Stretch ratio control circuit for multiple station web stretching apparatus

A control circuit for apparatus stretching a web between intake, discharge, and intermediate pulling stations (20-1, 20-2, 20-3). The circuit includes a first signal generator (32) for providing a signal to the intake discharge station (20-1) corresponding to the desired speed of the web at that station. A multiplier (40) alters the first signal in accordance with the amount of overall stretching, or stretch ratio, desired in the apparatus. The multiplier provides a second signal to the discharge station (20-3) corresponding to the speed of the web at the discharge station necessary to obtain the overall stretching. A second signal generator (48-66) subtracts a selected amount of the difference between the first and second signals from the second signal to provide a third signal to the intermediate pulling station (20-2) that distributes the overall stretching in the apparatus between intermediate zones formed by the adjacent pulling stations.

In processing many types of materials, the material must be elongated or 
stretched. For example, plastic films of polyethylene or polypropylene are 
stretched to orient the film and increase film strength in the direction 
of stretching. Stretching may also be used to determine the thickness, or 
gauge, of the film. Film can be stretched by passing it through a series 
of pull roll stations in which the film is moved faster at the downstream 
station than at the upstream station. 
Inasmuch as the stretching determines the film strength, gauge, and other 
significant properties, the amount of such stretching and its control is 
of considerable importance in establishing the properties of the finished 
product. 
The amount of stretch applied to the material is often stated in terms of a 
stretch ratio, that is, the ratio of the speed of the material at the 
downstream station to the speed of the material at the upstream station. 
In processing plastic films, stretch ratios of 5:1 or 8:1 are not 
uncommon. These are many times greater than the draw used in connection 
with materials such as paper that are typically stated in percentages, 
such as 2% or 5%. A stretch ratio of 5:1 on a percentage basis is 500%. 
To facilitate the processing of a film web, the stretching is typically 
carried out with three or more pull roll stations arranged in tandem along 
the web in a web stretching apparatus. The overall stretch ratio applied 
to the film is the ratio of the speed of the web at the discharge station 
of the apparatus to the speed of the web at the intake station of the 
apparatus. The overall stretch ratio is the product of the individual 
stretch ratios developed in the intermediate zones between each pair of 
adjacent pull roll stations in the apparatus. 
It is frequently necessary to change the stretch ratio developed in one or 
more zones of the apparatus. This may be done for a variety of reasons. 
For example, it may be carried out to limit the loading of one or more of 
the motors driving the pull roll stations or to equalize the loading of 
these motors. The amount of stretching that may be carried out on the web 
is related to its temperature. Changes in the temperature of the web at 
various points along its length requires changes in the stretching carried 
out in the various zones of the apparatus. 
Control circuitry has been developed for maintaining the stretch ratios as 
the line speed of the entire apparatus is increased or decreased. Control 
circuitry has also been developed by which when web speeds are altered in 
one zone of the apparatus to change the stretch ratio in that zone, 
compensating changes to the web speeds in all subsequent zones are made so 
that the stretch ratios developed in the subsequent zones remain 
unaffected. However, this causes the overall stretch ratio of the 
apparatus to change as the speed of the web at the discharge pull roll 
station will ultimately be altered. Since it is the overall stretch ratio 
of the apparatus that determines properties such as film strength and 
gauge of the finished product, changes in the overall stretch ratio may 
not be tolerable. 
To keep the overall stretch ratio constant with such a control circuitry, 
two adjustments must be made. One is the desired alteration of the stretch 
ratio in the particular intermediate zone. The second is a compensating 
adjustment in another zone so that the overall stretch ratio remains the 
same. The need to make two adjustments may adversely affect the constancy 
of the overall stretch ratio. Given typical processing speeds, large 
amounts of scrap material may be produced while the operator of the 
apparatus makes and checks these adjustments. And, there is always the 
possibility that during the course of the adjustments, the downstream pull 
roll station will drive the web at a slower speed than the upstream 
station. This will cause slack rather than stretching to appear in the 
web. Not only do the properties of the web suffer, the web may be broken 
if the slack is removed too rapidly. 
It is, therefore, the object of the present invention to provide a control 
circuit by which the stretch ratio in one intermediate zone of a 
multi-zone web stretching apparatus may be altered without altering the 
overall stretch ratio of the apparatus. More specifically, the present 
invention provides a control circuit by which, when the stretch ratio in 
one intermediate zone or zones is changed, a compensating adjustment is 
automatically applied to another intermediate zone or zones to maintain 
constant overall stretch ratio in the apparatus. The control circuit may 
thus be viewed as one for distributing the overall stretch ratio of the 
web stretching apparatus among a plurality of intermediate zones while 
maintaining the overall stretch ratio constant. Should it be desired to 
change the overall stretch ratio or the line speed of the apparatus, the 
control circuit will maintain the proportionate amount of the overall 
stretching occurring in the intermediate zones constant. The control 
circuit of the present invention also prevents slack conditions from 
occurring in the web. 
Briefly, the control circuit of the present invention inclues a first 
signal generator, such as a potentiometer, for providing a first signal to 
one of the intake or discharge pull roll stations corresponding to the 
desired speed of the web at that station. Typically, the first signal will 
be applied to the intake pull roll station. A multiplier, coupled to the 
first signal generator, alters the first signal in accordance with the 
amount of overall stretching desired in the apparatus and provides a 
second signal to the other of the intake or discharge stations 
corresponding to the speed of the web at the other station necessary to 
obtain the desired overall stretching of the web. The multiplier thus 
establishes the overall stretch ratio of the stretching apparatus. 
A second signal generator provides a third signal to an intermediate pull 
roll station that defines the intermediate stretching zone in the 
apparatus. The second signal generator is coupled to the first signal 
generator and to the multiplier for deriving a signal indicative of the 
overall stretching occurring in the apparatus as by subtracting the first 
signal from the second signal at a summing junction. A portion of that 
signal, selected in accordance with the desired distributor of the overall 
stretching between the intermediate zones, is subtracted from the second 
signal at a second summing junction to provide the third signal to the 
intermediate pull roll station to provide the web speed at that station 
necessary to obtain the desired distribution. Through the subtraction 
occurring in the second signal generator, the speed of the web at the 
intermediate station can never be greater than the intake station web 
speed or less than the discharge station web speed. Slack in the web is 
thus avoided.

FIG. 1 exemplarly shows a web stretching apparatus 10 as that for stretch 
orienting web 12 that may comprise a plastic film. Extruder 14 casts the 
plastic, such as polyethylene or polypropylene, on chill roll 16. A take 
off apparatus, diagrammatically shown as roll 18 removes the film from the 
chill roll and provides it to stretching apparatus 10. 
Stretching apparatus 10 includes a plurality of pull roll stations, three 
of which are indicated as 20-1, 20-2, and 20-3. In each pull roll station, 
web 12 may pass through a nip formed by a pair of pull rolls comprising 
driven roll 22 and idler roll 24. In actual practice, rolls 22 and 24 are 
usually arranged to provide an S-wrap to the web, similar to rolls 16 and 
18. Drive rolls 22 of pull roll stations 20-1, 20-2, and 20-3, are driven 
by motors 26-1, 26-2, and 26-3, respectively. 
To stretch web 12, the downstream pull rolls are driven faster than 
upstream pull rolls so that a tensile force is applied to the web between 
two adjacent pull roll stations. The amount of stretching to which web is 
subjected depends on the difference in the speeds of the web at the two 
adjacent stations. 
As noted above, the amount of stretching to which the web is subjected is 
usually defined in terms of a stretch ratio: that is, the ratio of the 
speed of the web at the downstream station to the speed of the web at the 
upstream station. 
The overall stretch ratio of stretching apparatus 10 is determined by the 
speed S.sub.3 of the web at the discharge pull roll station, i.e., pull 
roll station 20-3, and the speed S.sub.1 of the web at the intake pull 
roll station, i.e. pull roll station 20-1 and is defined as S.sub.3 
/S.sub.1. The overall stretch ratio determines film properties such as 
film gauge, film strength, and the like, and is established by the 
criteria for these properties. 
By controlling the operation of intermediate pull roll station 20-2, the 
overall stretch ratio may be distributed between zone A established 
between pull roll station 20-1 and pull roll station 20-2 and zone B 
established between pull roll station 20-2 and pull roll station 20-3. 
Thus, if pull roll station 20-2 is driven at the same speed as pull roll 
station 20-1, all of the stretching of web 12 will take place in zone B. 
If pull roll station 20-2 is driven at the same speed as pull roll station 
20-3, all of the stretching will take place in zone A. 
If pull roll station 20-2 is driven at some speed between the speeds of 
pull roll stations 20-1 and 20-3, part of the stretching will occur in 
zone A and part will occur in zone B. The distribution of the stretching 
will be determined by the speed of pull roll station 20-2. 
Also as noted above, it is often desired to control the amount of 
stretching occurring in each of intermediate zones A and B without 
altering the overall stretch ratio and the properties of the finished 
film. This may be desired to accommodate the temperatures of web 12 along 
its length, to adjust or equalize the loading of motors 26-1, 26-3, and 
26-3, or for other reasons. This distribution of the overall stretch ratio 
between zones A and B is carried out by control circuit 30 of the present 
invention. 
Control circuit 30 includes signal generator 32 providing a signal 
proportional to the desired speed S.sub.1 of web 12 at intake pull roll 
station 20-1 in conductor 34. The speed S.sub.1 of the web at intake pull 
roll station 20-1 is coordinated with the operation of extruder 14 and 
rolls 16 and 18 and is often termed the "line speed" of apparatus 10. 
Signal generator 34 is shown as a potentiometer in FIG. 1 for explanatory 
purposes. It will be appreciated that the signal generator may, in 
practice, be incorporated in the apparatus for casting web 12. 
The signal in conductor 34 is provided to motor 26-1 in conductor 36. While 
control circuit 30 is shown in simplified form in FIG. 1 to facilitate an 
explanation of its construction and operation, it will be appreciated that 
amplification and power control circuitry, not shown, will be provided at 
the input of motor 26-1 to control the motor responsive to the signal in 
conductor 36. 
The signal in conductor 34 is also provided in conductor 38 to the input of 
multiplier 40. Multiplier 40 determines the overall stretch ratio of 
stretching apparatus 10 by providing an output signal in conductor 42 to 
motor 26-3 that is a predetermined multiple of the input signal in 
conductor 38. For example, if a 5:1 stretch ratio is desired, a signal is 
provided in conductor 42 that will so drive motor 26-3 as to cause the 
speed S.sub.3 of web 12 at pull roll station 20-3 to be five times greater 
than the speed S.sub.1 of the web at pull roll station 20-1. In the 
simplified embodiment of control circuit 30 shown in FIG. 1, the magnitude 
of the signal in conductor 42 may be seen as five times the magnitude of 
the signal in conductor 38. In actual practice, the size of rolls 22 and 
24 at the two pull roll stations may be different, or motors 26 may have 
gearing associated with them that will cause the signal in conductor 42 to 
assume some other value. Appropriate scaling circuitry is provided in 
control circuit 30 to coordinate the signal levels within the circuitry. 
Multiplier 40 may be a semiconductor signal multiplier of the type 
available from Analog Devices Company of Norwood, Mass. as item AD534 or 
from RCA as item CA309ID. The amount of multiplication provided is 
determined by the magnitude of the input signal in conductor 44 from a 
signal generator 46, such as a potentiometer. When fractional 
multiplication is desired, a potentiometer between conductors 38 and 42 
will suffice, as shown in FIG. 4. 
As the overall stretch ratio of apparatus 10 is established by the amount 
of multiplication provided by multiplier 40, adjustment of only a single 
element, such as signal generator 46 is required to set that stretch 
ratio. And, as noted supra and infra, the overall stretch ratio 
established by the setting of signal generator 46 does not vary as the 
overall stretch ratio is distributed between zones A and B. 
The means used to generate the signal to intermediate pull roll station 
20-2 includes summing junction 48 connected to conductor 42 through 
conductor 50 and to conductor 38 through conductor 52. Summing junction 48 
provides an output signal in conductor 54 proportional to the difference 
between the signals in conductor 42 and conductor 38 or S.sub.3 -S.sub.1. 
The signal S.sub.3 -S.sub.1 corresponds to the amount of overall 
stretching occurring in apparatus 10. 
The signal in conductor 54 is passed through unity gain buffer amplifier 56 
to potentiometer R, the wiper of which provides a selected portion of the 
S.sub.3 -S.sub.1 signal in conductor 58 or R(S.sub.3 -S.sub.1). The signal 
may include all of the signal in conductor 54, none of that signal, or 
some intermediate portion thereof. 
The signal in conductor 58 and a signal in conductor 60 corresponding to 
the signal in conductor 42 or speed S.sub.3 of the web at pull roll 
station 20-3 are supplied to a second summing junction 62 that subtracts 
the signal in conductor 58 from the signal in conductor 60 and provides a 
difference signal in conductor 64 corresponding to S.sub.3 -R(S.sub.3 
-S.sub.1). The signal in conductor 64 is provided through unity gain 
amplifier 66 to motor 26-2 in conductor 68 to control the speed S.sub.2 of 
web 12 at pull roll station 20-2. 
In operation, potentiometer 32 is adjusted to establish the speed of motor 
26-1 and the speed S.sub.1 of web 12 at pull roll station 20-1. 
Potentiometer 46 is adjusted to operate multiplier 40 and establish the 
overall stretch ratio: that is, the ratio between the speed S.sub.3 of the 
web at pull roll station 20-3 and the speed S.sub.1 of the web at pull 
roll station 20-1 or S.sub.3 /S.sub.1. 
The adjustment of potentiometer R determines the speed S.sub.2 of web 12 at 
pull roll station 20-2 and the distribution of the overall stretch ratio 
between zone A and zone B by providing for the subtraction of a selected 
portion of the signal S.sub.3 -S.sub.1, corresponding to the overall 
stretching in apparatus, from the web speed S.sub.3 signal at summing 
junction 62. If potentiometer R is set at zero, there will be no signal in 
conductor 58. The signal in conductor 64 is that of conductor 42 
corresponding to S.sub.3 and the speed S.sub.2 of the web at pull roll 
station 20-2 is the same as the speed of the web at pull roll station 20-3 
so that all the stretching occurs between pull roll station 20-1 and 20-2 
in zone A. 
If potentiometer R is set to provide all of the signal S.sub.3 -S.sub.1 
from summing junction 48, the speed S.sub.2 of web 12 at pull roll station 
20-2 will be the same as the speed S.sub.1 at pull roll station 20-1 and 
all the stretching will occur between pull roll station 20-2 and pull roll 
station 20-3 in zone B. This is because the signal at summing junction 62 
will be S.sub.3 -[S.sub.3 -S.sub.1 ], that reduces to S.sub.3 -S.sub.3 
+S.sub.1, or simply to S.sub.1. 
If potentiometer R is set at some intermediate value, the overall 
stretching will be distributed between zone A and zone B in some desired 
manner in accordance with the setting of the potentiometer. For example, 
if potentiometer R is set to provide half the difference signal S.sub.3 
-S.sub.1 at summing junction 48 in conductor 58, equal amounts of 
stretching will be developed in zone A and in zone B. 
From the foregoing it will be appreciated that the distribution of the 
stretching between zones A and B is carried out without altering the 
overall stretch ratio of stretching apparatus 10 that is established by 
potentiometer 46. However, should it be desired to change the overall 
stretch ratio, control circuit 10 will maintain the proportionate amount 
of the overall stretching occurring in each of zones A and B constant. The 
same is true for changes in line speed. It will be further appreciated 
that the speed S.sub.2 of web 12 at pull roll station 20-2 can never be 
greater than speed S.sub.3 at pull roll station 20-3 or less than the web 
speed S.sub.1 at pull roll station 20-1. This insures that no slack can 
occur in web 12 that could interfere with proper handling or treatment of 
the web. 
While control 30 has been shown in FIG. 1 in conjunction with three pull 
roll stations, it can be employed in stretching apparatus having a greater 
number of pull roll stations, if desired. FIG. 2 shows a stretching 
apparatus 10A having five pull roll stations 80-1, 80-2, 80-3, 80-4, and 
80-5 driven by motors 82-1, 82-2, 82-3, 82-4, and 82-5. FIG. 2 also shows 
control circuit 30A. For control purposes, stretching apparatus 10A may be 
divided into two main control zones A and B. Zone A extends from pull roll 
station 80-1 to pull roll station 80-3. Zone B extends from pull roll 
station 80-3 to pull roll station 80-5. Each of zones A and B may be 
divided into subzones. Zone A is divided into subzone A-1 extending 
between pull roll station 80-1 and pull roll station 80-2 and subzone A-2 
extending between pull roll station 80-2 and pull roll station 80-3. Zone 
B is divided into subzone B-1 extending between pull roll station 80-3 and 
pull roll station 80-4 and subzone B-2 extending between pull roll station 
80-4 and pull roll station 80-5. 
In FIG. 2, elements common to FIG. 1 have been given the same identifying 
designation. Thus, potentiometer 32 provides a signal in conductors 34 and 
36 corresponding to the desired web speed S.sub.1 at intake pull roll 
station 80-1. The same signal is applied to multiplier 40 in conductor 38 
that provides a signal in conductor 42 to discharge pull roll station 80-5 
in accordance with the desired discharge web speed S.sub.5 to determine 
the overall stretch ratio S.sub.5 /S.sub.1 of stretching apparatus 10A. 
Summing junction 48, amplifier 56, potentiometer R, summing junction 62, 
and amplifier 66 to provide a signal S.sub.3 in conductor 68 to pull roll 
station 80-3 that distributes the overall stretch ratio of stretching 
apparatus 10A between zones A and B in a manner corresponding to that 
described above in connection with FIG. 1. 
Similar circuitry is employed to distribute the stretching occurring in 
zone A between subzone A-1 and subzone A-2. Thus, the signal corresponding 
to the speed S.sub.3 of the web at pull roll station 80-3 in conductor 68 
is provided to summing junction 84 in conductor 86. The signal 
corresponding to the speed S.sub.1 of the web at pull roll station 80-1 in 
conductor 52 is provided to summing junction 84 in conductor 88. The 
difference signal in conductor 90 is provided through unity gain amplifier 
92 and potentiometer A to summing junction 94 in conductor 96. Summing 
junction 94 also receives the signal corresponding to the speed S.sub.3 of 
the web at pull roll station 80-3 in conductor 98. The difference signal 
from summing junction 94 in conductor 100 is provided through unity gain 
amplifier 102 to motor 82-2 in conductor 104. 
Operation of potentiometer A distributes the stretching occurring in zone A 
between subzones A-1 and A-2 in the same manner as that by which 
potentiometer R distributes the stretching between zones A and B. 
The circuitry associated with potentiometer B also operates in the same 
manner to distribute the stretching occurring in zone B between subzone 
B-1 and subzone B-2 of zone B. 
FIG. 3 shows a further embodiment of the stretch ratio control circuit 30B 
of the present invention suitable for use with stretching apparatus 10B in 
which the stretching zones are defined in a slightly different manner. In 
stretching apparatus 10B, zone A is established by pull roll station 120-1 
and 120-2. Stretching zone B is established by pull roll station 120-2 and 
pull roll station 120-5. In stretching apparatus 10B shown in FIG. 3, zone 
B is further divided into zones B-1 and B-2 established by pull roll 
stations 120-2 and 120-3 and 120-3 and 120-5, respectively. Zone B-2 is 
then further subdivided into zones B-3 and B-4 established by pull roll 
station 102-3 and 102-4 and by pull roll station 120-4 and 120-5, 
respectively. The pull roll stations are driven by motors 122-1, 122-2, 
122-3, 122-4, and 122-5. 
As before, elements common to FIGS. 1 and 2 have been identified with 
similar designations in FIG. 3 so that elements 32 through 68 determine 
the overall stretch ratio S.sub.5 /S.sub.1 for stretching apparatus 10B in 
accordance with the setting of potentiometer 46. These elements also 
determine the distribution of the stretching between zone A and zone B by 
virtue of the output signal S.sub.2 in conductor 68. 
To distribute the stretching occurring in zone B between zone B-1 and zone 
B-2, the signal corresponding to the web speed S.sub.5 at pull roll 
station 120-5 in conductor 42 is provided to summing junction 124 in 
conductor 126. The signal corresponding to the speed S.sub.2 of the web at 
pull roll station 120-2 in conductor 68 is provided to summing junction 
124 in conductor 128. The output of summing junction 124 is provided 
through unity gain amplifier 130 and potentiometer B to summing junction 
132, the other input of which is connected to conductor 42 and web speed 
signal S.sub.5 by conductor 134. 
The output of summing junction 132 is provided through unity gain amplifier 
136 to conductor 138 to motor 122-3 driving pull roll station 120-3. 
Circuitry 124 through 138 operates to distribute the stretching occurring 
in zone B between zone B-1 and zone B-2. 
To distribute the stretching occurring in zone B-2 between zones B-3 and 
B-4, the web speed signal S.sub.5 in conductor 42 is provided to summing 
junction 140 in conductor 142. The speed signal S.sub.3 of the web at pull 
roll station 120-3 in conductor 138 is provided to summing junction 140 in 
conductor 144. The output of summing junction 140 is provided through 
unity gain amplifier 146 and potentiometer B-1 to summing junction 148 in 
conductor 150. Summing junction 148 also receives the web speed signal 
S.sub.5 in conductor 152. The output of summing junction 148 is provided 
through unity gain amplifier 154 to conductor 156 and to motor 122-4. 
Operation of potentiometer B-1 distributes the stretching occurring in zone 
B-2 between zones B-3 and B-4. 
The operation of control circuit 30B shown in FIG. 3 facilitates a 
prioritization of the stretching zones of apparatus 10B in a cascade-like 
fashion. Thus, by sequentially adjusting potentiometer R, potentiometer B, 
and potentiometer B-1, the stretching occurring in the individual zones 
can be established, beginning with the most upstream zone A, and 
continuing through to the most downstream zone B-4. The adjustment 
potentiometer R controls the stretching occurring in zone A and zone B. 
The adjustment of potentiometer B distributes the stretching occurring in 
zone B between zones B-1 and B-2. The adjustment of potentiometer B-1 
distributes the stretching occurring in zone B-2 between zones B-3 and 
B-4. 
It will be appreciated that various other modifications of the control 
circuitry of the present invention are possible. For example, while signal 
generator 32 has been shown as controlling the speed of the intake pull 
roll station and multiplier 40 controlling the speed of the discharge pull 
roll station, signal generator 32 may be used to control the latter and 
multiplier 40 used to control the former, as shown in FIG. 5. The 
circuitry 30C shown in FIG. 5 may be so arranged that the zones are 
cascaded from the most downstream stretching zone to the most upstream 
stretching in a manner opposite to that shown in FIG. 3. 
Various modes of carrying out the invention are contemplated as being 
within the scope of the following claims particularly pointing out and 
distinctly claiming the subject matter which is regarded as the invention.