Abstract:
A method for quality control of a stretch-blow-molded plastic container by inspecting its base, which method is easy to execute and is not highly susceptible to faults. Such inspection of the container bases includes determining quality features such as the surface area of an unstretched and/or only slightly stretched region of the base of the plastic container.

Description:
[0001]    The present application claims the benefit of priority of International Patent Application No. PCT/EP2006/008448, filed on Aug. 29, 2006, which application claims priority of German Patent Application No. 10 2005 044 206.4, filed Sep. 15, 2005. The entire text of the priority application is incorporated herein by reference in its entirety. 
     
    
     FIELD OF THE DISCLOSURE 
       [0002]    The present disclosure relates to a method for controlling the quality of a stretch-blow-molded plastic container and, preferably, for monitoring and controlling the stretch-blow-molding process. 
       BACKGROUND 
       [0003]    A method for quality control is known from U.S. Pat. No. 6,872,895. The known method determines the conditions at the base of a stretch-blow-molded plastic container as indication for a successfully and correctly executed stretch-blow-molding process. However, this known method only determines the mass or volume of the material in the base of the plastic container. To this end, the absorption characteristics of the whole base or of a selected area of said base are determined and, on the basis of this determination, conclusions are drawn with regard to the mass or the volume of the material which still exists in the base after the stretch-blow-molding process. The features taken into account in the absorption measurement comprise, however, also features, such as minor cloudiness or the like, which do not have any influence on the quality of the plastic container and which may corrupt the measurement. Meaningful absorption measurements are, moreover, comparatively complicated. An area-integrating absorption measurement with a non-spatially-resolving sensor is not very meaningful because of the non-linear characteristics of the law of absorption, and, when a spatially-resolving sensor is used, such measurements are extremely complicated, especially in the infrared light region. 
         [0004]    Furthermore, it is known e.g. from DE 199 14 028 to determine the quality of a stretch-blow-molded plastic container in that the outer contours of various areas of the container and the ratio of said contours to one another are determined in a transmitted light process. This method does, however, not allow a reliable detection of all deviations. 
         [0005]    Furthermore, a great variety of wall thickness determination methods of stretch-blow-molded plastic containers is known, one of said methods being known e.g. from DE-A-101 16 665. A wall thickness measurement must, however, be executed at many different points of the plastic container so as to be meaningful for the quality of the container in its entirety. The method described in this publication can be used for determining, analogously to the wall thickness, also other parameters, such as the density. The known method is therefore comparatively complex. 
       SUMMARY OF THE DISCLOSURE 
       [0006]    It is therefore the object of the present disclosure to provide a method for controlling the quality of a stretch-blow-molded plastic container as well as for monitoring and controlling the stretch-blow-molding process, which method is easy to execute and largely insusceptible to faults. 
         [0007]    The disclosure is based on the finding that, depending on the respective process control, the stretching of the base of a plastic container leads to different characteristic distributions of material in said base. These inhomogenities can be emphasized by special illumination according to the present disclosure and can be recorded in a spatially resolved manner with electronic camera technology, especially also in visible light. Making use of image processing algorithms according to the present disclosure, the characteristic distributions of material are measured and used for determining therefrom characteristic reference numbers, such as the surface area of an unstretched and/or only slightly stretched region of the base of the plastic container and/or the size and the position of the transition area between the unstretched and the stretched region. A parameter which is very meaningful for the quality of the stretch-blow-molding process and for the quality of the stretch-blow-molded plastic container can easily be provided in this way. The thus obtained information content is much higher than in the case of conventional methods, such as the determination of the section weight or of the wall thickness at individual points of the side wall. 
         [0008]    The characteristic distributions of material can be made visible with various methods, e.g. the wedge angle between the inner and the outer wall by means of directed light, material stresses with the aid of polarized light, or, in the case of deep-dyed container materials, also with the aid of absorption in the visible wavelength region. A combination of illuminations which are more or less strongly directed, illuminations with different wavelengths in the visible or infrared or ultraviolet wavelength region, or illuminations having different directions of polarization are imaginable as well. 
         [0009]    Depending on what is easier to realize, this method can be applied to the whole container base or to subareas thereof. 
         [0010]    The determination in the transmitted light process is specially preferred and is particularly suitable for light-transmissive PET plastic bottles. 
         [0011]    The characteristic reference numbers determined in the process are preferably used for controlling the stretch-blow-molding process by means of process parameter control. 
         [0012]    Since the method simultaneously represents an online quality control of all the containers produced, and since the product quality also depends on multiply provided production tools, such as the cavities of the stretch-blow-molding machine, the container quality measured is preferably statistically associated with the respective production tools. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0013]    In the following, embodiments of the present disclosure will be explained in detail on the basis of the drawings, in which: 
           [0014]      FIG. 1  shows a schematic representation of a quality control apparatus for use in the method according to the present invention, 
           [0015]      FIG. 2  shows a schematic representation of a surface area determined according to a first embodiment, 
           [0016]      FIG. 3  shows a schematic representation of a surface area determined according to a further embodiment, 
           [0017]      FIG. 4  shows a photographic representation of a surface area determined according to a further embodiment, and 
           [0018]      FIG. 5  shows a photographic representation of a surface area determined according to a further embodiment. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
       [0019]      FIG. 1  shows an embodiment of an apparatus  1  for executing the method according to the present disclosure. The apparatus comprises a control station  2 , an evaluation means  3  and a transport unit  4 . A plastic container to be inspected  9 —the container shown in  FIG. 1  is a plastic bottle consisting preferably of PET—is held through the transport unit  4  by a clamp element  10  such that its base and its mouth are free. 
         [0020]    The container  9  has a shape that is substantially rotationally symmetric with regard to a longitudinal axis M. The wall  13  of the container  9  comprises a mouth region  15 , which is provided with an external thread and a laterally projecting support ring  16 , and a substantially cylindrical body area  17 , which is located below the support ring  16  and which laterally projects beyond said support ring  16 , said body area  17  defining the largest outer diameter of the container  9 . The body area  17  terminates in a base  12 , which is implemented as a foot. On the outer side of the base  12 , an injection point  14  is provided. 
         [0021]    The transport star wheel  11  having the axis of rotation B comprises a plurality of the transport units  4  shown. The transport star wheel  11  is arranged after, preferably immediately after a conventional stretch-blow-molding machine, which is not shown in the drawing and which is used for producing the plastic containers  9 , but it may also be provided at some other location. However, it should preferably be possible to precisely associate each container  9  to stations of its production. In this stretch-blow-molding machine, preforms are subjected to blowing so as to establish their desired final shape; in so doing, said preform is stretched and enlarged while reducing its wall thickness. 
         [0022]    The transport star wheel  11  conducts the molded and stretched plastic containers  9  through the control station  2  for the purpose of quality control. 
         [0023]    The control station  2  is implemented as a transmitted-light examination unit and comprises a stationary light source  5 , preferably an optical unit  6 , a sensor  7 , preferably a camera (matrix camera) and an imaging objective  8 . The optical axis A of the optical components of the control station  2  is arranged in such a way that it coincides with the centre line M of a correctly shaped plastic container  9 , when said container  9  is being transported through the control station  2 . The light source  5  with the optical components which are preferably associated with said light source  5  are located below the base  12  of the container  9 , whereas the sensor  7  and the components associated therewith are arranged on the side of the mouth  15  of the container  9  and are directed into the mouth opening. The sensor  7  is connected to the evaluation means  3 . 
         [0024]    The optical unit  6  can comprise e.g. a stop, a directing foil and/or a lens system or other optical units which are able to generate directed, in particular strongly directed, so-called hard light. 
         [0025]    If such directed light is directed onto the base  12 , in particular in the area of the center line M, where the injection point  14  is located, a dark outer contour of the type shown in detail in  FIGS. 4 and 5  will appear around the injection point  14 . All the parts of the container base in the case of which the inner and the outer wall are not plane parallel, but define a wedge angle, whereby the directed light is deflected by diffraction and does not arrive at the camera, are imaged darkly. The image according to  FIG. 4  is obtained when strongly directed light is used, whereas the image according to  FIG. 5  is obtained, when less strongly directed light is used, in which case the outer contour  18  and the dark spot in the middle appear brighter so that further details can still be seen. 
         [0026]    It turned out that the surface area within the dark outer contour  18  is a measure for the quality of the stretch-blow-molding process, since the dark outer contour  18  encloses an unstretched and/or only slightly stretched region, the size of this region being a measure for the quality of the stretch-blow-molding process. Hence, a reference value for the surface area of region  18 , which indicates a container  9  of satisfactory quality, was ascertained. This reference value indicates that, on the one hand, the region  18  is not excessively large, i.e. the base  12  does not comprise an excessive amount of unstretched material which would then be missing at other locations, e.g. in particularly sensitive areas of the wall, and that, on the other hand, the base  12  does not comprise an excessively small amount of unstretched or only slightly stretched material, which may be a sign of irregular heating and of an increased wall thickness at some other point. 
         [0027]    For quality control, the surface area of said region  18  is determined, as shown in  FIG. 2 , either in that the total size  19  of the whole area enclosed by the shaded region  18  is determined. 
         [0028]    Furthermore, it is also possible to determine the surface area of the unstretched region by determining only the surface area of a subregion  20  and by extrapolating from this subregion  20  to the total surface area. The subregion  20  is preferably measured at a representative, meaningful location; in  FIG. 3 , a subregion  20  is shown, which extends in the form of a strip, symmetrically with regard to the center line M and the injection point  14 , across the whole unstretched region  18 . 
         [0029]    Additional quality features can be determined from the darkly imaged transition region between the unstretched or only slightly stretched region in the middle of the base and the stretched region at the periphery of the base. Features which are meaningful as regards the quality of the stretch-blow-molding process are the width, the position, the shape and the degree of shading of this region. 
         [0030]    The optical unit  6  may, however, also be implemented as a polarizer for generating polarized light. By means of the polarized light, material stresses in the base  12  can be made visible so that it is possible to extrapolate from the presence and absence of material stresses in the various regions of the base  12  to the size of the unstretched and/or only slightly stretched region  18 . 
         [0031]    The quality features determined or received from the sensor  7  are supplied to the evaluation means  3 . The evaluation means  3  compares the quality features with the reference value and determines whether the plastic container  9  measured satisfies the demands on quality or whether it must be rejected. It will be expedient when the reference value has the form of a tolerance range between upper and lower threshold values. 
         [0032]    If it has been determined that a plastic container  9  does not satisfy the demands on quality, the evaluation means  3  will also be able to intervene in the production process of the plastic container  9  and to control process parameters by way of example. Such process parameters can e.g. be the heating temperature in the individual heating zones. 
         [0033]    As modification of the embodiments described hereinbefore and shown in the drawings, the control station can be arranged at any other suitable location. The transport unit can have any structural design which allows the base to be inspected in a transmitted light process, e.g. it may also be a chainlike transport unit, or a transport unit which holds the containers below the mouth or the support ring (indicated by a dot and dash line in  FIG. 1 ).