Abstract:
A blow head for extruding a tubular plastic preform from an outlet gap between a fixed die core and a die body allows a change of the width of the outlet gap as a function of the circumference using light and small actuating devices, e.g., double-acting actuator cylinders, if the die body has its upper end, facing away from the outlet gap, pivotably mounted in the blow head and a pressure ring is positioned in the outlet gap, on which the actuating devices act.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
   This application claims priority under 35 U.S.C. § 119 to German Application No. 10 2004 028 100.9 filed on Jun. 9, 2004, entitled “Extrusion Blow Head” the entire contents of which are hereby incorporated by reference. 
   FIELD OF THE INVENTION 
   The present invention relates to an extrusion head used in blow molding that produces a tubular preform made of plastic and, in particular, to an extrusion head including an adjustable outlet gap. The extrusion head may include a fixed die core and a die body, wherein a pressure ring and actuators are engaged to change the width of the outlet gap as a function of the circumference. 
   BACKGROUND 
   In the production of a tubular preform, the gap of the outlet between the die core and the die body determines the wall thickness of the hollow body tube that is subsequently manufactured from the tubular preform in a blow mold device. If the hollow body is a technical molded part having a longitudinal axis deviating from a straight line (i.e., the molded part has at least one curve in the longitudinal direction), a preform extruded with a constant wall thickness over its circumference results in the molded part having a greater wall thickness (i.e., material accumulation) in the region of the smaller radius of curvature, as well as a thinner wall thickness in the region of the larger radius of curvature. 
   Typically, however, to ensure a tube of an appropriate strength, the wall thickness may not fall below a minimum wall thickness value at any point of the hollow body. To stay above this minimum value, it is necessary to consume more plastic material in the manufacture of the tube, thereby increasing the weight of the finished hollow body. In addition, in the event of small radii of curvature, the finished hollow body may have wrinkles on the inside and transverse marks on the outside in the region of the smaller radius of curvature as a result of the material accumulation, through which both the technical function (e.g., as an air conduction channel) and the appearance of the article suffer. 
   An extrusion head having die body displaceable by small amounts along a plane perpendicular to the longitudinal axis of the die core, wherein the head is configured to achieve a linear, stretched exit of the preform out of the outlet gap having a constant wall thickness around its circumference (and not a targeted setting of a varied wall thickness of the preform as a function of the circumference) is known from DE-A-21 28 901. For this purpose, the die body has a flange ring, positioned along the side facing away from the outlet gap, configured for transverse displacement between an upper and a lower retaining ring of the blow head. The flange ring is enclosed by a displacement ring, which is provided outside the extrusion blow head with two eyes (offset by 90° from each other) engaged by two pivotable arm levers connected via spindle drives. In this construction, great force is required to transversely displacement the die body in relation to the die core, because the force is introduced at the upper end of the die body, i.e., in a region in which the plastic melt is still under very high pressure (the plastic melt does exit out of the outlet gap practically without pressure, but has a pressure of 400 to 500 bar at the entry point into the blow head in current machines). 
   In addition, an extrusion blow head having a die body displaceable in two orthogonal directions using two eccentrically mounted displacement rings that enclose the die body, as well as levers engaging thereon, but is adapted to achieve a varied wall thickness of the preform as a function of the circumference is known from DE-C-195 37 132. This configuration, however, suffers from the same disadvantage discussed above, namely, it requires great force to displace the die body at a significant distance from the outlet gap. 
   OBJECTS AND SUMMARY 
   An object of the present invention is to provide an extrusion head used in blow molding wherein the change of the width of the outlet gap as a function of the circumference requires significantly less force and, accordingly simpler, smaller, and lighter actuation devices than in the known constructions. 
   This object is achieved according to the present invention by providing a blow head having a die body mounted so it is pivotable within the blow head. In comparison to a translational adjustment of the die body, this has the advantage of requiring significantly less plastic mass that must be displaced. Consequently, smaller actuating forces are necessary and the actuators are simpler, smaller, and lighter. The parts of the blow head which absorb the forces may also be implemented having lower wall thicknesses. The smaller space requirement allows the use of the present invention even in machines having multiple blow heads. 
   The actuating forces are especially small if, according to the preferred embodiment, the die body is mounted so it is pivotable in the blow head at its upper end (i.e., the end facing away from the outlet gap), and the pressure ring is positioned near the outlet gap. 
   The die body preferably has an external surface shaped like a spherical cap for its pivotable mounting, which is received in a complementary bearing shell in the blow head. 
   In addition, the die body may comprise an upper section having an external surface shaped like a spherical cap and a lower section connected to the upper section such that it is removable. Consequently, to refit the blow head for extruding a plastic preform having a different diameter and/or different wall thickness, the complete die body does not have to be replaced, but rather only its lower section. 
   In one embodiment of the invention, the bearing shell in the blow head may comprise a ball socket upper part and a ball socket lower part. Such a configuration is advantageous from the standpoint of manufacturing. 
   The actuators engaging the pressure ring may comprise hydraulic cylinders. Preferably, a minimum of three hydraulic cylinders offset by 120° from one another around the circumference are used. More preferably, four hydraulic cylinders are used, each two of which lie opposite in pairs, the two pairs being offset by 90° from one another. 
   At least two displacement sensors offset by 90° from one another around the circumference are assigned to the die body and used to determine the actual position of the die body. Each of the displacement sensors may comprise a feeler pin spring-loaded in the direction of the pressure ring and displaceable in respective cylinder holders. 
   The distribution and change of the wall thickness in relation to the circumference of the preform is oriented to the geometry of the hollow body subsequently blown full in the blow mold, and therefore typically changes over the length of the extruded preform. For this reason, the actuators are controlled via a computer, which receives the output signals of the displacement sensors and controls the actuators according to a predefined program as a function of the actual value signals of the displacement sensors. 
   The computer is typically a component of a machine controller which, among other things, may change the wall thickness of the preform as a function of its length during its extrusion according to a predefined program, through peripherally-symmetric change of the width of the outlet gap, i.e., by raising or lowering the die body in relation to the conical die core. 
   In an especially advantageous embodiment, the actuators comprise double-acting actuators and transmit pressure and tensile forces to the die body. Therefore, two actuators suffice for pivoting the die body so that the pressure ring is no longer clamped orthogonally. In comparison to the embodiment having four single-acting actuator cylinders, for example, the force necessary for pivoting is reduced by the absolute value transmitted by the diametrically opposite actuator cylinder onto the pressure ring and, in addition, by the orthogonal friction forces. The actuators used, therefore, may be smaller and lighter. Hydraulic cylinders or spindle drives are particularly suitable as actuators. 
   In order to decouple the movements of the actuators as much as possible, the actuators may be suspended in a plane which is orthogonal to the die core axis and which runs through the pivot point of a pivot bearing. This allows the die body to be pivoted in a plane using only one actuator. To avoid constraining forces, the actuators may be mounted in an articulated way on both ends. 
   The use of ball and socket bearings, comprising a spherical segment in a bearing shell shaped like a spherical cap, which are known per se, is especially advantageous. 
   Another embodiment of the invention operates according to the principle of wedge adjustment. In wedge adjustment, the die body is pivoted through the simultaneous insertion of a first wedge on one side, and the anti-parallel removal of a second wedge having a diametrically opposed relationship to the first wedge. Both wedges are guided synchronously, so that the pressure ring is not clamped. 
   In still another embodiment of the invention, the die body may be pivoted using cams which roll off one another. The sliding and frictional movement, which results in material abrasion, is replaced in the wedge adjustment through rolling. 
   The above and still further objects, features and advantages of the present invention will become apparent upon consideration of the following detailed description of specific embodiments thereof, particularly when taken in conjunction with the accompanying drawings wherein like reference numerals in the various figures are utilized to designate like components. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The drawings show an embodiment of a connector in accordance with the invention in a schematic view and in a longitudinal sectional view, wherein like reference numerals in the various figures are utilized to designate like components, and wherein: 
       FIG. 1  illustrates a cross-sectional side view of an extrusion head according to an embodiment of the invention. 
       FIG. 2  illustrates a close-up view of the die body and core of  FIG. 1 , showing the die body pivoted with respect to the core. 
       FIG. 3  illustrates a bottom view of the extrusion head of  FIG. 1 . 
       FIG. 4  illustrates the extrusion head of  FIG. 3 , showing the die body pivoted with respect to the core. 
       FIG. 5  illustrates a cross-sectional bottom view of an extrusion head according to another embodiment of the invention. 
       FIG. 6  illustrates a cross-sectional, side view of the extrusion head of  FIG. 5 . 
       FIG. 7  illustrates a partial cross-sectional view of an extrusion head according to another embodiment of the invention, showing a wedge adjustment. 
       FIG. 8  illustrates a partial cross-sectional view of an extrusion head according to another embodiment of the invention, showing a cam adjustment. 
   

   DETAILED DESCRIPTION 
   An extrusion head device according to an embodiment of the invention is explained with reference to  FIGS. 1-4 . As shown, the device includes a fixed head portion  100  coupled to a core carrier  101  supporting a core holder  1 . The flow of melted plastic (indicated by arrow K) is supplied (at a pressure of several hundred bar) from a typical extruder to the space enclosing the core carrier  101  via an adapter  102  and a channel in the head portion  100 . A guide ring  103  is secured to the bottom of the core carrier  101 . A guide bushing  104 , seated on the guide ring  103 , is connected to a baseplate  105 . The baseplate  105  may be raised and lowered in any conventional manner. For example, guide rods  106  may be used for setting and adjusting the wall thickness of the tubular preform extruded out of the blow head, ideally uniformly around the circumference. 
   The baseplate  105  supports additional parts of the blow mold. Specifically, a housing  7  may be attached to the baseplate  105  via an intermediate ring  6 . The housing  7  may enclose and support a ball socket upper part  8  and a ball socket lower part  9 . The upper and lower ball socket parts  8 ,  9  together form an annular, peripheral bearing shell for a die body  10 . The die body  10  may comprise an upper section  10   a  and a lower section  10   b . The upper section has an external surface shaped like a spherical cap configured to be received in the bearing shell  8 ,  9 . The lower section  10   b , moreover, may be removably connected to the upper section  10   a . The die body  10  may be surrounded about its periphery by a heating strip  11 . The die lower section  10   b  encloses a die core  2 , which is connected to the core holder  1  such that it may be removed. An outlet gap S remains between the die core  2  and the die body lower section  10   b.    
   The width of the gap S (and thus the thickness of a tube wall) can be adjusted independently of the circumference by raising or lowering the baseplate  105  using the rods  106 . In addition, the width of the gap S can be changed as a function of the circumference by pivoting the die body  10  in its bearing shell (as formed by the upper and lower ball socket parts  8 ,  9 ). To pivot the die body  10 , a pressure ring  12  is seated on the lower section  10   b  of the die body  10 , near its lower end and is fastened by a retaining ring  13 . Double-acting actuator cylinders  21  and  22  (actuator cylinder  22  best seen in  FIG. 3 ) engage the pressure ring  12 . The actuator cylinders  21 ,  22  are each supported on a holder  31  and are connected to the ring through pivot bearings  30 . The cylinders  21 ,  22  may be hydraulic, with the pistons of the cylinders  21 ,  22  having fluid applied to them via valves (not shown) controlled by the computer of a machine controller (not shown) to achieve courses of the width of the outlet gap S that change as a function of the circumference. 
   The particular, actual position of the die body  10  is measured using displacement sensors  41  and  42  (best seen in  FIG. 3 ). The displacement sensors  41 ,  42  send signals to the machine controller. The sensors  41 ,  42  and/or the computer compare the actual values to the setpoint values contained in a stored program as a function of the particular extruded length of the preform, and regulate the position of the die body  10  in relation to the die core  2  during the extrusion of the preform out of the gap S by activating the hydraulic valves so that the preform has the predefined course of the wall thickness around the circumference at every point, in addition to the aforementioned, conventional manner of varying wall thickness by raising or lowering the die body  10  using the rods  106 . 
   The cylinder holder  31  may be fastened to the housing  7  using a screw  35 . As indicated above, the actuator cylinder  21  is supported by a pivot bearing  30 , and is located between the cylinder holder  31  and the pressure ring  12 . Each cylinder holder  31  includes a guide pin holder  33  supporting an adjustable guide pin  23 . Each guide pin  23  is secured to the side of the housing  7  such that the pin is diametrically opposed to the cylinder holder  31 . The guide pin  23  may be axially displaced along a recess of the pressure ring  12 . The guide pin  23  and recess structure prevent twisting of the die body  10 , as well as prevent damage to the die body  10  or the die core  2  since it simultaneously forms a stop. With this configuration, the die body  10  can be pivoted in a desired direction and to any desired degree. For example, in  FIG. 2  the die body  10  is shown pivoted to the right. 
   Referring to  FIG. 3 , the double-acting actuator cylinders  21 ,  22  (positioned such that they are offset by 90°) and the guides  23  positioned diametrically opposite each of their respective cylinder are shown. Commercially available displacement sensors  41 ,  42  may be connected to the actuator cylinders  21 ,  22  using an angled support (not shown in  FIG. 3 , referenced as  40  in  FIG. 6 ). Each of the displacement sensors  41 ,  42  may comprise a displaceable feeler  41   a ,  42   a  that presses against a respective feeler pin  51 ,  52  attached to the piston rod via a feeler pin holder  46 . The feeler pin holders  46  are each penetrated by a pin  45  (seen in  FIG. 1 ). The pins  45  are each attached to an actuator cylinder  21 ,  22  and guide the feeler pin holders  46 . With this configuration, the die body  10  may be pivoted in a desired direction and to any desired degree, as illustrated in  FIG. 4 . 
     FIG. 5  shows an embodiment in which the radially oriented actuating forces are produced using small, single-acting actuator cylinders  61 ,  62 ,  63 , and  64  that are offset from each other by 90° around the circumference of the pressure ring  12  such that two diametrically opposed pairs are created. Each actuator cylinder  61 ,  62 ,  63 ,  64  comprises a piston  61   a ,  62   a ,  63   a ,  64   a , respectively, whose piston rod has its free end pressed against the pressure ring  12 . The piston  61   a ,  62   a ,  63   a ,  64   a  is seated in a respective cylinder housing  61   b ,  62   b ,  63   b ,  64   b  fastened (e.g., by a screw) to a respective cylinder holder  71 ,  72 ,  73 ,  74 . Each cylinder holder  71 ,  72 ,  73 ,  74  includes connection holes  71   a ,  72   a ,  73   a ,  74   a , respectively, for applying hydraulic fluid to the pistons  61   a ,  62   a ,  63   a ,  64   a.    
   Referring to  FIG. 6 , the width of the outlet gap S results from a maximum adjustment of the die body  10  in relation to the die core  2  and increases as a function of the circumference, from almost zero to a maximum value and then decreases again to nearly zero. To delimit the maximum adjustment path of the die body  10 , the pressure ring  12  has an annular bead  12   a , which presses against the relevant cylinder (in  FIG. 6 , it presses against cylinder  61 ) at the end of the adjustment path in order to avoid damage to the die body  10  caused by its striking the die core  2 . Displacement sensors (only one displacement sensor  41  is shown in  FIG. 6 ) may also be provided for each cylinder  61 ,  62 ,  63 ,  64 . Preferably, at least two sensors are provided. The displaceable feeler  41   a  illustrated presses against its feeler pin  51 , which is guided into the cylinder holder and whose other end is held against the mantle of the pressure ring  12  using a coiled spring  51   a.    
     FIG. 7  illustrates an extrusion head according to another embodiment of the present invention, showing the pivoting of the die body  10  using wedge adjustments. As shown, two double-acting hydraulic cylinders  221  and  222  (offset from each other by 90°) may generate the radial forces necessary to adjust the die body  10  via its lower section  10   b . These radial forces are transmitted using two rod pairs  203  between which the die body  10  is seated. The force is transmitted according to the wedge principle. Specifically, each rod pair has recesses on its interior shaped punctually symmetric to the axis of symmetry of the die body  10 , so that the interiors of the rods  203  form opposing wedge pairs. In operation, the die body  10  is pivoted through the simultaneous insertion of a first wedge on one side, and the anti-parallel removal of a second wedge having a diametrically opposed relationship to the first wedge. Both wedges are guided synchronously, so that the pressure ring is not clamped. 
     FIG. 8  illustrates an extrusion head according to another embodiment of the present invention, showing a cam adjustment mechanism. As shown, the extrusion head includes four mounted cams  200  set such that two diametrically opposed cam pairs are formed (the cams  200  are positioned such that each is offset by 90°). The cams  200  are rotatably attached to a fixed pressure ring  121  and are configured to roll oppositely in tandem on the die body lower section  10   b . The cams  200  may be generally egg-shaped and are mounted eccentrically. The cam pairs are each twisted by a double-acting hydraulic cylinder  221 ,  222  using one rod pair  202  each. Each rod pair  202  is connected to the housing of one of each of the hydraulic cylinders  221 ,  222  through a spacer  201 . The piston rods of the hydraulic cylinders  221  and  222  are fixed on the pressure ring  12 . 
   While the invention has been described in detail and with reference to specific embodiments thereof, it will be apparent to one skilled in the art that various changes and modifications can be made therein without departing from the spirit and scope thereof. Accordingly, it is intended that the present invention covers the modifications and variations of this invention provided they come within the scope of the appended claims and their equivalents.