Stock material or miscellaneous articles – Composite
Reexamination Certificate
2001-08-28
2004-02-10
Nakarani, D. S. (Department: 1773)
Stock material or miscellaneous articles
Composite
C428S475800, C428S476100, C428S476300, C428S476900, C428S516000, C428S523000
Reexamination Certificate
active
06689472
ABSTRACT:
BACKGROUND OF THE INVENTION
The invention relates to a method for manufacturing extrudable products, wherein the material to be extruded is fed in a powder, pellet or granulate form, by using one or more supply means, into a processing cavity consisting of the volumes of the grooves in the rotor, of the grooves in the stator and of the clearance between them.
The invention further relates to an apparatus for manufacturing an extruded plastic product, the apparatus comprising at least one rotor and at least one stator, and a feed gap between them, grooves being provided on the other side of the feed gap for pressing the material to be extruded out of the apparatus when the rotor is rotated, the cross-section of the groove being substantially unchanging, and a countergroove being provided on the other side of the feed gap at least over a distance of the gap, the thread of the countergroove being opposite in direction to the groove provided on the other side of the feed gap.
The invention also relates to a plastic product containing matrix plastic to which 1 to 30% of barrier plastic has been added.
FIELD OF INVENTION
It is very difficult to process plastics having a high molecular weight with a conventional extruder consisting of a long screw and a cylinder. The yield of the extruder remains very small, since temperatures easily rise too high due to heat generated by the friction. Polymers that are difficult to process include for example fluoroplastics and polyethylenes having a great molar mass and a molecular weight of over 200 000 g/mol, in very hard plastics even over 300 000 g/mol. There are a number of similar materials that are hard to process and they have in common a high molecular weight and therefore a low melt flow rate, a high melting point and in some cases a narrow processing window, in other words for example the decomposition temperature of the plastic is close to its melting point.
Extrusion is particularly difficult at low temperatures when the extrusion temperature is kept only a few dozen degrees above the crystalline melting point, i.e. the temperature is typically about 30 to 40° lower than normally in order that optional reactive components would not react too early. This situation is common especially when pipe grade chemically cross-linked pipes are produced. Problems occur since there are five different interconnected functions that are based on the action of the screw and that operate on the same axis and therefore at the same speed of rotation: feed, melting, mixing, homogenization and generation of pressure. In a conventional extruder, the screw is long and the ratio of the screw length to its diameter is most often about 20 to 30, and the screw comprises one, sometimes two or three, threads. The groove of the screw has been divided into different sections by varying the cross-section of the groove in the screw in such a way that the threads are often deep and positioned at long intervals from one another at the section where the supply takes place, and the cross-section is kept constant all the way in the melting zone but generally it is much bigger than at the outlet end. Thus, the material does not flow easily through the groove path and the end restriction causes a lot of frictional heat. In the mixing section the groove often has its own geometry, and in the end the groove becomes shallower and the generation of pressure begins. By means of this pressure a mass can be pressed even through a complicated tool. On the other hand, in a conventional extruder the total length of the groove around the screw is too long, in which case the ratio of the length of the groove to its cross-section becomes too great and it is therefore totally unsuitable for poorly flowable plastics.
U.S. Pat. No. 3,314,108 discloses an extruder comprising a conical rotor and conical stators provided in the exterior and interior of the rotor. The rotor comprises flat rectangular grooves for pressing the material to be extruded out of the extruder by rotating the rotor. However, with the aforementioned apparatus it is very difficult to process materials that are not easily workable. Also, the capacity will be limited.
EP 422,042 discloses an extruder comprising several conical stators and several conical rotors situated between them. The rotors and/or the stators are provided with calotte-shaped grooves for pressing the material to be extruded out of the extruder when the rotors are rotated. With this apparatus it is possible to manufacture very advantageously multilayer plastic pipes, but the processing of materials that are not easily workable creates a problem. Further, in the apparatus the moulding pressure is generated at the output end and the yield of the apparatus is therefore not entirely sufficiently good. Also, the material to be extruded is melted with heat that is provided from the outside, and it is therefore difficult to control the temperature, and the consumption of energy is relatively great.
U.S. Pat. No. 4,125,333 discloses an extruder comprising a long screw with threads, and a stator situated outside the screw and having same-handed threads. The same-handed threads of the stator produce a backflow, whereupon the material is mixed and the amount of heat generated by the friction easily increases to an uncontrolled level.
DE 2,558,238 also discloses an extruder comprising at its end a mixing section with either same-handed, opposite or direct grooves in the stator. Such an apparatus mixes the material very effectively, but it cannot be applied at all in equipment where the temperature of the material is to be adjusted accurately, since the amount of heat generated by the friction easily increases too much.
U.S. Pat. No. 3,712,783 discloses an extruder comprising a diverging feed zone. After the feed zone the material is pumped into a restriction zone. Thereafter the mass is pumped and caused to be extruded through an outlet. The structure of the apparatus is very complicated. The structure of the apparatus causes very high friction and because the extrusion pressure is raised near the outlet, the yield of the apparatus is very poor.
EP 0,678,069 discloses extruding multilayer pipes made of cross-linked polyethylene. The first step comprises extruding the centre layer of the pipe and thereafter the centre layer is coated with skins. The skins are used only for improving the flow characteristics of the pipe when the pipe is passing the heating tool. A special disadvantage of the apparatus is the need for a tool having spider legs, because the spider legs cause weld lines.
SUMMARY OF THE INVENTION
The purpose of the present invention is to provide a method and an apparatus with which it is relatively easy to also extrude poorly workable materials into a plastic product, and a plastic product with excellent properties.
The method according to the invention is characterized in that the cross-sectional area of said cavity decreases at least partially continuously along the axis of the extruder, and by the relative rotational movement of the stator and the rotor said material is forced to proceed along an x-axis, whereby the frictional heat caused by the shear melts a part of the material forming a bed consisting of mainly unmelted particles and some melt around them, enabling the formation of a processing cavity that is completely filled at a certain cross-section at a distance from the end of the apparatus, end hence a rise in the pressure to a level higher than needed for pushing the material through a die later on along the x-axis.
Also, the apparatus according to the invention is characterized in that after the feed section there is a shear zone, the countergroove being positioned substantially along the entire length of the shear zone, and that the cross-sectional area of the processing cavity, consisting of the volumes of the grooves in the rotor, of the grooves in the stator and of the clearance between them, decreases at least partially continuously along the axis of the extruder.
Further, the product according to the invention is characterized in that
Järvenkylä Jyri
Kirjavainen Kari
Sjöberg Michael
Ladas & Parry
Nakarani D. S.
Uponor Innovation AB
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