Plastic and nonmetallic article shaping or treating: processes – Forming continuous or indefinite length work – Shaping by extrusion
Patent
1992-09-30
1994-02-15
Thurlow, Jeffery
Plastic and nonmetallic article shaping or treating: processes
Forming continuous or indefinite length work
Shaping by extrusion
26421123, 264349, 366 76, 366282, 425204, 425205, 425209, B29B 728, B29C 4736
Patent
active
052864375
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
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 EP-A-0 008 712. 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 transferred into a horizontal rotating and homogenising heat-retaining silo, from which the extruder is fed continuously 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.
SUMMARY AND ADVANTAGES OF THE INVENTION
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 fluctuations of, for example, 10.degree. C. having no influence on the processing of the plastic.
Due to the low temperature of the mixture as obtained according to the method of the invention, preferably between 70.degree. and 90.degree. C., no agglomeration of the mixture occurs and the mixture remains readily flowable, which is in contrast to mixtures 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 extrusion line and per product.
Moreover, this method can be used for all possible extrusion applications. Even in the case of extrusion 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 continuously 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
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Kunststoffberater, vol. 28, No. 3, Mar. 1983, (Frankfurt/Main, DE) M. Weber, "Kontinuierliche PVC-Autbereitung auf der Verarbeitungsmaschine".
Thurlow Jeffery
Wavin B.V.
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