DE102008052609A1 - Plastic hollow body manufacturing method, involves correcting influence of deviation of molding process on material distribution in hollow body by intervention in molding process and/or intervention in preform-formation - Google Patents

Abstract

The method involves correcting a nozzle clearance, which forms a preform (2), and/or mass flow rate for preform-formation, when a measured value of net weight of a plastic hollow body (10) and a control variable deviates from a reference value. Additional deviation of molding process from a defined process flow, which is considered as a reference variable, is detected. Influence of deviation of the molding process on material distribution in the hollow body is corrected by intervention in the molding process and/or intervention in the preform-formation.

Description

The invention relates to a method for producing hollow plastic bodies by blow molding,
wherein preforms are formed from a plastic melt having a wall profile changing in the preform longitudinal direction and fed to an open blow mold,
wherein the preforms are expanded after closing the blow mold with a gaseous medium to form hollow bodies, and
wherein the preform formation is controlled and by means of an axial wall thickness layer control, the net weight of the hollow body and a second controlled variable, which indirectly describes the axial position of a selected portion of the wall profile of the preforms in a defined process flow of the molding process at the closing time of the blow mold, measured and the preforms forming nozzle gap and / or the mass flow rate for the preform formation are corrected if the measured values deviate from nominal values.

The existing of a hot, moldable plastic preforms are usually tubular and can be produced by continuous extrusion or in memory head systems cyclically. The material thickness of the preforms is often controlled in the preform longitudinal direction and in complex shaped hollow bodies also in the circumferential direction by a wall thickness program, which takes place during the preform formation. Corresponding methods are in EP 0 345 474 B1 . EP 0 776 752 B1 and EP 0 693 357 B1 described. The known methods try using markers, time and length measurements and by evaluating weight measurements to correct changes in the manufacturing process, resulting mainly from changing properties of the plastic melt. The known methods have proven effective in order to compensate for temperature-induced or material-related influences to a certain extent.

According to the known methods are primarily used to weight measurements determine and correct material-related deviations. For this purpose, the wall thickness control system setpoint values become product-dependent and specified according to the process. The specific weight is an integral measure, which conclusions on the existing in a portion of the plastic hollow body Quantity of material allowed. Exact statements about the concrete Material distribution is usually not possible.

In particular changes the blow molding process that occurs unconsciously or consciously for process adaptation can be made, the material distribution within significantly affect the preform and thus provide a significant Intervention in the procedure. that exiting from the die gap preform free moldable exposed to internal and external forces is. Due to the viscoelastic behavior of the plastic melt can the material distribution in the preform change, for example due to fluctuations in the melt temperature, the plastic material, the extrusion time, shear stresses acting on the melt, the orientation of the preform with respect to the blow mold and the like. Inaccuracies arise from the interplay of as thresholds, shrinking and Auslang designated viscoelastic deformation, in particular Measurements are also a function of time.

With Thresholds are called the effect that the diameter and the Wall thickness of emerging from the nozzle gap Preform itself due to a disorientation of the flow channel Aligned molecules changes. Under shrinking is understood to be the relaxation of the preforms after emergence from the nozzle gap. Relaxing the polymer chains of Plastic melt causes a shortening of the preforms, wherein the wall thickness of the preforms increases. Auslangen occurs as a result the dead weight of the preform. Under that with the extrusion increasing weight of the preform reduce diameter and wall thickness of the preform below the head exit. The elongation is essentially determined by the viscosity of the Plastic melt, the at a considered Vorlumenabschnitt hanging weight and the time that the volume section this weight is determined determined. The at the extrusion head observed exit velocity of the preform increases accordingly to. The described viscoelastic effects are in a complex Interaction. With longer extrusion time and higher melt temperature the extension becomes larger. Will the Extensions larger, so the swelling decreases at the head exit. For longer extrusion times, eventually the shrinkage at the bottom of the preform with larger Distance from the nozzle gap larger. Furthermore becomes the local viscoelastic flow behavior in particular Dimensions due to the local wall thickness profile profile in longitudinal and circumferential direction determined.

In addition to previously described method for wall thickness control is from the DE 692 13 307 T2 a method for the production of hollow plastic bodies by blow molding is known in which preforms are formed from a plastic melt and fed to a blow mold, wherein the position of the lower end of the preform is determined and wherein depending on the position of the preform the blow mold is closed and the extrusion process is terminated. According to the described method, the blow mold can be prevented from being closed too early or too late with respect to the expansion of the preforms. However, since the extrusion process is completed depending on the position of the lower edge of the preforms, a constant net weight of the formed hollow plastic body can not be ensured.

at the known method, which in particular the length using preforms as process-determining parameters, can also be the distribution of the plastic melt in the longitudinal direction the preforms are not further influenced.

In front In this background, the invention is based on the object Process for the production of blow-molded hollow bodies with which it is possible to specify default values for the weight and the wall thickness distribution of the hollow body to comply with tight tolerances. In particular, it should be with the method be possible in the manufacturing process, which occur unconsciously or deliberately made to adjust the procedure.

outgoing from a process for producing blow-molded hollow plastic bodies with the features described above, the object is achieved according to the invention solved that after the formation of the preforms from a Plastic melt during a molding process, the blow mold with a controlled positioning movement within a given Mold closing time is closed, in addition Deviations of the molding process from the defined process flow, which is used as reference size, recorded and their influence on the material distribution in the blow-molded Hollow body by an intervention in the molding process and / or be corrected by an interference with the preforming. Einfindungsgemäß is the monitoring of the molding process and the comparison with a defined process flow additionally to an axial wall thickness control provided the Net weight of the hollow body and a second controlled variable considered. By combining the different measures for process adaptation, a whole host of changes can be made the parameter in the process for producing hollow plastic bodies be detected and compensated. When determining the molding condition It may be useful over several Cycles provide an averaging to a swing of the scheme to avoid and only on level shifts with a control intervention to react.

To In particular, deviations that affect changes are considered the molding process during the closing of the Blow mold, changes in inflation conditions during the formation of the hollow bodies, changes in the extrusion conditions of the preform, changes in the operating point of the blow molding process and changes the wall thickness profile curve, the wall thickness control underlying. Since according to the invention deviations of the molding process with respect to a defined process flow These changes can be made by a customized Control of the positioning movement during the molding process be corrected immediately. The molding process includes the section of the blow molding process that begins with that Preforms have reached a defined length and / or a given extrusion time is reached and ends with the completely close the blow mold before the Preform through the gaseous medium in its final Shape is widened.

Of the The molding process itself usually begins as previously explained with the achievement of a defined preform length, the expiration of a defined extrusion time or the achievement of a defined profile point of the wall thickness control wherein the blow mold in a defined position, for example the position an opening limit or preferably a waiting position located. Subsequently, the blow mold completes within the mold closing time a closing movement, usually first a fast closing and then a slow closing he follows. Immediately before the closed position of the blow mold takes place over a comparatively short way a closing movement with an increased closing pressure and one typically increased closing speed, also called Lookup is called. The lookup serves mainly in addition formed by the increase of the closing pressure Squeeze the waste to the extent that it is after the blow molding process only through a thin web with the formed hollow body are connected and so easily separated. The described sequence of different closing movements is just an example. So you can without restriction during the closing process more or less Sections with different closing speeds be provided. The described sequence of closing speeds is typical for hydraulic or electric drives of Blow molding. For hybrid-driven blow molding is usually when switching from electric to hydraulic drive a additional change of the closing speed observed.

Finally, courses of the closing speed are possible in which no fixed predetermined areas with a respective constant closing speed, but continuous change tions of closing speed are provided.

According to one preferred embodiment of the molding process is provided that the preforms in the supply to the blow mold at its lower End are open, being during the molding process before the beginning of the closing movement, the open, lower end the preforms is spread by a spreading device. After spreading fingers the spreading device then reaches a predetermined spreading position have, the preforms are each of a closing device closed at its lower end. Depending on the application can be before the beginning of the spreading process and / or the closing device a certain delay may be provided. After closing The preform is usually within the scope of the described preferred embodiment a Vorblasvorgang provided, taking into account a delay time and a given preblow time is carried out. The delay time can be be chosen such that the pre-bubbles before, with or after the beginning of the closing movement begins. In addition to the Delay time on the one hand and the Vorblaszeit on the other In particular, the pre-blowing pressure and the pre-blowing volume are process parameters to take into account. The pre-blowing can be without restriction from below, via the extrusion head or over a pierced into the preform glass needle done.

Around to intervene in the molding process may be provided during the mold closing time the amount of gas contained in the preform to control. For example, as previously described be at an interval within the mold closing time introduce a gaseous medium into the preform, which expands the preform by pre-blowing, the lower End of the tubular preforms by means of a hose closing device is squeezed before the pre-blowing begins. A control of in each of the preforms contained gas amount is further also possible if a cutting device for separating the preforms is used, which are the preforms in each case during the separation process below, during the preform extrusion usually the supply of supporting air through the extrusion head is provided. Additionally or alternatively to a permanent, uniform supply of supporting air can while a temporally controlled support air supply done. It is worth noting that for the extrusion process optimal amount of support air may also be to an undesirably large in relation to the molding process Extension of the preforms can lead. The control the amount of gas contained in the preforms can basically and regardless of whether the preforms initially are open at its lower end and only through the hose closing device be closed or when using a cutting device closed during extrusion at its lower end include draining or exhausting gas from the preforms. The formation of open preforms using a hose closing device on the one hand and the formation of closed preforms in use a cutting device on the other hand differ in particular in that in the first case the amount of gas contained in the preform be precisely adjusted during the molding process can, while in the second case in the closed preform to introduce support air during its extrusion is that can not escape and that during the molding process in the parison contained amount of gas at least substantially miter. The same applies to both cases significantly different time intervals in which the in the preforms contained amount of gas can be controlled.

Around to allow a flexible adaptation of the molding process, is in accordance with a preferred embodiment of inventive method within the mold closing time a mold waiting time is provided, which is varied as a buffer time so that at a changing closing speed the mold closing time remains constant overall. The term Mold waiting time is thus within the scope of the present invention not on the time the blow mold is open and first the preform formation is awaited. The shape wait, While the blow mold rests, anywhere on the Mold closing time, especially at the beginning of mold closing time be provided. Furthermore, there is the possibility the Mold-waiting time as interruption of a fast or slow closing movement or between a fast and a slow closing movement provided. Furthermore, during a Mold closing time without restriction also several Mold waiting times can be provided. The shape waiting time results from this from the total mold closing time, subtracting the for the individual routes provided times.

If the blow molding process is operated with a dead time optimization or if negative shape hold time values are defined in the control of the positioning movement as part of the molding process adjustment, the mold wait time can not be used as a buffer. But if you still want to keep the mold closing time constant, so must be a suitable compensation between the sub-steps. For example, it is known from practice that the shock sensitivity determined in a drop test of a hollow plastic body is influenced to a particular degree by a change in the slow closing movement can. Accordingly, if the path for the slow closing movement is increased and / or the speed for the slow closing movement is reduced to improve the impact sensitivity, the resulting loss of time can be compensated by increasing the closing speed during the previous rapid closing operation. If necessary, the parameters for the pre-blowing, ie the time for the beginning of the pre-blowing as well as the pre-blowing time, must also be adapted during an adaptation. It may be expedient to keep the time interval from the beginning of Vorblasens and the end of Vorblasens until complete closing of the blow mold and / or the distance from the beginning of the pre-blowing and the end of the pre-blowing to the position of the fully closed blow mold. Even if the mold closing time is kept constant, it can not be ruled out that a change in the individual speeds of the molding process has a not insignificant effect on the wall thickness distribution of the preform or the wall thickness distribution of the hollow plastic body formed. Corresponding changes can be seen in the context of the wall thickness control based on the net weight and the second controlled variable, in which case, if appropriate, a corresponding compensation with respect to the wall thickness control can be performed.

Around To keep the mold closing time constant, the closing movement the blow mold preferably detected by displacement sensors and time measurements and controlled the closing movement on the basis of the measured values obtained. To achieve as described above, either the remaining closing path from the beginning of Vorblasens and the end of Vorblasens or still remaining closing time from the beginning of Vorblasens and The default values correspond to the end of the pre-blowing is preferably provided that the interval for the pre-blowing through Time or distance measurements is controlled. Finally, can be achieved by an appropriate control that during the mold closing process, the amount of gas contained in the preform Default values. A corresponding procedure results also in preforms, which in preforming on closed at the bottom.

at The combination of different regulations can be provided that in each case a separate control intervention is determined, in which case referred to the respective process step a superposition of the control interventions he follows. For example, the cam of a Wall thickness control superimposed correction values be determined separately from each other. According to one However, alternative embodiments can also rule interventions under common consideration of several controlled variables be determined. So it may be particularly appropriate be the net weight along with the corresponding slug weights to evaluate together, since the different weights of the educated Hollow body on the one hand and different sections of the On the other hand, preform can be assigned. So For example, it may be found that to compensate for a deviation the manufacturing process several control interventions possible are, and then the actually performed Control intervention taking into account several controlled variables is determined. Through a joint consideration of the controlled variables In particular, it is also possible, comparatively large Compensate for deviations by a control intervention.

The method according to the invention is basically suitable for correcting a large number of very different deviations in relation to a defined process sequence. Significant deviations from a defined process flow arise in particular when starting the blow molding process after an interruption. In particular, thermal effects which can relate to all steps of the process for producing hollow plastic bodies must be taken into account. With regard to the extrusion process, it should be noted, for example, that the temperature and the composition of the plastic melt may not have reached a stationary state immediately after starting up. Furthermore, the thermal expansion of the parts of the extruder and in particular the parts of the extrusion head may be associated with significant changes. With regard to the molding process, it should be taken into account that the adjusting movements usually take place hydraulically during the closing process, wherein the hydraulic oil is still comparatively cold and viscous immediately after the start of the blow molding process. In order to be able to compensate for the deviations with respect to the defined process sequence that typically occur during startup, it is provided according to a preferred embodiment of the invention to query when starting the blow molding process whether the blow molding process has reached a stable operating state. If this is not the case, the influence of changes in the molding process on the material distribution of the blow-molded hollow body can be corrected by an intervention in the molding process. Furthermore, it can also be provided, when the blow molding process starts, to correct the influence of changes in the extrusion process on the material distribution of the blow molded hollow bodies by intervention in the wall thickness program running during the preform extrusion, as long as the stable operating state of the blow molding process has not yet been reached. In principle, corrective interventions can be made both in the molding process and in the startup process be provided the extrusion process. Furthermore, to a certain extent, a change of the molding process caused by the startup can be carried out by an adaptation of the extrusion process and vice versa.

The opened by the method according to the invention Possibilities of a versatile, flexible and accurate Correction of deviations are also particularly beneficial when a frequent conversion of the used blow molding machine is provided. So it is common for a blow molding machine is used for the production of various products, in which case a conversion is required when changing the product. It should be noted that it is in the setup processes can come to mistakes and that between two production cycles make conscious or unconscious changes to a product can. According to the invention can be so in addition for a product-specific exchange of process data a needs-based Correction with the aid of the method according to the invention be performed. Of particular advantage is according to the invention, both deviations on the Einformprozess as well as batch-related changes of the Plastic raw materials are detected and corrected.

At the transition from development stage to production maturity, they are to be manufactured Blow molded parts optimized by experts. These professionals define after completion of the optimization work, the setpoints for an automatic process control. The professionals are specialists and usually not machine operators who later the Monitor production and correctively intervene when the Quality of the blow-molded parts does not meet the standard values equivalent. The machine operators are usually with the monitoring multiple machines entrusted at the same time and often do not have the specialist knowledge of the specialists, which optimizes the process control to have. If the machine operator detects a problem during production, So he tries this by an intervention in the blow molding process without compensating for the influence of this intervention can overlook the whole process. By an inventively designed automatic Process control is achieved that conscious and unconscious changes at the blow molding process by the machine operator not the quality Manufacturing affect the quality of the manufacturing process less dependent on the level of knowledge of the operators, that a comprehensive control of the manufacturing process take place and that can be an appointment of specialists after completing the Optimization phase is no longer necessary.

Of the the preforms forming nozzle gap is expediently with a wall thickness program running with the preform formation controlled, which is the volume of a preform required plastic melt in a given number (n) volume sections divided and these volume sections nozzle gap control values assigns one over the number (n) of the volume sections form plotted program curve. The design of the program curve takes into account the viscoelastic behavior of the plastic melt for one by a certain temperature, mass throughput, Material, nozzle gap, screw speed and if necessary further operating parameters specified operating point and contributes a sagging of the preforms in this operating point Bill. The operating parameters deviate from those of the design this can affect the material distribution in the preform have, for example, the distance between two preform sections, each with a maximum function are assigned to the program curve, opposite to the Interpretation based on distance changes. This bothers me especially when the distance between two preform sections, each associated with a functional maximum of the program curve are and when expanding the preforms to hollow bodies represent critical cross sections, change. Even minor changes This distance can be a significant loss of quality in terms of compression, warp and strength properties resulting from the hollow body result. To remedy to provide, the axial wall thickness control is taken into account the net weight of the hollow bodies and at least one second controlled variable operated. For further regulation the wall thickness control is preferably within the scope of the invention provided that in case of deviations of the molding process from the reference size the nozzle gap control values of the program curve additional Correction values are switched on. It is advantageous if the wall thickness control under Consideration of all correction factors and curves in such a way operated, that used for preforming Melt quantity remains constant. By the immediate adjustment of the wall thickness control can cause unwanted interference in the Vorformlingsextrusion or in the preform ejection and be avoided during the molding process. The correction values can determined empirically and / or on the basis of entered material data and / or based on the measured melt temperature and optionally further measured data and depending on the measured values be constantly adjusted.

Particularly advantageous is an embodiment of the method in which, in the event of deviations of the molding process, from the defined process sequence, on the one hand, the program curve of the nozzle gap control values is applied with correction values On the other hand, an intervention in the molding process takes place with the proviso that the mold closing time has a specified value defined by the defined process.

The According to the invention provided monitoring The molding process also allows deviations of the material distribution in the blow-molded hollow bodies of default values, if the deviations are due to changes in the molding process based. Corresponding relationships can both determined empirically as well as theoretically. A corresponding Linking changes of the molding process on the one hand and deviations of the material distribution in the blow-molded Hollow bodies on the other hand allowed by targeted intervention in the blow molding process and / or the extrusion process a direct Correction of the material distribution in the blow-molded hollow bodies. So can to correct deviations of the material distribution the time of Vorblasens, the duration of Vorblasens, the Vorblasdruck or preforming be changed, the said measures being implemented individually or in combination can be.

According to one preferred development of the method is provided that also deviations of the radial wall thickness distribution of the hollow body from a given distribution depending on a change of the molding process and corrected by an intervention in the molding process become. This can be provided in particular that a change the molding process as well as the change of a size, which on the image of a slug on the head of the hollow body and / or along an interface in the side region of the hollow body is characteristic, be established and that for correction the image of the slug during the molding process in the Vorformling contained gas quantity is changed. To one To produce desired slug image, the Time of Vorblasens and / or the duration of Vorblasens and / or the Vorblasdruck be corrected. Additionally or alternatively can the amount of gas contained in the preforms by venting or suction, whereby the venting or Absaugzeitpunkt and / or the duration of the venting or Absauggen or the suction pressure can be changed to the to produce desired slug image.

According to one another variant of the method according to the invention it is provided that the nozzle gap of a running with the preform formation Wall thickness program is controlled as described above, wherein for the axial wall thickness control controls the material distribution in the preform longitudinal direction captured by a third controlled variable and with default values is compared and wherein for the purpose of Ausregelns by viscoelastic effects caused deviation from the specification Nozzle gap control values of the program curve correction values be formed, the sagging of the preforms influence. Also in the context of such an embodiment is preferred provided that the program curve when applying the correction values adjusted so that the for preforming used amount of melt remains constant. The for determination of the correction value used for the third controlled variable can be based on entered material data and based on the measured melting temperature and optionally further measured data calculated and constantly adjusted depending on the measured values become.

The method according to the invention is intended to modify a wall thickness program provided for the wall thickness control by the superposition of different correction quantities and preferably also to adapt the inventively monitored molding process as needed. It should be noted that changes in the process for the production of plastic hollow bodies by blow molding are due in part to unforeseen disturbances and in part to intended user intervention. Intended interventions in the blow molding process can be carried out with the intention of compensating for any faults that have occurred or of further optimizing the blow molding process via the initially defined, intended process flow. Furthermore, interventions in the blow molding process are required, for example, even in the case of a geometric change in the blow mold, a different composition of the plastic melt or a change in the wall thickness specification. If these changes are to be maintained permanently, it is expedient not to maintain the changes as correction values or functions, but to adapt the program curve for the wall thickness control and / or the defined process sequence of the molding process, which is used as the reference variable according to the method according to the invention. For this purpose, it can be provided in a particularly advantageous manner that, given a predetermined change in the process parameters, a suitable intervention in the molding process and / or in the preform formation is automatically determined, wherein the program curve determined in consideration of the suitable intervention or the determined optimum process sequence of the molding process Default or reference size. It can be provided that a manual user input is requested for the acceptance of the values. In the context of such an adaptation can be provided, for example, the reference value for the mold closing time or the predetermined parameters for the duration and closing speeds in to change individual time intervals of the mold closing time.

The inventive method is both applicable in tubular preforms as well as flat Preforms that are in the blow mold during the blow molding process still united in the plasticized state. Dependent on The hollow body to be manufactured, the various control mechanisms be combined with each other.

Also There are no restrictions on preforming. In addition to a continuous extrusion of the preforms can in particular also a discontinuous formation by means of a storage head plant be provided.

to Further process optimization can be provided that at least a characteristic of the state of the plastic melt Measured value is determined for additional adaptation of the method, wherein the measured value in relation to a predetermined reference value is set, starting from the deviation between the measured value and the reference value for the preform using a numerical value Model the viscoelastic flow behavior of the plastic melt and assuming the spatial arrangement at least a significant profile point as a function of time be determined and that the particular spatial arrangement the at least one significant profile point with a reference position being compared, wherein for correcting deviations between the arrangement the at least one profile point and the reference position a control intervention takes place in the preforming and / or the blow molding process. Especially can be provided that by means of the numerical model at a predetermined change of the process parameters automatically a suitable compensation curve is determined by a given Control curve of the wall thickness control is superimposed.

According to one Another aspect of the present invention is advantageous when by intervention in the control individual control measures can be disabled. If at otherwise reproducible Process flow, for example, due to the numerical model or one of the previously described control variables a unexpectedly high deviation is determined, this may be an indication of an erroneous determination of measurement data or a be a different kind of disorder. Starting a compensation of the data obtained in this case does not make sense, so that then the procedure without the consideration of the numerical Model or the corresponding controlled variable. The embodiment described thus becomes an emergency operating mode provided, albeit with reduced accuracy, a allows continued production of hollow bodies. To the numerical model or one of the other controlled variables can disregard a user intervention and / or a causality query in the context of process control be provided.

in the The invention will be described below with reference to exemplary embodiments explained.

It show schematically:

1 a blow molding plant for the production of hollow plastic bodies with a continuous preform formation by screw extrusion,

2 an alternative embodiment of a blow molding plant for the production of plastic hollow bodies,

3 a control scheme for operating a blow molding machine,

4a and b path-time diagrams for the closing movement of a blow mold,

5 a blow-molded hollow body with an upper, a lower and a side Abfallbutzen and

6 Examples to illustrate the relationship between each one shown in a view square image and the corresponding, shown in cross-section wall thickness distribution of a blow-molded hollow body.

For the basic structure of in 1 and 2 blow molding plants each include a plasticizing unit 1 for the production of tubular preforms 2 as well as a facility 3 for blow molding with a blow mold consisting of molds 4 , a spreader 5 , which is shown rotated for better visibility by 90 °, and a hose closing device 6 , The tubular preforms 2 made of hot, malleable plastic are vertically down from the plasticizing unit 1 extruded and placed in a reproducible position to the open blow mold.

While according to the 1 the hose closing device 6 and the blow mold 4 to take over the preform 2 in an underhead position immediately below the extrusion head 7 are arranged, the shows 2 a device for continuous extrusion in which the preforms 2 After reaching a predetermined length, after a predetermined extrusion time or reaching a defined profile point of the wall thickness control immediately below the extrusion head 7 detached and from a gripper 8th to the device 3 with the blow mold 4 , the spreader 5 and the hose closing device 6 to be moved. To check the length of the preforms 2 a measuring device, for example a light barrier, can be provided. For reasons of clarity, corresponding measuring devices are not shown in the figures.

The according to the 1 directly and in accordance with 2 by means of the gripper 8th in the blow mold 4 introduced preform 2 is by means of the spreading device 5 fixed in the squish direction and by pressing the hose closing device 6 closed. After that, the blow mold 4 closed and the preform 2 with a gaseous medium, preferably air, which passes through a previously introduced from above or below in the preform glass mandrel 9 introduced through the extrusion head or through pierced needles, in the blow mold 4 to a hollow body 10 widened. At the head and at the bottom of the hollow body 10 waste garbage is produced 11 . 12 as waste from the blow-molded hollow body 10 be separated. The net weight GN of the hollow body 10 , the weight GUB of the bottom garbage 11 and the weight GOB of the upper Abfallbutzens 12 are measured and as controlled variables in a control scheme 13 used.

A blow-molded hollow body 10 with a head-side upper waste trash 12 , a bottom-side bottom trash 11 and a side, middle rubbish 14 is in the 5 shown. Also, the image of at least one of the waste can be recorded and used as a controlled variable.

According to the 1 The molding process begins with the preform 2 has reached a defined preform length, a predetermined extrusion time has expired or a defined profile point of the wall thickness control can be achieved. As explained above, the lower, open end of the preform with the spreading device then becomes part of the molding process 5 spread, with the preform 2 from the hose closing device 6 is closed after spreading fingers of the spreading have reached a spreading position. The preform 2 is thus kept down. After closing the hose closing device 6 begins the closing of the blow mold 4 , This is the entire facility 3 with the blow mold 4 and the hose closing device 6 still in the lower head position (blow mold 4 under extrusion head 7 ), so that further plastic melt nachgefördert from above. In the lower part of the preform 2 passing through the hose-closing device 6 is held, there are only minor changes in terms of existing there material volume. This area later significantly determines the shape and weight of the lower Abfallbutzens 11 , On the other hand, by the still nachgeförderte plastic melt and the sagging of the preform 2 as a result of its own weight and viscoelastic flow behavior, the net weight GN but above all the weight GOB and the image of the upper waste casing 12 as well as the weight GMB and the slug image of the middle Abfallbutzens 14 depending on the molding conditions, for example, the travel-time control of the blow mold 4 , influenced. Thus, if there is a change in the molding process, the throughput of plastic melt through the extrusion head may be appropriate 7 adapt. If, for example, an overall longer closing time is provided, it is expedient to reduce the throughput accordingly. When the blow mold 4 is closed, the entire facility 3 moved to a blowing position and the extrusion of a new preform 2 began.

According to the embodiment of the 2 the device remains 3 with the blow mold 4 , the spreader 5 and the hose closing device 6 permanently in the blowing position. It is to transport the preforms 2 from the extrusion head 7 to the blow mold 4 the gripper 8th , which may be part of a robot, for example. If a predetermined hose length is reached after a predetermined extrusion time or determined by appropriate measuring devices, for example in the form of light barriers, the gripper takes over 8th the preform 2 from the extrusion head 7 and separates him from it. The gripper 8th transports the preform 2 to the device 3 , wherein the spreading of the preform 2 at the bottom by means of the spreader 5 only takes place when the gripper 8th has reached its target position, in which case the preform 2 at its upper end by the gripper 8th is closed. Basically, the path-time behavior of the gripper 8th be integrated into the controller. Subsequently, as described above, the preform is then lowered through the hose closing device 6 closed and is as in the execution according to the 1 kept down. In the subsequent closing of the blow mold 4 However, in contrast to the embodiment according to the 1 no more plastic melt nachgefördert, with the preform 2 still deformed due to the sag. The sagging of the preform 2 In particular, depending on the closing speed of the blow mold, in addition to the net weight GN, the weight GOB and the slipper pattern of the upper waste casing are influenced 12 as well as the weight GMB and the slug image of the middle Abfallbutzens 14 ,

From the described embodiments of the 1 and the 2 it is immediately apparent that in addition to the known metho the influencing of the wall thickness control the exact position of the individual volume sections can be significantly changed by the closing process.

In addition to the measures described for compensating for the influence of changing molding conditions by an intervention in the molding process and / or preform formation, measures are also conceivable which mechanically influence the compensation of an occurring positional shift. For example, light barriers, which are the length of the preform 2 measure the distance between extrusion head and blow mold 4 , the distance between the hose closing device 6 and / or the spreading device 5 to the blow mold 4 or, if the separation of the preforms 2 is provided with a cutting device, the distance between the cutting device and the extrusion head 7 be automatically adjusted.

The preforms 2 become from the plastic melt with the preform longitudinal direction changing wall thickness profile, which at least one area 15 having an increased wall thickness formed. 3 shows the schematic representation of a control scheme. A preform extrusion is followed by a blow molding process which initially comprises the previously described molding process. According to the invention is a detection 16 the molding conditions and a subsequent comparison 17a the Einformbedingungen provided with a defined process flow, which is used as a reference size. If the deviations of the molding process are below a predetermined threshold, no further control intervention is provided. Subsequently, a causality query 17b intended. If the detected molding conditions deviate very far from the reference size and exceed a given maximum deviation, this can be an indication of a faulty measurement or a serious disturbance of the molding process. A control intervention may not be appropriate. When the maximum deviation is exceeded, different measures can be provided. For example, the hollow body formed in the respective blow molding process 10 sorted out, an error signal is output or even the entire process is stopped. It is conceivable for the different measures to provide different thresholds. Regardless of the specific embodiment of the method according to the invention, it is basically also advantageous to store the determined molding conditions and to make this information available to a user if required, for example, to carry out an error analysis or process optimization in retrospect. Otherwise, that is, if a rule intervention is provided due to the specific deviations, a query 18 whether the blow molding process is in an unstable operating state after starting up. If this is the case, a compensation control for the startup takes place 19 , If, on the other hand, the blow molding process has reached a stable operating state, a check is made 20 whether it is necessary for the compensation of the deviations found during the molding process to intervene in the wall thickness control by providing correction values. If this is not the case, a compensation regulation takes place 21 which causes a change in the parameters of the molding process. If an intervention in the wall thickness control system is required, can here also, as indicated by the dashed line, optionally a compensation control 21 respectively. According to the in 3 illustrated control scheme 13 After the blow molding process, the weight GOB of an upper waste casing becomes 12 , the weight GUB of a bottom garbage 11 and the net weight GN of the hollow body 10 certainly. First, the gross weight GB is calculated from the sum of the weights GOB, GN and GUB. From the gross weight results directly the total amount of the formation of a hollow body 10 used plastic melt. If the measured value IW of the gross weight GB deviates from a default value SW, the mass flow rate m is changed by changing the screw speed of the plasticizing unit 1 customized.

According to the in 3 illustrated control scheme, the nozzle gap s is regulated when the measured value IW of the net weight GN of the blow-molded hollow body 10 deviates from a predetermined target weight SW. To change the nozzle gap s, the nozzle gap set values 22 immediately increased or decreased.

As a further controlled variable, the weight GOB of the upper Abfallbutzens 12 compared with default values. If the deviations lie outside specified tolerances, suitable correction values or curves become 23 then determines the nozzle gap values to control preform extrusion by superposition 22 be superimposed. Furthermore, also in the previously explained test 20 during the monitoring of the molding process, the own correction value or curves 23 but then, if necessary, are to be matched with the correction values or curves due to a deviation of the weight GOB of the upper Gutzens.

For further control, according to the example in 3 illustrated control scheme 13 the Butzenbild BB at least one Abfallbutzens 11 . 12 , preferably the upper Abfallbutzens 12 certainly. If the badge picture BB outside of an adjustment is made 24 during the closing movement of the blow mold 4 in the preforms 2 contained gas quantity. Thus, for example, be provided in a pre-blowing of the preforms 2 increase or decrease the amount of gas intended. Alternatively, it may also be necessary to control the amount of gas gas from the preforms 2 let off or suck off. As indicated by the dashed line, with deviations of the slug image BB also correction values 23 be determined.

The relationship between the slug image and the corresponding wall thickness distribution in the blow-molded hollow bodies is in 6 for a few examples. A comparative analysis of in 6 The illustrated examples make it clear that the slug image can be used as a measure of the radial wall thickness distribution. In Example A of 6 Correspond to the figure and the wall thickness distribution in the cutting plane AA the default values. In Examples B and C, the slug image of the upper Abfallbutzens formed at the top of the hollow body 12 from the default value. The deviations correlate with irregularities in the radial wall thickness distribution of the blow-molded hollow body.

The 4a and 4b show exemplary path-time diagrams of the blow molding device 4 , The mold closing time t _total for the path between an opening limit position and a position in which the blow mold 4 is completely closed results from the individual times t ₁ , t _w , t ₂ , t ₃ and t ₄ , where t _1, the time for closing the blow mold from the opening limit position in a mold waiting position, t _w a mold waiting time at which the T _{2 is} the time for a fast closing, t _{3 is} the time for a slow closing and t _{4 is} the time for a lookup. After the blow mold 4 starting from the opening limitation position has reached a shape waiting position, first includes the mold waiting time t _w , in which the blow mold is not moved. The mold waiting time t _w can be provided, for example, to wait until the preform 2 has reached a predetermined length. Furthermore, the shape waiting time t _w can also be provided at any intervals of the total time t _total and serves as a buffer to allow an intervention in the molding process. During the fast closing, after a predetermined delay Δ _vb, the pre-blowing takes place, which takes place over the duration of the pre-blowing time t _vb . In the 4a dash-dotted lines a modified by a control intervention curve of the time-dependent closing path is shown, wherein the mold waiting time is increased. In order nevertheless to be able to comply with the predetermined mold closing time t in _total , the closing speed is slightly increased in the interval t ₃ . With regard to the pre-blowing, in particular the time interval between the beginning of the pre-blowing and the complete closing of the blow mold 4 on the one hand and the time interval between the end of the pre-blowing and the complete closing of the blow mold 4 decisive. Thus, starting from a constant mold closing time t _total, the delay for the pre-blowing Δ _vb and the pre-blowing time t _vb remain unchanged, so that the total amount of gas provided is essentially unchanged. By adjusting the closing speeds while the distance traveled during the pre-blowing path of the blow mold changes 4 ,

The 4b shows a path-time diagram in which no shape waiting time t _{w is} provided. With reference to a reference curve a, the curve b shows a course in which the process of slow closing over a longer period of time is to take place at a lower speed. Accordingly, the speed of the quick closing operation is increased. The curve c shows a closing time-optimized course, in which the speed of the fast closing process is increased and thus the mold closing time t is reduced _overall .

If the molding process permanently with a closing time optimized Course is to be operated, it is expedient to take this course as a new reference.

QUOTES INCLUDE IN THE DESCRIPTION

This list The documents listed by the applicant have been automated generated and is solely for better information recorded by the reader. The list is not part of the German Patent or utility model application. The DPMA takes over no liability for any errors or omissions.

Cited patent literature

• EP 0345474 B1 [0002]
• EP 0776752 B1 [0002]
• EP 0693357 B1 [0002]
• - DE 69213307 T2 [0006]

Claims (20)

Process for the production of plastic hollow bodies by blow molding, wherein preforms ( 2 ) is formed from a plastic melt with a wall profile changing in the preform longitudinal direction and an opened blow mold ( 4 ), wherein during a subsequent molding process, the blow mold ( 4 ) is closed with a controlled adjusting movement within a predetermined mold closing time (t _total ), wherein the preforms ( 2 ) after closing the blow mold ( 4 ) with a gaseous medium to hollow bodies ( 10 ), wherein the preform formation is regulated and, by means of an axial wall thickness control, the net weight (GN) of the hollow bodies ( 10 ) and a second controlled variable, which determines the axial position of a selected region of the wall profile of the preforms ( 2 ) at a defined process flow of the molding process at the closing time of the blow mold ( 4 ) indirectly, measured and the preforms ( 2 ) forming nozzle gap and / or the mass flow rate for the preform formation are corrected if the measured values deviate from nominal values, wherein additional deviations of the molding process from the defined process sequence, which is used as a reference, are detected and their influence on the material distribution in the blow-molded hollow bodies ( 10 ) can be corrected by an intervention in the molding process and / or by an intervention in the preform formation. A method according to claim 1, characterized in that during the mold closing time (t _total ), the amount of gas contained in the preform is controlled. Method according to claim 1, characterized in that for separating the preforms ( 2 ) a cutting device is used, the preform ( 2 ) is closed at the bottom during the separation process, whereby in the preform ( 2 ) controlled amount of gas is controlled. Method according to one of claims 1 to 3, characterized in that within the mold closing time (t _total ) a mold waiting time (t) is provided and this is varied as a buffer time so that at a changing closing speed, the mold closing time (t _total ) remains constant. Method according to one of claims 1 to 4, characterized in that the closing movement of the blow mold ( 4 ) is detected by displacement sensors and time measurements and controlled so that the mold closing time (t _total ) always remains constant. Method according to one of claims 1 to 5, characterized in that the interval for the pre-bubbles is controlled by time or distance measurements and in each case so is determined that either the remaining closing path from the beginning of the pre-blowing and the end of Vorblasens or still remaining closing time from the beginning of Vorblasens and correspond to the end of Vorblasens default values. Method according to one of claims 1 to 6, characterized in that during the mold closing operation in the preform ( 2 ) contained gas standard values. Method according to one of claims 1 to 7, characterized in that at a start of the blow molding process it is queried whether the blow molding process a stable operating condition has achieved, and that the influence of changes in the Einformprozesses on the material distribution of the blow-molded hollow body is corrected only by an intervention in the molding process, as long as the stable operating state of the blow molding process has not yet been reached is. Method according to one of claims 1 to 8, characterized in that is queried when starting the blow molding process, whether the blow molding process has reached a stable operating condition, and that the influence of changes in the extrusion process on the material distribution of the blow-molded hollow body ( 10 ) has been corrected by intervention in the wall thickness program which takes place during the preform extrusion, as long as the stable operating state of the blow molding process has not yet been reached. Method according to one of claims 1 to 9, characterized in that the nozzle gap is controlled by a run with the preform formation wall thickness program, which is the volume of a preform ( 2 ) plastic melt is divided into a predetermined number (n) volume sections and these volume sections nozzle gap set values, which form a over the number (n) volume segments applied program curve, and that in case of deviations of the molding process of the reference size the nozzle gap control values of the program curve correction values are switched. A method according to claim 10, characterized in that in addition an intervention in the molding process takes place with the proviso that the mold closing time (t _total ) has a value defined by the defined process flow. Method according to one of claims 1 to 11, characterized in that changes of the molding process based deviations of the material distribution in the blow-molded hollow bodies ( 10 ) are detected by default values and corrected by a change in the timing of the pre-blowing, a change in the duration (t _vb ) of the pre-blowing, a change in the Vorblasdruckes or by an interference with the preform formation, wherein the above measures can be implemented individually or in combination , Method according to one of claims 1 to 12, characterized in that deviations of the radial wall thickness distribution the hollow body of a given distribution in response to a change of the molding process and by an intervention in the molding process Getting corrected. Method according to one of claims 1 to 13, characterized in that a change of the molding process and the change of a size, which for the slipper image (BB) one at the head of the hollow body ( 10 ) and / or along an interface in the side region of the hollow body, it can be ascertained and that, during the shaping process in the preforms (BB), to correct the image of the slit (BB). 2 ) Amount of gas is changed. A method according to claim 14, characterized in that the timing of the pre-blowing and / or the duration (t _vb ) of the pre-blowing and / or the pre-blowing pressure are corrected to produce the desired slug image (BB). A method according to claim 15, characterized in that in the preforms ( 2 ) is reduced by venting or suction, wherein the venting or suction time and / or the duration of the venting or suction or the suction pressure to be changed to produce the desired slug image (BB). Method according to one of Claims 1 to 16, characterized in that the nozzle gap is controlled by a wall thickness program running with the preform formation, which subdivides the volume of the plastic melt required for a preform into a predetermined number (n) of volume sections and associates these nozzle sections with gap values a program curve plotted over the number (n) of volume sections form that for the axial wall thickness control, the material distribution in the preform longitudinal direction is detected by a third control variable and compared with a specification and for the purpose of Ausregelns caused by viscoelastic effects deviation from the default the Düsenspaltstellwerten the Program curve correction values are switched, which are the sagging of the preforms ( 2 ) influence. Method according to claim 17, characterized in that that the program curve when applying the correction values adjusted so that the for preforming used amount of melt (m) remains constant. Method according to claim 17 or 18, characterized that the correction values are based on entered material data and based on the measured melt temperature and optionally further Measurement data are calculated and constantly depending on the measured values be adjusted. Method according to one of claims 1 to 19, characterized in that at a given change the process parameters automatically a suitable intervention in the Einformprozess and / or in the preform formation is determined.