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Method and apparatus for making dimensionally stable thermoplastic tubular articles
| Details |
Inventors: Butcher, Peter E.; Dick, David A.; Nicholas, James W.; Robinson, Melvin E. R.; Staines, Glyn;
Assignee: Metal Box p.l.c. (Reading, GB2)
Primary Examiner:
Assistant Examiner:
Attorney, Agent or Firm:
For heat-setting tubular articles (such as can bodies) of an at least partly biaxially oriented crystallizable polymer, preferably a saturated linear polyester such as polyethylene terephthalate, e.g. for thermally processable food containers, a tube of the polymer is fitted over a mandrel and clamped at its ends to the mandrel at a temperature below the glass transition temperature of the polymer. The tube is then heated, e.g. by inserting a heater 28 into the interior of the mandrel, above the temperature to which it is to be heat-set (at least 60.degree. C. above for PET) whereupon it shrinks into contact with the mandrel but is restrained from axial or further radial shrinkage. The tube and mandrel are cooled to below the heat-set temperature and the tube may then be cut into can body sections by knives engaging in circumferential grooves. Contact between the tube and mandrel is released, e.g. by forcing compressed air out through small holes in the mandrel surface to allow the tube and mandrel to be separated. Tubular bodies with one closed end can be heat-set on a mandrel with a correspondingly shaped end. |
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DETAILED DESCRIPTION According to the present invention, a method of making a thermoplastic tubular article, including heat-setting the article so that it will be dimensionally stable up to a specific elevated temperature, characterised by the steps of fitting an at least partly biaxially oriented tube of a crystallisable polymer over a mandrel whose outer diameter is only slightly less than the internal diameter of the tube and clamping the tube to the mandrel to restrain axial shrinkage of the tube, while maintaining the temperature of the tube and mandrel below the glass transition temperature of the polymer, heating the tube to a temperature higher than the said specific elevated temperature so that the tube shrinks radially into contact with the mandrel but is restrained from further radial shrinkage, maintaining the tube at said higher temperature for a time sufficient to ensure the required degree of dimensional stability, cooling the tube and mandrel to a temperature below the said specific elevated temperature, and then releasing the contact between the tube and mandrel to allow them to be separated.
The heat-setting operation is thus effected independently of the forming operation and does not cause problems therein.
A residual linear shrinkage of up to 3% is usually found after heat-setting, but it can be eliminated by a further step of re-heating the article to the specific elevated temperature without internal restraint against shrinkage.
Such residual shrinkage may be used to eliminate or reduce head space in a container having a body heat-set by the method of the invention.
Where the polymer is a saturated linear polyester, the temperature to which the tube is heated on the mandrel is preferably at least 60.
degree.
higher than the specific elevated temperature, and in particular, where polyethylene terephthalate is used, the specific elevated temperature is preferably about 120.
degree.
C.
and the tube is heated to at least 180.
degree.
C.
on the mandrel.
The degree of dimensional stability which is achieved depends on the temperature to which the tube is heated on the mandrel (the "heat-setting temperature") and the time for which the tube is maintained at this temperature, and it may be adjusted to the particular requirements for which the tubular articles are made
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