting 88 and lie in planes parallel to the 
longitudinal axis of bore 36. The restricted region 124 lies along an arc 
of approximately 90.degree. about the screw section 86, and terminates in 
a final decompression region 128 that begins with a ramp-like section of 
the bottom wall 118 of groove 90. 
In similar manner, groove 92 also presents three distinct processing zones 
130, 132 and 134 substantially similar to corresponding zones 98-102, but 
disposed on opposite sides of the screw section 86 at 180.degree. around 
the perimeter of the same. Each of the zones 130-134 present, in 
sequential order, a compression region 104, 112, 122 respectively followed 
by a restricted region 106, 114, 124 having transverse bars 108, 116, 126, 
and a decompression region 110, 120, 128 wherein the root or bottom wall 
of the groove 92 is undercut to present a free incremental volume greater 
than any free incremental volume of the groove 92 within the corresponding 
restricted regions or compression regions. 
By observation of FIGS. 5-10, it can be appreciated that the grooves 90, 92 
within the respective processing zones 98-102, 130-134 have free 
incremental volumes of values within the compression regions 104, 112, 122 
which are greater than any incremental volume of the corresponding groove 
90, 92 in the next adjacent, downstream restricted regions 106, 114, 124 
in a direction radially outwardly from the respective bars 108, 116, 126. 
As an example, in FIG. 6 the incremental volume of groove 92 at location 
129 in compression region 104, as well as the incremental volume of groove 
92 at location 131 in decompression region 110, is greater than the 
incremental volume of groove 92 outwardly from bar 108 at location 133. 
In addition, the disposition and configuration of the rectangular, 
parallel, elements or bars 108, 116, 126 is such as to cause the passage 
areas or gaps radially outwardly of the bars to be smaller as the outlet 
40 of bore 36 is approached. In particular, FIG. 5 shows that an outer 
surface 136 of bar 108 is spaced slightly from the outer surface 138 of 
the adjacent portions of the flighting 88 while in FIGS. 7 and 9 the outer 
surface 140 of bars 116, 126 is substantially flush with the outermost 
surface 142 of adjacent portions of flighting 88 (in this regard, see also 
FIG. 4). The outer surfaces of bars of the restricted regions within the 
processing zones 130-134, as shown in FIGS. 6, 8 and 10, are substantially 
identical in disposition and configuration to the aforementioned surfaces 
136, 140 of bars 108, 116, 126 relating to the respective outer surfaces 
136, 140. 
The flighting 88 has an outer diameter which gradually decreases along the 
length of grooves 90, 92 such that the screw section 86 presents an 
overall conical configuration. Thus, the pressure exerted on materials 
passing through the extruder 20 increases as the materials sequentially 
advance through the compression regions and the restricted regions of each 
of the processing zones 98-102 and 130-134. Moreover, the bottom walls 118 
of grooves 90, 92 may be roughened within the corresponding compression 
region of each zone 98-102, 130-134 to further promote rolling and 
twisting of the materials as the same advance toward bore outlet 40. 
Each of the outwardly projecting bars 108, 116 and 126 extend completely 
across the reaches of corresponding grooves 90, 92 and lie in planes 
extending along the longitudinal axis of bore 36. Moreover, by comparison 
of FIGS. 2 and 4, it can be observed that the outwardly projecting bars 76 
within each of the restricted regions 48 of the grooves 54-60 of head 
section 30 are oriented in acute angular relationship relative to the 
direction of extension of the bars of screw section 86, including the four 
bars 126 of each restricted region 124 of the final, third processing zone 
102, 134 of screw section 86. As such, the stationary bars 76 of the final 
head section 30 are disposed in scissors-like fashion relative to the bars 
126 within the restricted region 124 of the third processing zones 102, 
134 of final screw section 86, which has been found to promote twisting 
and separation of the fibers of wood particles when the same are 
introduced into the barrel 22 of extruder 20. 
Operation 
It has been found that the extruder 20 in accordance with the present 
invention is particularly useful for processing wood materials, including 
wood particles, chips, shavings and sawdust and refining the same to a 
final product that is characterized as having a large number of 
individual, separated fibers each of relatively long length and with a 
narrow, transverse cross-sectional area. Preferably, the wood particles 
have a moisture content in the range of about 10% to about 50% by weight 
of water when introduced into the inlet 38, although better results have 
been observed when the moisture content of the wood particles is within 
the range of about 25% to about 30% by weight of water. In addition, the 
wood particles preferably have a length of about 1.5 mm. to about 38 mm. 
immediately prior to being introduced into the inlet 38 of bore 36. 
During operation of the extruder 20 to refine wood particles, the screw is 
preferably rotated at a speed within the range of about 100 r.p.m. to 
about 500 r.p.m., although better results can be observed when the screw 
is rotated at a velocity within the range of about 250 r.p.m. to about 350 
r.p.m. The particles of wood are advanced first through the sections 24, 
26 and then fed into the third head section 28 which compresses the 
particles somewhat due to the overall conical configuration of the screw 
section 84 and the bore 36 within head section 28. 
Next, the wood material reaches the first processing zones 98, 130 and is 
further compressed due to the increase in root diameter of the bottom wall 
118 between the flighting 88 within compression region 104 (see FIG. 5). 
The bottom wall 118, as described earlier, is preferably roughened to 
promote rolling of the fibers and cause the fibers to grind against each 
other and separate from adjacent fibers of the same particle. 
Subsequently, as the wood material reaches the restricted region 106, the 
particles are forced over the top of the bar 108 which presents a narrow 
area or gap through which the same can pass. Consequently, additional 
fiber separation occurs in area of the groove 90 adjacent the bars 108. 
Next, the particles approach the decompression region 116 wherein the bulk 
density of the material is decreased and the particles expand somewhat, 
which further facilitates rolling and mixing of the particles before next 
advancing to the compression region 112 of the second processing zone 100. 
Similarly, the particles during advancement through zones 100, 102 and 132, 
134 are subjected to processing similar to the processing occurring in 
zones 98, 130. In zones 100, 102, 132, 134 however, the wood material is 
exposed to higher pressures due to the fact that the same amount of 
material must flow through an increasingly smaller free incremental volume 
because of the tapered or conical profile of screw section 86. In 
addition, the flush disposition of the outer surfaces 140 of the bars 116, 
126 forces the particles of wood to pass through a smaller restricted 
opening than is presented outwardly of the bars 108 in the first 
processing zone 98. 
Importantly, the wood material when passing through the third processing 
zones 102, 134 is subjected to the action of bars 126 in the restricted 
regions 124 which move relative to the stationary bars 76 of the 
restricted regions 68 of head section 30. Bars 76, 126, being disposed in 
acute angular relationship relative to each other, function in a 
scissors-like fashion to facilitate additional twisting of the wood fibers 
without shortening the length of an excessive number of the same. 
In preferred embodiments of the invention, the free incremental volume of 
the bore 36 in the first head section 24 adjacent the inlet 38 is in the 
range of about 100 times to about 20 times the free incremental volume of 
the bore 36 surrounding the end of the final screw section 86 in the 
region adjacent outlet 40. Better results are observed, however, when the 
same, aforementioned free incremental volume adjacent the inlet 38 is in 
the range of about 70 times to about 50 times the free incremental volume 
adjacent outlet 40. In particularly preferred embodiments, the free 
incremental volume in the bore 36 adjacent inlet 38 is about four times 
the free incremental volume of bore 36 at the downstream end of the third 
head section 28, and in turn the latter free incremental volume is about 
15 times the free incremental volume in bore 36 in the region adajcent 
outlet 40. 
The particles of wood, during advancement through the bore 36, are 
compressed in the final restricted region 124 of the third processing 
zones 102, 134 to a bulk density in the range of approximately seven to 
approximately 20 times the bulk density exhibited by the wood particles 
when introduced through the inlet 38 of extruder 20. The temperature of 
the wood particles during passage through the barrel 22 is advantageously 
in the range of about 93.degree. C. to about 175.degree. C., although 
better results have been observed when the wood particles have a 
temperature of about 110.degree. C. If desired, moisture may be added to 
the wood prior to extrusion in order to increase steam generation during 
the refining process and soften the particles to further facilitate 
separation of the fibers. 
After the wood particles travel along the length of the three processing 
zones 98-102 and 130-134 of grooves 90, 92 respectively, the particles are 
discharged through outlet 40 which takes the form of an annular opening 
surrounding the downstream end portion of final screw section 86 and the 
adjacent, surrounding portions of the final head section 30 downstream of 
bars 76. 
The compression regions 104, 112, 122, and particularly the restricted 
regions 106, 114, 124 of each of the processing zones 98-102 and 130-134, 
along with the compression region 66 and restricted region 68 of the four 
grooves 54-60 in head section 30, are operable to restrict or choke the 
advancement of materials passing through the extruder 20, with the 
materials being subject to a greater compressive force in each successive 
processing zone. The materials are discharged directly through the annular 
outlet 40 into the atmosphere without the use of dies or other types of 
restricted orifices disposed on the downstream end of the final head 
section 30, which is particularly advantageous in that the refined wood 
particles do not readily flow around corners, as might otherwise be 
presented during passage of the materials toward an outlet die opening. 
Advantageously, the configuration of the extruder barrel 22 in combination 
with the configuration of the screw 78 causes the wood particles to rub 
against each other for proper rolling and twisting and produce a refined 
product having separated fibers of relatively long length. The "wood 
against wood" action promoted by extruder 20 causes significantly less 
wear on components of the latter in comparison to, for instance, disc 
refiners where "wood against metal" forces are presented. Additionally, it 
has been observed that the energy requirements of extruder 20 are 
significantly less than the energy that would be required for processing a 
similar amount of wood material by a disc refiner. 
EXAMPLE 
In this test, the first screw section 80 had a length of 6096 mm. and had 
flighting with an outer diameter of 3023 mm. adjacent inlet 38 which 
steadily decreased to 235.degree. at the end of the first section 80. The 
second or intermediate screw section 82 had a length of 4255 mm., and 
flighting having a straight, cylindrical configuration with a diameter of 
2350 mm. 
The extruder 20 was further equipped with a third screw section 84 having a 
length of 4636 mm., and an outer flighting diameter which tapered from 
2350 mm. to 1867 mm. The final screw section 86 had a length of 3480 mm., 
and had flighting which tapered from a diameter of 1867 mm. to 1499 mm. 
The extruder 20 in this respect was of a configuration substantially 
identical to that shown in FIG. 1, in that the screw 78 presented three 
processing zones each with compression regions, restricted regions having 
transverse bars, and corresponding, downstream decompression regions. In 
addition, the head sections 24-30 were substantially as depicted in FIGS. 
1-3. 
A quantity of wood particles, adjusted by addition of water to moisture 
content of 30% by weight and having an overall size ranging from 1.6 
mm..sup.2 to 0.08 mm..sup.2 in cross-sectional area, was fed into the 
extruder 20 at a rate of 3636 kg/hr. The screw 78 was rotated at a speed 
of 300 r.p.m. which caused the tip speed of the screw 78 in the third 
processing zones 102, 134 of the final screw section 86 to be about 173.7 
meters/minute. The load in kilowatts of the extruder was 350. The load on 
the extruder without the mixture present at 300 r.p.m. is about 60 kw so 
that the increased load required by processing of the wood material 
therein was about 290 kw. 
Temperatures of the mixture within the final or fourth head section 30 were 
approximately 112.5.degree. C. The compression ratio along the length of 
the first screw section 80 (i.e., the ratio of the free incremental volume 
of bore 36 adjacent inlet 38 to the free incremental volume of bore 36 
adjacent the end of the first screw section 80) was 3.5:1. In the 
transition area of the bore 36 between the downstream end of the first 
screw section 80 and the upstream end of the second screw section 82, an 
expansion discharge ratio of 1.5:1 was presented (that is, the free 
incremental volume in the bore 36 at the beginning of the second screw 
section 82 was 1.5 times the free incremental volume in the bore 36 at the 
downstream end of the first screw section 80). The compression ratio along 
the first 1905 mm. length of the second screw section 82 was 1.17:1, while 
the compression ratio along the entire length of the third screw section 
84 was 1.46:1. The compression ratio within bore 36 along the length of 
the fourth screw section 86 was 1.46:1 if the restriction elements or bars 
76, 108, 116 and 126 are removed. The overall net compression ratio of the 
bore 36 from inlet 38 to the downstream end of the third screw section 84 
was 4:1, while the compression ratio from the upstream end of the fourth 
screw section 86 to the outlet 40 was 15:1, whereby the total net 
compression ratio from the inlet 38 to the outlet 40 was 60:1. The 
material during passage through the extruder 20 was compressed in the 
third processing zones 102, 134 to a bulk volume of about 1/17 of the 
original bulk volume of material entering the extruder inlet 38. 
The extruded wood product was highly refined and exhibited a large number 
of individual, separated fibers each having a relatively long length and a 
relatively narrow transverse cross-sectional area. The fibers were cleanly 
separated and did not present a "fuzzy" appearance. 
In this connection, a degree of fibrilation leading to a somewhat "fuzzy" 
appearance may be desirable in certain senses. Specifically, the 
fibrilated ends of fibers may act to strengthen the resultant particle 
board, inasmuch as the fibrilated ends interact with each other during the 
compression step of board production, thereby increadsing the final 
overall strength of the board.