Abstract:
A method and an apparatus for blow-molding containers are disclosed. A parison made of a thermoplastic material is first subjected to a thermal treatment in the zone of a heating section along a conveying path. The parison is then shaped into the container within a blow mold under the effect of a blowing pressure. Once the container has been blow-molded, a wall thickness is measured on at least one vertical level of the container. A preset value for the wall thickness is fed to a controller as a desired value, and the measured wall thickness is fed thereto as an actual value. The controller presets the quantity of at least one parameter influencing the blowing process in accordance with a difference between the desired value and the actual value. More specifically, the controller presets the quantity of at least one parameter influencing the supply of blowing gas. The quantity of the parameter is preset on the basis of a blowing process simulation model implemented in the controller.

Description:
The present application is a 371 of international application PCT/DE2009/001267 filed Sep. 7, 2009, which claims priority of DE 10 2008 057 999.8, filed Nov. 13, 2008, the priority of these applications is hereby claimed and these applications are incorporated herein by reference. 
     BACKGROUND OF THE INVENTION 
     The invention concerns a method for blow molding containers, in which a preform made of a thermoplastic material is subjected to thermal conditioning along a conveyance path in a heating line and then molded into a container in a blow mold by the action of blowing pressure; in which, after the container has been blow molded, a wall thickness is measured at at least one height level of the container; in which an automatic control system is supplied with a preassigned value for the wall thickness as the setpoint value and with the measured wall thickness as the actual value; and in which the automatic control system presets a value of at least one parameter that affects the blowing process as a function of the difference between the setpoint value and the actual value. 
     The invention also concerns a device for blow molding containers made of a thermoplastic material, which has at least one heating line arranged along a preform conveyance path and at least one blowing station with a blow mold, and in which an automatic control system is used, which is connected with at least one sensor for determining a wall thickness of the container, and in which the automatic control system has at least one final control element for presetting the value of a parameter that affects the blowing process. 
     In container molding by the action of blowing pressure, preforms made of a thermoplastic material, for example, preforms made of PET (polyethylene terephthalate), are fed to different processing stations within a blow-molding machine. A blow-molding machine of this type typically has a heating system and a blowing system, in which the preform, which has first been brought to a desired temperature, is expanded by biaxial orientation to form a container. The expansion is effected by means of compressed air, which is fed into the preform to be expanded. DE-OS 43 40 291 explains the process-engineering sequence in this type of expansion of the preform. The aforementioned introduction of the pressurized gas comprises both the introduction of compressed gas into the developing container bubble and the introduction of compressed gas into the preform at the beginning of the blowing process. 
     The basic structure of a blowing station for container molding is described in DE-OS 42 12 583. Possible means of bringing the preforms to the desired temperature are explained DE-OS 23 52 926. 
     Various handling devices can be used to convey the preforms and the blow-molded containers within the blow-molding device. The use of transport mandrels, onto which the preforms are slipped, has proven especially effective. However, the preforms can also be handled with other supporting devices. Other available designs are grippers for handling the preforms and expanding mandrels, which can be inserted in the mouth region of the preform to support the preform. 
     The handling of containers with the use of transfer wheels is described, for example, in DE-OS 199 06 438 with the transfer wheel arranged between a blowing wheel and a delivery line. 
     The above-explained handling of the preforms occurs, for one thing, in so-called two-step processes, in which the preforms are first produced by injection molding and temporarily stored and then later conditioned with respect to their temperature and blown into containers. For another, the preforms can be handled in so-called one-step processes, in which the preforms are first produced by injection molding and allowed to solidify sufficiently and are then immediately suitably conditioned with respect to their temperature and then blow molded. 
     With respect to the blowing stations that are used, various embodiments are known. In the case of blowing stations that are arranged on rotating transport wheels, book-like opening of the mold supports is often encountered. However, it is also possible to use mold supports that can be moved relative to each other or that are supported in a different way. In stationary blowing stations, which are suitable especially for accommodating several cavities for container molding, plates arranged parallel to one another are typically used as mold supports. 
     Before a heating operation is carried out, the preforms are typically slipped onto transport mandrels, which either convey the preforms through the entire blow-molding machine or merely revolve within the heating system. In the case of vertical heating of the preforms in such a way that the mouths of the preforms are oriented vertically downward, the preforms are usually placed on a sleeve-like mounting element of the transport mandrel. In the case of suspended heating of the preforms, in which the mouths of the preforms are oriented vertically upward, expanding mandrels are usually inserted into the mouths of the preforms to clamp them tightly. 
     In carrying out container molding by blow molding, an essential task is to achieve a predetermined material distribution in the container wall. An important parameter for predetermining the material distribution that is obtained is the heat distribution realized in the preforms before the blow molding. 
     The heat distribution is typically realized in such a way that an even temperature level is produced in a circumferential direction of the preforms, while a temperature profile is produced in a longitudinal direction of the preforms. In addition, a suitable temperature profile through the wall of the preform from the outside to the inside is also predetermined. It can basically be assumed that regions of the preform with a lower temperature lead to thicker wall regions of the blow-molded container, while the warmer regions of the preform are stretched to a greater extent during the blow molding operation and thus lead to thinner wall regions of the blow-molded container. 
     The temperature of the preforms can be measured with so-called pyrometers. Exact wall thicknesses of the blow-molded containers can be measured with so-called wall thickness sensors, which operate, for example, optically or with the use of sound waves. 
     Other important parameters for controlling the material distribution in the blow-molded containers are the stretching speed, the assignment with respect to time of the stretching operation to the delivery of compressed gas, and the pressure distribution with respect to time in the expansion of the preform to the container. In particular, controlling the actual blowing pressure has been found to be difficult, because between a control valve for presetting the blowing pressure and the preform to be expanded there lies a flow path with variable passage cross section and throttles that affect the flow, and, in addition, the increase in the volume of the preform during the blow molding of the preform into the container causes a reaction on the developing pressure. On the other hand, the insertion of the stretch rod into the preform leads to a reduction of the available volume. Furthermore, there are relatively complex interactions among the individual parameters, and these interactions affect the actual material distribution that develops in the blow-molded container. 
     Due to the large number of parameters and interactions among the parameters, instead of actual automatic control, is often only possible to realize control based on the consideration of empirically determined and manually preset adjustments. Practically realized automatic controls typically relate to individual parameters without sufficient consideration having been given to the complexity of the blowing process. 
     SUMMARY OF THE INVENTION 
     The objective of the present invention is to improve a method of the aforementioned type in such a way that qualitatively high-quality container molding is supported with little mechanical engineering effort and at the same time high throughput rates are achieved. 
     In accordance with the invention, this objective is achieved in such a way that the value of at least one parameter that affects the supply of blowing gas is preset as a correcting variable by the automatic control system and that the value of the parameter is preset on the basis of a blowing process simulation model implemented in the automatic control system. 
     A further objective of the present invention is to design a device of the aforementioned type in a way that is conducive to high throughput rates with good product quality despite a simple design. 
     In accordance with the invention, this objective is achieved by designing the final control element to preset the value of a parameter that affects the supply of blowing gas and that the automatic control system comprises a blowing process simulation model for determining the value of the parameter as a function of a control deviation between the setpoint value and the actual value. 
     The automatic control of the blowing operation, taking into consideration a parameter that affects the supply of blowing gas and taking into account the interactions of individual influencing factors by the blowing process simulation model, allows qualitatively extremely high-grade container production, since preassigned setpoint values are maintained with low tolerances and interfering effects can be automatically controlled with a short time delay. The simulation model considers especially flow cross sections and flow resistance in the vicinity of the blowing gas supply as well as interactions among the developing pressure, the volume flow of blowing gas, and volume changes occurring as a result of the developing container bubble, the positioning of the stretch rod at a given time and other given influencing factors, for example, the temperature of the preforms, the temperature distribution in the wall of the preforms, and the blow mold temperature. Basically, it is possible to incorporate any other desired influencing factors in the blowing process simulation model. 
     High contour precision of the blow-molded containers can be achieved by measuring the wall thickness of the containers at several different height levels. 
     In one embodiment, it is provided that the blowing pressure is automatically controlled as a parameter. 
     It can also be provided that the volume flow is automatically controlled as a parameter. 
     Furthermore, it is also possible that in addition to the parameters affecting the supply of blowing gas, the heating temperature is automatically controlled. 
     Expanded control possibilities are made available by automatically controlling the heating temperature in at least one sectional length of the preform. 
     It is conducive to an easily comprehensible system structure if the automatic control is carried out in the form of a cascade control system. 
     In particular, taking dynamic characteristics into consideration, it has been found to be effective to automatically control the temperature in an outer closed-loop control system and the blowing gas parameter in an internal closed-loop control system. 
     An adaptive automatic control design is supported if the blowing process simulation model acts on the control characteristics of at least one controller. 
     In another embodiment, it is also possible for the blowing process simulation model to act on the controller input of at least one controller. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWING 
       Specific embodiments of the invention are schematically illustrated in the drawings. 
         FIG. 1  shows a perspective view of a blowing station for producing containers from preforms. 
         FIG. 2  shows a longitudinal section through a blow mold, in which a preform is stretched and expanded. 
         FIG. 3  is a drawing that illustrates a basic design of a device for blow molding containers. 
         FIG. 4  shows a modified heating line with increased heating capacity. 
         FIG. 5  shows a cross section through a heating element with a plurality of radiant heaters arranged one above the other and an associated preform. 
         FIG. 6  shows a cross section through a sensor system for measuring wall thicknesses of a blow-molded container. 
         FIG. 7  shows a schematic drawing of a blow-molding machine with a heating line, blowing wheel, pyrometer, and wall thickness sensor. 
         FIG. 8  shows a schematic drawing of an automatic control system design for automatically controlling the temperature of the preform and the wall thickness of the blow-molded containers. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       FIGS. 1 and 2  show the basic design of a device for molding preforms  1  into containers  2 . 
     The device for molding the container  2  consists essentially of a blowing station  3 , which is provided with a blow mold  4 , into which a preform  1  can be inserted. The preform  1  can be an injection-molded part made of polyethylene terephthalate. To allow the preform  1  to be inserted into the blow mold  4  and to allow the finished container  2  to be removed, the blow mold  4  consists of mold halves  5 ,  6  and a base part  7 , which can be positioned by a lifting device  8 . The preform  1  can be held in place in the area of the blowing station  3  by a transport mandrel  9 , which, together with the preform  1 , passes through a large number of treatment stations within the device. However, it is also possible to insert the preform  1  directly into the blow mold  4 , for example, with grippers or other handling devices. 
     To allow compressed air to be fed in, a connecting piston  10  is arranged below the transport mandrel  9 . It supplies compressed air to the preform  1  and at the same time produces a seal relative to the transport mandrel  9 . However, in a modified design; it is also basically possible to use stationary compressed air feed lines. 
     In this embodiment, the preform  1  is stretched by means of a stretch rod  11 , which is positioned by a cylinder  12 . In accordance with another embodiment, the stretch rod  11  is mechanically positioned by means of cam segments, which are acted upon by pickup rollers. The use of cam segments advantageous especially when a large number of blowing stations  3  is arranged on a rotating blowing wheel. 
     In the embodiment illustrated in  FIG. 1 , the stretching system is designed in such a way that a tandem arrangement of two cylinders  12  is provided. Before the start of the actual stretching operation, the stretch rod  11  is first moved into the area of a base  14  of the preform  1  by a primary cylinder  13 . During the stretching operation itself, the primary cylinder  13  with the stretch rod extended, together with a carriage  15  that carries the primary cylinder  13 , is positioned by a secondary cylinder  16  or by a cam control mechanism. In particular, it is proposed that the secondary cylinder  16  be used in such a way under cam control that a current stretching position is predetermined by a guide roller  17 , which slides along a cam track while the stretching operation is being carried out. The guide roller  17  is pressed against the guide track by the secondary cylinder  16 . The carriage  15  slides along two guide elements  18 . 
     After the mold halves  5 ,  6 , which are arranged in the area of supports  19 ,  20 , are closed, the supports  19 ,  20  are locked relative to each other by means of a locking mechanism  20 . 
     To adapt to different shapes of a mouth section  21  of the preform  1 , provision is made for the use of separate threaded inserts  22  in the area of the blow mold  4 , as shown in  FIG. 2 . 
     In addition to the blow-molded container  2 ,  FIG. 2  shows the preform  1 , which is drawn with broken lines, and also shows schematically a container bubble  23  in the process of development. 
       FIG. 3  shows the basic design of a blow-molding machine, which has a heating line  24  and a rotating blowing wheel  25 . Starting from a preform feeding device  26 , the preforms  1  are conveyed to the area of the heating line  24  by transfer wheels  27 ,  28 ,  29 . Heating elements  30  and fans  31  are arranged along the heating line  24  to bring the preforms  1  to the desired temperature. After sufficient heat treatment of the preforms  1 , they are transferred to the blowing wheel  25 , where the blowing stations  3  are located. The finished blow-molded containers  2  are fed to a delivery line  32  by additional transfer wheels. 
     To make it possible for a preform  1  to be blow molded into a container  2  in such a way that the container  2  has material properties that ensure a long shelf life of the foods, especially beverages, with which the container  2  is to be filled, specific process steps must be followed during the heating and orientation of the preforms  1 . In addition, advantageous effects can be realized by following specific dimensioning specifications. 
     Various plastics can be used as the thermoplastic material. For example, PET, PEN, or PP can be used. 
     The preform  1  is expanded during the orientation process by feeding compressed air into it. The operation of supplying compressed air is divided into a preblowing phase, in which gas, for example, compressed air, is supplied at a low pressure level, and a subsequent main blowing phase, in which gas is supplied at a higher pressure level. During the preblowing phase, compressed air with a pressure in the range of 10 bars to 25 bars is typically used, and during the main blowing phase, compressed air with a pressure in the range of 25 bars to 40 bars is supplied. 
       FIG. 3  also shows that in the illustrated embodiment, the heating line  24  consists of a large number of revolving transport elements  33 , which are strung together like a chain and are moved along by guide wheels  34 . In particular, it is proposed that an essentially rectangular basic contour be set up by the chain-like arrangement. In the illustrated embodiment, a single, relatively large-sized guide wheel  34  is used in the area of the extension of the heating line  24  facing the transfer wheel  29  and a feed wheel  35 , and two relatively small-sized guide wheels  36  are used in the area of adjacent deflections. In principle, however, any other types of guides are also conceivable. 
     To allow the closest possible arrangement of the transfer wheel  29  and the feed wheel  35  relative to each other, the illustrated arrangement is found to be especially effective, since three guide wheels  34 ,  36  are positioned in the area of the corresponding extension of the heating line  24 , namely, the smaller guide wheels  36  in the area of the transition to the linear stretches of the heating line  24  and the larger guide wheel  34  in the immediate area of transfer to the transfer wheel  29  and to the feed wheel  35 . As an alternative to the use of chain-like transport elements  33 , it is also possible, for example, to use a rotating heating wheel. 
     After the blow molding of the containers  2  has been completed, the containers  2  are carried out of the area of the blowing stations  3  by an extraction wheel  37  and conveyed to the delivery line  32  by the transfer wheel  28  and a delivery wheel  38 . 
     In the modified heating line  24  illustrated in  FIG. 4 , a larger number of preforms  1  can be heated per unit time due to the larger number of heating elements  30 . The fans  31  in this case feed cooling air into the area of cooling air ducts  39 , which lie opposite the associated heating elements  30  and deliver the cooling air through discharge ports, A direction of flow of the cooling air essentially transverse to the direction of conveyance of the preforms  1  is realized by the arrangement of the discharge directions. The surfaces of the cooling air ducts  39  opposite the heating elements  30  can provide reflectors for the thermal radiation. It is also possible to realize cooling of the heating elements  30  by the delivered cooling air. 
       FIG. 5  is a schematic drawing of a heating element  30  that is provided with a plurality of radiant heaters  41  arranged one above the other. With the use of the radiant heaters  41 , it is possible to produce a predetermined temperature profile in the direction of a longitudinal axis  42  of the preform  1 . When a stretching operation is being carried out, a stretch region  43  of the preform  1  is essentially subjected to a bilateral orientation. 
       FIG. 6  is a schematic drawing of a measuring device  44  with a plurality of sensors  45  arranged one above the other for detecting a wall thickness of the container  2 . The stretch region  43  of the preform  1  was shaped into an orientation region  46  of the container  2  as a result of the stretching and blowing operation. The stretch region  43  of the preform  1  has an initial length  47 , and the orientation region  46  of the container  2  has a product length  48 . The quotient of the product length  48  and the initial length  47  represents the realized stretch factor. 
     The container  2  has a longitudinal axis  49 , and the sensors  45  are arranged one after the other in the direction of this longitudinal axis  49 . The distance  50  between the sensors  45  is obtained as the distance  51  between the radiant heaters multiplied by the stretch factor. 
       FIG. 7  is a schematic drawing of a blow-molding machine  52  with a greatly simplified and highly schematic configuration compared to the drawing in  FIG. 3 . The drawing shows that a temperature sensor  53  for detecting a temperature of the preforms  1  is arranged near the heating line  24  downstream of the heating elements  30  in the direction of conveyance of the preforms  1 . It is advantageous for the temperature sensor  53  to be arranged as closely as possible to the blowing wheel  25  to allow temperature detection after thermal equalization processes have taken place within the wall of the preforms  1 . A pyrometer is an example of a temperature sensor  53  that can be used. In particular, it is possible to arrange several temperature sensors  53  one above the other in the direction of the longitudinal axis  42  of the preforms  1  in order to determine a temperature profile of the preforms  1 . It has been found to be especially advantageous to position a plurality of temperature sensors  53  at the various height levels of the radiant heaters  41  in order to be able to carry out direct automatic control of the individual radiant heaters  41 . 
       FIG. 7  also shows the arrangement of the measuring device  44  for determining the wall thickness of the containers  2 . For example, the measuring device  44  can be arranged in the vicinity of an extraction device  54 , which carries the blow-molded containers  2  away from the area of the blowing wheel  25 . 
       FIG. 8  is a schematic drawing of an automatic control system for the heating elements  30  and radiant heaters  41  in an outer closed-loop control system and for one or more parameters related to the delivery of blowing gas in an inner closed-loop control system. The automatic control system is designed as a cascade control system. An outer closed-loop control system detects the wall thickness  2  of the container  2  at a predetermined height level by means of the measuring device  44  downstream of the blowing station  3  and supplies this actual value to the input of a wall thickness controller  55 . The direct input value for the wall thickness controller  55  is the control deviation between a preset wall thickness and the actual wall thickness determined by measurement. An output value of the wall thickness controller  55  provides the setpoint value for an inner temperature closed-loop control system. 
     The difference between the output value of the wall thickness controller  55  and a temperature value of the preform  1  detected by the temperature sensor  53  at a predetermined height level is supplied to a temperature controller  56  as a direct reference value. An automatic control system of the type illustrated in  FIG. 8  is typically assigned to each of the radiant heaters  41 . 
     The innermost and thus fastest closed-loop control system of the cascade control system shown in  FIG. 8  includes one or more blowing gas controllers  57 . The blowing gas controller  57  can be designed, for example, to automatically control the pressure and/or the volume flow of the blowing gas. A control deviation between an actual value supplied by a sensor  58  and the given blowing gas parameter that is being automatically controlled, which is obtained as the output value of an associated controlled system  59 , is supplied to the blowing gas controller  57  as an actual value. 
     It is advantageous if at least one of the controllers  55 ,  56 ,  57  is designed with integral control action in order to avoid control deviations. In accordance with another automatic control variant, the automatic control system takes into consideration lag time behavior of the automatic control system on the basis of the conveyance distances of the preforms  1  and containers  2 . In this regard, it is taken into consideration that there is a known delay between a change in a correcting variable and a change in the output variable, which depends on the conveyance speed. 
     As an alternative to the realization of the automatic control system as a cascade control system of the type illustrated in  FIG. 8 , the automatic control system can be realized with any other desired automatic control structure. In the case of cascade types of automatic control, it has been found to be effective to automatically control rapidly variable process parameters in the inner closed-loop control systems and slowly variable process parameters in the outer closed-loop control systems. 
     At least one of the measured values delivered by the sensors  44 ,  53 ,  58  is supplied to a process model  60 . In addition, the process model  60  has one or more sensor inputs  61  that make it possible to consider additional measurement information regarding the blowing process. The process model  60  also has one or more model outputs  62  that make it possible to affect the automatic control behavior. In one embodiment, the control characteristics of at least one of the controllers  55 ,  56 ,  57  are varied via the model output  62 . In another embodiment, it is contemplated, alternatively or additionally, that the input value of at least one of the controllers  55 ,  56 ,  57  be influenced by the model output or outputs  62 . This influence can be brought about, for example, in addition to the influence brought about by the sensors  44 ,  53 ,  58 . It is also possible to replace at least one of the signals of the sensors  44 ,  53 ,  58  by a value available at the model output  62 . The process model  60  forms a simulation model. 
     The process model  60  makes it possible to take into account complex relationships among the individual process parameters during the execution of the automatic control. In particular, it is possible to take delays, lag times and nonlinearities into account. In addition, the process model  60  makes it possible for the automatic control to incorporate process variables that elude direction measurement or that can be measured only with great effort. 
     The container production can be automatically controlled, for example, on the basis of a predetermined pressure development for the blowing gas. If a comparison of the measured values with the values generated by the simulation model reveals deviations in at least one of the measured parameters, then, for example, the starting point for supplying the preblowing pressure can be varied for each of the production cycles, and/or it is possible to increase or decrease the speed of the stretching process in a suitable way. This can be done especially by presetting the given rate of insertion of the stretch rod into the preform  1  to be stretched.