Plastic and nonmetallic article shaping or treating: processes – Forming continuous or indefinite length work – Layered – stratified traversely of length – or multiphase...
Patent
1995-07-19
1996-11-12
Thurlow, Jeffery R.
Plastic and nonmetallic article shaping or treating: processes
Forming continuous or indefinite length work
Layered, stratified traversely of length, or multiphase...
26417117, 26417123, 26417124, 26417128, 2642096, 26421123, 264236, 4251311, 4251335, B29C 4706
Patent
active
055737200
DESCRIPTION:
BRIEF SUMMARY
FIELD OF THE INVENTION
The present invention relates to the extrusion of plastic products generally and more particularly to novel techniques for continuous extrusion of plastic products and novel plastic products.
BACKGROUND OF THE INVENTION
Continuous extrusion of plastic products, such as tubing and plastic coated cables often suffers from the consequences of high frictional resistance between the extruded plastic product and the static walls of the extruder die. This is particularly true with regard to the extrusion of thermally cross-linkable materials, wherein cross-linking occurs in response to heat applied in the extruder die. This will be understood from the following description.
Plastics are divided generally into two main categories: thermoplastic materials and thermosetting materials. Thermoplastic materials consist mainly of macromolecules formed of long chains of monomers, based on carbon and/or silicon atoms, which may or may not be branched, hub are of finite size,
Molecules of thermosetting materials consist of three-dimensional networks which, theoretically, may extend infinitely, Thermoplastic materials may be formed either by cross-linking monomers or by cross-linking of thermoplastic materials. For example, Bakelite (R) (phenolformaldehyde) is formed generally by cross-linking of monomers, whereas cross-linked polyethylene is formed by cross-linking of thermoplastic polyethylene.
Thermosetting materials are preferable to thermoplastic materials in many respects, such as, heat resistance, mechanical strength, and low creep. By way of example, when polyethylene is cross-linked, its heat resistance, wear, creep characteristics, and resistance to chemicals are improved.
Notwithstanding the advantages of thermosetting materials, there is a certain difficulty in using them. This difficulty emanates from their particular internal structure which does not permit melting by heat of the material in its final form. It is difficult, therefore, to work these materials by use of some of the known processes.
It is especially difficult to use these materials in extrusion. This is due to the fact that thermosetting materials flows as a solid material, thereby causing very high frictional resistance between the extruder die wall and the extruded product. The consequences of such high frictional resistance during extrusion include reduced throughput, increased wear on the extrusion die and the extruded product and consequent decreased quality of the extruded product.
In order to overcome this problem, the prior art teaches extrusion of these materials prior to their being cross-linked, and in conditions which do not cause cross-linking. Cross-linking is thus carried out at a later stage, after extrusion, that is to say, after the extruded product has left the extruder die.
Throughout the specification and claims, the term "extrusion" is used to mean the forming process that is performed on the extruded material prior to its leaving the extruder die.
There are many methods of bringing about cross-linking after extrusion, in accordance with the material, technology and technical preference of the manufacturer. Some of these methods involve cross-linking immediately after extrusion, while others are performed at a later stage.
In the rubber industry, vulcanization (cross-linking) of natural thermoplastic rubber has been known for many years by the addition of sulfur and the application of heat.
It is also known to cross-link polyethylene, for example, by first, grafting of silane groups onto the monomer chain, prior to or during extrusion and, thereafter, exposing the extruded product to humidity, thereby to bring about cross-linking.
An alternative method of cross-linking of extruded polyethylene products is by exposing the extruded product to Beta or Gamma radiation.
Further methods include the preparation of suitable compounds which cause cross-linking on exposure to heat for a preselected time period. These methods involve extrusion at a relatively low temperature and/or for a short time, thereby p
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Pipex Ltd.
Thurlow Jeffery R.
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