Abstract:
The invention concerns a process for the production of hollow bodies of thermoplastic material by extrusion blow molding. In the process according to the invention continuous extrusion of at least two preforms in web form of plasticized material is effected by means of a plurality of extrusion devices. The preforms are jointly shaped to form a hollow body in the still plastic condition, that is to say in the first heat, within a multi-part tool forming a mold cavity. In that case a wall thickness profile is imparted to at least one preform during the extrusion operation independently of the respective other preform by continuous nozzle gap adjustment during the extrusion operation.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims priority to German Application DE 10 2006 027 254, filed Jun. 9, 2006 and to U.S. Provisional Application 60/806,489 filed Jul. 3, 2006. 
    
    
     FIELD 
     The invention concerns a process for the production of hollow bodies of thermoplastic material by extrusion blow molding. 
     BACKGROUND 
     It is basically known for hollow bodies such as for example fuel tanks, drums or heating oil tanks to be produced from plastic material by extrusion blow molding. 
     In that case generally granulated thermoplastic material is plasticized in one or more extruders and fed to an extrusion head. The plasticized material is expelled from the extrusion head in the form of a tubular preform which is either removed at the extrusion head and introduced by means of manipulators into an opened mold tool or is extruded into the mold tool in a hanging condition directly at the extrusion head. The mold tool generally comprises two mold halves forming a mold cavity defining the external contour of the finished blow-molded article. The preform is expanded within the mold cavity by means of gas pressure and/or by means of a reduced pressure. The finished article is of an external contour corresponding to the contour which is predetermined by the mold cavity of the tool. 
     It is equally known for a tubular preform, after extrusion, to be cut up to the intended final length into one or more material webs and for those material webs to be transported by means of a gripper into position between the opened halves of a tool. 
     Finally it is also known for preforms in web form or band form to be expelled from extruders with wide-slot nozzles and for the preforms to be shaped in some manner in the first or second heat to afford the definitive configuration of the finished product. 
     When dealing with articles of a comparatively complicated external contour a tubular preform is generally extruded. In that situation the material which is extruded at the extrusion head in a hanging condition experiences a certain amount of stretch by virtue of its length-related weight. If no measures are taken to compensate for that stretch, that results in a differing wall thickness distribution on the finished product, in the extrusion direction. When dealing with molten materials of comparatively low viscosity that can be counteracted by means of discontinuous extrusion. 
     It is easy to envisage that articles of a comparatively complex external configuration experience different degrees of stretching upon expansion at various locations within the blow molding mold so that this factor alone results in differing wall thickness distribution. In the production of certain products different wall thicknesses on the finished product are desirable, whereas other products require different wall thickness profiles for reasons of stability in the finished product. Finally it is known for the wall thickness of the extruded preform to be varied in such a way that differing degrees of stretch are compensated upon expansion of the preform. Critical articles in that respect are transparent detergent containers, canisters with handles shaped therein and so forth. 
     For that purpose it is known for a wall thickness profile to be imparted to tubular preforms, more specifically both axially and also radially. That is implemented for example by adjustment of the annular gap at the extrusion head. In the case of very large products wall thickness control is also used to compensate for the lengthwise stretch of the tube in the extrusion procedure. 
     It is also desirable in the case of relatively large products such as for example in the case of fuel tanks of plastic material to impart to the preform a wall thickness which varies over the length and/or periphery thereof, so that differing degrees of stretch in the tool can be compensated or it is possible to achieve a different wall thickness distribution on the finished product. It is conceivable for example that, in the case of a fuel tank, a different wall thickness distribution is to be provided for reasons of stability in order to be able to better accommodate any impact-induced deformation. 
     Therefore the object of the present invention is to provide a process for the production of an extrusion blow molded container of thermoplastic material, which entails very substantial freedom in terms of variation in the wall thickness of the container in any direction. 
     SUMMARY 
     That object is attained by a process for the production of hollow bodies of thermoplastic material by extrusion blow molding, in which extrusion of at least two preforms in web form of plasticized material is effected by means of a plurality of extrusion devices, wherein the preforms are jointly shaped in the first heat, that is to say in the still plastic condition, within a multi-part tool forming a mold cavity to provide a hollow body and wherein a wall thickness profile is imparted to at least one preform during the extrusion operation independently of the respective other preform by continuous nozzle gap adjustment during extrusion. 
     Preferably at least two preforms in web form are extruded continuously. If the hollow body to be produced is for example a fuel tank, it may be advantageous for the preforms to be produced from a multi-layer co-extrudate with barrier layers for hydrocarbons, for example in the form of EVOH or the like. 
     In contrast to the previously known processes for wall thickness control in respect of tubular preforms, it is possible with the process according to the invention to produce a hollow body having two regions with respectively different wall thickness profiles. In the extrusion of a tubular hollow body, radial and/or axial wall thickness adjustment results in influencing the entire extrudate over the entire periphery thereof. With the process according to the invention two preforms respectively forming a hollow body can be influenced independently of each other, wherein it is possible to impart to at least one preform, during the extrusion operation, a wall thickness profile which is different from the respective other preform. That has the advantage that the process according to the invention also makes it possible to produce large and complex products involving any wall thickness variation in any direction. 
     Desirably wall thickness control is effected in each case in accordance with a predetermined program. 
     In a preferred variant of the invention it is provided that a wall thickness profile is imparted to the preforms or at least one preform in the extrusion direction and/or transversely with respect to the extrusion direction. 
     Desirably the preforms are expanded within the tool by an increased internal pressure and/or a reduced external pressure. 
     Preferably the shaping operation is effected within a three-part tool in a two-stage mode, wherein in a first step the preforms are respectively caused to bear against a contour forming a portion of the mold cavity by means of increased pressure and/or reduced pressure and are shaped out and in a second step the intermediate products respectively forming portions of the hollow body are joined together to provide the finished product. 
     Preferably the preforms are extruded from extrusion devices arranged in mutually parallel relationship and having wide-slot nozzles in a hanging condition (in the direction of the force of gravity). 
     Preferably extrusion of the preforms is effected synchronously in respect of the extrusion speed. In that respect the variation in the wall thickness of the preforms can be effected in dependence on the progress in respect of time of the extrusion operation. 
     In a particularly preferred feature the preforms are extruded between the opened portions of the tool which is closed around the preforms and moved away from under the extrusion devices. 
     Alternatively for example the preforms in web form can be respectively removed by means of a gripper at the extrusion device in question and transported to the tool. 
     In accordance with the invention it is provided that the preforms are respectively extruded between an outer mold and a central mold, that then the outer molds are closed against the central mold, the preforms are respectively caused to bear against the internal contour of the outer molds, the central mold is removed from between the outer molds and the outer molds are closed against each other in order to join together intermediate products which are disposed therein. 
     Although the above-described process is referred to as extrusion blow molding, the process is to be so interpreted that the preforms can also be caused to bear against the internal contour of the mold cavity by means of a reduced pressure. For that purpose it is known to provide suitable vacuum passages in the mold cavity at the internal contour of the tool. 
     It will be appreciated that the preforms can be extruded in a single-layer or multi-layer configuration, in which case for example the production of fuel tanks usually involves the use of six-layer extrudates which respectively include barrier layers for hydrocarbons. 
    
    
     
       DESCRIPTION OF THE DRAWINGS 
       The invention is described hereinafter by means of an embodiment by way of example illustrated in the drawings, in which: 
         FIG. 1  is a diagrammatic side view of an extrusion blow molding apparatus for carrying out the process according to the invention, 
         FIGS. 2-10  are cross-sections through the tool of the apparatus shown in  FIG. 1  at various steps of the process according to the invention, 
         FIGS. 11   a - 11   c  are respective views of the nozzle of the extrusion head, partly in section, of a first variant of the nozzle gap adjustment, and 
         FIGS. 12   a - 12   c  are views of a nozzle, partly in section, with a second variant of nozzle gap adjustment. 
     
    
    
     DETAILED DESCRIPTION 
     The extrusion blow molding machine  1  shown in  FIG. 1  comprises two extrusion heads  2  with respectively associated extruders which are not illustrated. The extrusion heads  2  are arranged above a three-part tool  3  which is movable both into the plane of the drawing and also out of same. The tool  3  includes two outer molds  3   a ,  3   b  and a central mold  3   c , wherein the outer molds  3   a ,  3   b  each have a respective cavity  4   a ,  4   b  which define the later external contour of the finished product  5 . (See  FIG. 10 ). 
     A preform  6  in web form of thermoplastic material is extruded in a hanging condition from each extrusion head  2 . In the described embodiment of the invention the preforms are extruded between the respectively open outer molds  3   a ,  3   b  and the central mold  3   c  which are shown in the opened position in  FIG. 1 . As is diagrammatically illustrated hereinafter (See  FIG. 3 ), the outer molds  3   a ,  3   b  are displaceable relative to each other and with respect to the central mold  3   c  by hydraulic drives within closing frame structures (not shown). Those details are known and are not shown here for that reason. 
     The entire assembly comprising the outer molds  3   a ,  3   b  and the central mold  3   c  can be displaced into and out of the plane of the drawing relative to the extrusion heads  2 . In addition the central mold  3   c  can be displaced with respect to the outer molds  3   a ,  3   b  into and out of the plane of the drawing. 
     For the sake of simplicity that direction of movement into and out of the plane of the drawing is referred to hereinafter as the Z-direction. 
     The direction of the closing and opening movement of the outer molds  3   a ,  3   b  is referred to hereinafter as the X-direction and the extrusion direction as the Y-direction. 
     The production cycle begins in the position shown in  FIG. 1  of the tool  3  beneath the extrusion head  2 . As has already been mentioned hereinbefore, each preform  6  is extruded between a respective outer mold  3   a ,  3   b  and the central mold  3   c . When the preform  6  has reached its full length, as is also shown in  FIG. 1 , the outer molds  3   a ,  3   b  are moved towards each other in the X-direction so that they clamp the preform against the central mold  3   c  (See  FIG. 3 ). 
     As the preforms are extruded continuously, the entire closed tool  3  is then moved away in the Z-direction from under the extrusion heads  2 , principally in order not to impede the discharge of the following extrudate. At the same time or thereafter an increased internal pressure is produced in the mold cavity  7 , for example by introducing compressed air or another suitable gas. (See  FIG. 4 ). As support or alternatively the preforms  6  can be caused to bear against the internal contour of the cavities  4   a ,  4   b  by means of reduced pressure. The respective cavity  4   a ,  4   b  corresponds to the external contour of the product  5  in that region. 
     Provided in the central mold  3   c  are component holders which can be extended for the purposes of positioning built-in fitment components into the product  5 , which holders will not be described in greater detail hereinafter. Built-in fitment components can be fixed to the inside wall of the product  5  with those component holders. When the product  5  is in the form of a fuel tank they can be for example valve mounting means or the like (See  FIGS. 3-6 ). 
     After the preforms  6  have been caused to bear against the inside wall of the respective cavity  4   a ,  4   b  and have been shaped out thereagainst, the outer molds  3   a ,  3   b  are moved away from each other in the X-direction (See  FIG. 7 ) and the central mold  3   c  is moved out in the Z-direction between the outer molds  3   a ,  3   b  so that the condition illustrated in  FIG. 8  is attained. The outer molds  3   a ,  3   b  are then closed against each other in the X-direction so that the edges  8 , of a flange-like configuration, of the respective intermediate product  9  are welded together (See  FIG. 9 ). The outer molds  3   a ,  3   b  are then opened and the product  5  can be removed (See  FIG. 10 ). 
     The process according to the invention now provides that the wall thickness of each preform  6  is varied in accordance with a predetermined program, that is to say a thickness profile is imparted to the respective preform  6  both in the Y-direction and also in the Z-direction. Extrusion of the preforms  6  from the respective extrusion head  2  is effected synchronously, wherein a wall thickness profile is imparted either to one preform or also in relation to the preforms in dependence on time (in relation to the cycle time or extrusion time). In that situation the wall thickness profiles of the preforms  6  are controllable independently of each other so that for example it is possible to obtain an article which for example is thicker at one side than at the other side, without in that respect extrusion of the one side being influenced by the wall thickness variation at the other side. 
     Possible configurations of the nozzle  10  of the extrusion heads  2  are shown in each of  FIGS. 11   a  and  12   a  respectively. They show a section through the nozzle region of the nozzle which is respectively in the form of a wide-slot nozzle. The nozzle  10  has a nozzle body  11  with, in the case of the embodiment shown in  FIGS. 11   a  through  11   c , two tool lips  13  delimiting a nozzle gap  12 . 
     Adjustment of one of the tool lips  13  in the extrusion direction, that is to say in the Y-direction, causes the nozzle gap  12  to be narrowed or enlarged and thus causes a change in the wall thickness of the preform  6  in question.  FIGS. 11   a  and  12   a  each show a section at a location of the nozzle body  11 . The man skilled in the art will appreciate that the nozzle body  11  is adjustable in portion-wise manner in the Z-direction so that portion-wise adjustment of the nozzle gap  12  is possible, thereby providing for a variation in the wall thickness of the preform  6  in the Y-direction and the Z-direction respectively. (See  FIGS. 11   b  and  11   c ). That portion-wise adjustment of the nozzle gap  12  can be implemented for example by the tool lips  13  being of a suitably pliable nature, with control elements which are not shown in  FIGS. 11   a  through  11   c  acting on the tool lips. 
     An alternative configuration of the nozzle body is shown in  FIGS. 12   a  through  12   c , in which a first tool lip  13 ′ is displaceable uniformly over the entire depth of the nozzle gap  12 , that is to say over the entire width of the preform, in the Z-direction, whereas a second tool lip  13 ″ is in the form of a counterpart lip which is oppositely deformable in portion-wise manner in the Z-direction and is thus superimposed on the adjustment of the nozzle gap  12  by the tool lip  13 ′. 
     Various variations in that adjusting mechanism are possible in accordance with the invention. 
     LIST OF REFERENCES 
     
         
           1  extrusion blow molding machine 
           2  extrusion head 
           3  tool 
           3   a ,  3   b  outer molds 
           3   c  central mold 
           4   a ,  4   b  cavity 
           5  product 
           6  preform 
           7  mold cavity 
           8  edges 
           9  intermediate product 
           10  nozzle 
           11  nozzle body 
           12  nozzle gap 
           13  tools 
           13 ′ tool lip 
           13 ″ counterpart lip