A double-worm press, especially an extrusion or plastifying press for thermoplastic materials and particularly high viscosity thermoplastics, has generally conical worms, at least the discharge zones of which are formed so that the material is treated generally adiabatically therein. For this purpose, the end of the discharge zone has a depth of the worm flight which is between 24 and 33% of the worm diameter, preferably between 26 and 30%, with the ratio of the depth of the flight (or height) to the diameter of the worm increasing progressively away from the discharge end, counter to the direction of feed of the material.

FIELD OF THE INVENTION 
My present invention relates to a double-screw extrusion press and, more 
particularly, to a double-worm press for the plastifying, mastication and 
extrusion of thermoplastic materials and particularly high viscosity 
thermoplastics. 
BACKGROUND OF THE INVENTION 
In the synthetic resin (plastic) industry, screw or worm presses have been 
developed for plastifying, masticating, displacing and extruding synthetic 
resin materials, particularly thermoplastics, and for mixing or otherwise 
treating these materials by mechanical and, where necessary, a combination 
of mechanical and thermal techniques. 
The development of worm or screw presses for this purpose will be discussed 
in greater detail below, although initially some definitions are in order. 
Thus, while reference may be made to extrusion presses herein, it should be 
noted that such presses are not limited to the production of extruded 
products since an extrusion press for preparing thermoplastic materials 
can also be employed in injection molding, blow molding and other 
applications common in the thermoplastics field. For example, the products 
may be extruded by a press through a die imparting the final shape or the 
extrusion press may simply prepare the feed for an injection-molding or 
blow-molding installation. 
Reference will also be made herein to the screws or worms of such presses. 
For the purpose of this description, a screw or worm will be considered to 
be an elongated body having one or more helical ribs or spiral ribs which 
may have a screw thread profile or can deviate from such profile and 
which, when rotated, tends to advance a thermoplastic material through a 
worm housing from an inlet end to a discharge end. Each such rib will be 
considered a "flight" by analogy with the ribs of worm conveyors. 
Naturally each flight will have a number of turns, the inter-turn spacing 
defining the pitch of the worm or screw. The outermost portion of each 
flight is referred to as the crest and low point between turns can be 
termed the root, the height or depth of the flight being equal to the 
radial distance between the root and the crest and being equal to (D-D'/2) 
where D is the outer diameter of the worm and D' is the root diameter. The 
flight height (or depth) can be represented by d. 
As noted, the profile of the flight may be similar to that of a screw 
thread and, in practice, this profile will be generally trapezoidal. Each 
turn will therefore have two flanks which are inclined outwardly toward 
one another, the flank facing in the direction of advance of the material 
being termed the pressure flank because it provides the forward impetus to 
the material, the other flank of each turn being referred to as the 
trailing flank. Each flank includes an angle with a perpendicular to the 
axis of the worm, this angle being termed the flank angle. 
For various configurations of worm presses, their applications, 
construction, drives and configurations, reference may be had to the 
publications discussed in the expanded consideration of the background 
below, and in the prior U.S. Pat. Nos. 3,913,897, 3,927,869, 3,929,322, 
3,969,956 and 4,047,705. Reference may also be made to the publications in 
the files of these patents and in the classes to which these patents have 
been assigned in the Manual of Patent Classification. 
Early use of screw or worm presses for polyolefins utilized single worm 
presses in which a worm or screw press having a cylindrical configuration 
primarily and one or more helical flights was driven to displace a 
granular or pulverulent polyolefin material, primarily polyethylene, from 
a hopper at the inlet end of the press to an outlet at the opposite end 
from which a continuous strand of the polyolefin emerged. 
The flight depth, pitch and configuration were so selected that the mass 
was subjected to considerable shear with heat being generated by the 
friction of the shear energy and compression. It was possible to develop 
these parameters so that, without external heating, the polyolefin mass 
reached the desired plastification temperature. 
Certain relationships were discovered at such early dates. For example, it 
was found that the stiffer the mass, the deeper the worm flight required. 
In practice, however, it was not possible to select the structural 
parameters which always resulted in the appropriate temperatures and thus 
a certain contribution to the energy was generally provided by controlled 
heating and by constriction of the flow of the material from the press or 
by increase of the residence time. 
The viscosity of a polyolefin, moreover, drops sharply in the region of the 
extrusion temperature and hence the shearing energy likewise varies along 
the length of the path. Excess temperatures can develop close to the 
discharge end because of the viscosity drop and in practice it is required 
to cool the discharge end because the depth of the flight cannot be 
reduced beyond the level required for displacement of the material. 
It has also been found that the handling of hard polyvinyl chloride (PVC) 
by single screw worm presses is only economical with very small machines 
of limited capacity. Hard PVC is not only stiffer than most polyolefins, 
it is also thermally unstable and reacts to overheating by adhesion to the 
worm, thermal decomposition and combustion with strong evolution of 
hydrogen chloride. 
Because of this, double-screw worm presses have been developed, such 
presses having two intermeshing counter-rotating worms. These presses have 
increased the rate at which hard PVC could be treated. The presses had a 
worm length of 10 to 12 times the diameter and, while the flight height, 
output per revolution and rotational speed were small, only a simple drive 
was required for the two parallel worms and productivity was increased. 
However, efforts to increase the speed, reduce the interaxial spacing of 
the worm, and the like to permit greater flight heights and outputs to be 
developed, fail for one or more reasons. For example, when the shafts of 
the worms were brought too closely together, extreme complex transmissions 
were required to drive the worms. 
In addition, excessive length, deep flights and large pitches resulted in 
weakening the worms such that the weak points were not their drive gears 
or bearings, but rather the worm shaft or body of the worm itself. 
The worms were incapable of effectively withstanding the high driving 
torque, enormous backpressure and non-uniform wear of the flights and the 
cylinder or housing in which the worms were rotated. 
Indeed, the worms themselves were weakened by the need to provide cooling 
channels through the cores of the worms and breakage frequently resulted. 
The depth of the flight was found to be limited by practical 
considerations, namely, the drive requirements and the strength and 
stability of the worm or its shaft. 
The excess energy developed in the handling of PVC at high speeds at the 
discharge zone are extracted by cooling of the cylinder and the worm if 
thermal decomposition of the PVC is to be avoided. This is also a handicap 
since cooling of the cylinder may be inconvenient, but cooling of the worm 
is extremely complex and, where the desired temperatures cannot be 
maintained, such cooling must be supplemented by control actions such as a 
reduction in the throughput of the press. All of these requirements 
detrimentally influenced the output of the apparatus and increased 
production costs. 
Thus the art has come forward with conical worms as a means of avoiding the 
disadvantages of the double-worm systems previously discussed. The double 
conical worm system permits the axes of the two worms to be inclined to 
one another and hence to have a progressively increasing spacing away from 
the discharge end of the press. The interaxial spacing in the region of 
the drive wheels and bearings can be comparatively large so that the drive 
system is relatively simple. The peripheral speeds at the upstream end of 
the worms, for a given angular velocity, are comparatively large while the 
peripheral speeds at the discharge ends are correspondingly small, thereby 
providing a more effective energy distribution, since most of the shear, 
compression and friction heat is developed at the upstream end while 
minimum shear energy is developed at the downstream end. 
Nevertheless experience has shown that excess energy may still develop at 
the downstream or discharge end and that such presses have not fully 
eliminated all of the disadvantages of the earlier systems. Attempts have 
been made in this direction (see German patent document DE-OS No. 24 46 
420 and Austrian Pat. No. 356,882) with only partial success. 
OBJECTS OF THE INVENTION 
It is the principal object of the present invention to provide an improved 
double-worm extrusion press, especially for high viscosity thermoplastics 
such as polyvinyl chloride, whereby disadvantages of earlier systems are 
obviated, the danger of overheating at the discharge end is precluded and 
a more effective preparation of the material can be insured. 
Still another object of this invention is to provide an improved 
double-screw extrusion press which operates with reduced wear of and 
strain upon the worms even with prolonged operations. 
SUMMARY OF THE INVENTION 
I have found, quite surprisingly, that under certain conditions a 
double-screw or double-worm extrusion press having generally conical worms 
rotated in opposite directions can be dimensioned so as to practically 
preclude wear during prolonged operations and at the same time delivers 
no, or practically no, excess energy to the thermoplastic materials in the 
discharge zone when the flight height or depth is 24 to 33%, preferably 26 
to 30%, of the outer diameter of each worm at the downstream end (in the 
flow direction) of the discharge zone. 
Furthermore, it is highly advantageous to provide the flights so that the 
ratio of the flight height or depth to the diameter decreases in the 
opposite direction and the pitch of each flight along the worm and the 
widths of the flights are such that the flank gaps between the worms 
progressively increase in this direction, i.e. in the direction in which 
the worms taper and continuously from the large base of each worm toward 
the tip thereof. 
For best results, the flank angles of the worms can be 10.degree. to 
25.degree. and can diminish continuously or constantly (monotonically) in 
the direction of the taper of the worm. 
Rather than flat or line contact between the meshing worms, I provide, in 
accordance with another feature of the invention, for the juxtaposed 
flanks of the two worms to include an angle with one another. The core or 
root of each worm should lie along a conical surface. 
The result is an extrusion press which suffers significantly less wear, 
even with long-term usage, that has hitherto been the case and, in 
addition, practically no excess heat is generated in the mass at the 
discharge zone so that complex cooling is avoided.

SPECIFIC DESCRIPTION 
The worm housing 10 of an extrusion press in accordance with the invention 
can receive a pair of generally conical worms 11, 12 whose shafts 13, 14 
are journaled in the housing at the material-feed end in bearings (not 
shown) and carry gears 15, 16 enabling the two worms to be counter-rotated 
when one is driven by the motor diagrammatically shown at 17. 
The opposite end of the housing has an outlet 18 from which the 
thermoplastic material may be extruded. 
The principles of operation and applications of the extruder are the same 
as those discussed in U.S. Pat. No. 4,047,705 and the other references 
mentioned. 
Each worm 11, 12 is formed with a spiral flight 21, 22 extending 
continuously over a length of the worm which may have a length between 10 
and 15 times the mean diameter thereof and rising from a root 23, 24 lying 
along a conical surface. 
The flight cross section is generally trapezoidal and has a flank angle 
.alpha. which can range from .alpha..sub.1 =25.degree. at the broad base 
of each worm to .alpha..sub.2 =10.degree. at the discharge end 25 of the 
worms, this flank angle being measured between the flank and a 
perpendicular 26 to the axis 27 of the worm. 
Juxtaposed flanks of the two worms, where the flights interfit, include 
acute angles .beta. with one another and in these regions, the flights of 
the two worms are separated by gaps G which can increase progressively in 
the direction of taper and material feed (arrows A and B) by about 40% 
over the length of the worm. 
In the discharge zone the flight depth or height d is in a ratio to the 
outer diameter D at the downstream end of the discharge zone such that 
d/D.times.100=24 to 33%, preferably 26 to 30%, i.e. the dimension d is 24 
to 33%, preferably 26 to 30%, of the dimension D. 
The surprising results which have been obtained were discovered based upon 
the development of new concepts in worm presses worked out from motors of 
the torque requirement over the length of the worm. 
It was discovered, for example, that the power development of a worm press 
with meshing counter-rotating worms in the discharge zone and hence the 
heat development therein is dependent upon the following factors: 
1. The hydraulic pressability; 
2. The friction resulting from pressing of the worm against the cylinders 
in the housing; 
3. The drag flow in the C-shaped material chamber; 
4. The shear between the periphery of the worm and the cylinder; 
5. The shear due to differential speeds between the meshing flanks as well 
as the core and flight periphery. 
Working with these factors it was discovered that it was necessary to treat 
a double-worm system as inherently capable of higher heat development than 
a single worm system. 
Based upon the fact that PVC undergoes a gel transformation under a 
temperature of 120.degree. to 140.degree. C. in a pressure build-up region 
of the discharge zone and the temperature in this zone can reach 
180.degree. to 190.degree. C., it was determined from the enthalpy of the 
PVC that the thermal requirements were about 30 W/kg of the PVC. 
By then utilizing different depths of the flight and rheological residence 
of the material of 300 bar, a coefficient of friction of 0.1 and a mean 
viscosity of 0.2 kps/cm.sup.2, it was discovered that an adiabatic 
operation could be obtained with extremely large flank heights. 
Such severe flank heights over the length of the worm weakened the latter 
and further investigation showed that the torque requirement of the worm 
amounted to 40 to 50% upstream of the discharge zone with the remaining 50 
to 60% in the 4 to 5 turns of the flight in the discharge zone and that 
this torque was converted largely to heat. 
The worm was then considered to be broken down into zones with the 
following torque distribution: 
Preplastification zone upstream of the discharge zone=40%; 
Discharge zone: 
First turn=20%; 
Second turn=17%; 
Third turn=13%; 
Fourth turn=10%. 
(The turns being counted from the upstream end.) 
Bearing these values in mind and that it was important to prevent 
overstressing of the steel of the worms toward the narrow end of the 
worms, it was discovered quite surprisingly that the entire zone could be 
made to operate in an adiabatic manner. For example, it was found to be 
essential for this purpose that of course, depending upon the viscosity 
and the material used, the flight depth should be between 1/4 and 1/3 of 
the flight diameter. Reduced depths can give rise to overheating while 
larger depths result in problems with the output and create the 
possibility of mechanical failure. 
Utilizing the principles of the invention, therefore, I am able to increase 
the economy of the steel used for the worms, reduce the energy 
requirements per kg of extruded product, increase the quantity of extruded 
product per revolution of the worm, carry out a more homogeneous 
plastification for a given energy consumption, maintain the uniformity of 
the output so that the latter does not vary in density, eliminate cylinder 
and worm cooling, and eliminate the need to control the feed of the 
material to the press.