Plastic and nonmetallic article shaping or treating: processes – With severing – removing material from preform mechanically,... – Forming continuous work followed by cutting
Patent
1989-09-27
1991-10-01
Thurlow, Jeffery
Plastic and nonmetallic article shaping or treating: processes
With severing, removing material from preform mechanically,...
Forming continuous work followed by cutting
264 407, 2642094, 2642095, 26421121, 264230, 264235, 264342R, 4253781, 425380, 425461, B29C 4786
Patent
active
050531743
DESCRIPTION:
BRIEF SUMMARY
The invention relates to a method of producing a shrinkable product of a thermoplastic raw material by extrusion.
Products produced by this method have a great variety of uses. Depending on the requirements in each particular case, the products may be sheetlike, bandlike or tubular in shape. Possible uses include finishing cable ends in electric installations, extending protective sheathings of district heating pipes, applying protective coating to large iron pipelines, providing sockets in polyethene pipes, joining iron, concrete or PVC pipes, binding of packages, and insulation and sealing in air conditioning techniques. Other applications of the invention are also possible.
To begin with, the associated prior art will be discussed from different points of view.
In cases where the product is intended for use in the form of a sheet or a band, it is customary to produce it directly into the desired shape by means of a flat die. In prior production methods, however, the final product has had undesired properties, such as variation in thickness, indefinite stresses, etc.
As a consequence, sheetlike products have been produced by first extruding the raw material into a pipe by means of an annular die, whereafter the pipe is cut open and spread out. Such applications are disclosed e.g. in Finnish Patent Specifications 25 288 and 45 014. Especially with regard to the latter one, which describes the method in more detail, it should be noted that the object is to produce unshrinkable products.
Such products are not suited for the uses described above. On the other hand, attempts have been made to utilize a similar method in the production of shrinkable bands or the like products, too.
Such methods are disclosed e.g. in Swedish Published Specification 335 081 and Finnish Patent Application 842837. Firstly, these solutions are rather complicated in the process-technical sense. For obtaining the desired end result, the raw material has to be cross-linked and, further, a laminate of two or more layers has to be formed before the product is usable. These measures themselves cause considerable costs and require expensive apparatus investments. In addition, the use of products produced by these methods is difficult as well as time-consuming. Products of this kind are marketed in Finland under the product names NEO COVER or RAYCHEM, too.
The object of the present invention is to provide a method which avoids the above-mentioned drawbacks and enables a simple production of a versatile product which is easy to use. This object is achieved by means of the method according to the invention which is characterized by the steps of
extruding the plastic raw material during extrusion through a die at a low speed in such a manner that the flow remains laminar, thus obtaining a longitudinal orientation of the molecules, and
rapidly cooling the plastic material after extrusion so that the longitudinal orientation of the molecules is maintained in a solidified state in the finished product.
The basic idea of the invention is that, surprisingly enough, it is possible to utilize the properties and behaviour of a thermoplastic raw material in conventional production methods utilizing a screw extruder comprising, alternatively, an annular or flat die. The die, however, differs slightly from the one used in the traditional production technique.
The die is adjusted in such way that the flow path of the raw material is as long as possible and the die slot is narrow. Furthermore, the production rate is kept at a lower level than usually, so that the flow remains laminar. It is also to be preferred that the molecular speed is kept as close to zero as possible along the die walls, whereby the raw material retains its adhesion to the walls. For improving the adhesion, the temperature of the terminal portion of the die is preferably kept at a level as low as possible. The molecules are thus straightened and oriented in the longitudinal direction. This results in that the material becomes anisotropic, i.e., certain properties are different in dif
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Muolon Oy
Thurlow Jeffery
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