Plastic and nonmetallic article shaping or treating: processes – With measuring – testing – or inspecting – Controlling rate of movement of molding material or its...
Reexamination Certificate
2001-08-29
2004-04-27
Eashoo, Mark (Department: 1732)
Plastic and nonmetallic article shaping or treating: processes
With measuring, testing, or inspecting
Controlling rate of movement of molding material or its...
C264S167000, C264S209300, C264S211120, C264S290200
Reexamination Certificate
active
06726863
ABSTRACT:
FIELD OF THE INVENTION
The invention relates to the production of tube from thermoplastic material, in particular from polyolefin plastic material, such as polyethylene. The invention also relates to the production of plastic tube in which the thermoplastic material is biaxially oriented, which process is known as the biaxial stretching process. The invention also relates to improvements to the process for the production of extruded tube from thermoplastic material, which process may form part of the production of biaxially oriented plastic tube. The invention additionally relates to the production of an improved joint between tubes made from biaxially oriented thermoplastic material.
The present invention relates in particular to the production of a tube from biaxially oriented thermoplastic material with an integrally formed socket at an end, so that tubes of this nature can be coupled to one another via socket joints, in order in this way to form a pipe, for example for transporting water, gas, etc.
BACKGROUND OF THE INVENTION
WO 95/25626 has disclosed a method for the production of biaxially oriented plastic tube, also known as a stretched tube. In this method, the stretched tube is of uniform cross section, i.e. has a uniform wall thickness and diameter, over its entire length, and is also uniformly stretched in the axial and tangential (circumferential) direction of the tube over its entire length. A method for providing a tube which has been produced in this way with a socket at one of its ends is known from WO 97/33739.
Another method for producing tube from biaxially oriented plastics material is known from GB 1 589 052. This method is based on a tube made from thermoplastic material which has not been subjected to biaxial orientation, which tube has a tube body with, at one end, an end part with a greater wall thickness than the tube body. The tube is placed in a die and is expanded by an internal pressure so that the plastics material of the tube is biaxially oriented. In the process, the end part is deformed to form a socket.
WO 98/13190 has described yet another method for the production of a tube with an integral socket from biaxially oriented thermoplastic material.
Despite all the developments in the field of the production of tubes from biaxially oriented thermoplastic material, and in particular in the field of forming a socket on a tube of this nature, load tests still show that the socket of a tube of this nature forms the critical part of the tube. This is because the tube has been found to rupture earlier at the socket than in the tube body, and therefore the socket constitutes an undesirable limitation on the mechanical strenght of the tube.
OBJECT OF THE INVENTION
The object of the present invention is to propose measures which make it possible to produce a tube of the above type with an integral socket at one or both ends. The invention also provides measures for improving the spigot of the tube, which is to be fitted into a socket.
SUMMARY OF THE INVENTION
For this purpose, the invention, according to a first aspect, provides a method wherein-by periodical variation of the ratio of the preform advancement speed, on the one hand, and the output of the extruder, on the other hand, between a plurality of different values-the wall thickness of the preform is periodically changed.
When the method according to the first aspect of the invention is carried out, an axial preform part with a different wall thickness from the preceding part of the preform is periodically formed in the section between the extruder die and the tube speed-control means, in practice in particular immediately downstream of the extruder die.
Surprisingly, in practice it has proven possible to control the process of biaxial stretching of the preform successfully despite the variation in the wall thickness of the preform which is to be forced over the mandrel. In particular, it has proven possible for a preform part with a greater wall thickness to be forced over the mandrel without this having undesirable effects on that part of the preform which has a smaller wall thickness and is located between the said thick preform part and the drawing device.
The method according to the first aspect of the invention enables a stretched tube of biaxially oriented thermoplastic material to be produced in a continuous process with axial tube parts which have varying wall thicknesses.
In practice, it has proven expedient for the maximum wall thickness of the preform to be 5-15% greater than the smallest wall thickness of the preform, as seen at a location immediately downstream of the extruder die. It will be clear that other values also lie within the scope of the invention.
Preferably, the transition from one wall thickness value to another wall thickness value is gradual. This is of benefit to the stability of the process.
In a preferred embodiment, the ratio between the advancement speed of the preform, which is determined by the tube speed-control means, on the one hand, and the output of the extruder, on the other hand, is to be at a substantially constant first value for a first period and to be at one or more values which differ from the first value for a second period, which is considerably shorter than the first period, which cycle is repeated continuously.
In practice, this means, as seen at a point downstream of the expansion mandrel-the stretched tube in each case has a part of great axial length with a uniform first wall thickness and associated diameter, which part is followed by a considerably shorter axial part of the tube in which the wall thickness differs from the said first wall thickness, in particular is of one or more greater values, as seen in the axial direction of the said shorter part. In particular, there is provision for the wall thickness-as seen in the axial direction-to vary between a plurality of values in the latter axial part, so that annular areas which adjoin one another and have different wall thicknesses can be distinguished in the relevant part of the stretched tube.
The method according to the first aspect of the invention can be implemented by periodically varying the output of the extruder, in which case the advancement speed of the preform which is determined by the tube speed-control means is kept substantially constant. This does require an extruder which can be adjusted within a suitable range in terms of its output.
However, the method according to the first aspect of the invention can also be implemented, as is preferred, by keeping the output of the extruder substantially constant and periodically varying the advancement speed of the preform which is determined by the tube speed-control means.
In a preferred embodiment of the method according to the first aspect of the invention, the stretched tube acquires substantially the same axial stretching over its entire length. To achieve this, in the preferred embodiment of the method it is in some cases sufficient to keep the advancement speed of the stretched tube downstream of the mandrel, which is determined by the drawing device, constant, so that the ratio of the advancement speed of the stretched tube downstream of the mandrel, on the one hand, and of the preform upstream of the mandrel, on the other hand, remains substantially constant.
In another preferred embodiment of the method, the advancement speed of the preform upstream of the mandrel, which is determined by the tube speed-control means, varies, and for this reason it is then necessary for the advancement speed of the stretched tube downstream of the mandrel, which is determined by the drawing device, to be varied periodically in such a manner that the ratio of the advancement speed of the tube downstream of the mandrel, on the one hand, and of the preform upstream of the mandrel, on the other hand, is kept substantially constant.
In a variant of the method according to the first aspect of the invention, there is provision for the tube parts with a greater wall thickness not to have the same level of axial stretching as an intervening tube
Prenger Jan Hendrik
van Lenthe Arjan Dirk
Visscher Jan
Blank Rome LLP
Eashoo Mark
Wavin B.V.
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