METHOD AND INSTALLATION FOR THE PRODUCTION OF PLASTIC PRODUCTS WITH THE AID OF A HOT MIXTURE OF PVC POWDER AND ADDITIVES, AND A HOT MIXER SUITABLE FOR USE WITH THIS METHOD

Abstract: A method is disclosed for the production of extruded plastic products by using an extruder (1) which has an extruder hopper (2) which is supplied with a hot mixture obtained by discontinuous mixing of PVC powder and additives in a hot mixing zone, with the supply of frictional heat, wherein a first amount of PVC powder is mixed with an excess of additives with the supply of frictional heat, after which the mixture thus obtained is homogenised in the same mixing zone with a supplementary amount of PVC powder, in such a way that the final mixture has the desired concentration of additives, after which the hot mixture obtained after homogenisation is fed directly to the extruder hopper (2). An installation for the production of plastic products by extrusion, comprising a discontinously operating hot mixer is also disclosed, as well as said hot mixer (3).

Description: Method and installation for the production of plastic products with the aid of a hot mixture of PVC powder and additives, and a hot mixer suitable for use with this method. The invention relates to a method for the production of extruded plastic products using an extruder which has an extruder hopper which is supplied with a hot mixture obtained by discontinuous mixing of PVC powder and additives in a hot mixing zone, with the supply of frictional heat. A method of this type is known. In this method the additives become predominantly soft or fluid at the elevated temperatures, as a consequence of frictional heat, as a result of which the said additives adhere very well to the PVC powder. Following the preparation of this mixture by discontinuous hot mixing, the mixture obtained is trans¬ ferred into a horizontal rotating and homogenising heat- retaining silo, from which the extruder is fed con¬ tinuously or discontinuously. With this method the additives can be supplied to the mixer from a central weighing installation. One drawback of this method is that a horizontal heat-retaining silo is needed in the extrusion line, while, moreover, a great deal of space is lost because a high vertical mixing installation is positioned above each extruder, as a result of which the building costs for a factory are appreciably increased. The aim of the invention is, now, to provide a method of the type mentioned in the preamble, with which method a horizontal rotating and homogenising heat- retaining silo is no longer needed, since the hot mixture obtained is fed directly to an extruder hopper of an extruder without the use of special installations for keeping the mixture hot, while, moreover, less space is required for installing a mixing zone above the extruder, so that the building costs for a factory are reduced. This aim is achieved according to the invention in that a first amount of PVC powder is mixed with an excess of additives with the supply of frictional heat, after which the mixture thus obtained is homogenised in the same mixing zone with a supplementary amount of PVC powder, in such a way that the final mixture has the desired concentration of additives, after which the hot mixture obtained after homogenisation is fed directly to the extruder hopper. When a method of this type is used, the hot mixture obtained after homogenisation can be fed directly to an extruder hopper, even temperature fluc¬ tuations of, for example, 10°C having no influence on the processing of the plastic. As a result of the lower temperature of this mixture, preferably between 70 and 90 "C, no agglomeration of the mixture occurs and the mixture remains readily flowable, which is in contrast to mix- tures at higher temperature such as are obtained with a single-step mixing method. In addition, this method has the advantage that it makes possible a high flexibility in the production, since an optimum formulation is now possible per ex- trusion line and per product. Moreover, this method can be used for all pos¬ sible extrusion applications. Even in the case of ex¬ trusion of corrugated drainage tubing, where, because of the thin wall of the tubing and the stretching of the plastic tubing to form the corrugations, malfunctions in extrusion immediately cause problems, malfunctions no longer occur when this method is used. Finally, smoother products are obtained with an equal yield compared with processing with cold mixtures, as a result of which a higher yield per extruder is possible. At the same time, an optimum grade can be obtained per extrusion line since the material flows can now be accurately monitored and controlled from raw material to end product. Finally, a fully integrated automation of the process is possible. It is pointed out that it is known per se to mix PVC powder with additives at elevated temperatures and to feed the resulting product con- tinuously directly to an extruder without the use of a horizontal rotating and homogenising heat-retaining silo. A disadvantage is, however, that with this method the supply of raw material to the hot mixer must also be continuous, in which case it has been found that a volumetric metering of the relatively small amounts of additive is insufficiently accurate and, therefore, use must be made of a much more expensive weight metering. Consequently, the total investment costs for an extrusion line are very high. In addition, it is also known to mix PVC powder with an excess of additives in a hot mixer, after which the mixture obtained is transferred to a cool mixer, where it is mixed with a supplementary amount of poly- vinyl chloride powder in order to obtain the desired concentration of additives in the final mixture, and in which cool mixer the mixture is cooled and homogenised, after which the powder mix is stored in hoppers, from which extruders are fed. A drawback is that with this method the energy consumption for the entire method is higher than in the case of the method according to the invention, since the homogenised mixture must be cooled and must then be fully heated up again for extrusion. In addition, the total investment in equipment and space is much higher. Expediently the temperature is raised to a temperature above 80 β C, preferably of 100 to 140 β C, during mixing, with the supply of frictional heat. The ratio of first amount of PVC powder to supplementary amount of PVC powder is between approxi- mately 2:1 and 1:2, but preferably identical amounts of first amount of PVC powder and supplementary amount of PVC powder are used, so that the ratio is about 1:1. The mixing and homogenisation is expediently carried out in a horizontal mixer, as a result of which relatively little space is required above the extruder for the installation of a hot mixer of this type. Although the hot mixture obtained after the homogenisation is usually fed to one extruder, it can be advisable for extruders with a low yield to fill the hot mixture into several extruder hoppers, with the aid of a distributing installation. In this case it is necessary that all these extruders use the same formulation. However, the extruder hoppers can also be filled in turn; in this case it is then possible to use dif¬ ferent formulations. In addition, the invention relates to an instal¬ lation as described in Claim 10. Finally, the invention relates to a hot mixer according to Claim 14. The invention will now be illustrated with reference to an exemplary embodiment with the aid of the appended figure, in which an installation for carrying out the method according to the invention is shown diagrammatically. The diagram shows an extruder 1 with an extruder hopper 2 and a hot mixer 3, which is positioned above this extruder and operates discontinuously, for mixing PVC powder and additives with the supply of frictional heat. The hot mixer 3 is provided with drive means 6 for driving the mixing elements 7 of the hot mixer. The drive means 6 can be set to various speeds of revolution. At a high speed of revolution, the mixing elements can consequently supply a large amount of frictional energy and therefore form a hot mixture of PVC powder and additives. The mixing elements 7 can then serve as homogenising elements at a low speed of revolu¬ tion, after which the hot mixture containing an excess of additives is homogenised with a supplementary amount of PVC powder in order to achieve the desired concentration of additives. The outlet 9 of the hot mixer opens directly into an extruder hopper 2. In order to fill extruder hoppers of several extruders which operate at low capacity, the hot mixer outlet 9 can expediently be provided with a distributing installation 10 to feed several outlets 9', 9" and 9' ". The hot mixer 3 is preferably a horizontal hot mixer, for example of a type which largely corresponds to a mixer as described in German Patent 27 30 283. The hot mixer is provided with an additive feed 4, which can be shut off, and with a PVC powder feed 5, which can be shut off. After feeding a specific amount of additives to a first amount of PVC powder fed through PVC powder feed 5, both the additive feed 4 and the PVC powder feed 5 are closed, after which, after mixing with the supply of frictional heat, the PVC powder feed 5 is opened again and the supplementary amount of PVC powder is fed in. Instead of using only original PVC powder, the installation is also provided with a feed 11 for broken or ground polyvinyl chloride, as a consequence of which broken or ground polyvinyl chloride to be reprocessed can be used. The installation can also be provided with a colorant feed 14 for feeding colorant from colorant reservoir 13. This colorant is appropriately fed to the mixer. If desired, direct feeding to the extruder is also possible. During mixing with the supply of frictional heat, the mixture is heated to a temperature of 110°C when use is made of a mixture of PVC powder with twice the amount of additives. During this operation the mixing elements 7 of hot mixer 3 are driven to a high speed of revolution (1100 rev/min) by the drive means 6, controlled by first control means 8. The speed of revolution of the mixing elements 7 of the hot mixer is then reduced to a low speed of revolution of 550 rev/min using second control means 8a. A second amount of PVC powder at ambient temperature, which amount is identical to the first amount of PVC powder already present in the mixer, is then fed to the mixer 3 and the mixture is homogenised for 1 minute at the low speed of revolution of 550 rev/min. During this operation the temperature of the hot mixture of PVC and the desired amount of additives falls to 80°C. The hot mixture is then fed directly, via hot mixer outlet 9, into extruder hopper 2 and from there directly into extruder 1 with extrusion head 18, which extrudes a tube 15, which is calibrated and cooled in calibrating bush 16 and cooler 16a and removed via draw- bench 17. During extrusion, no "plate out" deposition is observed, the energy consumption in the method according to the invention is very low and the mixing costs per extruder are low, while the hot mixture is outstandingly suitable for all possible applications, in particular tubing, including corrugated drainage tubing.

Claims: CLAIMS 1. Method for the production of extruded plastic products using an extruder which has an extruder hopper which is supplied with a hot mixture obtained by discon- tinuous mixing of PVC powder and additives in a hot mixing zone, with the supply of frictional heat, charac ¬ terised in that a first amount of PVC powder is mixed with an excess of additives with the supply of frictional heat, after which the mixture thus obtained is homo- genised in the same mixing zone with a supplementary amount of PVC powder, in such a way that the final mixture has the desired concentration of additives, after which the hot mixture obtained after homogenisation is fed directly to the extruder hopper. 2. Method according to Claim 1, characterised in that the ratio of first amount of PVC powder to sup¬ plementary amount of PVC powder is between approximately 2:1 and 1:2 and is preferably approximately 1:1. 3. Method according to Claim 1 or 2, characterised in that the mixing with the supply of frictional heat is continued until the mixture has a temperature of above 80°C and preferably a temperature of 100 to 140°C. 4. Method according to one or more of Claims 1-3, characterised in that the supplementary amount of PVC powder has a temperature of below 30°C and preferably ambient temperature. 5. Method according to one or more of Claims 1-4, characterised in that the final mixture has a temperature of between 70 β C and 90 β C. 6. Method according to one or more of Claims 1-5, characterised in that one hot mixer is added to one extruder. 7. Method according to one or more of Claims 1-5, characterised in that the hot mixture obtained after homogenisation is fed from the mixer to several extruders with the aid of a distributing installation. 8. Method according to one or more of the preceding claims, characterised in that the mixing and homogenisa¬ tion is carried out in a horizontal mixer. 9. Method according to one or more of Claims 1-8, characterised in that the PVC powder is at least par¬ tially replaced by broken or ground PVC to be reproces¬ sed. 10. Installation for the production of plastic products by extrusion, at least comprising an extruder (1) with extruder hopper (2) and a hot mixer (3), which operates discontinuously and has an additive feed (4), which can be shut off, and a PVC powder feed (5), which can be shut off, for mixing PVC powder and additives with the supply of frictional heat, the hot mixer (3) being provided with drive means (6) for driving mixing elements (7) of the hot mixer to a high speed of revolution with mixing of PVC powder and additives, characterised in that the drive means (6) can interact either with first control means (8) for driving the mixing elements (7) of the hot mixer to a high speed of revolution with mixing of PVC powder and additives, or with second control means (8a) for driving the mixing elements (7) to a low speed of revolution for homogenising a hot mixture of PVC powder and excess additives which is obtained following the action of the mixing elements (7) at a high speed of revolution, with a supplementary amount of PVC powder, in order to adjust the additives concentration in the final mixture to the desired value, and the outlet (9) of the hot mixer opens directly into an extruder hopper (2) of an extruder (1) . 11. Installation according to Claim 10, characterised in that the hot mixer outlet (9) is provided with a distributing installation (10) which is provided in order to feed several extruder hoppers (2, 2', 2") from one hot mixer. 12. Installation according to Claim 11 or 12, charac¬ terised in that the installation is provided with a feed (11) for broken PVC to be reprocessed. 13. Installation according to one or more of Claims 10-12, characterised in that the hot mixer (3) is a horizontal mixer. 14. Hot mixer (3) which has an additive feed (4) , which can be shut off, and a PVC powder feed (5), which can be shut off, and is provided with drive means (6) for driving mixing elements (7) of the hot mixer to a high speed of revolution with mixing of PVC powder and addi- tives, characterised in that the hot mixer (3) is pro¬ vided with first control means (8), which can interact with the drive means (6) to drive the mixing elements (7) to a high speed of revolution with mixing of PVC powder and additives, and with second control elements (8a) for driving mixing elements (7) for homogenising a hot mixture of PVC powder with excess additives, obtained following the action of the mixing elements (7) at a high speed of revolution, with a supplementary amount of PVC powder at a low speed of revolution, in order to adjust the additives concentration in the final mixture to the desired value.

Inventors: SEVERIENS HENDRIK, SEVERIENS, HENDRIK
Applicants: WAVIN BV, WAVIN B.V.
Application year: 1991
Main classification: B29B7
IPC codes: B29B7, B29K27, B29K105, C08J3