US 3225129 A
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Dec. 21, 1965 TAYLOR ETAL 3,225,129
METHOD OF MAKING MEMORY RE-SHAPED PLASTIC TUBES, ESPECIALLY FLUOROCARBON CYLINDER JACKETS Original Filed July 19, 1960 IN VEN T 0R5 John STaylor BY William Lfimves.
W, a Mm A TTORNEY United States Patent assignors to The Budd Company, a corporation of Pennsylvania- Contiuuation of application Ser. No. 43,844, July 19, 1960. This application June 26, 1962, Ser. No. 207,144 1 Claim. (Cl. 264230) This application is a continuation of pending application Serial No. 43,844, filed July 19, 1960, now abandoned and assigned to the same assignee as is this application.
This invention relates to a method of making memory re-shaped plastic tubes, especially fluorocarbon cylinder jackets, and method of making, and has for an object the provision of improvements in this art. Memory plastics include all of the fluorocarbon group which have memory after deformation, as by Working at normal or elevated temperatures and cooling to leave internal strains so as to be capable of returning toward original shape, usually after reheating. Polytetrafluoroethylene (PTFE or Teflon), polytrifiuorochloroethylene (PTFCE or Kel-F), etc. are of this group.
One of the particular objects of the invention is to make a tight-fitting liner or jacket for a cylinder, such as a tube or roll, which will have the low-friction surface and other characteristics of this material.
Another object is to make a tube of improved strength and freedom from porosity.
The above and other objects and advantages of the invention will be apparent from the following description of an exemplary embodiment, reference being made to the accompanying drawings, wherein:
FIG. 1 is a diagrammatic view showing multiple tape windings on a cylinder;
FIG. 2 is a diagrammatic view showing mechanical Working to enlarge and stress a tube;
FIG. 3 is a diagrammatic view showing pressure expansion of a tube; and
FIGS. 4 and 4a are side and end views showing a tube being shunk on a cylinder.
One of the particular uses of the invention is to provide a relatively thick tight shrunkon jacket for rolls which has a low coefiicient of friction and which will resist the sticking on of surface coatings in operation. For example, processing rolls in the textile industry need such a jacket to resist the accumulation of sizing and other fouling substances and to have long wear. There are many other uses.
Such rolls have been provided with PTFE by two methods in the past. By one method the rolls are spraycoated with a PTFE dispersion to form a relatively very thin jacket (about 0.001") which is then sintered. This gives a jacket which has only a short life and the rolls must be taken out of service and sent away for recoating. Moreover, the rolls may be warped at the temperature required for sintering. By another method a relatively very thick jacket is machined to fit the roll and secured thereon by fastenings. This is a difficult and expensive job, very wasteful of high-priced material and may get loose in service.
By the present method a jacket can be quickly installed at the textile mill by having spare jackets available and applying them by taking out the rolls, shrinking jackets thereon, and immediately putting the rolls back into use on the machine. The shrinkage temperature (up to about 400 F. for PTFE) is far below sintering temperature (about 621 F. for PTFE).
According to the preferred form of the present invention, a superior starting tube is made by helically cross- Patented Dec. 21, 1965 winding unsintered extruded and calendered PTFE tape, made for example according to the method disclosed in US. Patent No. 2,915,786 to Haroldson et a1. While the latter tape is disclosed as being very strong transversely for unsintered material as against practically zero cross strength for normally extruded strip, the transverse strength remains substantially less than the longitudinal strength.
FIG. 1 shows a tube being made this way. Here a plurality of layers, 10, 11, 12 and 13, of unsintered extruded and calendered tape are helically wound on a cylinder or mandrel 14 of metal, the layers having a small edge overlap. The preferred method is to helically crosswrap, as shown, but parallel wraps can be used. The tube so formed is sintered to integrate the edges and layers. During sintering the tape shrinks to the mandrel producing a tightly laminated tube. Shrinkage is inherent due to the calendering operation which occurs during the manufacture of the tape and the nature of the material.
An outer mold or rolling may be used during sintering if a very smooth outer surface is wanted. Another aid to smoothness would be to abut the edges of the final layer (the inner if used as an inner liner) instead of overlapping them like the inner layers.
The tape preferably is quite thin, say about 5 mils (0.005), so that the surface is fairly smooth after sintering even if a mold is not used. As many layers or wraps as wanted may be used; for cylinder jackets for textile rolls it has been found that four layers of 5 mil tape, giving a jacket 20 mils thick, gives good results. Where used for an inside jacket a thicker wall may be needed. After sintering the tube is removed from the cylinder, the cylinder having been coated with a suitable release agent (a silicone release agent such as Dow-Corning No. 20 mold release being one suitable type), and is ready for further treatment. Removal of the tube can be aided by forcing fluid (air will serve) between the tube and the mandrel or cylinder on which it has been formed.
This further treatment consists in deforming or changing the shape of the tube by force to put residual stresses in it. This makes it heat-unstable. The tube thereafter tends by memory to return to the sintered shape after a period of time. Return to or toward the original undeformed state is hastened by heating the material to a point above a critical temperature, which releases the locked-in elasticity of the material and leaves it in a heatstable condition.
It will be understood that the tube is cold-deformed from its initial diameter to a diameter intermediate its initial diameter and the final diameter. When the tube is to be a roller coating, the initial diameter will be less than and the intermediate diameter will be greater than the outer diameter of the roller; when the tube is to be a liner, the initial diameter will be greater than and the intermediate diameter will be less than the inner diameter of the item to be lined.
At room temperature (a cold-forming temperature) the tube is more difficult to stretch than at an elevated temperature but recovery is more complete at the temperatures available (300400 F.). Therefore, stretching at room temperature is a recommended procedure. If it is to be placed on a cylinder immediately, the deforming and jacketing operations can be performed at a relatively low temperature. If the tube is deformed at raised temperature, it tends to maintain its deformed shaped until again heated to a higher temperature than that at which it was deformed. The temperatures found suitable are from about 300 F. to 400 F., far below the sintering temperature of about 621 F. for PTFE.
FIG. 2 shows one method of deforming a large tube T to increase its diameter by rolling its Wall with rolls 15.
FIG. 3 shows an arrangement for deforming a tube T to increase its diameter by confining it in a cylinder 16 by stoppers 17 held tight, as by a rod 18, and expanding it by fluid pressure, asair, applied internally at an inlet The actions indicated in FIGS. 2 and 3 may be affected at room temperature or at any suitable elevated temperature below the sintering or melting temperature. Such temperatures can be established by placing the assembly in a heated enclosure. Cooling may be similarly controlled, with pressure still maintained, by placing in an enclosure with a regulated temperature.
FIGS. 4 and 4a illustrate how a deformed tube Tl,
indicated in broken lines as formed in FIGS. 2 or 3, is undeformed or restored to shrink it on a cylinder or roll 26 as a tight jacket T2. When jackets are formed and applied as described, they remain tightly in place in use and provide a durable low-friction surface. The jacket is especially good when made of cross-Wrapped unsintered extruded-calendered tape, as described.
While one embodiment of the invention has been described for purposes of illustration, it is to be understood that there may be various embodiments and modifications within the scope of the invention.
What is claimed is:
A method for forming a composite article having a rigid cylindrical member of a given diameter and a polytetrafluoroethylene tube memory fitted to an external surface of said member, said method comprising the ordered steps of: helically winding about a cylindrical mandrel having a diameter less than said given diameter, an un-. sintered extruded polytetrafluoroethylene tape exhibiting substantially all of its orientation and tensile strength in the lengthwise direction of said tape; sintering the polytetrafiuoroethylene tape Without completely relaxing its orientation to provide a tube having its tensile strength oriented helically about the major axis of the tube; removing tube from said mandrel; expanding said tube radially to a diameter greater than said given diameter, inserting said member into the interior of said tube of enlarged diameter, and heating and maintaining tube at a temperature sutficient to memory shrink the tube to said given diameter.
References Cited by the Examiner UNITED STATES PATENTS 2,027,961 1/1936 Currie 264-230 2,027,962 1/ 1936 Currie. 2,941,911 6/1960 Kumnick et al. 18-55 2,964,065 12/1960 Haroldson et al 1855 FOREIGN PATENTS 484,849 7/ 1952 Canada.
OTHER REFERENCES E. I. du Pont de Nemours & Co., Teflon, Tetrafluoroethylene Resins, Properties and Uses, pages 15 and 16, August 1957.
ROBERT F. WHITE, Primary Examiner.
ALEXANDER H. BRODMERKEL, Examiner.