Low shear free-flow extruder breaker plate

An extruder device having improved flow distribution, reduced shear and reduced surface area on which material can accumulate, including a breaker plate having a plurality of symmetrically arranged tapered holes provided therein. The holes are disposed on a series of V-shaped concentric centering grooves the diameters of which progressively increase, albeit by diminishing increments, over the adjacent inner circles. Each of the holes disposed on a particular centering groove is of equal diameter; however, the diameters of the holes become progressively smaller as the diameters of the centering grooves on which they are disposed increase. The holes nearest the center of the plate have the largest diameters, while those nearest the periphery of the plate have the smallest diameters. A compact symmetrical array of holes is provided, with the holes cooperating with the centering grooves to present a knife-like edge to the plastic material, with the interstitial spaces between the holes being inclined toward the holes.

FIELD OF THE INVENTION 
This invention relates to apparatus for extruding plastic insulating 
material onto a moving wire. More specifically, the invention relates to 
an apparatus for extruding insulation material which includes a breaker 
plate of improved design to be interposed between a screw which delivers 
the plastic insulating material and a crosshead device which applies the 
insulating material onto the wire. 
BACKGROUND OF THE INVENTION 
In applying plastic insulating material to a wire to be insulated, the 
plastic material is delivered to a crosshead device which applies the 
insulating material to the wire by means of an extruder. The extruder 
includes a screw which forces the plastic material against a breaker plate 
having a plurality of holes therein. The plastic material is extruded 
through the holes in the breaker plate into a conduit or adapter through 
which the plastic flows into the crosshead device. The breaker plate 
functions to create a back pressure in the extruder and to shear the 
plastic material as it passes therethrough. 
An extruder having a prior art breaker plate is disclosed in U.S. Pat. No. 
3,121,255. The prior art breaker plates typically comprised a flat disc 
having a number of holes of equal diameter, with each hole having a 
constant diameter along its length. The consequence of this configuration 
is that there are a number of flat surfaces between the holes on the side 
of the breaker plate facing the screw. These flat surfaces provide "dead 
spots" on which the plastic insulating material tends to accumulate, 
decompose with time and temperature, and eventually smolder or burn. This 
degraded material is ultimately passed through the breaker plate with the 
plastic flow and contaminates the insulating layer applied by the 
crosshead. In addition, these prior art breaker plates impart a relatively 
high shear force to the plastic material which is detrimental to certain 
shear sensitive plastics, such as polyvinylchloride (PVC), 
polybutylterephthalate (PBT), and vulcanized insulated polypropylene 
(VIP). Because the holes are of uniform diameter and profile, there is a 
tendency for a greater volume of the plastic material to flow through the 
radially outermost portions of the breaker plate than the radially inner 
portions because the aggregate hole surface area is greater in the 
outermost portions than in the inner portions. This results in an uneven 
flow distribution into the crosshead which can affect the uniformity of 
the insulation layer applied to the wire. 
Moreover, the uneven flow distribution through the breaker plate adversely 
affects the efficiency of the system since the buildup of degraded 
material requires more frequent cleaning of the breaker plate, thereby 
limiting the effective production cycle between cleanings. In addition, as 
mentioned above, the relatively high shear forces presented by the dead 
spaces on the breaker plate and the constant diameter holes pose a 
potential detrimental effect on certain shear-sensitive plastics. 
SUMMARY OF THE INVENTION 
The present invention is directed to an extruder device having improved 
flow distribution, reduced shear and reduced surface area on which 
material can accumulate. A preferred embodiment of the present invention 
comprises a breaker plate having a plurality of symmetrically arranged 
tapered holes provided therein. The holes are tapered so as to have a 
greater diameter at their upstream ends (facing the extruder screw) than 
at their downstream ends (facing the crosshead). The holes are disposed on 
a series of concentric V-shaped concave circular centering grooves, the 
diameters of which progressively increase, albeit by diminishing 
increments, over the adjacent inner circular groove. In other words, the 
V-shaped concave circular centering grooves are arranged such that 
adjacent grooves in the series become progressively closer to one another 
as their diameters increase. Each of the holes disposed on a particular 
centering groove is of equal diameter. However, because the centering 
grooves become closer to one another as their diameters increase, the 
diameters of the holes become progressively smaller as the diameters of 
the centering grooves on which they are disposed increase. The result of 
this arrangement is that the holes nearest the center of the plate have 
the largest diameters, while those nearest the periphery of the plate have 
the smallest diameters. The V-shaped grooves form alternating concentric 
"ridges" and "valleys," with the holes centered on the valleys. As a 
consequence, a compact symmetrical array of holes is provided, with each 
centering groove and the grooves adjacent thereto presenting a knife-like 
edge to the plastic material, with the spaces between the holes being 
concave and slanting generally in the direction of flow. 
The flow is balanced and centralized by the progressive increase in hole 
size in the direction toward the center of the breaker plate. The shape of 
the surfaces between the holes minimizes the buildup of plastic material 
on the breaker plate. This configuration, along with the tapered holes, 
also results in reduced shear of the plastic which minimizes degradation 
of shear-sensitive materials. Thus, longer production runs can be achieved 
before shutdown is required to service the extruder. 
With the foregoing and other advantages and features of the invention that 
will become hereinafter apparent, the nature of the invention may be more 
clearly understood by reference to the following detailed description of 
the invention, the appended claims and to the several views illustrated in 
the attached drawings.

DETAILED DESCRIPTION OF THE INVENTION 
Referring now to the drawings in detail, there is illustrated in FIG. 1 a 
prior art breaker plate mounted in an extruder device. As shown in FIG. 1, 
an extruder device, generally designated by the numeral 10, includes a 
barrel 12 in which a screw 14 is disposed. An annular space or channel 16 
is provided between the screw 14 and the wall of the barrel 12. Disposed 
downstream of the screw 14 is a breaker plate 18 having a plurality of 
straight or untapered holes 20 formed therein. Downstream of the breaker 
plate 18 is a conduit 22 which feeds a crosshead device (not shown) 
through which a wire (also not shown) is passed in order that insulating 
material can be applied thereto. As shown in FIG. 1, the breaker plate 18 
is subject to undesirable flow distribution and accumulation of insulating 
material thereon, as described above. 
As shown in FIG. 1, a typical prior art breaker plate 18 is disc-like in 
shape. The plurality of holes 20 provided in the prior art breaker plate 
18 are uniform in diameter and are untapered. The consequence of this 
configuration is that flat stagnation points or dead spaces 21 exist 
between the holes 20. The flat spaces are essentially perpendicular to the 
direction of extrusion, with the result that these dead spaces 21 are 
potential sites for buildup of material to be extruded. In addition, 
because of the mating of the screw 14 and channel 16 with breaker plate 
18, the flow of plastic material is directed outward toward the outer 
portions of breaker plate 18, through the outer holes 20a therein. Thus, 
the central portion 18a of breaker plate 18 has reduced flow. The straight 
untapered holes 20 also result in high shear of the plastic material being 
extruded. 
FIG. 2 illustrates a breaker plate 118, according to the present invention, 
mounted in an extruder device. As shown in FIG. 2, an extruder device, 
generally designated by the numeral 110, includes a barrel 112 in which a 
screw 114 is disposed. An annular space or channel 116 is provided between 
the screw 114 and the wall of the barrel 112, similar to that in FIG. 1. 
Disposed downstream of the screw 114 is the breaker plate 118 having a 
plurality of tapered holes 120 formed therein on a plurality of concentric 
V-shaped concave centering grooves (not shown), in accordance with the 
present invention. Downstream of breaker plate 118 is a conduit or adapter 
122 which feeds a crosshead device 124 through which an advancing wire W 
is passed in order that insulating material can be applied thereto. 
In operation, plastic is delivered to the screw 114 and is forced down the 
channel 116 to the breaker plate 118. There the plastic is extruded 
through the holes 120 into the adapter 122 and into the crosshead 124, 
where it is applied to the advancing wire W. 
FIGS. 3 and 4 show a breaker plate, designated generally by the numeral 
200, according to the present invention. As shown in FIGS. 3 and 4, a 
plurality of concentric V-shaped concave centering grooves 202, 204, 
206,208, 210, 212,214, 216 are provided having diameters which increase in 
the direction radially outward from the center 201 of breaker plate 200. 
Each centering groove has a plurality of holes 203, 205, 207, 209, 211, 
213, 215, 217, centered on and arranged circumferentially around the 
respective corresponding centering groove to thereby form a ring of holes. 
Each of these holes tapers from a larger diameter to a smaller diameter in 
the direction from the upstream side 218 facing the extruder screw to the 
downstream side 220 leading to the crosshead device. As shown in FIG. 3, 
each hole on a centering groove touches the other holes on the same 
centering groove tangentially. In addition, the holes on adjacent 
centering grooves are arranged to be in close proximity to each other, 
thereby limiting the spaces on the breaker plate 200 between holes. As 
explained below, this results in a knife-like edge being presented to the 
plastic on the upstream or inlet side of breaker plate 200. 
The close spacing of holes combined with the V-shaped centering grooves, as 
shown in FIG. 3, results in reduced or minimized accumulation of plastic 
insulating material on the spaces 222, 224, 226, 228, 230, 232, 234, 236 
between the holes. The variation in hole size, as shown, results in 
equalization of pressure across the breaker plate 200 and a more uniform 
flow distribution through the holes. The tapered holes reduce the shear on 
the plastic flowing through breaker plate 200, thereby reducing the 
degradation of shear-sensitive insulating materials. 
The V-shaped centering grooves and holes are formed in the breaker plate 
200, as follows: 
A conical depression 201a (shown in phantom because it is ultimately bored 
or reamed out) is made at the center of the breaker plate 200. The apex of 
the conical depression is formed at essentially a right angle (although 
other angles could be formed). The outer diameter of the conical 
depression 201a corresponds to the outer diameter (upstream diameter) of 
tapered hole 201 which is bored or reamed in the breaker plate 200. This 
outer diameter of depression 201a or hole 201 forms the inner diameter of 
a V-shaped centering groove 202 which is then cut into the breaker plate 
200. The centering groove 202 width corresponds to the upstream diameter 
of the tapered holes 203 which are to be bored or reamed in groove 202. 
The depth of the centering groove 202 is determined by the maximum depth 
of cut permissible to form an essentially right angle V-shaped groove 
corresponding to the chosen diameter of hole 203. As a consequence, a 
plurality of concentric V-shaped grooves are cut in breaker plate 200. The 
V-shaped grooves are essentially right angle grooves with the groove 
widths and groove depths decreasing as the grooves are cut in each 
radially outward increment. 
The V-shaped grooves result in a series of alternating concentric ridges R 
and valleys V on the surface of breaker plate 200. Once the centering 
grooves have been cut, then the tapered holes are bored or reamed in each 
centering groove. The holes are centered on the valleys of the centering 
grooves. The width of each centering groove determines the upstream 
diameter of each tapered hole. Once all of the centering grooves and 
corresponding tapered holes are formed in the breaker plate 200, the 
upstream surface which is presented to the extrudate is one of knife-like 
ridges and holes which have valleys at the points of tangency. The 
interstitial spaces between holes are all slanted away from a ridge toward 
a valley. Thus, no surfaces which are perpendicular to the direction of 
flow of the extrudate are presented on the upstream side 218 of the 
breaker plate 200. 
The amount of taper (shown as angle .alpha., FIG. 4) for the holes in each 
successive concentric ring of holes varies due to the change in diameter 
of the holes. Center hole 201 may preferably have a taper of approximately 
4.6.degree. with holes 217 having a taper of approximately 2.0.degree.. 
The remaining holes have tapers which are within this range but which 
decrease for each ring in the radially outward direction. 
As will be seen in FIG. 3, the spacing between centering grooves (the 
increment in centering groove diameters) is one-half of the diameter (the 
radius) of the holes on the centering groove plus one-half of the diameter 
(the radius) of the holes on the adjacent inner centering groove. Thus, 
the hole diameters and the centering groove diameters are inversely 
related. 
The breaker plate according to the present invention is made from materials 
known in the art to be suitable for use in insulation extrusion. 
Typically, stainless steel or tool steel are acceptable materials. 
Although a certain presently preferred embodiment of the invention has been 
described herein, it will be apparent to those skilled in the art to which 
the invention pertains that variations and modifications of the described 
embodiment may be made without departing from the spirit and scope of the 
invention. Accordingly, it is intended that the invention be limited only 
to the extent required by the appended claims and the applicable rules of 
law.