Plastic and nonmetallic article shaping or treating: processes – With measuring – testing – or inspecting – Controlling fluid pressure in direct contact with molding...
Patent
1998-04-29
2000-11-28
Silbaugh, Jan H.
Plastic and nonmetallic article shaping or treating: processes
With measuring, testing, or inspecting
Controlling fluid pressure in direct contact with molding...
264 401, 264 407, 2642094, 2642095, 264564, 264565, 425135, 425140, 425141, 4253261, 425392, 4253871, B29C 4792, B29C 4916, B29C 4978, B29C 5528
Patent
active
061531324
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF INVENTION
This invention relates to the manufacture of oriented plastics tubes, and in particular to the manufacture of tubes having a high degree of orientation in the circumferential direction.
International Patent Application No. WO 90/02644 describes a process for the manufacture of thermoplastics made, for example, of unplasticised polyvinyl chloride (uPVC) which have a degree of orientation in the circumferential direction that improves properties such as resistance to hoop stresses, and renders the tubes particularly suitable for transmission of water. The process described in that patent application comprises: temperature profile about the tube suitable for expansion of the tube to cause molecular orientation of the polymer, preferably to a temperature of 85-115.degree. C. for uPVC, more preferably 90-100.degree. C. pressure to the tube that is limited at its downstream end by a plug that is inflatable or otherwise expandable to maintain pressure within the expansion zone; and expanded configuration.
At the downstream end of the expansion zone, the expanded tube passes through a circular passage in a sizing device which limits diametrical expansion of the tube to that which allows the expanded tube to slide through the sizing device. After the tube has passed through the expansion zone and while it is being cooled, the tube may contract in diameter by a small amount, e.g. a few millimeters or less (so-called "snap-back") as the internal pressure is reduced, to give the tube its final, expanded, diameter.
In the case of tubes that are intended to be used as water pipes, and which must therefore be joined end-to-end, it is necessary for the diameter of the expanded tube, and especially its external diameter, to be controlled in order that the tube will be able to fit accurately and tightly within a coupling device or within an additionally expanded end section of an adjacent tube so as to prevent leakage. We have found, however, that control of the expansion process using conventional feedback control, i.e. by measuring the final expanded diameter and wall thickness of the tube, does not control the properties of the resultant tube sufficiently accurately, nor allow stable operation of the process itself.
SUMMARY OF INVENTION
According to the present invention, there is provided a method of expanding a tube of plastics material in which a longitudinally travelling tube is progressively diametrically expanded by internal fluid pressure in an expansion zone between an upstream plug which fits closely within the bore of the substantially unexpanded tube and a downstream plug which fits closely within the bore of the tube after the expansion step, wherein expansion of the tube is controlled in response to one or more parameters which directly or indirectly indicate variation in the relative velocity at which the material is delivered to and is removed from the expansion zone.
Control of the process in response to changes in the relative velocities is preferably achieved by varying the fluid pressure in the expansion zone. In one preferred form, this is done by controlling leakage of the expansion fluid past the expandable downstream plug, by varying the diameter of the expandable plug. In the most preferred case, in which the downstream plug is inflatable, the leakage is controlled by controlling the pressure of inflation fluid in the plug.
The variation in relative velocity into and out of the expansion zone is preferably measured indirectly, and may be measured by any one or more of the following methods: zone and the expanded tube downstream may be measured and compared. Preferably the upstream measurement is taken at a point substantially immediately before the expansion zone. This measurement will detect increases in expansion of the tube at the expansion zone as this will be accompanied by an increase in the velocity of material entering the expansion zone as the outward bulging will start to draw material from the upstream side of the upstream plug. In practice, the downstream ve
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Chapman Peter Glanville
Wallace Allan Kenneth
Poe Michael I.
Silbaugh Jan H.
Uponor Innovation AB
Vinidex Tubemakers Pty. Limited
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