Plastic and nonmetallic article shaping or treating: processes – Mechanical shaping or molding to form or reform shaped article – Bending or twisting of work
Patent
1981-12-31
1983-12-27
Czaja, Donald E.
Plastic and nonmetallic article shaping or treating: processes
Mechanical shaping or molding to form or reform shaped article
Bending or twisting of work
264235, 264285, 264558, B29C 1702
Patent
active
044229992
DESCRIPTION:
BRIEF SUMMARY
The invention relates to thermoforming articles and is more particularly concerned with methods of manufacturing articles having a predetermined shape or form and manufactured from heat mouldable tubular material.
It is well known that thermoplastic tubular material can be bent with a bending die at suitable temperatures followed by cooling to set the material in the bent configuration. Such a process suffers the disadvantage that the finished articles are lacking in thermal stability, i.e. when the service conditions result in the article being reheated to temperatures approaching the temperature at which they were initially bent there is a tendency for articles to resume the original shape or form of the tubular material.
In general there have been two principal ways of manufacturing thermally stable predetermined shaped articles from tubular material that can be thermoformed. The first of these methods comprises injection moulding in which the article is formed in an openable mould. For a hollow article the mould comprises a casing and a core, the mould being designed so that the core can be removed from the moulded article. At some stage in such moulding, the mould is heated. In the other way of manufacture, the article is shaped from material while the material is in a plastic pr pliable state and then the shaped article is baked or at least heated in an autoclave or oven.
Each of the above principal ways of manufacture have substantial disadvantages. Thus, the cost of injection moulding apparatus is sufficiently high to justify such expenditure only when very large numbers of articles are to be manufactured. Further it is really only economical and practical to mould relatively small articles. This is equally relevant to a manufacturer using an autoclave since the cost of a large volume autoclave can only be justified either for large production volumes or for high value large articles.
Neither of the conventional manufactures is particularly suitable for low volume production of relatively large awkward shaped articles. The present invention addresses this difficulty.
According to the present invention there is provided a method of annealing tubular articles of heat mouldable material comprising heating the articles to the annealing temperature for a time to effect the annealing thereof and cooling the articles to set by passing a fluid therethrough while the articles are restrained in the predetermined shape or form.
On releasing the constraint on the finished article it remains in the predetermined shape or form and exhibits a high degree of thermal stability.
The invention also provides a method of manufacturing an article having a predetermined shape or form from a tube of heat mouldable material comprising bending the tube to the predetermined shape, heating the tube to the annealing temperature of the material by passing heated fluid through the tube, maintaining the flow of heated fluid for a period of time sufficient to anneal the tube and cooling the tube while still constrained in the bent shape.
Preferably the tube is cooled by flowing cooling fluid through the tube after the flow of heating fluid is terminated. Preferably both fluids are the same substance.
When the tube is not sufficiently flexible at ambient temperature to be readily bent to the predetermined shape or form a heated fluid can be flowed through the tube to heat the tube before bending to a temperature at which the tube is flexible enough to be fitted to a jig structure defining the predetermined shape or form. The fluid which is flowed through the tube must have a boiling point higher than the desired process temperature and can conveniently be oil.
Suitable heat mouldable or thermoformable materials are polyethylene, vinyl polymers such as polyvinyl chloride, polyvinyl acetate or polyamides.
The tubular blank can, for example, comprise a tube and can be shaped either by winding it onto a former to form a coil or by threading it onto or fixing it to a jig or like support structure.
Two examples of ways of performing the
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Czaja Donald E.
Thompson W.
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