DE3536329C2 - - Google Patents

Description

The invention relates to a method and a device to keep the wall thickness constant during simultaneous Ex trusion of several blown films, tubes or tubular Strands of plastic, where the ex Truders plastic supplied in sufficient quantities and the finished plastic at the outlet of the extruder strands are continuously withdrawn.

Methods and devices of this type for extrusion individual blown films, pipes or tubular strands of art Material are already known, such as from DE-PS 19 64 386th and DE-OS 33 04 865 can be found.

According to DE-PS 19 64 386, the amount of plastic granules lats in front of the extruder in a measuring cylinder by means of an elec level impulses giving and determining the time interval of the impulses with one of the deductions speed and the weight of the plastic strand per Length unit dependent setpoint value compared and one from it resulting signal is used to control the granulate feed tion used on the measuring cylinders.

This allows the extrusion or withdrawal speed for the plastic strand are influenced so that its wall strength kept constant within prescribed limits becomes. Here one takes advantage of the fact that the  metrologically recorded extruder feed with the extruder butt is linearly related in time. With this he enough regulation can be compared to the usual wise used, unregulated devices also one considerable savings in the consumption of the plastic to reach.

In practical use, the respective Cross section of the extruded plastic strand with the arithmetically predetermined middle wall is a good approximation strength will be preserved. The absolute wall thickness of the extru Died plastic strand can, however, compared to the nerisch 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 Adherence to a certain average wall thickness speaking meter weight on the extruded plastic strand cannot be ensured according to this state of the art be that its absolute wall thickness is in all places cross-section of the arithmetically specified, mean Wall thickness corresponds.

Recognizing these shortcomings, the DE- OS 33 04 865, when extruding blown films, Tubes or tubular strands of plastic a rain to achieve with which arises from the arithmetically given, average wall thickness an absolute, minimal wall strength results. To do this, extrude behind the extruder ten plastic strand a wall thickness measurement with a cross to the movement direction extending component leads and thereby 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  Measurement in front of the extruder compared and followed up on this leads, and then with the corrected or calibrated direct average 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 measured middle wall behind the extruder strengthening in the sense of aligning the latter to the first re 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 rela tiv detected wall thickness provides absolute measurements over which the extruder is controlled to a minimum wall thickness. Such a thing The process also shows DE-OS 33 11 345.

With the method and devices according to DE-OS 33 04 865 and DE-OS 33 11 345, however, can only work properly with the Ex ensure a single strand of plastic. In contrast, the operation of extruder systems is not hereby possible, which for the simultaneous extrusion of several Ren blown films, tubes or tubular strands of art are designed because the calibration of the relative Measure wall thickness for each individual plastic strand difficulties and consequently similar shortcomings Lichen occur, as in DE-PS 19 64 386 in Ex trudge a single strand of plastic. Also the US-PS 45 15 738 leaves no hin point to keeping the layer constant strength of the individual mixture paths or art strands of a multilayer profile recognize bandes, but only one Mass control of the individual tracks according to the specified ratio and regulation of the entire wall thickness of the profile tape.

The aim of the invention is to address these shortcomings men as well as a method and an apparatus for its passage to specify management with which the calibration of the rela tive wall thickness distribution for the various plastics 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 by weighing determined and on the other hand the length of each of extruded plastic strand is measured that from the weighing and the length measurements each have a manipulated variable forms and by means of this a regulation of the meter weight for the associated plastic strand via the screw rotation number and / or the withdrawal speed is caused that continue to measure the wall thickness on each plastic strand and with it the weight of the plastic material the right lative strand weight is determined using these values the individual wall thickness recordings are calibrated and then the absolute strand weights are determined, and that close Position similar to the individual weight determinations size the centering and / or mass distribution at the outlet each extrusion head or tool can be controlled.

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

According to claim 3, however, there is also the possibility speed that the weighing of the plastic discontinuously Lich carried out and its transport to the material intake in its speed by the time interval between on consecutive weighings 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 can be driven by all to measure the wall thickness placed devices brought to the same basic setting will.  

The Ver according to the invention can be used particularly advantageously Realize driving style 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, the wall thickness Material acquisition has approximately the same material temperatures sen.

In any case, according to a further embodiment Invention according to the teaching of claim 7 is possible speed with several parallel length measurements and wall strengths the centering and / or mass distribution several parallel extrusion heads simultaneously and evenly can be controlled.

However, it has proven itself in terms of process technology also if according to claim 8 the length measurement on each art Fabric strand of the wall thickness detection carried out below becomes. Advantageously, according to claim 9, the wall thicknesses recorded on each plastic strand between potash brator and cooling section are carried out while the Län same measurement in or behind the material take-off is taken.

After all, from a procedural point of view, too important that according to claim 10 the calibration of the wall thicknesses solution 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 is continuously fed on a scale and the art Strands of material behind the extruder outlet by pulling off devices are transportable, according to An Say 11 mainly by leaving 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 are connected to his computer, by means of its position sizes can be generated, on the one hand, the screw rotation number of extruder and / or the withdrawal speed for the individual plastic strands as well as the Zen tration and / or mass distribution at the exit of each strand press head or tool is controllable.

According to claim 12, this device is further distinguished characterized in that the wall thickness gauges each the potash brator for the plastic strand concerned are while the length measuring devices are behind the deduction direction 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 match one have the basic setting. If the wall thickness kenmeßgeräte according to claim 14 to ultrasonic measuring devices delt, these are due to matching ultrasonic speeds preset.

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

Finally, according to claim 16 in the event that the Extrusion heads or tools matching outlet Have geometry, still suggested that all Plastic strands a common trigger device arranges and thereby the melt throughput through the individual Extrusion heads or tools only by actuation Damage elements can be influenced within the same is.

The invention is in the following description of a Darge in the drawing presented embodiment explained in more detail. It shows

Fig. 1 in a schematically simplified representation of an ex truder multi-extruding 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 to 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 , the multi-extrusion tool, for example. Triple-strand press tool 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 with plastic granules from a measuring cylinder 4 , while the measuring cylinder 4 can in turn be metered with the plastic granules 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 controlled by the level barrier 7 .

Each extrusion head 2 a, 2 b , 2 c of the multiple extrusion tool 2 is a non-contact wall thickness measuring device advises 8 a , 8 b , 8 c , these wall thickness measuring devices 8 a , 8 b , 8 c, for example, as ultrasonic measuring devices are designed, 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 further components, the measuring and dosing system according to the block diagram according to FIG. 2 also has a computer 9 which functions as a function of the settings of the jacket weights per unit length of the plastic strands, e.g. B. the tubes 3 a , 3 b , 3 c or the like and from 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 Abzuggeschwin speed of the tubes 3 a , 3 b , 3 c or the like in each case is determined by a tachometer generator 11 a , 11 b , 11 c .

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 as control pulses to the extruder discharge 16 or to the extruder 1 .

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 of 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 over the entire cross-sectional distance by guided measurement of the wall thickness measuring devices 8 a , 8 b , 8 c corresponds to a sample measurement determining the wall thickness distribution. All the measurement values obtained are given in 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 in a comparison stage 17, which the target value time went on, the proceeds from the computing ner 9 to the time stage 12 and as a function of SET development of Weight per unit length and the trigger is formed 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 set point time, which means that it is automatically calibrated. The overall structure of the extrusion system can be seen as a block diagram of FIG. 3. It can be seen that the wall thickness measuring devices 8 a , 8 b , 8 c are assigned to the tubes 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 19th a , 19 b , 19 c. At a distance behind the cooling sections 19 a , 19 b , 19 c , the extraction device 20 or its individual extraction devices 20 a , 20 b , 20 c assigned to the individual tubes 3 a , 3 b , 3 c or the like are provided, behind them in turn Length measuring devices 21 a , 21 b , 21 c cooperate with the individual tubes 3 a , 3 b , 3 c or the like, be in front of these, in particular for carrying out sample cuts, through the saws 22 a , 22 b , 22 c .

The length measuring devices 21 a , 21 b , 21 c detect the length expressed in the time unit of each individual tube 3 a , 3 b , 3 c or the like and carry out 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 from the measured values coming from the length measured values 21 a , 21 b , 21 c , a manipulated variable is formed in the computer 9 which determines the screw speed and / or the withdrawal speed of the pulling devices 20 and 20 a , 20 b , 20 c influences the same and thereby controls the meter weight for the individual tubes 3 a , 3 b , 3 c or the like.

By means of the individual wall thickness gauges 8 a , 8 b , 8 c on the tubes 3 a , 3 b , 3 c or the like detected wall thickness, the relative strand weight per unit length is also determined in the computer 9 via the weight of the processed plastic.

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

A prerequisite for the optimal functioning of the extrusion system is on the one hand that the calibration or basic setting of all the tubes 3 a , 3 b , 3 c or the like for ordered wall thickness measuring devices 8 a , 8 b , 8 c is the same, that is to say during use of ultrasonic measuring devices they must be set to the same ultrasonic speed. On the other hand, it is also a prerequisite that the geometry of the tubes 3 a , 3 b , 3 c or the like molded plastic strands are the same or their dimensions are chosen so that in the area of wall thickness measurement approximately the same temperatures in the extruded at the same time plastic strands prevail.

Since the same calibration or basic setting of the wall thickness gauges 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 by influencing the withdrawal speed on each individual tube, the minimum wall thickness can be optimally regulated.

During the simultaneous extrusion of tubes 3 a , 3 b , 3 c or the like, it is possible to move all these tubes 3 a , 3 b , 3 c or the like with a corresponding withdrawal speed, that is to say by means of a common withdrawal device 20 and then by means of their minimum wall thickness Operation of damming elements within the multiple extrusion die 2 or the extrusion heads 2 a , 2 b , 2 c to be regulated by 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 wind thickness measuring devices 8 a , 8 b , 8 c and (G 1 r + G 2 r ... Gnr) stands for the relative weights of the individual tubes 3 a , 3 b , 3 c or the like.

The weighing of the plastic 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 discontinuously and to control the speed of the transport of the material by the time interval between successive weighings.

While in the manufacture of tubes 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 tube circumference, they are arranged in the production of blown films by means of the extruder 1 and trained that they perform a reciprocating scanning motion across the width of the respective blown film.

Claims (16)

1. A method for keeping the wall thickness constant while extruding several blown films, tubes or tubular strands ( 3 a , 3 b , 3 c) made of plastic, in which the intake of the extruder plastic is supplied in sufficient quantity and at the exit of the extruder the finished plastic strands ( 3 a , 3 b , 3 c) are continuously drawn off, characterized in that
that on the one hand the amount of plastic taken in by the extruder ( 1 ) in the time determined by weighing (at 6 ) and on the other hand the length of each individual extruded plastic strand ( 3 a , 3 b , 3 c) is measured ge (at 21 a , 21 b , 21 c) ,
that from the weighing (at 6 ) and the length measurements (at 21 a , 21 b , 21 c) a manipulated variable is formed (at 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) is effected via the screw speed and / or the take-off speed,
that the wall thickness on each plastic strand ( 3 a , 3 b , 3 c) is detected (at 8 a , 8 b , 8 c) and the relative strand weight is determined with it via the weight of the plastic (at 9 ),
that the individual wall thickness recordings (at 8 a , 8 b , 8 c) are calibrated via 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 controlled by means of the individual weight determinations. 2. The method according to claim 1, characterized in that the weighing (at 6 ) of the plastic and its transport to absorb material (at 4 ) is carried out continuously ( Fig. 1). 3. The method according to claim 1, characterized in that the weighing (at 6 ) of the plastic is carried out discontinuously and its transport to the material intake (at 4 ) is regulated in its speed by the time interval between successive weighings (at 6 ). 4. The method according to any one of claims 1 to 3, characterized in that the wall thickness detections (at 8 a , 8 b , 8 c) all plastic strands ( 3 a , 3 b , 3 c) are driven with matching 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) with dimensions and / or cross-sections are squeezed out in the wall thickness detection (at 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 (at 21 a , 21 b , 21 c ) and wall thickness measurements (at 8 a , 8 b , 8 c ) the centering and / or mass distribution of several parallel extrusion heads ( 2 a , 2 b , 2 c) is controlled simultaneously and evenly. 8. The method according to any one of claims 1 to 7, characterized in that the length measurement (at 21 a , 21 b , 21 c) on each plastic strand ( 3 a , 3 b , 3 c) of the wall thickness detection (at 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 (at 8 a , 8 b , 8 c) on each plastic strand ( 3 a , 3 b , 3 c) between calibrator ( 18 a , 18th b , 18 c) and cooling section ( 19 a , 19 b , 19 c) is carried out, while the length measurement (at 21 a , 21 b , 21 c) of the same in or behind the material take-off (at 20 ; 20 a , 20 b , 20 c) is made ( Fig. 3). 10. The method according to any one of claims 1 to 9, characterized in that the calibration of the wall thickness detection (at 8 a , 8 b , 8 c) with the relationship G = K · ( G 1 r + G 2 r ... + Gnr ) is 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) stands for the relative weights of the individual tubes 3 a , 3 b , 3 c or the like. 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 by means of 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 the extruder ( 1 ) provided extrusion head ( 2 a , 2 b , 2 c) or tool ( 2 ) has 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 subordinate, which are connected to a computer ( 9 ) connected to the scale ( 6 ), by means of whose manipulated variables can be generated, on the one hand the screw speed of the extruder ( 1 ) and / or the speed of the extraction device ( 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 controllable. 12. The apparatus according to claim 11, characterized in that the wall thickness measuring devices ( 8 a , 8 b , 8 c) each the Ka librator ( 18 a , 18 b , 18 c) for the relevant plastic strand ( 3 a , 3 b , 3rd 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 have and the assigned wall thickness measuring devices ( 8 a , 8 b , 8 c) have a matching basic setting. 14. The device according to any 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 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 , 3 c) have cross-sectional dimensions at their outlet, in which 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) at least approximately the same material temperature can be achieved. 16. The device according to one of claims 11 to 14, characterized in that all plastic strands ( 3 a , 3 b , 3 c) are 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 ) exclusively by actuating dam elements in the individual extrusion heads ( 2 a , 2 b , 2 c) or in the extrusion tool ( 2 ) be influenceable.