Extrusion control

An extrusion process is controlled by feeding the material to be extruded from a supply station (5) to an extruder (1) and repeatedly weighing the supply station to calculate the throughput of the extruder. The initial line speed necessary to produce a predetermined weight/meter of extrudate is calculated and the line speed is adjusted accordingly. Subsequently, the throughput and the line speed are increased simultaneously, such that the weight/meter is maintained substantially constant until any one of a plurality of parameters such as line speed, screw speed, motor load current, extrudate temperature, melt pressure, etc reaches a predetermined maximum value. Thereafter either the line speed or extruder throughput is adjusted such as to maintain the weight/meter of the extrudate substantially constant at the desired value.

BACKGROUND AND SUMMARY OF THE INVENTION 
This invention relates to a method and apparatus for controlling an 
extrusion process such that the weight/meter of extrudate can be 
accurately controlled. 
The invention is especially concerned with the extrusion of a tubular 
covering of rubber or thermoplastics material on to a core, in which the 
linear speed of the extrudate as finally formed is equal to the speed at 
which the core passes through the machine. When extruding sections not 
formed as tubular coverings on a core, the linear speed of travel of the 
extrudate as finally formed is considered to be the speed at which it is 
taken up, e.g. by a capstan, an endless belt haul-off, or a take-up drum, 
from the extrusion machine after it has been cooled. This speed and the 
core speed when tubular coverings are being formed on a core will herein 
each be referred to generically as the "line speed". 
It is known to attempt to control the weight/meter of extrudate on a cable 
by measuring the diameter of a cable and varying the extrusion process to 
attempt to maintain the diameter constant. This suffers from the 
disadvantage that the diameter of a cable is not easy to accurately 
determine until the extrudate has been cooled. Furthermore where a core of 
non-circular cross-section is employed the diameter of the cable produced 
may not be an accurate reflection of the weight/meter of extrudate applied 
thereto. 
It is therefore proposed to provide an extrusion process in which the 
weight/meter of extrudate can be more accurately controlled. 
Accordingly a method of controlling an extrusion process comprises the 
steps of feeding the material to be extruded from a supply station to an 
extruder; 
repeatedly weighing the supply station to calculate the through-put of the 
extruder; 
calculating the initial line speed necessary to produce a predetermined 
weight/meter of extrudate; 
adjusting the line speed directly to said initial line speed; 
subsequently increasing the throughput of the extruder and the line speed 
simultaneously, such that the weight/meter of the extrudate is maintained 
substantially constant, until any one of a plurality of measured 
parameters reaches a predetermined maximum value; and 
thereafter adjusting the line speed or extruder throughput in response to 
the calculated throughput of the extruder such as to maintain the 
weight/meter of the extrudate substantially constant at said predetermined 
weight/meter. 
Conveniently the supply station is weighed by a load cell, typically by 
being freely suspended therefrom. Repeated measurement of the combined 
weight of the supply station and material therein will allow calculation 
of the weight of material leaving the supply station and hence the 
throughput of the extruder. Measurement of the weight of material entering 
the extruder is a direct indication of the performance of the extruder. 
Alternative systems which monitor the volume of material entering the 
extruder are inherently less accurate due to the property that the flow of 
material from the supply station is not regular. 
The supply station is preferably a container in which the material to be 
extruded is held. Alternatively, where the material to be extruded is in 
strip form rather than in the form of granules, powder etc, the supply 
station is conveniently a platform such as a pallet or the like. 
It is envisaged that the feed to the extruder will be such that it is choke 
fed and the throughput of the extruder varied by varying the screw speed 
thereof. However it is equally conceivable that the extruder may be starve 
fed and its throughput controlled, additionally or alternatively to the 
screw speed, by varying the amount of material fed to the extruder. It 
will be appreciated that whether the extruder is choke fed and its 
throughput is monitored, or whether the extruder is starve fed and its 
throughput is metered, repeated weighing of the supply station will allow 
the throughput to be calculated. 
Conveniently the throughput of the extruder and the line speed are 
simultaneously increased in an incremental manner, separated by periods in 
which only one of the throughput or line speed is varied such as to 
maintain the weight/meter of the extrudate substantially at the 
predetermined weight/meter. This incremental increase allows any 
discrepancy which may be introduced into the weight/meter of the extrudate 
by the simultaneous increase of the throughput and line speed to be 
corrected. Each increment is preferably preset to subsist for a 
predetermined time period. Additionally the time periods of respective 
increments conveniently vary according to a preset progression. For 
example a decreasing incremental length is envisaged such that more 
frequent corrective action is taken at relatively higher line speeds. 
Conveniently the method includes the additional step of reducing the 
extruder throughput and the line speed, once at least one of said measured 
parameters has reached its predetermined maximum value, until said at 
least one parameter again falls below the predetermined maximum value. 
This "backing-off" of the production rate is to avoid a situation in which 
the production rate is maintained whilst said parameter continues to rise 
above its predetermined maximum value. By reducing the throughput and line 
speed until the parameter once again falls below maximum, the possibility 
of the parameter considerably overshooting its maximum is removed. 
The measured parameters conceivably include the line speed, the screw speed 
of the extruder, the load current of the motor driving the extruder screw, 
the melt pressure at the extruder head and the temperature of the 
extrudate emerging from the extruder. Other parameters may be measured and 
monitored where appropriate to the manufacturing process. 
The method of the present invention is particularly suited to the 
production of variable density extrudate. By the term variable density 
extrudate is herein meant material which is expanded by a gas, normally 
nitrogen, to form an extrudate which is foamed, or is solid material which 
has cavities or segments therein. The gas is either injected into the 
extrudate or is produced chemically by the addition of a gas producing 
compound to the material to be extruded. The expansion continues until the 
extrudate is stabilized by cooling at which point the gas produced 
cavities are "frozen" into the extrudate. 
Accordingly the method conveniently includes the steps of measuring at 
least one additional parameter which is either a suitable dimension or the 
capacitance of the stabilized extrudate, and adjusting the temperature 
profile of the extrudate in response to said measured additional parameter 
in order to maintain said additional parameter substantially constant. The 
suitable dimension is typically the diameter of the extrudate. The 
temperature profile of the extrudate is conveniently adjusted by changing 
the longitudinal position of a cooling means, adapted to stabilize the 
extrudate. The cooling means is conceivably a quenching bath containing 
coolant such as water. Alternatively cooling is achieved by means of one 
or more jets adapted to spray air, water or other coolant on to the 
extrudate. 
The invention further resides in an extrusion apparatus controlled by the 
method described herein. In particular the extrusion apparatus comprises 
an extruder; a supply station adapted to support material to be extruded; 
supply means adapted to feed the material to the extruder; weighing means 
for repeated weighing of the supply station; drive means for controlling 
the line speed; means for measuring the line speed; sensors for sensing a 
plurality of parameters; and electronic processing means adapted to: 
I. receive repeated signals from the weighing means and calculate the 
throughput of the extruder, 
II. calculate an initial line speed necessary to produce a predetermined 
weight/meter of extrudate, 
III. supply signals to the drive means to adjust the line speed to said 
initial line speed, 
IV. compare the plurality of sensed parameters against predetermined 
maximum values for these parameters, and if all are below said maximum 
values supply signals to the drive means and to the extruder to increase 
the line speed and the throughput of the extruder simultaneously, such 
that the weight/meter of the extrudate is maintained substantially 
constant, and 
V. supply signals to the drive means, once at least one of said sensed 
parameters is at its maximum value, to adjust the line speed or extruder 
throughput in response to the calculated throughput of the extruder such 
as to maintain the weight/meter of the extrudate substantially constant at 
said predetermined weight/meter. 
Conveniently the electronic processing means is a microprocessor or 
microcomputer.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
Referring to FIG. 1 an extruder shown generally at 1 includes a heated 
barrel 2 within which is an extruder screw 3 driven by a motor 4. A supply 
station in the form of a hopper 5 is freely suspended from a load cell 6. 
The hopper 5 feeds thermoplastic material to be extruded in the form of 
PVC granules 7 into the barrel 2. A metallic core 8 is fed into the 
extruder 1 and the core is coated with an extrudate to form a cable 9. The 
cable 9 emerging from the extruder is fed to a cooling bath 10 and a pair 
of rollers 11 driven by a motor 12 hauls off the coated cable 9 onto a 
winding apparatus (not shown). 
A microprocessor 13 receives electronic signals from the load cell 6 via 
line 14. A plurality of parameters are sensed and fed to the 
microprocessor 13. For example, the microprocessor is in communication 
with the haul-off motor 12 by means of lines 15 and 16 in order to monitor 
and control the haul-off line speed. 
Additionally the microprocessor 13 is in communication with the motor 4 by 
means of line 21 and receives signals therefrom via lines 17 and 18 
regarding the screw speed and motor load current respectively. Finally a 
temperature sensor 19 senses the extrudate melt temperature and sends 
electronic signals to the microprocessor via line 20. 
The setting up of an extrusion run will now be described with particular 
reference to FIGS. 2 and 3. Data regarding the desired weight/meter of the 
extrudate and the maximum values for the line speed, melt temperature, 
screw speed and motor load current are input to the microprocessor 13 via 
input line 22. 
At time A the extruder 1 is started or accelerated up from `tick over` to 
run at a relatively low `initial` throughput. At this time the rollers 11 
are not running and the line speed is hence zero. The microprocessor 13 
receives measurements from the load cell 6 via line 14, determines the 
throughput of the extruder at `initial throughput` and calculates the line 
speed required to achieve the desired weight/meter as previously input. It 
sends signals via line 16 to the haul-off motor 12 to activate the rollers 
11 at the calculated line speed as shown at time B. The cable is therefore 
immediately produced at the desired weight/meter thereby reducing the 
amount of scrap cable commonly produced at the start of a production run. 
At time C the microprocessor starts to increase the production rate. It 
sends a signal via line 21 to the motor 4 to increase the screw speed to 
correspondingly increase the extruder throughout. Simultaneously the 
microprocessor 13 sends a signal via line 16 to the motor 12 to increase 
the line speed by means of rollers 11. 
After a preset time increment C-D the microprocessor 13 sends signals to 
the motors 4 and 12 to halt the increase in production rate. It again 
calulates the throughput of the extruder from the measurements of the load 
cell 6 and sends signals to the motor 12 to adjust the line speed to 
correct any discrepancy in the weight/meter of extrudate. The increase in 
production rate is further continued in increments E-F, G-H etc. with 
correcting intervals F-G, H-I therebetween. 
The microprocessor 13 continuously monitors the other parameters against 
their maximum values input previously via line 22. At time J one of the 
parameters, for example the extrudate melt temperature as measured by 
sensor 19, reaches its predetermined maximum value. The microprocessor 
therefore again sends signals halting the increase in the production rate. 
From time K the microprocessor starts to decrease the extruder throughput 
and line speed until at time L the melt temperature as sensed by sensor 19 
again falls below its predetermined maximum value. The production run then 
continues at this production rate until a signal is given to end the run. 
By increasing the production rate until one of the parameters reaches its 
maximum, the production run is effectively run at its optimum rate. 
Thereafter, optimization of the process is maintained by maintaining 
constant the weight/meter of the extrudate. 
It will be seen that the production run is controlled such that, even when 
the rate of production is being scaled up, the weight/meter of extrudate 
of the cable produced is never more than a small degree from the desired 
value. The degree of accuracy required during the sealing up of a 
production run can be achieved by setting the lengths of the increments in 
which production is increased. 
It will be appreciated that two or more extrusion machines as described 
herein may operate in tandem, the output of one machine being used as the 
core 8 to be fed to another machine. In particular, an arrangement is 
envisaged in which several such machines each extrude an insulating 
covering onto a wire, the wires from the machines each being fed to a 
single secondary extrusion machine which combines them within an extruded 
outer sheath to form a cable. Where two or more extrusion machines run in 
tandem, the line speed must be the same for each. Therefore where the line 
speed is tied to the throughput of one extruder, any adjustments to 
maintain the desired weight/meter at other extrusion machines must be 
carried out by varying the extruder throughput. 
FIG. 4 shows extrusion apparatus equivalent to that of FIG. 1, which is 
capable of accepting feed material in the form of a strip 50. The strip 50 
is supported on a pallet 51, itself mounted on a gyroscopic weighing 
device 52. Signals from the weighing device 52 are fed to the 
microprocessor 15 via line 53. 
The strip 50 is fed to the extruder 1 by two sets of driven pulley wheels, 
54 and 55, between which is an unpowered pulley wheel 56 and an 
accumulator shown generally at 57. The pulley wheels 54 are driven at a 
faster rate than the wheels 55 such that the strip feed is picked up from 
the pallet 51 faster than it is fed to the extruder, the additional strip 
material being temporarily stored in the accumulator 57. 
The accumulator consists of an accumulator pulley 58 freely movable between 
two positions P and O. At position P the drive to the pulley wheels 54 is 
switched in and strip 50 is taken up from the pallet and stored in the 
accumulator, by the downward movement of the pulley 58. When the 
accumulator pulley 58 reaches position Q the drive to the pulleys 54 is 
switched out, halting the take up of strip from the pallet 51 and allowing 
the weighing device 52 to make a static measurement of the weight of the 
strip material thereon. The strip is still fed to the extruder 1 by the 
pulley wheels 55 from the stored strip in the accumulator 57. When the 
accumulator pulley 58 reaches position P, the drive to pulley wheels 54 is 
switched in again and the cycle is repeated. It will be seen from the 
above that a static weighing measurement can thus be achieved, whilst 
maintaining a continuous feed to the extruder 1. 
Positioned between the pulley wheels 54 and the strip 50 on the pallet 51 
is a tension release device seen generally at 59. This device comprises an 
unpowered pulley wheel 60 movable by means of a hydraulic cylinder 61 
between two positions X and Y. When the drive to the pulley wheels 54 is 
switched in, the tension pulley 60 is moved to position Y. When the 
accumulator 57 is full and the drive to pulley wheels 54 is switched out, 
the tension pulley 60 is moved downwardly to position X, thereby 
eliminating any tension in the strip 50. This allows the gyroscopic 
weighing device 52 to make a true measurement of the static weight of the 
strip 50 on the pallet 51. As before, this repeated weighing of the input 
to the extruder 1 allows the microprocessor 13 to control accurately the 
weight/meter of extrudate applied to the core 8. 
With reference to FIG. 5 an apparatus is shown suitable for the extrusion 
of a variable density extrudate. The apparatus is similar to that of FIG. 
1 and like components are designated with like reference numerals. The 
major difference from the apparatus of FIG. 1 is that the cooling bath 10 
is movable longitudinally of the extruded cable 9. The cooling bath 10 is 
mounted on a track (not shown) and driven by means of a motor 30 connected 
to the bath via line 31. The microprocessor 13 is in communication with 
the cooling drive motor 30 by means of lines 32 and 33. A capacitance 
gauge 34 is positioned immediately downstream of the cooling bath 10. The 
gauge 34 sends information regarding the capacitance of the cable 9 to the 
microprocessor via line 35. 
The apparatus operates in a manner substantially similar to that described 
above with reference to Figure 1, with the exception that the 
microprocessor 13 additionally governs the position of the cooling bath 
10. A chemical blowing agent, added to the thermoplastics material 7 in 
the hopper 5, produces a gas on heating which expands the extrudate into a 
foamed construction. The extrudate is stabilized by cooling in the bath 10 
when the expanding foam is "frozen" in position. Signals from the gauge 
34, representing the capacitance of the cable 9, are fed to the 
microprocessor 13 via line 35. The microprocessor accordingly adjusts the 
position of the bath 10, by means of signals sent to the motor 30, in 
order to maintain the capacitance of the cable substantially constant. 
As the weight/meter of extrudate on the cable is maintained substantially 
constant as hereinbefore described, control of the capacitance of the 
cable automatically governs the other important parameter, the diameter of 
the cable. There is accordingly no necessity to independently measure the 
diameter of the cable. Indeed known control systems which attempt separate 
control of both diameter and electrical capacitance often lead to conflict 
within the controlling mechanism resulting in instability and an 
unreliable extrusion line. 
It will be appreciated that in an alternative embodiment the capacitance 
gauge 34 can be replaced with an optical diameter gauge which is then used 
to supply signals for the microprocessor to control the position of the 
cooling bath 10. In a similar manner it will be appreciated that various 
methods of controlling the temperature profile of the cooling extrudate 
may be employed. In addition to cooling baths, troughs or jets, the 
microprocessor may directly control heating means within the extruder in 
order to adjust the temperature at which the extrudate leaves the barrel 
2.