Abstract:
The invention relates to an extrusion head for producing a tubular parison ( 22 ) in order to manufacture large-volume, blow molded plastic hollow bodies. The inventive extrusion head has an adjustable ring-shaped tube outlet nozzle for selectively adjusting the nozzle opening ( 20 ) in order to alter the wall thickness of the exiting parison ( 22 ). In order to provide a multiple adjustability of the extruded tube cross section, the invention provides that the extrusion head comprises at least three separate nozzle/mandrel gap adjustment elements (D  0= mandrel, DS I, DS II, DS III) which are differently profiled and exchangeable. The elements can be individually and/or simultaneously brought into working contact with the exiting parison ( 22 ) in the mandrel gap ( 20 ) from inside and/or outside, whereby at least two of the adjustment elements (D  0= mandrel, DS I, DS II, DS III) are configured such that they can be adjusted. To this end, the at least two adjustment elements are each equipped with a corresponding adjusting drive.

Description:
BACKGROUND OF THE INVENTION 
     The invention relates to an extrusion head for producing a tubular parison for the manufacture of blow-molded plastic hollow bodies, including an adjustable ring-shaped tube outlet nozzle with annular nozzle/mandrel gap adjustment elements which permit a targeted adjustment of the nozzle gap for varying the wall thickness of the exiting parison. 
     Typical examples of such blow-molded parts of thermoplastics, in which customarily a targeted wall thickness adjustment is carried out in longitudinal direction of the tube and in circumferential direction of the tube, include large-volume containers such as, for example, canisters, lidded drums, bung drums or inner receptacles for pallet containers. During ejection of the tubular parison used for such blow-molded parts, the wall thickness of the tube is so adjusted by a rotation-symmetrical nozzle/mandrel gap control mechanism in longitudinal direction of the tube that the wall is slightly thinner toward the lower end and slightly thicker toward the upper end in direction of the discharge valve, because the tube as a consequence of its own weight lengthens by itself when suspended to thereby automatically realize an even axial wall thickness. Superimposed thereto is a partial wall thickness control in circumferential direction which is realized by a double nozzle/mandrel gap control mechanism in the tube in those tube zones which are provided in the blow-molded hollow body for the head and bottom areas at greatly varying degrees of elongation—particularly in the areas which are offset by 90° to the partition plane of the blow mold halves. 
     With double nozzle/mandrel gap control mechanisms (=two control choices) known heretofore, the wall thickness of such blow molded parts can be controlled over their length with respect to: 
     uniform radial wall thickness 
     radial and circumferential wall thickness 
     radial wall thickness with axially extending thick and thin areas, and 
     partial longitudinal thick/thin areas. 
     Various constructions of extrusion heads are known for such tube adjustments by means of nozzle/mandrel gap control, for example from German Pat. No. 26 54 001, U.S. Pat. No. 3,114,932 or U.S. Pat. No. 1,107,628. Although for standard applications, simple conventional or the double nozzle control mechanisms suffice, they lack the capacity to carry out additional influences of the wall thickness. A simple extrusion head is further disclosed in British patent specification 1,107,628 for producing teeth or ribs in the extruded tube for bottles blow-molded from thermoplastic material. By means of V-shaped grooves and teeth formed in the ejector die, the blown bottles should receive in particular a decorative look. The central mandrel can be adjusted axially so that the ejected tube may or may not be acted upon by the teeth. A further adjusting option or possible influence to permit a targeted wall thickness adjustment is, however, not possible. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to obviate the mentioned drawbacks of the prior art and to improve the extrusion head of the type as described above with a double nozzle/mandrel gap control so as to permit a further additional superimposed wall thickness adjustment of the ejected tube for large-volume plastic containers such as 220 liters drums and—particularly also for asymmetric technical parts such as e.g. plastic fuel tanks (KKB) or for special applications in large cylindrical blow molded parts. 
     This object is attained in accordance with the invention by providing at least three separate nozzle/mandrel gap adjustment elements (D  0 =mandrel, DS I, DS II, DS III) which are separately controlled, differently profiled, and adjustable, and which individually or/and simultaneously can be brought into active engagement in the nozzle gap with the exiting parison from inside and outside. 
     Since the tube outlet nozzle is equipped with a triple nozzle/mandrel gap control mechanism, special thick areas can be so adjusted at particular locations of the tube depending on the blow molded hollow body in dependence on the configuration of the third nozzle/mandrel gap adjustment element, or further adjustment elements and on the duration of the intervention or impact on the extruded parison, that the final blow molded hollow body exhibits only in the desired wall regions different or thicker wall thickness in comparison to the minimum wall strength of the typical wall. 
     A canister can, for example, be provided with stiffening ribs only at the vertical corner areas. In a KKB, the walls can also be made specifically thicker and thus more stable, e.g. in the side areas of the top bottom or at those areas at which connection stubs of fuel pipes are blow-molded thereon, or measuring instruments for a fuel tank level gauge are to be installed. In bung drums or lidded drums, it is possible to adjust a ribbed wall construction with uniform thick-thin distribution only in the vertical wall areas through a targeted material redistribution while, for example, maintaining the existing drum weight, thereby measurably improving the stiffness of the container, and in particular, the stacking strength when charged with hot contents. This is especially of particular importance in connection with large-volume drums (220 liters). Remarkably, the steps for quality improvement of the blow-molded hollow plastic body are realized exclusively on the exiting parison itself and not through constructive redesign of the blow mold. 
     As shown by way of example only, the novel triple or multi-nozzle/mandrel gap adjustment elements according to the novel invention results advantageously in a variety of new possibilities of application for large-volume blow-molded plastic parts of all types (e.g. automobile accessories or the like). 
     Compared to a conventional extrusion head with heretofore dual nozzle/mandrel gap adjustment elements, the provision of an extrusion head with the novel triple nozzle/mandrel gap adjustment elements according to the invention can be realized comparably inexpensively and does not require any investment in machines. The adjustment elements are moved only over very short distances, and only simple control motors are sufficient, and no deformation of the adjustment elements is experienced. 
     The process according to the invention for producing blow-molded hollow plastic bodies by a blow mold machine with extruder, extrusion head with circumferential distributor and respective blow mold, and carrying out a particular adjustment of the wall thickness of the exiting parison during ejection of the tubular parison from the extrusion head through adjustment of the nozzle/mandrel gap, is characterized in particular by realizing a random, circumferentially varying thick-thin adjustment of the tubular parison through a sequential or simultaneous impact of three differently profiled, separately adjustable nozzle/mandrel gap adjustment elements. This multiple adjusting capability of the tube for large-volume drums is unique and results in a quality of the drums that has been unattainable to date. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention will now be explained and described in more detail by way of exemplified embodiments illustrated only schematically in the drawings, in which: 
     FIG. 1 shows a partially sectional illustration of an extrusion head according to the invention; 
     FIGS. 2 to  6  show the extrusion head according to FIG. 1 with different operational positions of the adjustment elements; 
     FIGS. 7 to  9  show a modified embodiment of an extrusion head with different operational positions of the nozzle/mandrel gap adjustment elements; 
     FIG. 10 shows a further modified embodiment of an extrusion head; 
     FIG. 11 shows a fourth modified embodiment of an extrusion head; 
     FIGS. 12 to  15  show a fifth modified embodiment of an extrusion head with different operational positions of the nozzle/mandrel gap adjustment elements; 
     FIG. 16 shows a lidded drum as blow-molded hollow body; 
     FIG. 17 shows a fassett (canister of upright size) as blow-molded hollow body; 
     FIG. 18 shows a canister (jerrican) as blow-molded hollow body; 
     FIG. 19 shows a tubular parison with cross sections; and 
     FIG. 20 shows a cross sectional illustration of a schematically illustrated plastic fuel tank (KKB). 
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     FIG. 1 shows an extrusion head  10  with fragmentary illustration of three adjustable nozzle/mandrel gap adjustment elements D  0 , DS I and DS II in the basic position “nozzle gap closed”. Arranged centrally in the extrusion head  10  is an axially adjustable mandrel holder  12  with an underside which has secured thereon an easily detachable and exchangeable truncated cone shaped mandrel  14  as first nozzle/mandrel gap adjustment element Dzero (=D  0 ). To the outside, the extrusion head is enclosed by a housing  16 . Accommodated in the housing  16  is a hollow cylindrical storage space  18  in which the molten plastic material fed from one or several extruders has been circumferentially distributed. The storage space  18  ends in a circular ring-shaped nozzle gap  20  which is demarcated on the inside by the mandrel  14 , and the first nozzle/mandrel gap adjustment element D  0  and on the outside by a stationary nozzle ring part DF and two adjustable nozzle/mandrel gap adjustment elements, namely the nozzle slide  1 =DS I and the nozzle slide  2 =DS II. Like the adjustable mandrel  14 , also the axially adjustable adjustment elements DS I and DS II are secured to the extrusion housing for easy detachment and exchange. The axial adjustment and exact positioning of the adjustable nozzle/mandrel gap adjustment elements may be realized e.g. hydraulically or electromotorically. Furthermore, the housing-fixed nozzle ring part DF is likewise secured to the extrusion head in a manner that permits easy detachment and exchange. 
     This is of particular importance in order to realize during a product change or replacement of the subsequent blow mold also a rapid exchange of the ring parts and adjustment elements of the nozzle gap, which are suited to the product at hand and accordingly profiled. 
     In the extrusion head  10  shown in FIG. 1, all nozzle/mandrel gap adjustment elements are returned to their basic position “nozzle gap closed”, i.e. the mandrel D  0  has traveled to the uppermost position, and the nozzle slide=adjustment elements DS I and DS II have traveled to the lowermost position. The respective lengths of path=adjustability of the adjustment elements, are indicated by the respective arrows. In the illustrated exemplified embodiment, the nozzle gap defining surfaces of the fixed nozzle ring part DF and the adjustment element DS I are profiled, and the nozzle gap defining surfaces of the mandrel D  0  and of adjustment element DS II have smooth circumference, as further described with reference to the following Figures. 
     In FIG. 2, only the mandrel  14  is slightly moved downwards (see arrow). The nozzle gap  20  is demarcated at the outside by the lower inner circumferentially smooth edge  24  of the nozzle slide DS II, and at the inside by the mandrel  14 . The discharged tube  22  is evenly thin about its circumference. In the partial section of the tube  22 , as illustrated therebelow in FIG. 2 a , a small arrow indicates the potential thickness of the tube when the mandrel travels to the lowermost position and the nozzle gap is fully open. 
     In FIG. 3, the adjustment element DS I together with the adjustment element DS II is slightly moved upwards (see arrow), so that these two adjustment elements are not in operative engagement at that point with the exiting tubular parison  22 . The nozzle gap  20  is now demarcated by the mandrel  14  and the profiled housing-fixed ring part DF. The tube exiting the nozzle gap is no longer uniformly thick in circumferential direction, but is slightly thinner in two opposing regions than the respective tube regions respectively arranged offset by 90° thereto (FIG. 3 a ). Such a double-oval adjustment of the nozzle gap or oval wall thickness adjustment in areas of the tube is typical for blow molded parts with flat top plate and bottom plate, whereby, the two opposing tube sections  26  with greater wall thickness are so guided between the open blow mold halves that the horizontal container wall areas, which are offset by 90° to the mold partition plane, are blow-molded therefrom with the greatest stretching degrees and blow paths of the plastic material. This feature thus serves as a measure to attain a uniform wall thickness in the finished container, so that the container wall is not thinner in the comer areas with high degrees of stretching or orientation in comparison to the remaining vertical wall parts. FIG. 3 illustrates a sectional view through the thinner tube area (and extrusion head) which bears in the blow mold in the mold partition plane upon the blow mold wall. 
     FIG. 4 shows the same position of the nozzle/mandrel gap adjustment elements compared to FIG. 3, but at a 90° rotated section through the extrusion head and thus through a thicker wall zone  26  of the tube  22 , as clearly illustrated by the partial sectional view through tube  22  therebelow as seen in FIG. 4 a . To the left, next to the profiled ring part DF, there is indicated a developed view of the known wavy profile of the inner nozzle surface of the ring part DF. 
     In FIG.  5  and FIG. 6, the profiled nozzle slide DS I is moved downwards and acts on the exiting plastic tube, while the nozzle slide DS II still remains in its previous upper position outside a zone of influence. The profile of DS I includes a circumferential tooth profile, as clearly shown therebelow in FIG. 5 a  by the partial sectional illustrations of the exiting tube  22 . The teeth  30  of the profile form a thinner tube wall thickness, and a greater tube wall thickness with outwardly projecting ribs is formed in the tooth gaps  32 . FIG. 5 shows the section through the extrusion head and the tube  22  in the area of a thinner tube wall (tooth action  30 ), and FIG. 6 shows a slightly offset section in the area of a thicker tube wall (tooth gap action  32 ), with pronounced formation of longitudinal ribs  28  (FIG. 6 a ). The profile in the fixed nozzle ring part DF, as well as in the nozzle slide DS may be expanded slantingly outwardly or simply parallel to the surface of the mandrel  14 . FIG. 6 indicates by way of the small arrow below the nozzle guide DS II, the depth by which the teeth  30  of the tooth profile of the nozzle slide DS I are able to act on the tube  22  from outside. To the left of the nozzle slide DS I is a schematic illustration, in developed view, of the tooth profile of the DS I with teeth  30  and tooth gaps  32 . In this embodiment, the lowermost non-profiled nozzle slide DS II, serves actually only as a smoothening element or to cover the profiles of the ring part DF or/and to cover the nozzle slide DS I. 
     FIG. 7, FIG.  8  and FIG. 9 illustrate a modified embodiment of an extrusion head according to the invention, with the special profile (=tooth profile) formed on the nozzle slide DS II instead of on the nozzle slide DS I, while the nozzle slide DS I has a smooth circumferential surface in direction of the nozzle gap. In FIG. 7, both nozzle slides DS I and DS II are moved into their uppermost position (outside of zone of influence), and only the oval-profile of the housing-fixed ring part DF and the adjustable mandrel  14  act on the tube  22  in the nozzle gap  20 . The small partial sectional views  7   a  and  7   b  on the side indicate on the left ( 7   a ) a thinner tube wall thickness (in mold partition plane FT) and on the right ( 7   b ) a thicker tube wall thickness (90° with respect to the mold partition plane FT). In FIG. 8, both nozzle slides DS I and DS II are moved downwards. The DS I has a lower smooth circumference and covers the oval-profile of the fixed ring part DF, thereby rendering it ineffective. The tooth profile of the lower nozzle slide DS II acts in the nozzle gap  20  on the exiting tube  22  and produces the profiled tube formation with the longitudinal ribs  28 , as illustrated in the small partial sectional illustration in FIG. 8 a.    
     FIG. 9 shows a positioning of the adjustment elements in which no profile is effective, but rather only a circumferentially even change of the wall thickness can be carried out by an axial displacement of the mandrel  14 . In accordance with the wall thickness diagrams FIGS. 9 a, b , and  c , which illustrate for each adjustment element a separate control program, as shown in the lower left hand side of FIG. 9, the control of adjustment elements D  0 , DS I and DS II for adjusting a desired wall thickness over the length L of the exiting tube is effected at the blow form machine. Diagram  9   a  shows—as does the partial sectional view FIG. 9 f  to the right of the ejected tube  22 —an even increase of the wall thickness from bottom to top, by opening the nozzle gap  20  through axial displacement of mandrel  14  in downward direction. The added wall thickness for the top plate and bottom plate of the drum, as shown in diagram  9   b , is realized by the oval profile in the fixed nozzle ring part DF which is cleared when the non-profiled and smooth nozzle slide DS I travels upward. At that point, the tooth profile nozzle slide DS I is hereby not effective. For adjustment of the tooth profile in the tube—e.g. for longitudinal ribs in the vertical wall region of the bung drum as shown to the right thereof in FIG. 9 d  and  e —the nozzle slide DS II is moved downwards for active engagement, whereby also the nozzle slide DS I moves downwards, thereby covering again the profile of the fixed ring part DF. In order to maintain an even thickness of the tube, the mandrel  14  is also moved simultaneously slightly downwards, and the nozzle gap opened as needed. For clarification, it should be noted that the nozzle slide with the novel tooth profile of the nozzle slide realizes in general only a redistribution of the plastic material in the nozzle gap, whereby the free cross sectional area of the nozzle gap may remain constant. 
     Another embodiment of an extrusion head according to the invention is shown in FIG. 10, whereby no profiled housing-fixed ring part is provided and the nozzle slide DS I is also not profiled but exhibits a circumferentially smooth inner surface towards the nozzle gap  20 . The known oval profile is formed in the nozzle slide DS II, with an exchangeable mandrel  34  having a special configuration. At its outer circumference, the mandrel  34  is provided in direction of the nozzle gap  20  with a plurality of ribs  14 *, which are evenly spaced from one another in a star-shaped manner, and interposed grooves. Disposed at the lower end of the mandrel head  34  is the axially adjustable adjustment element D  0 * which is provided exteriorly with correspondingly formed ribs extending upwardly and uniformly spaced from each other for precisely filling in flush engagement the grooves and intermediate spaces between the ribs  14 *. When the star-shaped slide=adjustment element D  0 * occupies its uppermost basic position, the ribs of star-shaped slide D  0 * extend in the nozzle gap  20  flush with the ribs  14 * of the mandrel  34 , so that the interlocking ribs of the mandrel head  34  form a smooth conical surface in the nozzle gap. In contrast thereto, when the star-shaped slide D  0 * travels downwards, the intermediate spaces between the ribs and the grooves of the mandrel head  34  are cleared and the stationary ribs  14 * act in the nozzle gap on the exiting tube  22 , thereby forming a tooth profile in the inner surface of the tube in dependence on the geometry of the ribs  14 *. An up and down movement of the smooth non-profiled nozzle slide DS I allows, in general, an even adjustment of the wall thickness, while the oval profile of the tube is effected by the respectively profiled nozzle slide DS II, whereby the nozzle slide DS II travels into a lower position, with the minimum wall thickness of the tube being adjusted by the nozzle slide DS I. Then, DS II remains in position, and DS I travels slightly upwards, whereby the oval profile in DS II is cleared, and the tube is formed with respectively thicker walls in the two diametrically opposing zones. This construction of the extrusion head permits a best possible superimposed action of the various adjustment elements upon the tube. 
     A further embodiment is shown in FIG. 11, illustrating a modification of the extrusion head shown in FIG. 6, with a ring sleeve as nozzle slide DS III being secured inside the nozzle head DS II for adjustment in circumferential direction. This ring sleeve=DS III has at its bottom side a same tooth profile as the nozzle slide DS I disposed on the inside; see FIG. 11 a . Additionally, the nozzle slide DS II is slightly slanted inwardly at the bottom. When downwardly moving the nozzle slide II together with ring sleeve DS III so as to be flush with the nozzle slide I, a rotation of the ring sleeve enables a covering or opening of the teeth of the nozzle slide DS I from the side. In this way, the formation of the rib width on the exiting tube  22  can be continuously modified or randomly adjusted. A simultaneous up and down movement of the nozzle slide DS II together with the ring sleeve DS II also allows the height of the ribs  28  (FIG. 11 b ) to be continuously modified and adjusted. 
     Finally, FIG. 12, FIG. 13, FIG.  14  and FIG. 15 show a further advantageously configured extrusion head. The even wall thickness about the circumference is adjusted with the adjustment elements D  0 =mandrel  14 . A profiled housing-fixed ring sleeve is not provided; however, if needed, may be provided for installing a further special profile. The nozzle slide DS I has the tooth profile and is provided with a greater stroke of its adjustment drive. Furthermore, the nozzle slide DS II is provided with the oval profile and of split configuration i.e. it is made in this case of two 180° half ring parts which are guided and secured on the nozzle slide DS I for displacement in radial direction. In FIG. 12, all adjustment elements have traveled into the basic position “nozzle slide closed”. In FIG. 13, the nozzle gap is opened, with mandrel  14  having traveled downwards. Likewise, the nozzle slide DS I together with DS II have traveled axially downwards, with the attached nozzle slide DS II acting with its oval profile in the nozzle gap  20  on the exiting tube. In FIG. 14, both half ring parts of the nozzle slide DS II have traveled radially outwards and are no longer in an active engagement. As shown in FIG. 15, the nozzle slide DS I is now slightly moved further downwards until its tooth profile or special profile is in active engagement. The advantage of this construction is the need for only a single lifting drive for the nozzle slides DS I and DS II. The short radial movement of the DS II—half rings could be realized, for example, by an electric spindle motor or a respective small drive. This configuration is suited e.g. particularly for retrofitting existing extrusion heads. 
     The adjustment elements of all embodiments are basically attached to the extrusion head in an easily exchangeable manner. The nozzle slide with the tooth profile may certainly also be modified depending on the desired profile requirement of the hollow body to be blow-molded (cf. FIGS. 19,  20 ). 
     FIG. 16 illustrates a 220 liter lidded drum  36  having longitudinal ribs which are formed in the vertical wall zones and evenly spaced from one another but which do not extend into the bottom region, as shown in the sketched partial sectional illustration below in FIG. 16 a  with constant drum wall thickness. 
     Exemplified embodiment: A 220 l plastic drum (e.g. Vanguard lidded drum) with a drum body weight of approximately 7.5 kg has a largest diameter in the drum wall or foot ring of about 575 mm. The swelling parison exiting the nozzle gap has a diameter of about 270 mm with a wall thickness of about 12 to 25 mm. By means of the adjustment elements, the wall thickness of the tube can be adjusted partially or along sections in this thickness range or adjusted at even greater range. Hereby, the particular adjustment element D  0 * is advantageously provided with a rectangular tooth profile, whereby the diameter of the adjustment element D  0 * amounts to about 190 mm, and the inner and outer ring edges, interacting with the ejected tube, have alternately about 60 grooves which are half-round as viewed in cross section, and a complementary number of rectangular teeth, with the width of the grooves being about 4 mm and the width of the teeth about 5 mm. The radial depth of the grooves is thereby about 10 mm. The configuration of the rectangular teeth is hereby of importance; a toothed engagement with pointed saw teeth is unsuitable for large-volume containers according to the invention with a diameter of about 585 mm and a weight of about 8 to 10 kg plastic material, because the extension of the thinner areas in circumferential direction of the drum body must be wider in any case than the thicker areas (=internal ribs) in the axial drum wall of the finished blow-molded hollow body. 
     FIG. 17 shows a fassett  38  in which only the vertical wall zones are reinforced by longitudinal ribs. This longitudinal ribbing is clearly illustrated in the partial sectional illustration in FIG. 17 a  through the container wall. 
     FIG. 18 shows a canister  40  formed only in the corner regions with two longitudinal ribs which extend, however, to the top plate and bottom plate, as indicated in the partial sectional view below in FIG. 18 a . A longitudinal and cross-sectional view of a tubular parison  22  with special profile for a plastic fuel tank KKB  42  is shown in FIG. 19, with FIG. 20 showing flash pieces still attached thereto. 
     The extrusion head according to the invention with three separate adjustment systems is particularly suitable to fabricate and produce such special profiles with partial material accumulation (cf. section A—A, B—B and C—C in FIG. 19 a, b, c ) as required for KKB  42  in the area of a stub opening (cf. section D—D in FIG. 20 as shown in FIG. 20 a ).