Stock material or miscellaneous articles – Hollow or container type article – Polymer or resin containing
Reexamination Certificate
2000-02-18
2003-09-30
Mulvaney, Elizabeth (Department: 1774)
Stock material or miscellaneous articles
Hollow or container type article
Polymer or resin containing
C428S034100, C264S127000, C264S209100, C425S289000, C521S145000
Reexamination Certificate
active
06627277
ABSTRACT:
This application is the national phase under 35 U.S.C. §371 of PCT International Application No. PCT/JP98/03592 which has an International filing date of Aug. 12, 1998, which designated the United States of America.
TECHNICAL FIELD
The present invention relates to a polytetrafluoroethylene tube which has a small wall thickness deviation and an apparatus and a process for the production of such a tube.
BACKGROUND ART
As a production process of a polytetrafluoroethylene tube, a paste extrusion process has been hitherto known. In that process, a paste material is prepared by adding a suitable organic solvent called as a lubricant into a fine powder of a polytetrafluoroethylene (hereinafter, referred so as also “PTFE”) produced by emulsion polymerization; the paste material is preformed into a cylinder so as to obtain a billet; the billet is charged into an extruder which extrudes the billet into a tube form while pressing using a ram; and then the lubricant is removed from the tube by heating or extraction followed by sintering the tube so as to finally obtain a predetermined tube (see, for example, Fluoro-Resin Handbook edited by Takaomi Satokawa published by Nikkan-Kogyo-Shinbun-sha (1990)).
A cross-section of an end portion a ram extruder used for the above process is schematically shown in
FIG. 1
from which portion a tube is extruded. The extruder
10
comprises a cylinder
12
, a master die
14
and a die orifice
16
, which form a space
18
. A mandrel
20
and a core pin
22
which is attached to the mandrel are coaxially located within the space
18
. A billet is supplied into such a space of the extruder. When the billet is extruded by a ram (not shown) from the upper side to the lower side of the figure, a tube is obtained from an annular outlet
24
of the die orifice. A lubricant is removed from the tube, which is then sintered so as to finally produce a predetermined tube.
When a shaped article is produced in the form of a tube from the polytetrafluoroethylene in the above described process, the mandrel
20
and the core pin
22
have to be so located that their axes are aligned with a center axis of the space
18
which is formed by the cylinder
12
, the master die
14
and the die orifice
16
. However, one end of the core pin
22
cannot be supported at a level of an outlet
24
of the die orifice
16
, so that the core pin is cantilevered at a lower end of the mandrel
20
to which the core pin is attached, and such mandrel is cantilevered at its upper end. Therefore, it is not easy at all to a center the core pin
22
with the die orifice
16
around at the level near the outlet
24
of the die orifice
16
. In fact, it is extremely difficult and almost impossible to substantially align a center axis of the core pin
22
with a center axis of the die orifice
16
. As a result, a wall thickness of a currently and commercially available PTFE tube, especially one which has a lager diameter (for example, a tube having an inner diameter of not smaller than 10 mm), fluctuates largely. That is, wall thickness deviation phenomenon occurs in that a thickness in a radial direction (wall thickness) of the tube across its cross-section which is perpendicular to a longitudinal direction of the tube is different depending on a radial orientation.
DISCLOSURE OF THE INVENTION
Therefore, it is an object of the present invention to minimize the wall thickness deviation of the tubular article produced by paste extrusion of the PTFE.
The present inventors have made extensive studies so as to achieve the above object, and found that dimensioning a core pin such that at least a portion thereof can bend (or deflect) due to a pressure applied upon the extrusion provides an automatic centering function with an extruder (namely, a function to automatically align an axis of a core pin with an axis of a die orifice), so that the above object is achieved.
Generally, an elongated object which is supported at a single point is placed in a fluid which is flowing, the object moves in such a direction that a resistance which acts on the object is likely to be minimized. That is, when a supported portion of the object is able to freely rotate, the elongated object so moves that its longitudinal direction is to be parallel to a flowing direction of the fluid; and when the supported portion is constrained, the object itself bends such that the resistance is minimized. The present inventors have found that when a phenomenon similar to the above phenomenon occurs in the paste extruder for the PTFE, the core pin is so located that resistance acting on the core pin is minimized, so that the core pin is centered with the die orifice, whereby a polytetrafluroethylene tube having a minimized wall thickness deviation is obtained.
Generally, in an extruder, a mandrel is mechanically fixed at its one end which is opposed to a die orifice outlet while aligned with a cylinder, and a core pin is fixed to the mandrel at the other end thereof. The reason why a tube extruded while pressing in a longitudinal direction of the mandrel by a ram using such extruder has the wall thickness deviation is that the core pin is decentered because of a certain cause even if the mandrel is firmly fixed (namely, the core pin becomes not concentric with the die orifice, and for example, a center axis of the core pin is inclined relative to a center axis of the die orifice), and such decentered state is enlarged on an outlet side of the die orifice so that the center of the core pin is greatly deviated from the center of the die orifice. Upon such deviation, on a side where a thinner wall thickness of the tube is formed, the core pin forms an angle which is smaller than 180° with respect to a direction of an overall resin flow in a cylinder caused by ram pressing, namely a direction of a center axis of the cylinder (namely, the angle being formed between the mandrel and the core pin). To the contrary, on a sidle where a thicker wall thickness of the tube is formed, the core pin forms an angle which is larger than 180° with respect to the direction of the overall resin flow (namely, the angle being formed between the mandrel and the core pin).
In the above situation, the angle which is smaller than 180° produces a resistance against the resin flow, the core pin tries to move so as to reduce the resistance (namely, the core pin tries to approach the center axis of the die orifice). If the core pin could move so as to reduce the resistance, as a result of this the center of the core pin can substantially approach the center of the die orifice at a level of the outlet end of the die orifice. That is, the core pin is able to move so as to minimize the wall thickness deviation.
For example, in the case where the center of the core pin is deviated from the center of the die orifice at a level near the die orifice outlet due to a certain reason before the extrusion, whether the center of the core pin can or cannot return to the center of the die orifice (namely, becoming a concentric state) or approach the center of the die orifice at the level near the die orifice outlet depends on whether the core pin can or cannot bend (or deflect) under the extrusion conditions. Thus, the present invention resides in that shaping the core pin of the extruder such that it bends more easily than the conventional (or prior art) core pin so that the center of the core pin easily returns to the center of the die orifice around the core pin, which leads to the improved centering condition, whereby the wall thickness deviation of the extruded tube is improved.
Thus, in the first aspect, the present invention provides an extruder which produces a polytetrafluoroethylene tube by extruding a polytetrafluoroethylene through an annular portion which is formed by a core pin and a die orifice around the core pin, characterized in that the core pin comprises a mandrel connection end which is connected to a mandrel and a die orifice outlet end which is located at a level of a die orifice outlet, and the core pin has between its both ends a portion of which cross-
Kawachi Shoji
Murakami Shinji
Uchida Tatsuro
Daikin Industries Ltd.
Ferguson L.
Mulvaney Elizabeth
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