Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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1~ ME7r~OD FOR eONTROLLI~G TElli~ DIA~qETER
OF TUBUI~R F:~LMS M~ IN BL~W MOULDING PI~NTS
The invention relates to a method Eor controlling the
diameter of tubular films made in blow moulding plants. The
apparatus generally used for blow moulding a tubular film
comprises an extruder with a film blowing head; an internal
cooling means for the blown film; a calibrating means, a means
for laying ~lat the tubular film, and a means for film take-up.
In this apparatus, each of the following variables can
be independently controlled: the internal cooling air flow; the
speed at which the extruder supplies plastic material to the
blowing head; the temperature of the extruded plastic material;
the take-up speed of the extruded film; the level of external
cooling; and the position of the calibrating means to maintain
preset parameters.
Essential requirements in the manufacture of blown
films of thermoplastic material include maintaining a
substantially uniform width of the manufactured tubular film web
when laid flat, and a substantially uniform ~egree of orientation
of the plastic molecules drawn during the blowing of the tubular
film. The location of what is called the frost line has a
decisive influence on the film~quality, since it influences both
the degree of orientation within the plastic, and the diameter
of the manufactured blown film. Further, the location of the
frost line is of practical importance, as marring can occur when
the frost line lies too high relative to the calibrating means,
so that the supporting members of the calibrating means leave
undesired marks. However, when the frost line is too low
relative to the calibrating means, the dreaded "bubble pumping"
can occur, which can lead to the blown film breaking off.
Accordingly, the location of the frost line should preferably be
kept constant, both relative to the discharge nozzle and relative
to the height o~ the calibrating means.
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For control of the height of the frost line various
methods have already been described.
In most blow moulding plants the location of the frost
line is only visually monitored by the operating personnel.
According to the experience of the operating personnel the height
of the calibrating means, the amount of internal cooling air
introduced or discharged and/or the output of the extruder are
adjusted corresponding to the observed height of the frost line.
Such adjustments are, however, subject to considerable sources
of error, as they depend on the skill and the experience of the
operating personnel.
Methods for controlling blow moulding plants are
described in ~erman patents 27 21 609 and 28 31 212, where, in
particular for achieving a uniform predetermined film width, the
location of the frost line is measured, and corresponding to the
measured deviations of the frost line, controlling elements are
actuated for influencing the film thickness and/or the diameter
of the tubular film. A particular problem of these known methods
lies, however, in the devices and sensors for detecting the frost
line.
For detecting the frost line thermosensors are known,
which detect the film temperature in areas below or above the
frost line, by measuring the infrared radiation emitted by the
hot plastic film. Such thermosensors are, however, very
sensitive to soiling, so that malfunctions can occur. Finally,
thermosensors are unsuited for measuring very thin transparent
films.
Methods are also known where on the basis of the energy
balance calculated from a consideration of the used raw material,
the film output, the width and thickness of the film, as well as
the cooling air temperatures and further influencing variables,
the location of the frost lirle is calculated. This method is,
however, inaccurate, as it depends on frequently unknown, and
orefore largely unassessable, raw materia] characteristics.
Furthermore, the influence of the heat transfer coefficient from
air to the ~ilm can only be detected with some difficulty. As
a result, it is therefore not possible to make a calculation
which is accurate enough for practical purposes.
This invention seeks to provide a method for
controlling the diameter of tubular film, which can be performed
in practice with good results, easily and without failure,
without having to measure accurately the position of the frost
line.
In accordance with the method of this invention, in the
vicinity of the inflection point of the blow-up region, the
distance of the tubular film from a stationary sensor is measured
and, a first control circuit responds to deviations of the
distance from a predetermined setpoint by appropriately
increasing or decreasing the flow of internal cooling air. The
diameter of the tubular film above the frost line is measured,
and a second control circuit, in respon~e to deviations o~ the
diameter from a predetermined setpoint, correspondingly adjusts
any or all of the following variables: the temperature of the
extruded material; the rate at which all extruder supplies
plastic material to the blowing head; the film take-up speed; and
the flow of external cooling air.
The invention is first of all hased on the knowledge
that a certain shape of bubble in the blow-up region of the
tubular film results in a certain ratio of the longitudinal and
transverse extension speeds of the tubular film. This ratio
determines the degree of orientation. A desired degree of
orientation, which leads to the desired film quality, can
therefore be obtained by setting a certain shape of bubble in the
blow-up region, which can first be tested and determined
empirically. During the manufacture of the blown film, if a
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~sired shape of bubble is ~aintained in the blow-up region, lt
can be assumed that films of constant quality are manufactured,
with the same width and with the same degree of orientation.
.
By means of the method in accordance with the invention
it is possible without directly measurin~ the height of the frost
line not only to adjust the width of the manufactured plastic
tube to a desired value, but in addition the desired degree of
orientation of the plastic molecules can be achieved, because the
same degree of orientation is always achieved when the associated
shape of bubble is maintained.
The invention is furthermore based on the knowledge
that the shape of bubble in the blow-up region can, in essence,
be determined by the location of only the inflection point of the
sinusoidal path of the film in the blow-up region. It can thus
be assumed that the shape of bubble is always the same when the
inflection point is located at the same point.
The method in accordance with the invention thus first
of all ensures that the location of the :inflection point is kept
constant, and, therefore, that the tubular film is blown up with
the same degree of orientation. When deviations are detected,
the corresponding first control circuit therefore influences the
amount of the exchanged internal cooling air, which Gan easily
be ef~ected by controlling the corresponding air blowers.
Moreover, the shape of bubble and the diameter of the
manufactured tubular film are also influenced by the height of
the frost line. So, when the same diameter of the manufactured
tubular film is achieved with the same shape of tubular bubble
in the blowup region, it can be assumed that the ~rost line has
also maintained its desired location. So, to maintain the proper
height of the frost line, the diameter of the manufactured
tubular film is monitored by means of a second control circuit.
When this diameter changes, the temperature of the extruded
plastic material, and/or the speed at which the extruder supp1ies
astic ma~erial to the blowing head and/or the film take-up
speed and/or the external cooling are correspondingly increased
or decreased in accordance with the measured deviation.
The method in accordance with the invention is
therefore characterized in that it provides for a simple control
by detecting easily controlled variables.
The method in accordance with the invention requires
two control processes performed in parallel with each other,
which, due to differently actuated controlling elements, hardly
influence each other disadvantageously, so that one need not fear
that the control circuits might work against each other.
Moreover, the controlled variables can be adjusted-to each other
such that a negative influencing is avoided.
~ y means of the method in accordance with the invention
the outer wall of the blown film is measured in the blow-up
region on at least a first measuring point located below the
frost line, where the supply and discharge of internal cooling
air is controlled such that the outer delimination of the blown
film is kept constant at the first measuring point, and that the
outer wall of the blown film is measured on a second point
located above the frost line, and the measured value is
constantly maintained at the predetermined setpoint by means of
an appropriate control circuit.
The distance of the tubular film from a stationary
sensor can also ~e measured in the area between the inflection
point and the frost line.
The measurement may also be carried out with a
plurality of sensors at the first measuring point.
I'he diameter of the blown film above the frost line can
be measured in the vicinity of the calibrating means, but also
at the point at which the tubular film is laid flat.
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In the control process in accordance with the invention
there are thus provided at least two independent measurement
sensors for scanning the outer delimitation of the bubble, or the
film laid flat. At least the first of these is a contactless
sensor means.
The sensor at the first measuring point below the frost
line has the function of activating the circuit which regulates
the filling of the tubular bubble, i.e. the eontrol of the supply
and discharge of the internal cooling air in such a way that the
bubble maintains a constant distance from the sensor at the first
measurlng polnt.
When the height of the frost line changes due to
e~ternal influences, the shape of the bubble also changes. By
means of the first control circuit, the outer delimitation of the
blown film is kept constant in the blow-up region at the first
measuring point.
The shape of the bubble will, however, also change when
the outer delimitation of the blown film at the second measuring
point no longer corresponds with the setpoint. Such a deviation
is also caused by the displacement of the frost line. When the
measured value is kept constant also at the second measuri.ng
point~ the frost line, too, remains constant if the shape of the
bubble in the blow-up region is otherwise unchanged.
An embodiment of the invention will subsequently be
described by means of the drawings, wherein
Figure 1 represents a schematic illustration of a blow
moulding apparatus in a side view,
Figure 2 represents the blow-up region of the blown film
with proper location of the frost line,
Figure 3 represents the blow-up region of the blown film
with a downwardly shifted frost line, and
Figure 4 represents the blow-up region of the blown film
with proper -location of the frost line.
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In the blow moulding appaxatus shown in Figure 1, the
tubular die of the film blowing head 2 is supplied with molten
plastic material by the extruder 1. The molten plastic material
leaves the film blowing head 2 in an upward direction in the -Eorm
of a film tube, and is then inflated to from a film bubble. The
blowing head 2 is supplied with internal cooling air for the film
from line 3; air is then withdrawn from the film bubble through
the suction line 4, and then is discharged.
Above the frost line 7 a calibrating means 8 is
provided. Below the frost line 7 the first measuremen~ pick up
6 is disposed for measuring the outer delimitation of the blown
film at a first measuring point, which lies below the frost line,
but above the inflection point of the bubble curve, relative to
a stationary point.
The outer delimitation of the blown film is furthermore
measured at a second measuring point located above the frost line
7, at which the second-measurement pick-up 5 is disposed. The
second measuring point 5 lies below the calibrating means 8.
The measured values of the first and second measuring
point are supplied to the computer 9 and are processed there.
The computer 9 controls the supply and discharge of the internal
cooling air and the rate at which the extruder 1 supplies hot
plastic material to the blowing head 2.
The measured value of the first measuring point 6 is
supplied to the bubble controller 10, which controls the supply
and discharge of the amount of internal cooling air.
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In Figures 2, 3, and 4, the control method isillustrated in more detail. At the lower measuring point 6, a
contactless operating measurement pick-up is provided, so that
no marks are left on the still hot and soft plastic film. As can
be seen, the pick-up 6 lies below the frost line 7, but somewhat
doove the inflection point 11 of the bubble curve 12~ The upper
scanning poin-t 5 lies above the frost line 7, and below the
calibration means 8.
The sensor 6 below the frost line 7 has the function
o~ "bubble control", i.e. the control of the inlet and outlet air
such that the bubble 12 at the measuring point 6 has a constant
distance x from the measurement pick-up 6, and thus usually also
has a constant diameter.
Figure 2 shows the "normal state", where the frost line
7 is at the proper location. The distance x at the lower
measuring point 6 is kept constant by the bubble controller 6.
The inlet and outlet air is thus controlled by means of the
bubble controller 10 such that the distance x is kept constant.
At the upper measuring point a certain bubble diameter is
detected by the sensor 5.
When as a result of external influences the frost ~ine
7 moves downward as it is shown in Figure 3, the shape of the
bubble also changes. It is then in the position shown in Figure
3 by a continuous line. The bubble controller 10 again controls
the filling level of the bubble such that the distance x remains
constant. As a result, the bubble becomes smaller; the diameter
of the bubble thus decreases. This is detected by the measuring
element 5 above the frost llne 7.
By means of an appropriate control circuit the frost
line is then adjusted until the sensor 5 again detects the proper
bubble diameter in the manner shown in Figure 9. Then, the frost
line 7 is again located at the proper height.
The second control circuit operating in dependence on
the upper measuring point 5 changes the output and/or the
temperature of the extruder, and/or the amount of cooling air
such that the outer delimitation of the blown film remains
constant at the upper measuring point 5.
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Instead of the sensor 5 below the calibrating means 8
there can also be used a sensor which detects the width of the
film after it has been laid flat.
In these Figures only one sensor is provided at each
point, thus simplifying both the Figures and the description of
them. In practise, however, more than one sensor can be used,
with an appropriate control system adapted to process all of the
data received from them, and to adjust the film making conditions
accordingly.
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