METHOD AND APPARATUS FOR MOULDING PLASTIC

Abstract: A method and an apparatus for moulding plastic, and a plastic product produced with the method. The plastic material to be moulded is supplied to an injection space (7), and the apparatus comprises means for compressing the plastic material into a shape determined by the mould (16). The material is supplied to the injection space (7) by means of at least one conical inner stator (1) and at least one conical rotor (2) provided rotatably outside the stator. The injection press can thus be provided with a short and simple structure. When the material is supplied via a substantially tubular injection space (7), it is possible to provide a fibre-oriented plastic product produced by means of a mould.

Description: Method and apparatus for moulding plastic The invention relates to a method for moulding plastic, in which method plastic material is supplied to an injection space with injection means comprising at least one conical stator and at least one conical rotor, the rotor being rotated in such a way that as the rotor rotates, it supplies plastic material to be moulded, so that the plastic material passes through the injection space and is compressed into a shape determined by a mould. The invention also relates to an apparatus for moulding plastic, the apparatus comprising an injection space, at least one conical stator and at least one conical rotatable rotor, the stator and the rotor forming means for supplying plastic material via the injection space, and means for compressing the plastic material into a shape determined by a mould. The invention further relates to a plastic product produced with the method according to the invention. DE 3,814,571 discloses a piston engine injection press. This injection press comprises a plasticizing screw in which the material to be moulded is plasticized and with which it is supplied to a container via a valve. The container comprises an associated piston which can be moved with an actuator, so that when the container contains a sufficient amount of the material to be moulded, the material is moulded by means of the piston. The apparatus is cumbersome due to its deconcentrated structure. It is also difficult to adjust the apparatus, which is therefore not suitable for producing complicated technical products. The mass to be injected is also rather non-homogenous . WO 85/03027 discloses an injection press wherein a screw is placed inside a cylindrical body. The material to be melted is supplied with a feed worm to a feed hopper and with the feed hopper to the space between the screw and the cylindrical body. The screw can be rotated, whereupon the screw thread transports the material to be melted forward and extracts new raw material from the hopper. Further, heating resistors are positioned around the body, so that when the screw transports the material forward, the raw material melts by the action of the heating resistors and the heat resulting from friction. At the same time as the screw rotates it also moves backwards, whereupon molten mass fills the space in front of the screw head. When a sufficient amount of material has been transported to the front of the screw, the rotation of the screw iε terminated and the screw is pushed forward, whereupon the molten raw material flows via an injection aperture into the mould. This equipment is rather large and therefore expensive due to the length of the screw. Further, the batches of material injected at a time are rather small. Also, the orientation in the material of the product to be injected cannot be determined in any way. WO 93/19923 discloses an injection press comprising a cylindrical body with a cylindrical screw movable axially and rotatably in it. The material to be supplied is fed with a feed hopper into the exterior of the screw. The screw, and partly the body, comprises threads with which the material to be supplied can be moved towards the front of the injection press. The interior of the screw comprises a core enabling the adjustment of the volume of the container to which the material to be injected is conducted. When the container contains a sufficient amount of the material to be injected, the material is pushed through a nozzle into the mould by means of the screw or a combination of the screw and the core. The apparatus is complicated and therefore difficult and expensive to manufacture. Further, the length of the screw and the core increases the manufacturing costs of the apparatus. Also, the orientation of the material of the product to be moulded cannot be controlled in any way with this apparatus. US 4,519,976 and EP 0,619,172 disclose an injection press comprising a conical rotor outside which there is a conical stator, so that when the rotor rotates, it supplies material to be moulded to the injection space. The conical structure does provide the injection press with a short and simple construction, but in the injection-moulding apparatus according to the aforementioned references it is very difficult to arrange the heating and/or cooling functions of the material to take place from the inside of the cone formed by the supply means, i.e. through the rotating rotor. Also, it is difficult to provide high moulding pressures in the feed opening between the rotor and the stator described in the references. Further, it is not possible to manufacture oriented products with the structure according to the EP reference, and the arrangement according to the US reference requires the use of complicated and cumbersome mould tools. The purpose of the present invention is to provide a method and an apparatus that do not comprise the aforementioned drawbacks. A further purpose of the invention is to provide a fibre-oriented plastic product produced with a mould. The method according to the invention is characterized in that the rotor is placed outside at least one stator. Further, the apparatus according to the invention is characterized in that at least one rotor is placed on the outside of at least one stator. Further, the plastic product prepared with the method according to the invention is characterized in that the material of the plastic product is substantially oriented. The essential idea of the invention is that the means for supplying material to the injection space include a conical rotor and a conical stator that are placed in such a way that the rotatable rotor is positioned outside the stator. The idea of an embodiment is that there is also another stator provided outside the rotatable rotor, so that the material to be moulded flows both inside and outside the rotor. The idea of a preferred embodiment is that a piston with which the material supplied to the injection space is ejected into the mould is positioned in the centre of the apparatus. The idea of another preferred embodiment is that a mandrel is placed inside the piston, whereupon the material to be fed from between the rotor and the stators is supplied to a substantially tubular space between the mandrel and the frame, and outside the mandrel there is a cylindrical piston with which the material to be pressed is moulded. The idea of a third preferred embodiment is that in the middle of the means for supplying the material there are blow means for compressing the material by means of compressed air into the shape determined by the mould. The invention has the advantage that when the material is supplied with conical stators and a rotor, the injection press can be provided with a short and simple structure. When the rotor is placed outside the stator, the means for controlling the temperature of the material to be moulded can be placed easily in the middle of the injection press to warm the material to be moulded from the inner stator. Also, it is easy to place the supply means for supplying the material to be moulded in the inner stator, so that the injection press will have a simple and compact structure. When another stator is provided outside the rotor, the material to be moulded can be made to flow both inside and outside the rotor, so that the rotor in a way floats inside the mass to be moulded, and high moulding pressures can be generated advantageously. Further, by positioning the mandrel inside the piston the orientation of the material to be fed, oriented by means of the stators and the rotors, can be maintained when the material is inserted into the substantially tubular space between the mandrel and the frame. Further, the orientation can be maintained by moulding the product with a substantially tubular piston, thus producing fibre- oriented injection-moulded products. The invention will be described in greater detail in the accompanying drawings, in which Figure 1 is a schematic side view of the cross- section of an apparatus according to the invention, Figure 2 is a schematic side view of the cross- section of a second apparatus according to the invention, Figure 3 is a schematic side view of the cross- section of a third apparatus according to the invention, Figure 4 is a schematic side view of the cross- section of a fourth apparatus according to the invention, and Figure 5 is a schematic side view of the cross- section of a fifth apparatus according to the invention. Figure 1 shows an apparatus consisting of a conical stator and a conical rotor 2 provided rotatably outside the stator. The rotor 2 is rotated with a motor 4. The speed of rotation of the cone can be adjusted with a gear system 3. The speed of rotation of the rotor 2 can also be adjusted without the gear system 3, for example by using an electric motor with an adjustable rotational frequency as the motor 4. Further, it is not necessary to adjust the rotational frequency of the rotor 2, but it can be set to a suitable level. The motor 4 may also be for example a hydraulic motor or some other known apparatus suitable for the purpose. The material to be injected is supplied via a supply conduit 5 to the space between the rotor 2 and the stator 1. The material to be supplied is fed into the supply conduit 5 with a feeder 6. The supply conduit 5 can be easily positioned through the stationary stator 1, so that the structure of the apparatus is simple and compact. The surfaces of the rotor 2 and/or the stator 1 positioned against one another comprise screw threads, which are not shown in the accompanying figure for the sake of clarity. By means of these screw threads the rotor 2 supplies, as it rotates, material to be injected to an injection space 7. The injection space 7 is cylindrical. When the injection space 7 contains a sufficient amount of the material to be injected, a cylindrical piston 8 placed in the middle of the injection press ejects the material through a nozzle 10. The piston 8 is operated with an actuator, such as a hydraulic motor 9, shown schematically in Figure 1. Instead of the hydraulic motor 9, the actuator may also be for example a spindle motor or some other construction known per se. A mould to which the material to be injected is directed is placed in association with the nozzle 10. This mould is not shown in the accompanying figure for the sake of clarity. It is very easy and simple to provide the stator 1 with means 11 for controlling the temperature of the material to be injected, the means being used to effectively heat and/or cool the material to be injected from the inside. Figure 2 shows another apparatus according to the invention. The reference numerals in Figure 2 correspond to those in Figure 1. Outside the rotor 2 there is a conical outer stator 12. The material to be injected is then supplied to the injection space 7 both inside and outside the rotor 2. The rotor 2 thus floats in the material to be moulded, so that it is possible to generate high moulding pressures in a rather simple manner. The material to be injected is supplied via supply conduits 5 to the interior and the exterior of the rotor 2. Via the first supply conduit 5, the material to be injected can be supplied to the space between the rotor 2 and the outer stator 12. The material to be supplied is fed into the first supply conduit with a first feeder 6, which may be for example a feed worm, a pump or some other feeder known per se. Via the second supply conduit 5, the material to be moulded is supplied to the space between the rotor 2 and the inner stator 1 through openings 13 provided in the rotor. The material to be moulded is fed into the second supply conduit 5 with a second feeder 6 corresponding to the first feeder 6. Only one supply conduit is also possible, whereupon the material to be moulded can be fed along the single supply conduit both into the interior and the exterior of the rotor 2 in a manner known per se . The interior and the exterior of the rotor 2 comprise screw threads, which are not shown in the accompanying figure for the sake of clarity. By means of the screw threads the rotor 2 supplies, during its rotation, the material to be injected to an injection space 7. The injection space 7 is cylindrical and bordered by the inner surface of the outer stator 12. When the injection space 7 contains a sufficient amount of the material to be injected, a cylindrical piston 8 placed in the middle of the injection press ejects the material through a nozzle 10. A mould to which the material to be injected is directed is placed in association with the nozzle 10. This mould is not shown in the accompanying Figure 2 for the sake of clarity. Figure 3 shows a third apparatus according to the invention. The reference numerals in Figure 3 correspond to those in Figures 1 and 2. The piston 8 is hollow, whereupon a cylindrical mandrel 14 is placed inside it so that the injection space 7 is formed by a substantially tubular space bordered by the mandrel 14 and the outer stator 12. The definition ' substantially tubular' covers in connection with the present application for example such alternatives that the injection space 7 has an elliptic or asymmetrical cross- section or that the size or shape of its cross-section varies. The injection space 7 is designed in such a way that the material supplied by the stators 1 and 12 and the rotor 2 flows into the injection space 11 maintaining the helical orientation provided by the stators 1 and 12 and the rotor 2. A closing means 15, which prevents the material to be injected from flowing into the mould too early, is placed at the end of the injection space. When the injection occurs, the closing means 15 is opened so that when the hollow cylindrical piston 8 moves, the material to be moulded can be pressed from the injection space 7 into the mould 16. The nozzle 10 of the apparatus is annular so that the orientation of the material to be moulded can also be maintained inside the mould 16. There are no weld lines in the product provided with the mould 16, either. For the sake of clarity, the mould 16 is only shown schematically and its shape may be selected freely. The accompanying Figure 3 shows schematically a mould 16 which provides for example elbow pieces for pipes. Figure 4 shows a fourth apparatus according to the invention. The reference numerals in Figure 4 correspond to those in Figures 1 to 3. At least the outer surface of the inner stator 1 is cylindrical after the end of the rotor 2. At a corresponding point, at least the inner surface of the outer stator 12 is cylindrical. The injection space 7 is thus formed in the space that is situated between the outer stator 12 and the inner stator 1 and that begins from the end of the rotor 2 and ends in the nozzle 10 at the end of the outer stator 12 and the inner stator 1. When the rotor 2 rotates it carries the plastic material to be moulded into the injection space 7. The rotor provides the radial orientation of the fibres in the plastic material, the orientation remaining in the material when it is transported into the substantially tubular injection space 7. When more material is supplied with the rotor 2, the new material ejects the old material from the nozzle 10. The exterior of the nozzle 10 comprises mould parts 16a and 16b, which can be moved in the manner shown by arrows A to encircle the plastic 18 to be moulded and away from it. When a sufficient amount of the plastic 18 to be moulded has been extracted from the nozzle 10, the mould halves 16a and 16b are directed close to the plastic 18 to be moulded. Air is thereafter blown to the interior of the plastic 18 to be moulded with blow means 17 at a high positive pressure, so that this positive pressure causes the plastic 18 to be moulded to compress against the edges of the mould formed by the mould halves 16a and 16b, whereupon the object will receive its final shape. The orientation of fibres in the plastic 18 to be moulded also remains substantially unchanged. The blow means 17 are arranged to blow the air from between the stators 1 and 12 and the rotor 2. Figure 5 shows a fifth apparatus according to the invention. The reference numerals in Figure 5 correspond to those in Figures 1 to . For the sake of clarity, the supply conduits and the feeders are not shown. A mandrel 14 is provided as an extension to the inner stator 1. In such a case, a substantially tubular injection space 7 is formed between the mandrel 14 and the outer stator 12. The mandrel 14 is provided with an extension cone and the outer stator 12 is provided with a similar shape in the corresponding place, so that the diameter of the injection space 7 increases at the extension cone. The material to be moulded is thus provided with axial orientation as it flows in the injection space at the orientation cone. The material to be moulded is pressed into the mould 16 with a substantially tubular piston 8 in the same manner as shown in Figure 3, to obtain oriented moulded plastic products. The mandrel 14 can be rotated, if desired, with rotation means 20 in order to provide radial orientation. If there is no need to rotate the mandrel 14, it can be formed into a stationary construction together with the inner stator 1. Figure 5 shows grooves 19 provided on the outer surfaces of the inner stator 1 and the rotor 2. The rotor and stator surfaces positioned one against the other are preferably provided on one side wit i a screw thread and on the other side with a counterg: ove that is opposite in direction to the screw thread and that has a smaller width than the groove of the screw thread and a smaller gradient, so that the countergrooves are positioned closer together than the grooves of the screw thread. The rotor and stator surfaces positioned one against the other comprise especially preferably on one side a screw thread and on the other side a substantially direct groove, i.e. a groove with a gradient of 90°, so that this direct groove improves the forward supply of the material to be moulded during normal use. On the other hand, if the forward flow of the material is prevented in the injection end for example by means of the piston 8, the rotor 2 can still be rotated continuously since the material to be moulded can flow backwards in the apparatus along the direct grooves, and the pressure of the material cannot rise too high near the output end of the apparatus. In this manner, rapid production of for example small articles is made possible when the rotor 2 rotates at all times at a constant speed. Further, this kind of groove construction is very advantageous in a conical arrangement, since the material to be moulded cannot be destroyed, because the return path of the material in the feed zone is short and it passes the grooves having the shape of the screw thread. The apparatus of Figure 5 is also provided with cross-linking means 21 with which the material to be moulded can be first cross-linked and then pressed into the mould 16 with the piston 8. Therefore, the apparatus is particularly applicable for the manufacture of cross¬ linked polyethylene and injection-moulded objects made of this material. The cross-linking means 21 can be placed either in the direct section of the inner stator 1 or along the extension cone. In the situation shown in Figure 5, the material to be moulded is supplied to the injection space 7 at a temperature that is lower than the cross-linking temperature of the material, and the cross-linking agent is made to react by raising the temperature of the materials by means of the cross- linking means 21. The material is thereafter pressed into the mould 16 with a substantially tubular piston 18. It is therefore possible to produce injection- moulded objects with permanent orientation that has been fixed effectively by the cross-linkage. With the apparatus according to the invention, it is also possible to provide multilayer injection-moulded objects, in which case different materials are supplied to different sides of the rotor 2 and both layers of material are pressed into the mould 16 simultaneously. The different layers can also be provided with different degrees of orientation for example by maintaining different temperatures for the materials on different sides of the rotor 2 or by supplying polyethylene to one side of the rotor 2 and polyethylene and peroxide to the other side, so that the peroxide cross-links the other side which therefore remains oriented, and the other side is substantially non-oriented. Such a piece can be used for example as a connecting sleeve one surface of which is welded onto the surface of the object to be joined. The drawings and the related description are only intended to illustrate the inventive idea. The details of the invention may vary within the scope of the claims. Therefore, the surfaces of the outer stator 12 and the inner stator 1, excluding the surfaces situated against the rotor 2, may have either a conical, cylindrical or some other shape, depending on the needs of manufacturing technology, for example, or some other requirements. Further, ere may be more than one rotor and more than two state s, whereupon it is possible to supply material to the injection space in several layers, thus providing multilayer products.

Claims: Claims 1. A method for moulding plastic, in which method plastic material is supplied to an injection space (7) with injection means comprising at least one conical stator (1, 12) and at least one conical rotor (2) , the rotor (2) being rotated in such a way that as the rotor (2) rotates, it supplies plastic material to be moulded, so that the plastic material passes through the injection space (7) and is compressed into a shape determined by a mould (16, 16a, 16b) , c h a r a c¬ t e r i z e d in that the rotor (2) is placed outside at least one stator (1) . 2. A method according to claim 1, c h a r¬ a c t e r i z e d in that there is at least one stator (12) outside the rotor (2) . 3. A method according to claim 1 or 2, c h a r a c t e r i z e d in that the material is supplied with injection means to a substantially tubular injection space (7) . 4. A method according to claim 2 and 3, c h a r a c t e r i z e d in that different materials are supplied to different sides of the rotor (2) in order to prepare a multilayer injection-moulded piece, and that both layers are pressed into the mould (16) simultaneously. 5. A method according to claim 3 or 4, c h a r a c t e r i z e d in that the material is cross-linked in the injection space (7) and thereafter pressed into the mould (16) with a substantially tubular piston (8) . 6. A method according to claim 2, c h a r a c¬ t e r i z e d in that at least the outer surface of the inner stator (1) is cylindrical after the end of the rotor (2) , and that at a corresponding point at least the inner surface of the outer stator (12) is cylindrical, whereupon the inner stator (1) and the outer stator (12) encircle a substantially tubular injection space (7) , so that when the rotor (2) rotates it supplies material to the injection space (7) simultaneously pushing material out from the injection space, and that a mould (16a, 16b) is placed around the material and air is passed to the interior of the plastic material to be moulded, whereupon the plastic material to be moulded can be compressed against the edges of the mould (16a, 16b) by means of positive pressure. 7. A method according to claim 6, c h a r a c- t e r i z e d in that the air is supplied to the interior of the plastic material to be moulded from between the means for supplying the plastic material . 8. A method according to claim 3, 4 or 5, c h a r a c t e r i z e d in that the plastic material is injected into the mould (16) via an annular nozzle (10) . 9. A method according to any one of claims 1, 2, 3, 4, 5 or 8, c h a r a c t e r i z e d in that from the injection space (7) the plastic material is compressed into the mould (16) with a piston (8) positioned in the middle of the means for supplying the plastic material. 10. A method according to claim 8, c h a r a c- t e r i z e d in that when plastic material is supplied to the injection space (7) , the nozzle (10) is closed with a closing means (15) , and before the plastic material is compressed into the mould (16) the nozzle (10) is opened. 11. A method according to any one of the preceding claims, c h a r a c t e r i z e d in that the stator (1, 12) and the rotor (2) comprise grooves (19) wherein the plastic material is made to flow in the material supply stage towards the injection space (7) , and in the material moulding stage the material flows backwards in either grooves (19) . 12. An apparatus for moulding plastic, the apparatus comprising an injection space (7) , at least one conical stator (1, 12) and at least one conical rotatable rotor (2), the stator (1, 12) and the rotor (2) forming means for supplying plastic material via the injection space (7) , and means for compressing the plastic material into a shape determined by a mould (16, 16a, 16b) , c h a r a c t e r i z e d in that at least one rotor (2) is placed on the outside of at least one stator (1) . 13. An apparatus according to claim 12, c h a r a c t e r i z e d in that the apparatus comprises at least one stator (12) provided outside the rotor (2) . 14. An apparatus according to claim 12 or 13, c h a r a c t e r i z e d in that the injection space (7) is substantially tubular. 15. An apparatus according to claim 14, c h a r a c t e r i z e d in that the apparatus comprises cross-linking means (21) for cross-linking the plastic material, and a substantially tubular piston (8) for pressing the material into the mould (16) . 16. An apparatus according to any one of claims 12 to 14, c h a r a c t e r i z e d in that at least the outer surface of the inner stator (1) is cylindrical after the end of the rotor (2) , and that at a corresponding point at least the inner surface of the outer stator (12) is cylindrical, whereupon the inner stator (1) and the outer stator (12) encircle a substantially tubular injection space (7) , and that the apparatus comprises blow means (17) for blowing air to the interior of the plastic material to be moulded so as to compress the plastic material to be moulded against the edges of the mould (16a, 16b) by means of positive pressure. 17. An apparatus according to claim 16, c h a r a c t e r i z e d in that the blow means (17) are positioned in the middle of the means for supplying the plastic material. 18. An apparatus according to any one of claims 12 to 15, c h a r a c t e r i z e d in that a piston (8) , arranged to compress the material to be injected into the mould (16) , is positioned in the middle of the means for supplying the plastic material. 19. An apparatus according to claim 14, c h a r a c t e r i z e d in that the nozzle (10) of the apparatus, via which the plastic material is injected into the mould (16) , is annular. 20. An apparatus according to claim 19, c h a r a c t e r i z e d in that a closing means (15) is placed in association with the nozzle (10) for closing and opening the nozzle (10) . 21. An apparatus according to any one of claims 12 to 20, c h a r a c t e r i z e d in that the rotor (2) and the stator (1, 12) comprise grooves (19) for making the plastic material flow more efficiently, the grooves (19) being opposite in direction and the gradient of the countergroove being equal to or less than 90°. 22. A plastic product produced with the method according to claim 1, c h a r a c t e r i z e d in that the material of the plastic product is substantially oriented. 23. A plastic product according to claim 22, c h a r a c t e r i z e d in that the plastic product is formed without weld lines. 24. A plastic product according to claim 22 or 23, c h a r a c t e r i z e d in that the plastic product comprises at least two different layers that have been compressed into the mould simultaneously. 25. A plastic product according to claim 24, c h a r a c t e r i z e d in that the different layers are provided with different degrees of orientation.

Inventors: JAERVENKYLAE JYRI, KIRJAVAINEN KARI, JAERVENKYLAE, JYRI, KIRJAVAINEN, KARI
Applicants: CONENOR OY, JAERVENKYLAE JYRI, KIRJAVAINEN KARI, JAERVENKYLAE, JYRI, KIRJAVAINEN, KARI
Application year: 1997
Main classification: B29C45
IPC codes: B29C45, B29C49