DE3536329A1 - Process and device for maintaining a constant wall thickness in the extrusion of blown films, pipes or tubular profiles made of plastic - Google Patents

Abstract

In the simultaneous extrusion of a plurality of blown films, pipes or tubular profiles made of plastic, the maintenance of constant wall thicknesses of all plastic profiles is effected. In this process, plastic material is fed in an adequate amount to the feed of the extruder, whilst the finished plastic profiles are constantly taken off at the discharge of the extruder. It is essential in this that on the one hand the amount of the plastic material taken from the extruder per unit time is determined exclusively by weighing and on the other hand the length of each individual plastic profile forced out by the extruder is measured, that a reference parameter is formed in each case from the weighing of the length measurements and, by means of this reference parameter, a control of the weight per metre for the associated plastic profile is brought about via the screw speed and/or the take-off rate, that the wall thickness of each plastic profile is also recorded and, by means of this, the relative profile weight is determined from the specific weight of the plastic material, that, via these values, the individual wall-thickness recordings are calibrated and then the absolute profile weights are determined and that, finally, the centring and/or weight distribution at the discharge of each extruder head or extruder die is controlled via the reference parameters analogous to the individual weight determinations.

Description

The invention relates to a method and a device to keep the wall thickness constant while extruding of several blown films, tubes or tubular Strands of plastic at the time of extruder feed Plastic material supplied in sufficient quantities and at the exit of the extruder the finished plastic strands are constantly deducted.

Methods and devices of this type for extruding individual Blown films, pipes or tubular strands of plastic are already known, for example from DE-PS 19 64 386 and DE-OS 33 04 865 can be found.

According to DE-PS 19 64 386, the amount of plastic granules before the extruder in a measuring cylinder by means of an electrical Level barrier giving impulses determined and doing so the time interval of the impulses with one of the withdrawal speed and the weight of the plastic strand per Length unit dependent setpoint value compared and one from it resulting signal is used to regulate the granulate feed used on the measuring cylinders.

This allows the extrusion or withdrawal speed for the plastic strand are influenced so that its wall thickness kept constant within prescribed limits becomes. Here one takes advantage of the fact that the  metrologically recorded extruder feed with the extruder output is linearly related in time. With the hereby achieved Regulation can be compared to the usual used, unregulated devices also a considerable savings in the consumption of the plastic material to reach.

In practical use, the respective Cross section of the extruded plastic strand with a good approximation is the calculated average wall thickness be preserved. The absolute wall thickness of the extruded Plastic strand can, however, computationally predetermined, average wall thickness within a certain bandwidth up and down, so that the usability of the plastic strand, in particular when he used to form preforms for blow molding should be used, is permanently impaired. In spite of Compliance with a certain, average wall thickness Weight per meter on the extruded plastic strand cannot be ensured according to this state of the art be that its absolute wall thickness at all points of his Cross-section of the arithmetically given, mean Wall thickness corresponds.

Recognizing these shortcomings, the DE- OS 33 04 865, when extruding blown films, Pipes or tubular strands of plastic a scheme to achieve with which the calculated, medium wall thickness an absolute, minimal wall thickness results. For this purpose, extrusion is carried out behind the extruder Plastic strand a wall thickness measurement with a cross movement component running to the scanning direction and thereby the over the full running distance of the Measured values resulting from movement integrated into a mean value, whereupon this mean value with the mean value from the  Compare the measurement in front of the extruder and track it, and it will then be corrected or calibrated Mean value continuous direct wall thickness measurements on Plastic strand executed and the resulting Actual values constantly as control connection to the extruder and / or abandoned the strand deduction.

The advantage here is that the regulation on a before Extruder calculated average wall thickness and one average wall thickness determined by measuring technology behind the extruder in the sense of aligning the latter with the former takes place. This is how calibration is carried out of the wall thickness measuring system acting behind the extruder, in such a way that this is only relative after the first detected wall thickness provides absolute measured values over which the extruder is controlled to a minimum wall thickness.

With the method and the device according to DE-OS 33 04 865 However, proper operation can only be done when extruding of a single strand of plastic. In contrast, the operation of extruder systems is not hereby possible, which for the simultaneous extrusion of several Blown films, pipes or tubular strands made of plastic are designed because the calibration of the relative Wall thickness measurement for each individual plastic strand is considerable Difficulties and consequently similar shortcomings occur as in DE-PS 19 64 386 when extruding of a single strand of plastic.

The aim of the invention is to overcome these shortcomings and a method and an apparatus for carrying it out specify at which or with which the calibration of the relative Wall thickness distribution for the different plastic strands is ensured.  

The procedural solution to this task is described below the characterizing part of claim 1 according to the invention achieved that on the one hand the amount of extruder in the Unit of time recorded plastic material by weighing determined and on the other hand the length of each of extruded plastic strand is measured that from of the weighing and the length measurements each form a manipulated variable and by means of this a regulation of the meter weight for the associated plastic strand over the screw speed and / or the withdrawal speed is caused to continue to measure the wall thickness on each plastic strand and with it the relative weight of the plastic material Strand weight is determined using these values the individual wall thickness recordings are calibrated and then the absolute strand weights are determined, and that finally Analogue manipulated variables above the individual weight determinations the centering and / or mass distribution at the outlet each extrusion head or tool can be regulated.

According to the invention it is provided that according to claim 2 Weighing of the plastic material and its transport to the material intake is carried out continuously.

On the other hand, according to claim 3, there is also the possibility of that the weighing of the plastic material is discontinuous carried out and its transport for material intake in its speed by the interval between successive Weighing is regulated.

It is particularly important in the method according to the invention that according to the teaching of claim 4, the wall thickness detections all plastic strands with matching calibration be driven by all used for the wall thickness detection Devices brought to the same basic setting will.  

The type of process according to the invention can be used particularly advantageously realize if all according to claim 5 Plastic strands with the same geometry are pressed out. Technically, however, it is possible according to claim 6 also that all plastic strands with dimensions and / or cross-sections are pressed out when measuring the wall thickness have approximately the same material temperatures.

In any case, according to a further embodiment Invention according to the teaching of claim 7, the possibility that with multiple parallel length measurements and wall thickness measurements centering and / or mass distribution several parallel outlets simultaneously and evenly can be regulated.

However, it has proven itself in terms of process technology also if according to claim 8 the length measurement on each plastic strand of the wall thickness detection carried out below becomes. Advantageously, according to claim 9, the wall thickness detection on each plastic strand between the calibrator and cooling section are carried out during the length measurement the same in or behind the material deduction becomes.

After all, from a procedural point of view, too important that the calibration of the wall thickness detection with the relationship

G = K · ( G 1 r + G 2 r ... Gnr )

is carried out, where G = ( G 1, G 2.... Gn ) corresponds to the absolute strand weights, K is the common calibration factor for all wall thickness measurements and ( G 1 r + G 2 r ... Gnr ) the relative weights of the individual Plastic strands correspond.

An apparatus for performing the method in which the plastic granules to the extruder feed in portions or the plastic strands can be fed continuously via a scale behind the exit of the extruder through extraction devices are transportable, is characterized by claim 11 mainly from the fact that everyone at the exit of the extruder provided extrusion head or tool own wall thickness measuring device and own length measuring device are subordinated, which are also connected to the scale Computer are connected, by means of its manipulated variables can be generated, on the one hand, the screw speed of the extruder and / or the withdrawal speed for the individual plastic strands as well as the centering and / or mass distribution at the exit of each extrusion head or tool is adjustable.

According to claim 12, this device is further distinguished characterized in that the wall thickness gauges each the calibrator subordinate for the relevant plastic strand are while the length measuring devices are behind the trigger device for the respective plastic strand.

A further development feature of the invention is based on claim 13 on the fact that the extrusion heads or tools for all Plastic strands have the same exit geometry and the wall thickness measuring devices assigned to them a matching one Have basic setting. If it is the wall thickness gauges according to claim 14 is ultrasonic measuring devices, are these on matching ultrasound speed preset.

According to claim 15, it is also within the scope of the invention that the individual extrusion heads or tools at different Geometry for the different plastic strands Have cross-sectional dimensions at their exit  those in the extruded plastic strands in the area of Wall thickness gauges at least approximately the same material temperature is achievable.

Finally, according to claim 16 in the event that the Extrusion heads or tools with matching outlet geometry have, yet suggested that all Plastic strands assigned a common take-off device and thereby the melt throughput through the individual Extrusion heads or tools only by actuation can be influenced by damming elements within the same is.

Further features and advantages of the invention result from the following description of one shown in the drawing Embodiment. Show it

Fig. 1 in a schematically simplified representation of an extruder with multiple extrusion head or tool and an associated measuring dosing

Fig. 2 is a block diagram of the measuring and dosing system in association with the extruder and the multiple extrusion head or tool and

Fig. 3 is a schematic overview of the structure of the overall system.

In Fig. 1 of the drawing, an extruder 1 is shown , which is a multiple extrusion die, for example. Triple extrusion die 2 or more, for. B. three extrusion heads 2 a , 2 b and 2 c , with the help of which pipes 3 a , 3 b , 3 c or tubular strands and blown films can be made of plastic.

The material feed of the extruder 1 can be fed from an attached measuring cylinder 4 with plastic granules, while the measuring cylinder 4 can again be metered with the plastic granulate from a storage container 5 by means of a scale 6 , for example a belt scale.

The control of the scale 6 and thus the quantity metering of the plastic granulate into the measuring cylinder 4 is effected by the level barrier 7 assigned to it. This level barrier 7 regulates the level of the plastic granulate in the measuring cylinder 4 between two limit values. An actual value time is determined by the distance between the pulses delivered by the level barrier 7 , which is proportional to the quotient of the weight set on the scale 6 and the total output of the extruder 1 . The actuation of the scale 6 z. B. the running speed of the belt scale is regulated by the level barrier 7 .

Each extrusion head 2 a , 2 b , 2 c of the multiple extrusion tool 2 is followed by a non-contact wall thickness measuring device 8 a , 8 b , 8 c , these wall thickness measuring devices 8 a , 8 b , 8 c being designed, for example, as ultrasonic measuring devices, which preferably work according to the pulse-echo principle or as a reflectoscope and together with the measuring cylinder 4 , the balance 6 and the level barrier 7 form a measuring and dosing system for the extruder 1 .

As a further component, the measuring and dosing system according to the block diagram according to FIG. 2 also has a computer 9 which, as a function of the settings of the jacket weights per unit length of the tubes 3 a , 3 b , 3 c or the like and their withdrawal speed, forms a setpoint time .

To adjust the jacket weight per unit length of each tube 3 a , 3 b , 3 c or the like, actuators 10 a , 10 b , 10 c , for example. Potentiometers, while the withdrawal speed of the tubes 3 a , 3 b , 3 c or the like each of a tachometer generator 11 a , 11 b , 11 c is determined.

The actual value time delimited by the level barrier 7 and the setpoint time formed by the computer 9 pass via a time stage 12 to an evaluation stage 13 , into which an output voltage is in turn fed by a tachometer generator 14 , which is coupled to the extruder screw and thus supplies a voltage proportional to the screw speed.

The difference between the setpoint and actual value time is given by the evaluation stage 13 in a time-voltage converter 15 , the output variables of which are fed to the extruder discharge 16 or to the extruder 1 as a control connection.

The said multi-injection molding tool 2 and the spray heads 2 a, 2 b, 2 c downstream Wandstärkenmeßgeräte 8 a, 8 b, 8 c are used first to start-up of the extruder 1, so, the onset of discharge of the plastic tubes 3 a, 3 b, 3 c or the like from the spray heads 2 a , 2 b , 2 c , the measurement of the wall thickness distribution over the entire cross-sectional distance thereof. This cross-sectional distance corresponds to the full circumference of pipes or tubular strands, while in the case of blown films it is determined by their width.

In each case, the first measurement of the wall thickness measuring devices 8 a , 8 b , 8 c carried out over the entire cross-sectional distance corresponds to a sample measurement that determines the wall thickness distribution. All of the measurement values obtained are put into the computer 9 , which in turn forms an average of all these measurement values by integration. This mean value is then given from the computer 9 to a comparison stage 17 , which also has the setpoint time applied, which goes from the computer 9 to the time stage 12 and is formed as a function of the setting of the tube weight per unit length and the take-off speed. The output variable of the comparison stage 17 is then used to correct the wall thickness measuring devices 8 a , 8 b , 8 c in the direction of the setpoint time, ie to calibrate this automatically. The overall structure of the extrusion system can be seen as a block diagram in FIG. 3. It can be seen that the wall thickness measuring devices 8 a , 8 b , 8 c are assigned to the pipes 3 a , 3 b , 3 c or the like in an area which is between the calibrators 18 a , 18 b , 18 c and the cooling sections 19 a , 19 b , 19 c . At a distance behind the cooling sections 19 a , 19 b , 19 c , the extraction device 20 or its components 20 a , 20 b , 20 c assigned to the individual tubes 3 a , 3 b , 3 c or the like are provided, behind which in turn length measuring devices 21 a , 21 b , 21 c interact with the individual tubes 3 a , 3 b , 3 c or the like before they run through the saws 22 a , 22 b , 22 c , in particular for performing sample cuts.

The length measuring devices 21 a , 21 b , 21 c record the length of each individual pipe 3 a , 3 b , 3 c or the like expressed in the unit of time and feed their measured values to the computer 9 in parallel with the measured values coming from the scale 6 .

From the measured values coming from the scale 6 and the measured values coming from the length measuring devices 21 a , 21 b , 21 c , a manipulated variable is formed in the computer 9 , which determines the screw speed and / or the take-off speed of the take-off device 20 or the components 20 a , 20 b , 20 c influences the same and thereby regulates the weight per meter for the individual tubes 3 a , 3 b , 3 c or the like.

The wall thickness measured by means of the individual wall thickness measuring devices 8 a , 8 b , 8 c on the tubes 3 a , 3 b , 3 c or the like also determines the relative strand weight per unit length in the computer 9 via the weight of the processed plastic material.

Since these values are used in the computer 9 to calibrate the individual wall thickness detections, the absolute strand weights of the individual pipes 3 a , 3 b , 3 c or the like can then be determined and, via these individual weight determinations, analog manipulated variables that are present at the output of the computer 9 The centering and / or mass distribution at the outlet of the multiple extrusion tool 2 or each individual extrusion head 2 a , 2 b , 2 c can then be effectively regulated.

A prerequisite for the optimal functioning of the extrusion system is on the one hand that the calibration or basic setting of the wall thickness measuring devices 8 a , 8 b , 8 c assigned to all tubes 3 a , 3 b , 3 c or the like is the same, that is to say when using ultrasonic measuring devices must be set to the same ultrasonic speed. On the other hand, it is also provided that the geometry of the tubes 3 a, b 3, 3 c, or the like molded plastic strands with each other is equal to or but their dimensions are chosen so that in the region of the wall thickness measurement prevail approximately equal temperatures in the same extruded plastic strands.

Since the same calibration or basic setting of the wall thickness measuring devices 8 a , 8 b , 8 c results in the same common calibration factor K for all pipes 3 a , 3 b , 3 c or the like, the wall thickness distributions determined for each pipe 3 a , 3 b , 3 c or the like can be optimally controlled by influencing the withdrawal speed on each individual tube, the minimum wall thickness.

When tubes 3 a , 3 b , 3 c or the like are extruded at the same time, it is possible to move all of these tubes 3 a , 3 b , 3 c or the like at a corresponding take-off speed, that is to say by means of a common take-off device 20 , and then actuate the minimum wall thickness to regulate damming elements within the multiple extrusion die 2 or the extrusion heads 2 a , 2 b , 2 c , thereby varying the melt throughput.

The calibration of the wall thickness detection in the computer 9 is with the relationship

G = K · ( G 1 r + G 2 r + ... Gnr )

carried out, in which G = ( G 1, G 2... Gn ) stands for the absolute strand weights, K is the same common calibration factor for all wall thickness measuring devices 8 a , 8 b , 8 c and ( G 1 r + G 2 r . ... Gnr ) for the relative weights of the individual tubes 3 a , 3 b , 3 c or the like.

The weighing of the plastic material and its transport to the material holder of the extruder can not only be carried out continuously, as shown in FIG. 1, by means of a belt scale 6 . Rather, it is also possible to carry out the weighing of the plastic material discontinuously and to regulate its speed at which the material is taken up by the time interval between successive weighings.

While in the production of pipes 3 a , 3 b , 3 c or tubular strands the wall thickness measuring devices 8 a , 8 b , 8 c work with a circular movement around the pipe circumference, they are arranged and designed in the production of blown films by means of the extruder 1 , that carry out a reciprocating scanning movement across the width of the respective blown film.

Claims (16)

1. A method for keeping the wall thickness constant during the simultaneous extrusion of several blown films, tubes or tubular strands of plastic, in which plastic material is supplied in sufficient quantity to the intake of the extruder and the finished plastic strands are continuously drawn off at the exit of the extruder,
characterized by
that on the one hand the amount of plastic material taken up by the extruder ( 1 ) in the unit of time is determined by weighing ( 6 ) and on the other hand the length of each individual extruded plastic strand ( 3 a , 3 b , 3 c ) is measured ( 21 a , 21 b , 21 c ),
that from the weighing ( 6 ) and the length measurements ( 21 a , 21 b , 21 c ) a manipulated variable is formed ( 9 ) and by means of this a regulation of the meter weight for the associated plastic strand ( 3 a or 3 b or 3 c ) the screw speed and / or the withdrawal speed is effected,
that the wall thickness on each strand of plastic ( 3 a , 3 b , 3 c ) is also detected ( 8 a , 8 b , 8 c ) and the relative strand weight is determined with it via the weight of the plastic material ( 9 ),
that the individual wall thickness measurements ( 8 a , 8 b , 8 c ) are calibrated using these values and then the absolute strand weights are determined,
and that finally the centering and / or mass distribution at the outlet of each extrusion head or tool ( 2; 2 a , 2 b , 2 c ) are regulated via the individual weight determinations analog control variables. 2. The method according to claim 1, characterized in that the weighing ( 6 ) of the plastic material and its transport to the material holder ( 4 ) is carried out continuously ( Fig. 1). 3. The method according to claim 1, characterized in that the weighing (6) batchwise of the plastic material and its transport is regulated for receiving material (4) in its speed by the time interval between successive weighings (6). 4. The method according to any one of claims 1 to 3, characterized in that the wall thickness detections ( 8 a , 8 b , 8 c ) all plastic strands ( 3 a , 3 b , 3 c ) are driven with the same calibration. 5. The method according to any one of claims 1 to 4, characterized in that all plastic strands ( 3 a , 3 b , 3 c ) are pressed out with the same geometry. 6. The method according to any one of claims 1 to 4, characterized in that all plastic strands ( 3 a , 3 b , 3 c ) are pressed with dimensions and / or cross sections, which in the wall thickness detection ( 8 a , 8 b , 8 c ) have approximately the same material temperatures. 7. The method according to any one of claims 1 to 6, characterized in that with several parallel length measurements ( 21 a , 21 b , 21 c ) and wall thickness measurements ( 8 a , 8 b , 8 c ) the centering and / or mass distribution of several parallel outlets ( 2 a , 2 b , 2 c ) is regulated simultaneously and evenly. 8. The method according to any one of claims 1 to 7, characterized in that the length measurement ( 21 a , 21 b , 21 c ) on each plastic strand ( 3 a , 3 b , 3 c ) of the wall thickness detection ( 8 a , 8 b , 8 c ) is subsequently carried out ( Fig. 3). 9. The method according to any one of claims 1 to 8, characterized in that the wall thickness detection ( 8 a , 8 b , 8 c ) on each plastic strand ( 3 a , 3 b , 3 c ) each between calibrator ( 18 a , 18 b , 18 c ) and cooling section ( 19 a , 19 b , 19 c ) is carried out while the length measurement ( 21 a , 21 b , 21 c ) is carried out in or behind the material take-off ( 20; 20 a , 20 b , 20 c ) will ( Fig. 3). 10. The method according to any one of claims 1 to 9, characterized in that the calibration of the wall thickness detection ( 8 a , 8 b , 8 c ) with the relationship G = K · ( G 1 r + G 2 r ... + Gnr ) is carried out. 11. An apparatus for performing the method according to any one of claims 1 to 10, in which the plastic granules can be fed to the extruder feed in portions or continuously on a balance and the plastic strands behind the exit of the extruder can be transported by extraction devices, characterized in that everyone at the exit of the Extruder ( 1 ) provided extrusion head ( 2 a , 2 b , 2 c ) or tool ( 2 ), its own wall thickness measuring device ( 8 a , 8 b , 8 c ) and its own length measuring device ( 21 a , 21 b , 21 c ) are connected, which are connected to a computer ( 9 ) also connected to the scale ( 6 ), by means of which manipulated variables can be generated, on the one hand the screw speed of the extruder ( 1 ) and / or the take-off speed ( 20; 20 a , 20 b , 20 c ) for the individual plastic strands ( 3 a , 3 b , 3 c ) and on the other hand the centering and / or mass distribution at the outlet of each extrusion head ( 2 a , 2 b , 2 c ) or . Tool ( 2 ) is adjustable. 12. The apparatus according to claim 11, characterized in that the wall thickness measuring devices ( 8 a , 8 b , 8 c ) each the calibrator ( 18 a , 18 b , 18 c ) for the relevant plastic strand ( 3 a , 3 b , 3 c ) are subordinate, while the length measuring devices ( 21 a , 21 b , 21 c ) are behind the extraction device ( 20; 20 a , 20 b , 20 c ) for the respective plastic strand ( 3 a , 3 b , 3 c ) ( Fig. 3). 13. Device according to one of claims 11 and 12, characterized in that the extrusion heads ( 2 a , 2 b , 2 c ) or the extrusion tool ( 2 ) for all plastic strands ( 3 a , 3 b , 3 c ) have the same exit geometry and the wall thickness measuring devices assigned to them ( 8 a , 8 b , 8 c ) have a matching basic setting. 14. Device according to one of claims 11 to 13, characterized in that the wall thickness measuring devices ( 8 a , 8 b , 8 c ) are ultrasonic measuring devices which are preset to a matching ultrasonic speed. 15. Device according to one of claims 11, 12 and 14, characterized in that the individual extrusion heads ( 2 a , 2 b , 2 c ) of the extrusion tool ( 2 ) with different geometry for the different plastic strands ( 3 a , 3 b , 3rd c ) have cross-sectional dimensions at their outlet, in which at least approximately the same material temperature can be achieved in the extruded plastic strands ( 3 a , 3 b , 3 c ) in the area of the wall thickness measuring devices ( 8 a , 8 b , 8 c ). 16. Device according to one of claims 11 to 14, characterized in that all plastic strands ( 3 a , 3 b , 3 c ) assigned a common extraction device ( 20 ) and thereby the melt flow rate in the extrusion heads ( 2 a , 2 b , 2 c ) or in the extrusion tool ( 2 ) can only be influenced by actuating baffle elements in the individual extrusion heads ( 2 a , 2 b , 2 c ) or in the extrusion tool ( 2 ).