Abstract:
The invention relates to a method and a device for the production of blow-molded two- or multi-layer containers made of thermoplastic, and a plastic container produced accordingly, having view strips, particularly for storage and for transport of contents. To this end, molten plastic material is deflected in an extrusion head ( 10 ) from the inner, natural-colored layer ( 34 ) of the tubular preform ( 18 ) into the outer layer ( 36 ) in a process technical step, and incorporated there, whereby a view strip ( 24 ) is formed in the colored outer plastic layer ( 36 ).

Description:
CROSS-REFERENCES TO RELATED APPLICATIONS 
     This application is the U.S. National Stage of International Application No. PCT/EP2008/007627, filed Sep. 5, 2008, which designated the United States and has been published as International Publication No. WO 2009/040009 and which claims the priority of German Patent Application, Serial No. 20 2007 013 318.9, filed Sep. 21, 2007, pursuant to 35 U.S.C. 119(a)-(d). 
     BACKGROUND OF THE INVENTION 
     The invention relates to a method and an apparatus for producing blow-molded two-layer or multilayer containers made of a thermoplastic material and plastic containers produced with this method having viewing strips, in particular for storing and transporting liquid contents. 
     Such a plastic container has typically vertical container walls with a horizontal container lid and base, wherein a gas- and liquid-tight closable fill and drain opening is disposed at least in the container lid or the container cover. 
     The plastic container can be formed as a canister, fassett, bunged barrel with lid and clamping ring, or as a large volume interior container of a pallet container, wherein this type of container typically includes a bottom drain fitting. 
     Frequently, plastic containers—with respect to multilayer containers, at least the outer layer—are colored, for example blue bunged barrels or black plastic containers colored with carbon black with an electrically dischargeable outer layer. 
     It is desirable to be able to identify at any time the respective fill or drain level of the container of plastic containers having a colored outer layer. For this purpose, colored plastic containers are typically provided with a narrow vertical viewing strip made of natural color (virgin) plastic, through which the respective level of the contents in the container can be ready identified. However, producing plastic containers with a viewing strip by blow molding is somewhat complex and requires a corresponding additional structural configuration of the extrusion head (continuous or discontinuous) and a corresponding separate extruder for the material of the viewing strip. 
     Another field of application concerns the storage and transport of flammable liquids. For example, when filling or emptying a pallet container (DE 196 05 890 A1) or more generally, when stirring liquids in plastic containers for the purpose of mixing and the like, electric charges generated by friction between the liquid and the inner container surface are discharged to ground via an electrically conducting outside coating of the inner container and the metallic support frame (steel pallet). This prevents an electric discharge between the plastic in a container and the outer frame formed by a metal lattice, which may cause sparking capable of igniting flammable content of the pallet container and of explosive mixtures of gases and vapors in enclosed spaces. Disadvantageously, conventional pallet containers do not have a viewing strip, so that the fill level cannot be identified with optical monitoring due to the black color of the inner plastic container. 
     DE 38 44 605 discloses a pallet container with at least one viewing strip is made of a translucent or transparent plastic material, which forms a unit with the black colored plastic material; a level indicator is provided next to the viewing strip. The conventional complex machine configuration is required for producing the plastic container with inserted viewing strip. 
     DE 41 36 766 discloses a multilayer container wall with an outer colored container layer with an interrupted layer region in form of a strip. The strip-shaped interrupted layer region is materially connected with the colored plastic material of the container wall. The corresponding apparatus for discontinuous production of a tubular preform for blow molding the containers is described in the published document DE-U-91 07 812. 
     DE 202 06 436 another pallet container with a multilayer inner container, with a permanently antistatic outer layer made of carbon black with at least one integrated viewing strip made of transparent plastic material. For example, EP 0 333 956 B1 describes an apparatus for producing plastic containers with a viewing strip embodied in an extrusion storage head. 
     A drawback common to all conventional multilayer embodiments is the provision of a continuous viewing strip made of the colorless plastic material of the inner layer. Both the apparatus and the method for fabricating a viewing strip of this type are quite complex. For example, when associating the viewing strip and the volume graduation, the circumferential position of the viewing strip of the inner container must be exactly matched to the circumferential position of the volume graduation. Moreover, an exact vertical orientation of the viewing strip on the inner container is difficult to maintain due to different degrees of expansion when the inner container is manufactured by blow molding. 
     SUMMARY OF THE INVENTION 
     It is therefore an object of the present invention to provide a corresponding apparatus and a method, which allows a simple and cost-effective manufacture of a colored plastic container with a viewing strip, wherein an additional extruder with lines carrying the molten plastic material for the viewing strip is no longer required. In addition, the apparatus of the invention should have a simple design and should be constructed so that it can be installed in existing multilayer extrusion heads even at a later date. 
     This object is attained in accordance with the invention by an extrusion head for producing blow-molded multilayer hollow plastic bodies, with a housing, on which two or more fittings for extruders are provided, with molten plastic materials being supplied through the fittings, the molten elastic materials being distributed along the circumference through two or more circumferential distributors to form a two-layer or multilayer tubular preform, wherein the preform flows through a cylindrical reservoir and can be elected through a controllable annular nozzle, wherein the ejected tubular reform is provided with a viewing strip, wherein an additional fitting is provided on the side of the housing ( 12 ) below the circumferential distributor and above the annular nozzle, with a hollow tube being inserted through the fitting into the cylindrical reservoir, wherein the hollow tube has on the inside of the reservoir an upward pointing inlet opening and on the outside of the reservoir a downward pointing outlet opening, through which plastic material can be deflected and incorporated from the inner layer in the colored outer layer for providing the viewing strip. 
     According to the invention, colorless plastic material is partially removed with a suitable device from the inner layer in the annular gap of the extrusion head shortly before the material exits from the annular nozzle, is deflected and rerouted in the radial direction and introduced into the separate outer layer (transfer of virgin material from the inner layer to the outer layer). 
     The subsequent dependent claims recite advantageous modified embodiments of the invention. 
     With respect to the apparatus, the invention includes an extrusion head for producing blow-molded multilayer hollow plastic bodies, with a housing provided with two or more fittings for extruders, wherein molten plastic materials are supplied through the fittings. The molten plastic materials are distributed through two or more circumferential distributors along the circumference to form a two-layer or multilayer tubular preform. The preform flows through a cylindrical reservoir and can be ejected through a controllable annular nozzle, whereby the ejected tubular reform is provided with a viewing strip. An additional fitting is provided on the side of the housing below the circumferential distributor and above the annular nozzle. A hollow tube is inserted through the fitting into the cylindrical reservoir, whereby the hollow tube an upwardly pointing inlet opening facing the inner surface of the reservoir and a downwardly pointing outlet opening facing the outer surface of the reservoir. Plastic material can hereby be deflected from the inner layer to the colored outer layer for incorporating the plastic material from the inner layer in the colored outer layer for providing the viewing strip. 
     In an embodiment of the invention, a helical screw with a drive is provided inside the hollow tube for transporting plastic material. The quantity of plastic material withdrawn from the inner layer of the tubular preform can then be easily controlled and changed. The helical screw can be driven, for example, with a hydraulic motor or an electric motor. 
     According to another embodiment of the invention, the hollow tube may be pulled out of the housing and the corresponding additional fitting may be constructed to be closable with an applied lid or solid plug. In this way, depending on the requirements, the extrusion head of the invention can be used to produce plastic containers with or without viewing strip. 
     Advantageously, the hollow tube with the helical screw and the drive may be configured as a retrofit kit that can be installed in any existing extrusion head. The extrusion head can be configured as a continuous tubular head (or Conti-Head) or as discontinuous extrusion head with injection piston (=storage head, Accu-Head). Manufacturers of blow-molded plastic hollow bodies can then retrofit their existing blow molding machines in a simple and cost-effective manner so that—depending on the requirements—different plastic containers, such as plastic bunged barrels or lidded barrels, can be manufactured with or without a viewing strip, without requiring costly retrofitting with an additional extruder. 
     According to another embodiment of the invention, the hollow tube with the helical screw may be configured for movement in the radial direction and may advantageously also be provided with a corresponding drive for movement in the radial direction. 
     With respect to the method, according to the invention, molten plastic material is deflected inside the cylindrical reservoir of the extrusion head from the inner layer to the colored outer layer and incorporated in the colored outer layer for providing the viewing strip. Optionally, deflection and incorporation of the molten plastic material from the inner layer in the colored outer layer for providing the viewing strip may be performed only sequentially (discontinuously) such that the viewing strip is disposed only in the vertical wall of the finished blown container and not in the lid and bottom of the finished blown container. 
     Additional advantages: Deflecting the material from the inside to the outside inside the tubular wall of the preform eliminates the typical continuous separation lines (vertical separation lines in the container wall) in the wall of the finished blown product, and hence also eliminates the disadvantageous effects on the performance data (e.g. drop test, hydraulic interior pressure test, etc.) of the finished container. The inner surface of the tube or of the finished container remains undisturbed and is not separated. Containers having different volumes from about 60 liters to about 1000 liters can be produced with an extrusion head according to the invention with an inserted hollow tube and a driven helical screw, for instance canisters, drums or large volume plastic inner containers for pallet containers. An additional extruder or an additional mass storage device (for sequential operation) is then no longer required. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWING 
       The invention will now be described in more detail with reference to an exemplary embodiment schematically illustrated in the drawings. 
         FIG. 1  shows an extrusion head according to the invention with schematically indicated transverse conveyor for the melt, 
         FIG. 2  shows the lower part of an extrusion head according to the invention with transverse melt conveyor on an enlarged scale, 
         FIG. 3  shows a plastic lidded barrel with viewing strip, and 
         FIG. 4  shows a large volume plastic inner container with viewing strip. 
     
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
       FIG. 1  shows with the reference numeral  10  an extrusion head for producing blow-molded two-layer plastic hollow bodies embodied as a storage head (Accu-Head) with a housing  12 , on which (not shown in the partial view) two connections for two extruders are provided. Two circumferential distributors  14 ,  16  are supplied with molten plastic material from the connected extruders. The two circumferential distributors  14 ,  16  are constructed as concentric cylinder sleeves with built-in helical channels for circumferentially distributing the plastic material flows that are received from the extruders as a full strand. The circumferential distributor  16  is covered by an additional cylinder sleeve  42 . The three concentric cylinder sleeves  14 ,  16 ,  42  with interior, or covered, helical channels are fixed in position relative to one another and form together the so-called ejection piston. The ejection piston encompasses a central mandrel  46  and is supported in the housing for vertical movement and provided with a corresponding ejection drive. 
     The plastic material, which is completely distributed about the circumference, exits at the bottom end of the cylinder sleeves or of the ejection piston as a two-layer tubular preform  18 , flows through a cylindrical reservoir  20  in which a viewing strip is applied during the extrusion process, and is then ejected from the extrusion head  10  through a controllable annular nozzle  22 . In the discontinuous operating mode of the storage head, the tubular preform  18  is sequentially ejected from the reservoir  20  by the ejection piston through the controllable annular nozzle  22 . The central mandrel  46  is also movably arranged in the housing  12 . By lowering the cone-shaped mandrel tip, the nozzle gap of the annular nozzle  22  opens and widens; when the cone-shaped mandrel tip is raised, the nozzle gap narrows and can close completely. The extrusion head  10  can, of course, also be designed as continuous extrusion head with continuous tube discharge. According to the present invention, a particular device for redistributing the material is arranged inside the reservoir  20 , below the circumferential distributors  14 ,  16  and above the controllable annular nozzle  22 . Redistribution of the material is performed by a hollow tube  28  such that, as viewed in the radial direction from the inside to the outside, plastic material is initially withdrawn from the inner layer  34 , which consists of original natural material, of the almost finished preform  18 , deflected in the radial direction and pushed to the outside, where it is then again deflected to flow in the axial direction and introduced into the outer colored layer  36  of the preform  18  as a transparent viewing strip. 
     As a result of the partial redistribution of the material, slight changes in the layer thickness occur in a local region about the introduced viewing strip (decrease in the thickness of the inner layer and corresponding increase of the outer layer), which is immaterial for the finished product and has no disadvantageous effect on the mechanical performance (inside pressure test, drop resistance, stack loading) of the container. 
     With particular container designs, for example, closed bunged barrels with high stretching rates of the tubular preform in regions that are offset by 90° with respect to the mold separation plane in the injection mold halves, it is desirable to arrange the viewing strips in a particular circumferential position. To this end, the lower housing part  48 , in which the hollow tube  28  for providing the viewing strip  24  is arranged, is rotatably attached on the housing  12  of the extrusion head, so that the viewing strip in the ejected preform can be positioned exactly in the desired position, for example at the mold separation plane. The lower housing part  48  can also be equipped with a stepping motor for rotating the lower housing part in both rotation directions and thereby adjust the position of the viewing strip with reference to the blow-mold and its position in the blown container, respectively. 
     In an exemplary actual design of medium size, the hollow tube  28  inserted in the lower housing part  48  has an outside diameter of about 45 mm and extends into the reservoir  20  to a depth of about 50 mm. The upper opening  30  has a width of about 30 mm and about the same length, wherein the lower opening  32  is about 35 mm long, but only 10 mm wide.  FIG. 2  shows in more detail the lower part of the extrusion head with a transverse melt conveyor implemented as a helical screw  38 . The helical screw  38  is preferably designed as a double-lead screw and has a length of only about 55 mm. The hollow tube  28  is completely sealed to the outside at the driveshaft behind the helical screw  38  by corresponding sealing rings. 
     With colored containers having a two-layer or three-layer wall structure, the outer layer colored with admixed dye pigments or with carbon black is relatively thin, since dye pigments or carbon black are relatively expensive. Advantageously, the colored outer layer has a thickness of only 5 to 20%, preferably about 10%, of the container wall. 
     The layer thickness of the inner layer made of new material (HD-PE High Density Poly Ethylene) should not be thinner than about 50%. With a three-layer wall structure, the middle layer has a layer thickness between 20% and 60% and is typically made of high density polyethylene, whereby recycled pellets or ground material can be used (colorless or natural color HD-PE). 
     For use in explosive areas or with flammable content, the plastic containers are provided—through addition of carbon black—with a black, electrically conducting outer layer. This outer layer should also be as thin as possible to save costs. 
       FIG. 3  shows in form of an exemplary embodiment of a blow-molded container produced with the method of the invention a 220 liter plastic lidded barrel  50  with a viewing strip  52  and measuring graduation  54  in the vertical barrel wall.  FIG. 4  shows a different exemplary embodiment in form of a large-volume plastic inner container  56  with viewing strip  58  and a measuring graduation  60  for a pallet container with a capacity of 1000 liters. 
     The viewing strip should have—when viewed in the circumferential direction—a width of about 10 mm to 20 mm, so that the respective the level of the liquid contents can be readily identified at any time.