Plastic and nonmetallic article shaping or treating: processes – Mechanical shaping or molding to form or reform shaped article – Stretching or stretch forming
Patent
1984-11-09
1986-12-30
Silbaugh, Jan
Plastic and nonmetallic article shaping or treating: processes
Mechanical shaping or molding to form or reform shaped article
Stretching or stretch forming
264522, 264550, 264322, 425384, B29C 5108, B29C 5110, B29C 5142
Patent
active
046327991
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to methods and apparatus for forming tubular articles, each with one closed end, from thermoformable polymer materials, particularly but not exclusively high molecular weight saturated linear polyester materials such as polyethylene terephthalate.
2. Discussion of the Prior Art
Various proposals have been made in the past for forming such articles by the technique of plug-forming, in which an annular region of a sheet of the polymer material is clamped and the portion of the sheet within the clamped region is heated to a temperature above the glass transition temperature but below the annealing temperature of the polymer material, and a plug heated to a temperature in the same range is forced through the plane of the sheet at a position centred within the clamped region so as to draw the polymer material into the form of a tubular body with a closed end. The speed of travel of the heated plug has generally been quite slow, e.g. of the order of 0.6 to 0.8 cm per second, resulting in a long production cycle time.
It is an object of the present invention to provide methods and apparatus for forming such articles reliably and effectively in relatively short process times.
SUMMARY OF THE INVENTION
According to the present invention, a method of forming a tubular article with a closed end from a thermoformable polymer material, particularly a high molecular weight saturated linear polyester material, comprises the steps of clamping an annular portion of a sheet of the polymer material, pre-heating at least the portion of the sheet within the annular clamped portion to a temperature above the glass transition temperature of the polymer but below its melting point, and forcing a punch through the plane of the sheet, centrally within an annular die, with a clearance between the punch and die, so as to draw the sheet into tubular form without exerting compressive stresses on the drawn sheet between the side of the punch and the die, wherein the punch is maintained at a temperature below the glass transition temperature of the polymer and is moved at a speed of at least 1/2 meter per second to effect the drawing. By the glass transition temperature is meant the glass transition temperature of the amorphous and unoriented polymer. When the thermoformable polymer material is a high molecular weight saturated linear polyester material, the temperature to which the sheet should be pre-heated is below that at which rapid crystallisation of the polyester material begins.
It has surprisingly been found that the use of a relatively cold punch moving at high speed forms the tubular articles in a reliable and effective manner, so long as sufficient clearance is maintained between the punch and die to avoid compressive stresses, while much shorter process cycle times, e.g. of the order of 1 second, can be achieved.
In commercial production where the cycle is repeated at a high rate, the punch is preferably cooled to maintain it at a temperature not exceeding the glass transition temperature of the polymer, preferably not exceeding 35.degree. C. Typically, the punch is moved at a speed not exceeding 5 meters per second to effect the drawing. When using polyethylene terephthalate, the portion of the sheet within the die is preferably pre-heated to a temperature of 90.degree. C. to 110.degree. C. The drawing operation imparts orientation to the polymer material. It is found that the hoop strength of the wall of the tubular article improves as the temperature of the sheet is increased, whereas the axial strength decreases. On the other hand, the thickness of the wall of the tubular article is more uniform when the drawing is effected at lower temperatures. The best combination of relatively uniform thickness and both axial and hoop strength is found when the drawing is effected in this range from 90.degree. C. to 110.degree. C., more particular at 100.degree. C.
Preferably a radiused flange is formed at the mouth of the tubular article by means of a co
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patent: 2911681 (1959-11-01), Distler
patent: 3586748 (1971-06-01), Ayres
patent: 3757718 (1973-09-01), Johnson
Dick David A.
Folland Rickworth
Smith Desmond P.
Staines Glyn
Metal Box p.l.c.
Silbaugh Jan
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