METHOD AND APPARATUS FOR BLOW MOULDING

Abstract: The invention relates to a method and apparatus for blow moulding. In the blow moulding apparatus, an extruder (1) feeds material into a connecting channel (9) to form an annular parison. The annular parison is fed into a mould and air is blown into the parison. In the connecting channel (9) there is a regulating cavity (10) with a variable volume, the volume being regulated by moving a regulating piston (11) of the apparatus back and forth. When increasing the volume of the regulating cavity (10), more material is left in said regulating cavity (10), the exit flow decreases and thus the wall of an article becomes slightly thinner. When contracting the regulating cavity (10), more material is extruded from the apparatus, and the wall of the article to be manufactured becomes thicker.

Description: METHOD AND APPARATUS FOR BLOW MOULDING The invention relates to a method for blow moulding, in which method an annular parison is formed, the annular parison is fed into a mould and air is blown into the parison. The invention further relates to a blow moulding apparatus comprising an extruder to form an annular parison, means for feeding the annular parison into a mould and means for blowing air into the parison. In the blow moulding process, hollow articles, such as bottles, are manufactured. In blow moulding, an annular parison is fed into a mould and air is blown into the parison in order to produce an article whose outer surface is shaped by the mould. The wall thickness of the article is determined by the width of a die opening in the blow moulding apparatus. If the wall thickness of the article needs to be varied on different sections of the article, a cone- shaped object is arranged in connection with the die, which object is moved back and forth to change the width of the flow outlet of the die. The regulation of the width of the die opening affects the counterpressure of the material in the apparatus, whereby the optimal operation of the apparatus is disturbed by pressure fluctuation. Said cone-shaped object cannot be applied in connection with a curved die, whereby in case of holding the mould in the vertical position preferable for the filling of the mould, the extruder of the apparatus must also be held in the vertical position, and the treatment, disassembly and cleaning of the extruder become fairly difficult. The object of the present invention is to provide a method and an apparatus to avoid the above disadvantages. The method of the invention is characterized by feeding material into a connecting channel to form the annular parison and there being a regulating cavity in the connecting channel, the volume of which regulating cavity is regulated in such a manner that when increasing the volume of the regulating cavity, more material is left in the regulating cavity, whereby exit flow decreases, and when decreasing the volume of the regulating cavity, more material is extruded from the apparatus, and the exit flow increases. The apparatus of the invention is further characterized in that the extruder includes a connecting channel, to which material is arranged to be fed by means of feeding means and in which connecting channel there is a regulating cavity, whereby the extruder is provided with a regulating piston ar- ranged to move in a reciprocating manner to regulate the volume of the regulating cavity in such a manner that when moving the regulating piston away from the regulating cavity, the volume of the regulating cavity increases and more material is left in the regulating cavity and thus the exit flow decreases, and when pushing the regulating piston towards the regulating cavity, the volume of the regulating cavity decreases and thus more material is extruded from the apparatus. The essential idea of the invention is that a connecting channel of the blow moulding apparatus, to which channel the extruder of the apparatus feeds material, is provided with a regulating cavity with a variable volume, which volume is regulated by moving the regulating piston of the apparatus back and forth. When the volume of the regulating cavity is increased during the blow moulding process, more material is left in said regulating cavity and the exit flow decreases. When pushing the regulating piston towards the regulating cavity to contract the regulating cavity, the piston concurrently extrudes more material out of the apparatus, and thus the exit flow increases. When the piston is substantially stationary, the exit flow equals the average flow determined by the die opening. The idea of a preferred embodiment is that the control means of the regulating piston have been integrated into the structure of the blow moulding apparatus. The idea of another preferred embodiment is that the die of the apparatus is arranged as curved. The idea of a third preferred embodiment is that the regulating piston is rotated during the process. The invention provides the advantage that the amount of exit flow and preferably the thickness profile of the wall of an article to be manufactured can be regulated quickly, simply and efficiently, and in such a manner that pressure fluctuations are not produced in the extruder of the apparatus. By integrating the control means of the regulating piston into the structure of the extruder, a durable and compact structure is accomplished. The invention also allows the use of a curved die in the apparatus, whereby the mould can be in the vertical position and yet the extruder in the horizontal position, whereby the treatment, disassembly and cleaning of the extruder are easily implemented. By rotating the regulating piston, the circumferential orientation of the material can be improved at the final stage of the extrusion, and the radial width of the article to be blow-moulded can also be regulated. The invention is described in greater detail in the attached drawing, in which Figure 1 shows a schematic, cross-sectional side view of an apparatus of the invention and Figures 2a and 2b show schematic, cross-sectional frontal views of alternative solutions for a regulating piston to be used in the apparatus of Figure 1. Figure 1 shows a cross-sectional side view of the blow moulding apparatus of the invention. The apparatus comprises an extruder 1 and a die 2 connected to the end of the extruder. By means of the apparatus the material is fed as an annular parison into a mould and air is blown into it in a manner known per se to form the walls of the article into a shape determined by the mould. In Figure 1 , the mould is shown schematically by broken lines. The extruder 1 comprises an outer stator 3 and an inner stator 4, and as adapted between them, there are an outer rotor 5 and on its inner side, an inner rotor 6. Between the outer rotor 5 and the inner rotor 6 there may also be a central stator 7. Helical grooves are adapted to the rotors and/or the stators, and when the rotors 5 and 6 are rotating, they feed the material fed between them further forwards. For the sake of clarity, means for feeding material between the rotors and the stators and rotating means of the rotors are not shown in the attached figure. Most preferably, the rotors 5 and 6 and the stators 3, 4 and 7 are formed as cone-shaped at least on their facing surfaces in such a manner that feed gaps that have a shape of a convergent cone are formed between the rotors and the stators. Thus, the structure of the apparatus be- comes relatively short and steady without any separate support means of the stators. On the inner side of the inner stator 4, a mandrel 8 is also arranged. The material flows from the feed gaps shaped as convergent cones to a connecting channel 9. The end section of the connecting channel 9 forms a regulating cavity 10. Outside the mandrel 8, a regulating piston 11 is arranged. By moving the regulating piston 11 in the axial direction of the extruder 1 , the volume of the regulating cavity 10 can be regulated. The wall thickness of the article to be manufactured is mainly determined by the size of a die opening 12 of the die 2. By moving the regulating piston 11 in Figure 1 to the right, the volume of the regulating cavity 10 can be increased. Thus, material to be extruded by the extruder 1 accumulates in the regulating cavity, and at the same time the amount of material flowing out de- creases, and preferably by suitably regulating the simultaneously supplied air flow, which makes the wall of the article to be extruded thinner. When the regulating piston 11 in Figure 1 is moved to the left, the regulating cavity 10 contracts and at the same time the regulating piston 11 extrudes the material in the regulating cavity 10 out of the apparatus, whereby the amount of material flowing out increases and preferably by suitably regulating the air flow that is supplied simultaneously, the wall of the article to be manufactured thus becomes temporarily thicker. The regulating piston 11 can be made to move very fast, whereby the thickness profile of the wall of the article to be manufactured can be regulated very quickly during the process. In the manufacturing process, air is blown into the article through an air passage 13 in a manner known per se. For the sake of clarity, any other means for supplying air are not shown in the attached figure, except for the air passage 13. The die 2 can be made to have a curved shape so as to change the direction of the material flow. Most preferably, the direction of the material flow is turned by 90°, whereby the mould can be in the vertical direction and its filling direction is from the top to the bottom, and the extruder 1 for its part can be located in the horizontal direction. The regulating piston 11 can be hydraulically moved back and forth. Thus, when the regulating piston 11 needs to be moved backwards, i.e. to the right in Figure 1 , pressure fluid is supplied through a pressure fluid channel 14 to the first pressure fluid chamber 15. At the same time pressure fluid is exhausted through the second pressure fluid channel 16 from the second pressure fluid chamber 17. Correspondingly, when the regulating piston 11 needs to be moved forwards, i.e. to the left in Figure 1 , pressure fluid flows are reversed in the opposite direction. Means for supplying and controlling pressure fluids are obvious for a person skilled in the art, and for the sake of clarity they are not shown in the attached figure. By integrating the control means of the regulating piston 11 in the manner shown in Figure 1 , a durable and compact structure is created for the inner stator 4 in the structure of the blow moulding apparatus. Rotating means 18 are adapted to the extruder 1 to rotate the regulating piston 11. The rotating means 18 may comprise e.g. an electric motor rotating a cogwheel by means of an axle, whereby notches fitted to the cogwheel are adapted to the regulating piston 11 , and thus when the cogwheel is rotating, it thus rotates the regulating piston 11. Further, any other solution known per se can be used as the rotating means 18 of the regulating piston 11. By rotating the regulating piston 11 , the circumferential orientation of the material to be extruded can be efficiently increased, because the effect of the orientation on the material is realised at the final stage of the extrusion. In the cone-shaped extruder 1 , the centricity of all its stators 3, 4 and 7 and thus the thickness symmetry of the wall of the material to be extruded can be thermally regulated very easily. Several, at least three heating elements 19, e.g. electric resistors, are adapted to each stator 3, 4 and 7 as equally distributed around the circumference of a concentric circle. The heat- ing element 19 is capable of regulating the temperature of each stator section and thus of regulating the shape of the stator by utilizing heat expansion of a material. This way the centricity of the stators can be regulated in a very efficient and precise manner. In Figures 2a and 2b, the regulating piston 11 has an asymmetrical cross-section. In Figure 2a, by rotating the regulating piston 11 the location can be moved where the regulating cavity 10 is at its largest, i.e. the wall thickness variation of the article can thus be regulated in the direction of the circumference. By means of the regulating piston 11 of Figure 2b, the walls can be made in some places thinner, which thin structures form in the thicker structures of the regulating piston 11. Naturally, a regulating piston having a different shape has also a different kind of effect on the thickness profile when the regulating piston 11 is moved back and forth. A regulating piston 11 can also be made as replaceable, whereby a different kind of regulating piston 11 is used when manufacturing different articles. The drawing and the description thereof are only intended to illustrate the inventive idea. The details of the invention may vary within the scope of the claims. The regulating piston 11 can thus be adapted to the extruder 1 by means of threads, whereby the regulating piston 11 is moved back and forth by rotating, i.e. a back and forth movement and rotating can be imple- mented by one means. Further, the direction of rotation of the regulating piston 11 can be constantly changed, i.e. the regulating piston 11 may rotate in an oscillating manner. The oscillating distance may be suitably chosen and it can also be clearly shorter than a full round.

Claims: CLAIMS 1. A method for blow moulding, in which method an annular parison is formed, the annular parison is fed into a mould and air is blown into the parison, characterized by feeding material into a connecting channel (9) to form the annular parison and there being a regulating cavity (10) in the connecting channel (9), the volume of which regulating cavity (10) is regulated in such a manner that when increasing the volume of the regulating cavity (10), more material is left in the regulating cavity (10), whereby exit flow decreases, and when decreasing the volume of the regulating cavity (10), more material is extruded from the apparatus, and the exit flow increases. 2. A method as claimed in claim 1, characterized by regulating the volume of the regulating cavity (10) by means of a regulating piston (11 ) by moving the regulating piston (11 ) back and forth in such a manner that when decreasing the volume of the regulating cavity (10), the regulating piston (11) is pushed towards the regulating cavity (10), and when increasing the volume of the regulating cavity (10), the regulating piston (11) is moved away from the regulating cavity (10). 3. A method as claimed in claim 1 or 2, characterized by leading the annular parison through a curved die (2) in such a manner that the direction of material flow changes. 4. A method as claimed in claim 3, c h a r a c t e r i z e d by the direction of material flow changing about 90° in the die (2). 5. A method as claimed in any one of the claims 2 to 4, characterized by rotating the regulating piston (11) during the blow moulding process. 6. A method as claimed in claim 5, characterized by the regulating piston (11) having an asymmetrical cross-section, and when rotating the regulating piston (11), the wall thickness variation of an article is regulated in the direction of the circumference. 7. A method as claimed in any one of the preceding claims, characterized by the means feeding material into the connecting channel (9) being substantially cone-shaped. 8. A blow moulding apparatus comprising an extruder (1) to form an annular parison, means for feeding the annular parison into a mould and means (13) for blowing air into the parison characterized in that the extruder (1) includes a connecting channel (9), to which material is arranged to be fed by means of feeding means and in which connecting channel (9) there is a regulating cavity (10), whereby the extruder (1) is provided with a regulating piston (11) arranged to move in a reciprocating manner to regulate the vol- ume of the regulating cavity (10) in such a manner that when moving the regulating piston (11) away from the regulating cavity (10), the volume of the regulating cavity (10) increases and more material is left in the regulating cavity (10) and thus the exit flow decreases, and when pushing the regulating piston (11) towards the regulating cavity (10), the volume of the regulating cavity (10) decreases and thus more material is extruded from the apparatus. 9. An apparatus as claimed in claim 8, characterized in that the means for moving the regulating piston (11 ) are integrated into the structure of the blow moulding apparatus. 10. An apparatus as claimed in claim 8 or 9, characterized in that the apparatus comprises a curved die (2) to turn the direction of material flow. 11. An apparatus as claimed in claim 10, characterized in that the die (2) is arranged to turn the direction of material flow about 90°. 12. An apparatus as claimed in claim 10 or 11, character- i z e d in that the extruder is arranged substantially in the horizontal direction. 13. An apparatus as claimed in any one of the claims 8 to 12, characterized in that the apparatus comprises rotating means (18) to rotate the regulating piston (11). 14. An apparatus as claimed in any one of the claims 8 to 13, characterized in that the regulating piston (11 ) has an asymmetrical cross-section. 15. An apparatus as claimed in any one of the claims 8 to 14, characterized in that the means of the extruder (1 ) for feeding material into the connecting channel (9) are substantially cone-shaped.

Inventors: KIRJAVAINEN KARI, KIRJAVAINEN, KARI
Applicants: KIRJAVAINEN KARI, NEXTROM HOLDING SA, KIRJAVAINEN, KARI, NEXTROM HOLDING S.A.
Application year: 1999
Main classification: B29C47
IPC codes: B29C47, B29C49