DE102008052611B3 - Plastic hollow body producing method, involves detecting variations of wall thickness distribution of hollow bodies, and correcting variations by intervening molding process or by changing outflow speed of plastic melt from die gap - Google Patents

Abstract

The method involves widening a tubular preform (2) with a gaseous medium to hollow bodies (8), and controlling formation of the preform. Net weight of the bodies and a control variable are measured, where the variable indirectly describes an axial position of a selected region of a wall profile of the preform before closing of a blow mold (4). Variations of wall thickness distribution of the bodies are detected, and are corrected by intervening a molding process or by changing outflow speed of a plastic melt from a die gap, where the speed causes embossment of the preform.

Description

The invention relates to a method for producing hollow plastic bodies by blow molding,
wherein preforms are supplied from a plastic melt having a changing in the preform longitudinal direction wall profile of an open blow mold,
wherein in a subsequent to the preform formation blow molding process, the blow mold is closed and the preforms are expanded with a gaseous medium to form hollow bodies and
wherein the preform formation is controlled and, by means of an axial wall thickness control, the net weight of the hollow bodies and a second controlled variable indirectly describing the axial position of a selected area of the wall profile of the preforms of the blow mold, and in case of deviations of the measured values from nominal values of the preform molding die gap and / or mass flow rate for preforming.

The existing of a hot, moldable plastic preforms are usually tubular and can be produced in cycles by continuous extrusion or in a storage well plant. 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 proceeds with 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. They are sufficient to compensate for temperature-related or material-related influences. However, operational practice shows that with the known method it is not yet possible to completely compensate for changes in the production process. Such changes can not be avoided in practice. If the blow-molded hollow body z. B. fail the fall test, the blow molding process, such as the closing speed of the blow mold is changed in practice as an immediate measure often. A change in the closing speed of the blow mold can have an immediate effect on the wall thickness distribution in the blower, since on the one hand with the Einformgeschwindigkeit also the position of the preform with respect to the cavity changes and, secondly, the influence of a Vorblasvorgangs performed during the closing phase on the weight distribution in Hollow body changes.

To consider is also that the preforms molded with a wall thickness profile which is in preform longitudinal direction and often also changes in the circumferential direction and that in the optimization of a plant for the production of blow-molded Plastic hollow bodies set the axial and radial wall thickness distribution of the preforms so will that the preforms after the blow molding process in all places have the desired wall thickness. This assumes that in particular those preform sections, which are subject to a particularly strong strain, to themselves in the design of intended sections in the cavity of the blow mold invest. For positional deviations of the preform with respect to design position provided for in the design and / or deviations the current preform diameter of that during the optimization phase determined preform diameter comes the point considered on the preform at another location in the blow mold for concern and leads thus a change the axial and radial wall thickness distribution, with quality losses, z. B. in the form of a hollow body distortion, lower compression values and lower strength properties the finished hollow body connected is.

Of the Invention is based on the object, a process for the preparation of blow-molded hollow bodies indicate with which it is possible is, default values for the weight and the wall thickness distribution of the hollow body with to comply with tight tolerances. In particular, it should be with the method possible be, influences on the wall thickness distribution as a result of changes in the production process to balance effectively and for that Ensure that the axial wall thickness control is optimal can work.

Starting from a method for producing blow-molded plastic hollow body having the features described above, the object is achieved in that detected in addition to the deviations of the axial wall thickness deviations of the radial wall thickness distribution of the hollow body of a given distribution and by an intervention in the blow molding process or by changes correcting the swelling of the preforms influencing exit velocity of the plastic melt from the nozzle gap. According to the invention, an additional position control is provided in combination with an axial wall thickness control system based on two control variables, which acts on the diameter of the preform at the closing time of the blow mold. For this purpose, a further controlled variable must be detected, which allows conclusions to be drawn as to whether the radial wall thickness distribution as a result of a faulty preform diameter deviates from a distribution specified as the nominal value. There the control acts on the preform diameter and thus at the same time on the distance of the preform to the cavity of the blow mold, this control is referred to in the context of the invention as radial wall thickness control. In a continuous extrusion of the preforms, the radial wall thickness control preferably acts on the blow molding process, wherein z. B. the closing speed of the blow mold and / or the blowing process can be changed or regulated. In the context of the invention, the blow molding process is understood to be the process beginning with a closing movement of the blow mold until the blow molded hollow body has been formed. The blow molding process in the sense of this definition comprises molding the preforms into the blow mold, possibly closing the lower end of the preforms, controlling the amount of gas optionally controlling the amount of gas during the closing movement of the blow mold, the subsequent expansion of the preforms in the closed blow mold, and Demolding the blow-molded hollow body from the blow mold.

If in the context of the invention, the formation of tubular preforms is provided this in the preform formation in a conventional manner be open or closed at the bottom. At the education from first At its lower end, open preforms are usually below the blow mold a hose closing device provided, which reaches the preform as described above a predetermined length or after a predetermined extrusion time at its lower Closing ends, then subsequently the blow molding process is performed. A more precise control of the amount of gas contained in the preforms is in such an embodiment only after closing the Preforms at their lower end immediately before the implementation of the Blow molding process possible, since before through the bottom opening a free gas exchange can take place. In the extrusion of closed Preforms, wherein the preforms immediately below the Extrusion head separated from each other by a cutting device and each closed at its lower end, the in The gas contained in the preforms, in particular by the extrusion head blown in support air, do not escape and thus essentially determines the blow molding process amount of gas contained in the preforms. It turns out that additionally to the optionally provided blowing or draining, for example with nozzles or Needles during supporting air provided in the entire preform formation the preform remains trapped and so a key size down is the amount of gas contained in the preforms.

Provided the preforms cyclically in the ejection mode are formed, the radial wall thickness control preferably acts on the exit velocity of the plastic melt from the die gap and consequently to the process step of preforming. Through a change or control of the exit velocity, the discharge operation easily changeable within the given cycle times is and technically can be easily realized, the so-called swelling influenced the preforms. Thresholds are the effect that the diameter of a preform emerging from the die gap changes as a result of disorientation of the molecules aligned in the flow channel. A higher ejection speed leads to a bigger swelling and thus to a larger diameter of the preforms. With a lower output speed can preforms be formed with smaller diameters. Regardless of whether the radial Wall thickness control on the blow molding process or for example by change the exit velocity of the plastic melt on the process step preforming is applied, the radial wall thickness control becomes expediently suitably on the control loops of the axial wall thickness control be matched. Advantageous embodiments of the method according to the invention are described below.

In particular, if the lower end of the preforms is squeezed off in the blow molding process by means of a tube closing device or a cutter is used to separate the preforms, which closes the preforms below, the intervention in the blow molding process is an effective means for deviations of the radial wall thickness distribution of the hollow body of a eliminate predetermined distribution. Preferably, the amount of gas contained in the tube during the closing operation of the blow mold, in a time interval between the closing time of the hose closing device and the cutting device and the time from which the blow mold is closed, corrected to deviations of the radial wall thickness distribution of the hollow body of the eliminate predetermined distribution. The time between the closing time of the hose closing device or the cutting device and the time from which the blow mold is closed, is process-dependent and is usually a few seconds in practice. The invention is based on the finding that the length of the squeezing seam formed at the top of the blow-molded hollow body is dependent on the swelling behavior of the plastic melt and the amount of gas contained in the preform during the closing of the blow mold, and that this relationship can be used to correct the squeezing seam length in that it corresponds to a default value. Furthermore, can the slug image of an upper waste casing formed at the head of the hollow body and the slug image lateral waste slugs which arise along the seam in the side region of the hollow body, by the amount of gas contained in the tube during the closing of the blow mold, are affected. To change this amount of gas, the person skilled in the art has several options for conducting the process which can be used individually or in combination.

in the Time interval between the closing point of the hose closing device or a cutting device, which the preforms during the separation process closes below and the time from which the blow mold is closed, a gaseous medium, Preferably, compressed air is introduced into the tubular preform, which the preform widens. The timing of the pre-blowing and / or the The duration of the pre-blowing and / or the pre-blowing pressure can be corrected be to deviations of the radial wall thickness distribution of the hollow body of to eliminate a given distribution.

in the It is also within the scope of the invention that the components contained in the preform Gas quantity in the time interval between the closing time, z. B. the hose closing device and the time from when the blow mold is closed, by venting or Suction is reduced, with the reduction or suction time and / or the duration of venting or suction or the suction pressure can be corrected to Deviations of the radial wall thickness distribution of the hollow body of to eliminate a given distribution. Become accordingly for preforms that during the extrusion are closed at the bottom, deviations of the radial Wall thickness distribution of the hollow body over the support air affected.

pre-blowing and bleeding / aspiration can each other be combined. An execution the method according to the invention provides that the preforms are first widened by pre-blowing and that when closing the blow mold part of the preforms included in the Gas by venting or suction is removed. According to one Another method variant, the pre-bubbles is carried out at intervals and the preforms are vented between two Vorblasintervallen.

at all these process variants, the timing of Vorblasens, the time of venting or aspirating and / or the length the time intervals and / or the Vorblas- or suction pressure varies be to the while the closing process of Blow mold in the preform to change the amount of gas and to deviations of the radial wall thickness distribution of the hollow body of to eliminate a given distribution.

Further it is possible, the closing movement to correct the tuyere to deviations of the radial wall thickness distribution of the hollow body to eliminate from a given distribution.

The activities The radial wall thickness control can basically be done manually. Preferably, however, the radial wall thickness control is in Integrated automatic process control. Here is a Size to capture measured by deviations of the wall thickness distribution automatically and as a measurand one Supplied to the control circuit, the when closing the Blow mold contained in the preform gas quantity and / or the closing movement the blow mold controls. The control loop must with the axial wall thickness control be matched. The vote is preferably carried out by that the length the preforms is detected and that deviations of the radial Wall thickness distribution of the hollow body of a predetermined distribution by an intervention in the blow molding process only be corrected if the length of the preforms is concerned Measured value is within a specified tolerance range.

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 later monitor the production and correctively intervene if the quality of the blow-molded parts is not corresponds to the default values. The machine operators are in the Usually with the supervision 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 oversee the whole process can. 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 of manufacture affect the quality of the Manufacturing process less of the knowledge of the machine operators depends 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.

To detect deviations of the radia len wall thickness distribution of the hollow body of a given distribution are preferably used Abfallbutzen, which are formed during blow molding and removed after molding of the hollow body. The slug image of the waste slugs and in this context in particular the length and position of the Quetschnähte, especially the squeeze on the head of the blow-molded hollow body, represents an important quality feature. If the Quetschnähte differ in terms of length and location of default values, so is an uneven wall thickness distribution of the hollow body go out. In the method according to the invention, therefore, a variable is preferably measured for detecting deviations of the radial wall thickness distribution of the hollow body, which is characteristic of the slug image of a top Abfallbutzens formed at the head of the hollow body and / or a lateral Abfallbutzens formed along a seam. Various methods are available for recording the size relevant to the Butzenbild. It can, for example, a characteristic dimension of Butzens measured and used as a controlled variable. Furthermore, the crimp seam length of the slug formed above the blow mold and / or a slug formed on the side of the blow mold can be detected as a characteristic size of the slug image. The metrological detection can also be simplified if the blow-molded hollow body are provided with one or more markings and the slug image is compared to these marks with a reference deposited as a slug image. The inner surface of the blow mold can be provided with engravings, which are formed during blow molding as marks on the blow-molded hollow bodies. The engravings are expediently mounted in areas which are adjacent to the parting plane of the blow mold.

From on the hollow bodies Adhesive waste can digital images are created, which are deposited with a reference Butzenbild be compared. It is also within the scope of the invention that the preforms at certain times, for example directly after extrusion of a preform, are detected by imaging and that the pictures are compared with a sub-reference picture become. The pictures can computerized be evaluated. The digital image analysis can be in a control loop, which when closing the blow mold in the preform contained air quantity and / or the closing speed of the blow mold controls, integrate. The technical imaging of the preforms offers the advantage, even when molding into the blow mold the necessary measures to be able to determine the correction of the ornamental image.

In the blow mold can Also sensors are used, which include the formation of pieces of slug in Capture sections along the squeeze seam. The amount of gas, the while closing the blow mold is contained in the preform and / or the closing speed the blow mold, according to specification the sensor signals are controlled. The signals of the sensors can also be used to faulty blow molded products, their Butzenbild at critical points deviates from default values, out of the process be rejected or marked as faulty.

Instead of or additionally to the slipper picture can also other sizes for Detection of deviations of the radial wall thickness distribution of the hollow body of a given distribution and used as a controlled variable for the radial Wall thickness control can be used. The wall thickness is expediently at least two, preferably opposite or offset by 90 degrees to each other points, the hollow body measured. Furthermore, can the preforms with circumferentially spaced markings be provided, wherein the distance of the markers to the blow-molded hollow is measured and where these measurements to detect deviations the radial wall thickness distribution compared with reference values become.

For certain applications, eg. As in the production of plastic fuel containers whose shape is approximated to a saddle fall in the blow molding medium or lateral Butzenabschnitte. The weight of a middle or lateral Butzens can be used as a measure of the radial wall thickness distribution. A further embodiment of the method according to the invention therefore provides that each of the blow-molded hollow bodies, an upper waste slug, a lower waste slug and a middle Abfallbutzen be removed, that the weight of the average Abfallbutzens is determined and that the weight of the average Abfallbutzens or the ratio of the Weight of the average Abfallbutzens and the net weight of the hollow body for detecting deviations of the radial wall thickness distribution are compared with reference values. Correspondingly, a lateral slit is used. From a central slug one speaks, if the slug is limited to a central region of the hollow body formed, while a lateral slit extends along one side of the hollow body. In principle, individual slugs can also merge into one another. In order to achieve a good reproducibility for the evaluation of slug weights and / or images, markings can be generated which characterize the material sections to be used for the evaluation. In order to achieve an assignment of the areas to the individual slugs and to be able to easily remove the slugs, webs with a low material thickness can also be produced the.

The Axial wall thickness control is based on at least two controlled variables. Next The net weight of the blow-molded hollow body may be the weight as a further variable one of the hollow bodies separated lower Abfallbutzens, the preform length and / or the extrusion time and / or the extrusion rate for preforming be used. Furthermore, there is a possibility that the preforms while preform formation with at least two preform longitudinal directions spaced marks are provided and that the distance between the marks as a controlled variable for the axial wall thickness control is used. After all it is possible, that the preforms in a cross-sectional area in which the wall profile the preforms have a critical area, they are marked that the markings on the blow-molded hollow bodies are detected and that the location of the hollow bodies measured markings as a controlled variable for the axial wall thickness control is used.

The Control scheme is expediently structured that the gap width of the nozzle gap changed if the net weight of the blow-molded hollow body of deviates from a required target weight, and that in case of a deviation the second controlled variable is the mass flow rate the plastic melt or the extrusion time is adjusted. The Adjustment of the shot weight takes place in the case of a continuous Extrusion of the melt, for example, by a change the screw speed or the extrusion time (at constant mass flow rate) and in a cyclic preform formation in the ejection operation by a change of the Ejection volume.

Of the the preforms forming nozzle gap is from a wall thickness program running with the preform formation controlled, which is the volume of plastic melt required for a preform divided into a predetermined number (n) volume sections and These volume sections nozzle gap control values assigns one over the number (n) of volume segments plotted program curve form. The design of the program curve takes into account the viscoelastic Behavior of the plastic melt for one by one particular Temperature, mass flow, material, nozzle gap, screw speed and optionally further operating parameters specified operating point and carries a sag the preforms at this operating point. Dodge the operating parameters This may be influenced by the values underlying the interpretation on the material distribution in the preform have the consequence that for example, the distance between two preform sections, each associated with a functional maximum of the program curve are facing each other the distance underlying the interpretation changes. Even if the extruded or ejected Preform the desired Weight and in relation to a preform section the right one Location in the blow mold can be viscoelastic Cause effects that the distance of the relevant preform section to changes another reference point within the preform. This disturbs in particular, when the distance between two preform sections, the each associated with a functional maximum of the program curve and when expanding the preforms to hollow bodies critical cross sections represent, changes. Even minor changes that distance can significant quality losses in terms on the compression values, distortion and strength properties of the resulting manufactured hollow body for Episode. To remedy this, the axial wall thickness control becomes preferably with a further controlled variable, so with a total of three controlled variables operated. The material distribution in the preform longitudinal direction is determined by a third controlled variable recorded and compared to a preset. For the purpose of Ausregelns a be caused by viscoelastic effects deviation from the specification the nozzle gap control values the program curve correction values applied, which is the sagging of the Influence preforms. Preferably, the program curve is included an adjustment of the correction values thereby adjusted so that the for the Vorformlingsbildung used amount of melt remains constant. The Correction values can have positive and negative values. By the immediate adjustment in the wall thickness program undesirable disorders avoided in the preform extrusion or in the preform ejection become. The correction values are determined empirically or based on of entered material data and the measured melt temperature and optionally further measured data and depending on the readings constantly customized.

The measurement of the material distribution in the preforms can be done in various ways. In the context of the invention, it is possible to work with parameters which only provide information about the material distribution in a cross section of the preforms or about the average material distribution in a preform section. These include weight measurements on one or more hollow body sections, wherein the deviation of at least one weight measurement from a predetermined desired value is used for the determination of the correction values. Another possibility for measuring the material distribution in the preforms is that at least one mark is attached to the exiting from the die gap preforms whose position on the completely ex truncated or ejected preform or is compared to the hollow body with a desired position. You can also work with multiple markers and capture the distance between the markers on the fully extruded preform or the hollow body. Finally, it is possible to carry out wall thickness measurements on the hollow bodies and / or the preforms and to determine the correction values in accordance with these measurements. Furthermore, the preform length and / or the exit velocity of the preforms can be measured with sensors, e.g. As photocells are detected.

to Detection of material distribution in the preforms can the blow-molded hollow body further by means of a radiation source, for example a light source become. The distribution of the intensity with which the rays of the Penetrate hollow body, lets you draw conclusions the wall thickness distribution too. The intensity distribution can be, for example recorded image-technically and compared with a reference image and be evaluated. Based on the determined deviation of the controlled variable is a Manual or automatic control intervention possible.

in the Frame of the invention may also be provided by the preforms while preforming or directly from the hollow bodies formed to record a thermal image after demoulding. With respect to the present invention, the particular arises Advantage that on the basis of temperature differences on the preforms, which are due to a different cooling, the changes the wall thickness estimated can be. So is on areas with a small wall thickness due to the unfavorable Volume-to-surface ratio a stronger one Cooling observed than in areas of greater wall thickness. By taking a thermal picture can so wall thickness deviations determined directly and with the inventive method be compensated.

Of Furthermore, there is also the possibility of one for the Condition of the plastic melt characteristic reading for additional Adjusting the method to determine the reading with a given reference value is set in relation and where, starting from the deviation between the measured value and the reference value, for the preform Using a numerical model, the viscoelastic flow behavior the plastic melt and starting from the spatial arrangement at least of a significant profile point as a function of time become. The so determined spatial Arrangement of the at least one profile point can then with a Reference position, and then based on the deviations a suitable correction factor or a suitable correction curve can be determined.

The inventive method is basically suitable for a defined process flow Correct variety of different deviations. substantial Deviations from a defined process flow arise in particular at a start of the blow molding process after a break. To be considered are especially thermal effects that affect all Refer steps of the process for the production of plastic hollow bodies can. For example, with regard to the extrusion process, that immediately after starting the temperature and the composition the plastic melt possibly not a stationary one yet State have reached. Furthermore, the thermal expansion the parts of the extruder and in particular the parts of the extrusion head with significant changes be connected. With regard to the molding process, it should be borne in mind that the adjusting movements usually take place hydraulically during the closing process, the hydraulic oil immediately after starting the blow molding process still comparatively cold and viscous. To those typically occurring during startup Balance deviations in relation to the defined process flow to be able to is according to one preferred embodiment of the invention provided when starting of the blow molding process to inquire whether the blow molding process is stable Operating state has reached. If this is not the case, can the influence of changes of the molding process on the material distribution of the blow-molded hollow body be corrected by an intervention in the molding process. Of Further can also be provided when starting the blow molding process the influence of changes the extrusion process on the material distribution of the blow-molded hollow body by engaging in that during to correct the preform extrusion running wall thickness program, as long as not the stable operating condition of the blow molding process is reached. in principle can when starting corrective interventions both in the molding process be provided as well as in the extrusion process. Furthermore can to some extent also a change in the molding process caused by the startup be carried out by an adaptation of the extrusion process and vice versa.

The possibilities afforded by the method according to the invention of a versatile, flexible and accurate correction of deviations are also particularly advantageous if a frequent conversion of the blow molding systems used is provided. So it is customary that a blow molding machine for the formation of blow-molded hollow body for the production ver a variety of products is provided, in which case a conversion is required in a product change. It should be remembered that errors can occur during the set-up processes and that conscious or unconscious changes can occur between two production cycles of a product. According to the invention, in addition to a product-specific exchange of the process data, a need-based correction can thus be carried out with the aid of the method according to the invention. It is particularly advantageous that, according to the invention, both deviations from the molding process and also batch-related changes in the plastic raw material are recognized and corrected.

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. So can For example, the control curve of a wall thickness control superimposed correction quantities be determined separately from each other. According to one alternative embodiment can However, also rule interventions under common consideration of several controlled variables determined become. So it may be particularly useful, the net weight Evaluate together with the corresponding truss weights, because the different weights of the formed hollow body on the one hand and various sections of the preform, on the other hand can be. For example, it may be that, to compensate for a Deviation of the manufacturing process several control interventions are possible where then the actually performed control intervention considering several Controlled variables determined becomes. By a common consideration of the controlled variables it also possible in particular comparatively large Compensate for deviations by a control intervention.

According to one Another aspect of the present invention is advantageous when by intervention in the control individual control measures can be disabled. For example, if otherwise reproducible process history of the numerical model or one of the previously described controlled variables 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. A compensation based on the determined data is in this Case does not make sense, so 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 control variables is disregarded can be a user intervention and / or a causality query be provided in the context of process control.

The inventive method can be used with tubular preforms as well at areal Preforms that are in the blow mold during the blow molding process are still combined in the plasticized state to form hollow bodies. In dependence the hollow body to be manufactured can the different control mechanisms are combined. In particular, with the control mechanisms and an adaptation of a Radical wall thickness control done.

If the recorded controlled variables very much far from the reference size and exceeding a given maximum deviation may be an indication be on a faulty measurement or a serious fault. A control intervention may not be appropriate. at the passing the maximum deviation can different measures be provided. For example, in the respective manufacturing process formed hollow body sorted out, an error signal is output or even the whole Process to be stopped. It is conceivable for the different activities to provide different thresholds.

in the The invention will be explained below with reference to exemplary embodiments. It show schematically:

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

2 a control scheme for the operation of in 1 illustrated blow molding plant,

3 and 4 further embodiments of the control scheme,

5 a blow molding machine operating in the ejection mode for the production of plastic hollow bodies,

6 a control scheme for the operation of in 5 illustrated blow molding plant,

7 a blow-molded hollow body with a waste slug and a lower waste slug,

8a the blow-molded product with an optimal slug image,

8b the blow-molded product with a non-optimal slug image,

9a an open blow mold with an array of sensors for detecting the slit width of the upper Abfallbutzens and

9b the view AA 9a and

10 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 Blow mold shown 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 will be vertically down from the plasticizing unit 1 extruded and in a reproducible position to the open blow mold 4 brought. The lower end of 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 squeezed off. After that, the blow mold 4 closed and the preform is with a gaseous medium, preferably compressed air, which by a previously introduced from above or below in the preform mandrel 7 or introduced by pierced needles, in the blow mold 4 to a hollow body 8th widened. At the head of the hollow body and at the bottom of the hollow body waste garbage arise 9 . 10 as waste from the blow-molded hollow body 8th be separated. The net weight GN of the hollow body 8th , the weight GUB of the bottom garbage 10 and the weight GOB of the upper Abfallbutzens 9 are measured and as controlled variables in a control scheme explained below 11 used.

A blow-molded product with a head-side upper waste slug 9 , a bottom-side bottom trash 10 and in the embodiment, a middle waste slug 12 is in 7 shown. Also, the slug image of the garbage is recorded and as a control variable for the manufacturing or control process 11 used.

The preforms 2 are made of a plastic melt with a change in preform longitudinal wall profile, which is at least one extreme value 13 has formed. Preform formation is regulated. According to the in 2 illustrated control scheme 11 is by means of an axial wall thickness control 14 the net weight GN of the hollow bodies 8th and a second controlled variable, which is the axial position of the extreme value 13 the wall profile of the preforms 2 indirectly before the closing of the blow mold, measured. If the measured values IW deviate from the setpoint values SW, the nozzle gap s forming the preforms and / or the mass throughput m for the preform formation are changed. According to the in 2 illustrated control scheme, the nozzle gap s is regulated when the net weight GN of the blow-molded hollow body 8th deviates from a desired target weight SW. As the second controlled variable another weight value is used. The second controlled variable used is the total weight or gross weight GB, which results from the sum of the weight values GUB, GOB of the waste slugs and the hollow body net weight. If the total weight GB (= shot weight) deviates from a preset value, the mass flow rate m is adjusted by a change in the screw speed. Alternatively, it is also possible to change the cycle time or the extrusion time with constant mass throughput.

The relationship between the slug image and the corresponding wall thickness distribution in the blow-molded hollow bodies is in 10 for a few examples. A comparative analysis of in 10 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 10 Correspond to the figure and the wall thickness distribution in the cutting plane AA the default values. In Examples B and C, the slippery image gives way 18 of the head of the hollow body formed upper Abfallbutzens 9 from the default value. The deviations correlate with irregularities in the radial wall thickness distribution of the blow-molded hollow body.

The nozzle gap s 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 of volume sections and associates these nozzle sections with gap values which form a program curve plotted over the number of the volume segment. At the in 2 illustrated control scheme includes the axial wall thickness control 14 a third controlled variable, which indirectly detects the material distribution in the preform longitudinal direction and compares it with a specification. As a third controlled variable, which allows conclusions about the material distribution in the axial direction of the preform, the weight value GOB of the upper Abfallbutzens 9 used. The weight value IW deviates from the upper one Abfallbutzens from a default value SW, are for the purpose of Ausregelns the Düsenpaltstellwerten 15 the program curve correction values 16 connected, which influence the sagging of the preforms.

In combination with the described axial wall thickness control is another control loop 17 provided with the deviations of the radial wall thickness distribution of the hollow body 8th be detected by a predetermined distribution and corrected by an intervention in the blow molding process. At the in 2 illustrated control scheme is used to detect deviations of the radial wall thickness distribution of the hollow body 8th a size measured, which for the slipper image 18 of the head of the hollow body formed upper Abfallbutzens 9 and / or a lateral Abfallbutzens formed along a seam 12 is characteristic. For example, a characteristic dimension of Abfallbutzen 9 . 12 measured and used as a controlled variable. Further, the squish length of the upper waste casing and / or a waste casing formed on the side of the blow mold may be detected as a characteristic quantity. If the badge picture 18 is deviated from a default value is via a slug image control 19 acted upon the blow molding process. For example, the closing speed of the blow mold 4 be corrected or the amount of gas during the closing of the blow mold 4 contained in the preform, be changed.

The slug image control 19 relevant control circuit 17 and the control loops of axial wall thickness control 14 must be coordinated. According to the in 2 shown control scheme is the Gap image control a query 20 upstream of whether the length VFL of the preforms lies in a predetermined tolerance field. The slug image control 19 is activated only if the measured value relating to the length VFL of the preforms lies within a predetermined tolerance range TOL.

For detecting the image of the slap 18 the expert can resort to various procedures. It can be measured a characteristic dimension of Abfallbutzens and used as a controlled variable. As a characteristic size, for example, the Quetschnahtlänge the upper Abfallbutzens 9 suitable. To simplify the measurements, the hollow bodies 8th be provided with one or more markings in blow molding, wherein the slug image 18 at these markings 21 is compared with a background image deposited as a reference ( 8a and 8b ). Furthermore, of the adhering to the hollow body slugs 9 . 10 . 12 created digital images and these are compared with a deposited as a reference slit image. The images can be evaluated computer-aided.

At the in 2 The control scheme is assumed that the blow molding plant has a gravimetric flow rate control of the melt flow and that the closing process of the blow mold 4 is triggered when a certain wall thickness profile point of the profile curve reached or a defined extrusion time has expired. This in 3 illustrated control scheme differs from the previously described embodiment in that the preform length VFL of a photocell at a predetermined position, for example below the blow mold 4 or below the hose closing device 6 , is recorded. In this case, the machine cycle time t _{m is} independent of the time t _p at which the wall thickness program is executed. The time t _p at which the wall thickness program is executed is also referred to below as wall thickness cycle time. Differences between the wall thickness cycle time t _p and the machine cycle time t _m can lead to position shifts of the preforms 2 relative to the blow mold 4 lead either because the wall thickness program is terminated too early or the wall thickness profile remains longer on the last profile point of the profile curve. To remedy this, the in 3 illustrated control scheme the wall thickness cycle time with the current machine cycle time t _m in the program element 22 compared and in case of deviations, the wall thickness cycle time t _p in the program element 23 customized. Further, the mold waiting time of the blow mold is detected, and in case of deviations of the mold waiting time t _w from a target value SW, the mass flow rate m in the program element 24 corrected. The fact that the preform length VFL is controlled by a photocell, one can assume that the preform length VFL always the basic requirements for the image quality control 19 enough. Therefore, the interpretation of the lower Butzgewichtes GUB with the slug image control 19 be linked. In principle, the control works as described in the previous example. It is known that the detection of the preform length VFL by means of a photocell is defective due to an uneven preform lower edge. In the 3 The control shown makes it possible to eliminate these disadvantages by taking the lower plug weight as the integral of the lower preform end. If the bottom slug weight deviates from the setpoint, although the slug pattern corresponds to the default values, the control can adjust the delay time t _vz for the photocell until the integral dimension for the correct hose length corresponds to the setpoint. Thus, problems, such as occur in an uneven hose separation can be compensated.

This in 4 illustrated control scheme is used for blow-molded hollow body, in particular plastic fuel tank whose shape is approximated to a saddle. In the production middle slug sections fall on. The extent of the middle Butzenabschnittes and consequently also its weight allows a conclusion as to whether the radial wall thickness distribution of the hollow body 8th Default values or not. In addition to the net weight GN of the hollow body 8th as well as the weights GUB, GOB of the lower and upper Abfallbutzens also the average slug weight GMB is recorded and set to the net weight of the hollow body in proportion. If the ratio GMB / GN deviates from the mean slug weight and the net weight of default values, the timing of the pre-blowing becomes after closing the hose closing device 6 and / or the duration of the pre-blowing and / or the pre-blowing pressure corrected. If the ratio GMB / GN of the weight of the mean Butzens and the net weight of the hollow body is greater than the default value, less blown, that is, the injected gas g is increased in the present embodiment. If the value is smaller than the preset value, the Vorblasvorgang is amplified, that is, the injected gas g is reduced.

To mutually disturbing influences on the control circuits 14 . 17 To avoid, at the in 4 illustrated control method interpretation of the net weight GN of the Butzenbildregelung 19 decoupled. The scheme is structured so that regardless of the image of the bust 18 the net weight GN has to be reached. Then, with regard to the GMB / GN ratio, the pre-blowing is corrected and, if necessary, the net weight is adjusted again.

Of course, instead of a middle Butzens 25 also a possible existing side slug 12 be used. Furthermore, horizontally arranged markings can be used as a controlled variable.

In 5 is a working in the discharge operation blow molding shown. It has an extrusion head 26 on, which is designed as a storage head and with a storage space 27 and a vertically adjustable hollow piston 28 Is provided. In the lower part of the blow mold is schematically a feed pipe 29 shown for the supply of blown air. With the system, for example, hollow body made of containers open on one side, which are formed with the bottom up. The upper waste trash 9 is separated from the tank bottom.

According to the in 6 shown control scheme are in the course of axial wall thickness control 14 the net weight GN of the hollow bodies 8th and the weight of the lower Abfallbutzens 10 recorded as controlled variables. If the net weight GN deviates from a default value, the nozzle gap width s is changed. The weight value GUB of the lower part 10 or the total weight of the weight of the lower Butzens and the net weight of the hollow body are used as a controlled variable, which indirectly the axial position of the extreme value 13 the wall profile of the preforms 2 before closing the blow mold 4 describes. When the weight GUB of the lower Abfallbutzens 10 or the total weight of the weight of the lower Abfallbutzens 10 and the net weight GN of the hollow body 8th deviates from the default value, the discharge volume is changed and optionally the capacity of the plasticizing unit 1 customized. If the weight reading IW is greater than the setpoint SW, the ejection volume is reduced and vice versa.

By means of another control loop 17 for radial wall thickness control additional deviations of the radial wall thickness distribution of the hollow body 8th captured by a given distribution. As a controlled variable, the width of the upper Abfallbutzens is used. If the measured value IW deviates from a default value SW, then, according to the in 6 illustrated control scheme, the exit velocity V _{A of} the plastic melt corrected from the nozzle gap. The exit velocity V _A affects the swelling of the preforms emerging in the melt state 2 and on the location of the wall thickness points. A higher exit velocity V _A causes a greater swelling and leads to a larger Butzenbreite. If the exit velocity V _{A is} reduced, the swelling of the preforms decreases 2 and the neck width becomes smaller. The control loop 17 acts by a discharge control 30 on the preforming. It has to be taken into consideration that with the ejection speed also that of the plasticizing unit 1 the quantity added changes. To adjust the delivery rate, an adjustment of the discharge volume and / or the speed may be required. This in 6 illustrated control scheme is appropriate for blow molding, which are operated without hose closing device. Such systems without hose closing device are used in particular for the production of barrels. A preform adaptation by pre-blowing is less suitable in blow-molding plants without a hose-closing device.

A change in the swelling due to a changed exit velocity V _A affects the length VFL of the preforms. A larger swelling leads to shorter preforms. If the swelling is reduced, longer preforms result. The balancing of the position of the preforms at different exit velocities V _A through a change in the nozzle gap s is not satisfactory possible. For this reason, according to the in 6 illustrated control scheme in the program element 31 Another correction factor, which acts on the wall thickness control, introduced.

The Butzenbreite the upper Abfallbutzens can by sensors 32 be captured in the blow mold 4 are arranged. The arrangement of these sensors 32 is in the 9a and 9b shown. There are several juxtaposed sensors 32 intended. Feedback on how many sensors are activated makes the width of the upper waste bin 9 certainly. As sensors 32 For example, pressure sensors, proximity sensors, temperature sensors and the like can be used.

Claims (31)

Process for the production of plastic hollow bodies by blow molding, wherein preforms ( 2 ) made of a plastic melt with a wall profile of an open blow mold that changes in the preform longitudinal direction ( 4 ) are supplied, wherein in a subsequent to the preform formation blow molding process, the blow mold ( 4 ) and the preforms with a gaseous medium to hollow bodies ( 8th ) and wherein the preform formation is controlled and by means of an axial wall thickness control ( 14 ) the net weight (GN) of the hollow bodies ( 8th ) and a second controlled variable, which determines the axial position of a selected region of the wall profile of the preforms ( 2 ) is measured indirectly before the closing of the blow mold, measured and, in the case of deviations of the measured values of nominal values, the nozzle gap forming the preforms and / or the mass flow rate (m) for the preform formation are changed, characterized in that additionally deviations of the radial wall thickness distribution of the hollow bodies ( 8th ) from a predetermined distribution and by an intervention in the blow molding process or by changing the swelling of the preforms ( 2 ) influencing exit velocity of the plastic melt can be corrected from the nozzle gap. Method according to claim 1, characterized in that the lower end of the preforms ( 2 ) in the blow molding process by means of a hose closing device ( 6 ) is squeezed and that the amount of gas, during the closing of the blow mold ( 4 ) in the preform ( 2 ) is included in a time interval between the closing time of the hose closing device ( 6 ) and the time from which the blow mold ( 4 ) is closed, corrected for deviations of the radial wall thickness distribution of the hollow body ( 8th ) from a given distribution. 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 of the separation process, and that the amount of gas which is produced during the preform extrusion in the preform ( 2 ) is corrected, in order to deviations of the radial wall thickness distribution of the hollow body ( 8th ) from a given distribution. A method according to claim 2 or 3, characterized in that in the time interval between the closing time of the hose closing device ( 6 ) or cutting device and the time from which the blow mold ( 4 ) is introduced, a gaseous medium is introduced into the preform which the preform ( 2 ) when closing the blow mold ( 4 ) by pre-blowing, wherein the timing of the pre-blowing and / or the duration of the pre-blowing and / or the pre-blowing pressure are corrected to eliminate deviations of the radial wall thickness distribution. A method according to claim 2 or 3, characterized in that in the preform ( 2 ) amount of gas in the time interval between the closing time of the hose closing device ( 6 ) or cutting device and the time from which the blow mold ( 4 ), is reduced by venting or aspiration, wherein the venting or aspiration timing and / or the period of venting or aspiration or the suction pressure are corrected to eliminate drifts in the radial wall thickness distribution. A method according to claim 4 and 5, characterized in that during the period of a first sub-interval, a gaseous medium in the preform ( 2 ) is introduced and that during the period of a second sub-interval, the gaseous medium is sucked off or drained. Method according to one of claims 1 to 6, characterized in that the closing movement of the blow mold ( 4 ) is corrected to deviations of the radial wall thickness distribution of the hollow body ( 8th ) from a given distribution. Method according to one of claims 1 to 7, characterized in that a variable for detecting deviations of the wall thickness distribution is automatically measured and fed as a measured variable to a control loop, which when closing the blow mold ( 4 ) in the preform ( 2 ) contained gas quantity and / or the closing speed of the blow mold ( 4 ) controls. Method according to one of claims 1 to 8, characterized in that the length of the preforms ( 2 ) is detected and that deviations of the radial wall thickness distribution of the hollow body ( 8th ) are corrected from a predetermined distribution by an intervention in the blow molding process, if the length of the preforms ( 2 ) is within a predetermined tolerance range. Method according to one of claims 1 to 9, characterized in that for detecting deviations of the radial wall thickness distribution of the hollow body ( 8th ) of a predetermined distribution a size which for the image of slipper ( 18 ) one at the head of the hollow body ( 8th ) formed upper Abfallbutzens ( 9 ) and / or a lateral Abfallbutzens formed along a seam ( 12 ) is characteristic, is measured. A method according to claim 10, characterized in that at least one characteristic dimension of the garbage ( 9 . 12 ) is measured and used as a controlled variable. A method according to claim 10, characterized in that the crimping seam length of the upper Abfallbutzens ( 9 ) and / or a Abfallbutzens formed on the side of the blow mold ( 12 ) as a characteristic size of the slug image ( 18 ) is detected. Method according to one of claims 10 to 12, characterized in that the hollow body ( 8th ) are provided with one or more markings during blow molding and that the slug image ( 18 ) is compared at these markings with a stored as a reference location of the markers. Method according to one of claims 10 to 13, characterized in that of the on the hollow bodies ( 8th ) sticking garbage ( 9 . 10 . 12 ) created digital images and these with a reference 18 ). Method according to one of claims 10 to 14, characterized in that the preforms ( 2 ) and that the images are compared with a stored reference value. A method according to claim 14 or 15, characterized in that the images are evaluated computer-aided and that the digital image analysis in a control loop ( 17 ), which when closing the blow mold ( 4 ) in the tubular preform ( 2 ) and / or the closing speed of the blow mold ( 4 ) controls, is integrated. Method according to claim 10, characterized in that that sensors are used in the blow mold, the forming from Butzenstücken in sections along the squeeze seam. Method according to one of claims 1 to 9, characterized in that for detecting deviations of the radial wall thickness distribution of the hollow body ( 8th ) of a predetermined distribution wall thickness measurements on the blow-molded hollow bodies ( 8th ) be performed. A method according to claim 18, characterized in that the wall thickness at at least two, preferably arranged at right angles to each other points of the hollow body ( 8th ) is measured. Method according to one of claims 1 to 9, characterized in that the preforms ( 2 ) are provided with circumferentially spaced marks and that the distance of the markings on the blow-molded hollow bodies is measured and the measured values for detecting deviations of the radial wall thickness distribution are compared with reference values. Method according to one of claims 1 to 9, characterized in that of the blow-molded hollow bodies ( 8th ) one upper waste slug ( 9 ), a lower waste slug ( 10 ) and a middle or side waste slug ( 12 ), that the weight (GMB) of the middle or side Abfallbutzens (GMB) 12 ) and that the weight (GMB) of the middle or side waste ( 12 ) or the ratio of the weight (GMB) of the middle or side Abfallbutzens ( 12 ) and the net weight (GN) of the hollow body ( 8th ) are compared with reference values for detecting deviations of the radial wall thickness distribution. Method according to one of claims 1 to 21, characterized in that as a second controlled variable for the axial wall thickness control, the weight (GUB) of one of the hollow bodies ( 8th ) separated lower Abfallbutzens ( 10 ), the preform length, the extrusion time or the exit velocity of the preforms ( 2 ) is used for preforming. Method according to one of claims 1 to 21, characterized in that the preforms ( 2 ) are provided with at least two markings spaced apart in the preform longitudinal direction during the preform formation, and that the distance between the markings is used as a control parameter for the axial wall thickness control ( 14 ) is used. Method according to one of claims 1 to 21, characterized in that the preforms ( 2 ) in a cross-sectional area in which the wall profile of the preforms ( 2 ) an extreme value ( 13 ), that the markings on the blow-molded hollow bodies ( 8th ) and that the position of the on the hollow bodies ( 8th ) measured marks as a controlled variable for the axial wall thickness control ( 14 ) is used. Method according to one of Claims 1 to 24, 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, that for the axial wall thickness control ( 14 ) the material distribution in the preform longitudinal direction is detected by a third controlled variable and compared with a specification and that for the purpose of Ausregelns caused by viscoelastic effects deviation from the specification of the nozzle gap control values of the program curve correction values are switched, which influence the sagging of the preforms. Method according to claim 25, characterized in that that the program curve when applying the correction values is adjusted so that the for the preform formation used remains constant. Method according to claim 25 or 26, characterized that the correction values are based on entered material data and based on the measured melt temperature and optionally further Measurement data calculated and depending on the readings constantly be adjusted. Method according to one of Claims 25 to 27, characterized that the weight measurements on one or more hollow body sections carried out and that the deviation is at least one weight measurement from a predetermined setpoint for the determination of the correction values is used. Method according to one of claims 25 to 27, characterized in that at the exiting from the die gap preforms ( 2 ) at least one marking is applied whose position on the completely extruded or ejected preform or on the hollow body ( 8th ) is compared with a target position, wherein the length deviation from a predetermined target value is used for the determination of the correction values. Method according to one of claims 25 to 27, characterized in that on the hollow bodies ( 8th ) and / or the preforms ( 2 ) And the correction values are determined in accordance with these measurements. Method according to one of claims 1 to 30, characterized in that of the preforms ( 2 ) and / or are absorbed by the hollow bodies formed immediately after demolding thermal images.