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Патент USA US3031365

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April 24, 1962
M. BALKIN' ETAL
3,031,357
METHOD OF‘ MAKING A POLYTETRAFLUOROETHYLENE GASKET ENVELOPE
Filed April 21, 1958
BY
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ORNEYS
United States Patent 0
l
.1
1C6
3,031,357v
Patented Apr. 24, 1962
2
the polytetra?uoroethylene shrinks slightly and closely
3,031,357
METHOD OF MAKING A POLYTETRAFLUORO
ETHYLENE GASKET ENVELOPE
embraces the mandrel and ?nally removal of the man
drel, still between its cover plates, from the oven for
Marl: Balkin and Denys G. Turner, Newcastle-upon-Tyne,
England, assignors to George Angus & Company Lim
?nal cold pressing which effects rapid cooling.
envelopes thereof from polytetra?uoroethylene which
envelope.
A typical production of a polytetra?uoroethylene en
velope according to the invention will now be described
with reference to the accompanying drawings, in which:
Filed Apr. 21, 1958, Ser. No. 729,958
FIG. 1 is a plan view of a ring mandrel having tape
Claims priority, application Great Britain Apr. 25, 1957
wound thereon.
1 Claim. (Cl. 156—-189)
10
FIG. 2 is a fragmentary radial cross-section of a taped
This invention relates to envelope gaskets and its ob
mandrel between plates for pressing, and
ject is to provide an improved method of making the
FIG. 3 is a radial section through a completed gasket
ited, Newcastle-npon-Tyne, England
can be processed in a manner to be described.
Polytetra-?uoroethylene powder of line particle size is
Envelope gaskets are used to provide seals in joints 15 mixed with about 20% by weight of a volatile liquid,
and the like at which only a relatively low holding pres
such as solvent naphtha, and slightly compressed to a
sure can be used, such as in equipment made from
cylindrical preform. The preform is placed in an ex
porcelain, glass or glass-lined, and consist each of a re
trusion press and tape is formed by extrusion through a
silient ?ller ring, such as of rubber, enclosed in an en
die. The tape may then be allowed to air dry or passed
velope which is of deep channel cross-section so as to 20 through an oven to evaporate the volatile liquid. Al
sheath and protect the ?ller ring. Polytetra?uoroethyl
ternatively, the tape while still moist with the liquid may
ene is generally used for the envelope because it is chem
be used for wrapping a mandrel as described below and
ically inert, resistant to heat and, although ?exible, is
capable of holding a high polished surface not easily
the liquid be removed, by air drying or moderate heat
ing, at any suitable stage prior to the ?nal heat treatment.
dented or scratched.
25 So long as the tape has not been fused by heating, the
Existing methods of making polytetra?uoroethylene
liquid will migrate through and evaporate from super
gasket envelopes include moulding from powder or ma
posed layers of the tape even after cold pressing.
chining from moulded blocks. Both these methods are
A suitable thickness of extruded tape for the manu
relatively expensive and are excessively so for large size
facture or" gasket envelopes is 0.002" and the width of
gaskets which may be up to two feet in diameter. To 30 the tape, although small in relation to the circumference
avoid the expense of moulding or machining, large size
of the envelope to be made, is conveniently proportional
envelopes are made from sintered and peeled strip or
to the size of envelope to be made. For example, for
tape. For example, a cylinder is ?rst moulded and sin
making an envelope of 8" internal diameter a suitable
tered followed by peeling to a suitable thickness. The
width of tape is 1”.
peeled strip is cut to length, the ends joined together to 35 As shown by FIG. 1, to make a plane circular envelope
form an endless band and the borders are then mechan
the tape 1 is wrapped toroidally around a plane ring
ically stretched and ?nally folded down over an inner
mandrel 2 with a half width overlap of successive turns
?ller ring. Large envelope gaskets made in this way
and reversal of direction of winding each time winding
have the serious disadvantage of a pronounced joint,
once around the mandrel is completed. A convenient
which impairs sealing in service, and many attempts have
mandrel for making an envelope of internal diameter
been made to provide a completely jointless envelope for
8” is a steel ring of the same internal diameter, or slight
the manufacture of larger-sized gaskets.
ly larger to allow for shrinkage, 1/2" wide and .080"
According to the present invention, a method of mak
thick. The mandrel should preferably be chromium
ing a gasket envelope of polytetra?uoroethylene material
plated so as to avoid discoloration of the polytetrafluoro
comprises winding thin tape of the material around a 45 ethylene during sintering. The edge 3 of the mandrel,
mandrel in overlapping layers until a su?icient thickness
see FIG. 2, which is to form the closed edge of the en
has been built up and then subjecting the taped mandrel
velope, i.e. the base of the deep channel section, should
to pressure and to heat to cause merging together of the
layers into a homogeneous envelope. By thin tape is
be rounded, so as to avoid sharp bends in the cross-sec
tion of the eventual envelope, but the other edge may
meant tape of a thickness which is a sub-multiple of the 50 be square, this facilitating cutting-ohc of one edge of
thickness of material required to form the ?nished en
the sheath from which the envelope is formed, as de
velope so that the difference in thickness at the edge of
scribed later. For making such an 8" envelope, four
an overlap in the wound tape is relatively small and dis
lengths of tape, each 6 feet long, 1" wide and 0.002"
thick, may be used, each length being suf?cient for one
The tape is wound toroidally on to a ring mandrel, 55 complete winding of the mandrel with a half overlap be
each turn overlapping the previous turn and the direc
tween successive turns. This gives four complete lay
tion of winding being reversed in successive layers.
ers of tape 1*‘, 1“, 1° and 1‘1 as indicated in FIG. 2.
After the taped mandrel has been pressed, and heated,
The unsintered polytetra?uoroethylene tape is su?i
the original layers of tape form a complete, smooth-sur
, ciently cohesive to permit the handling involved in wind
faced sheath enclosing the mandrel. To convert this 60 ing and is slightly tacky so that the turns and layers
sheath into a gasket envelope, one peripheral edge, either
adhere together well. In this respect, the winding of the
the inner or the outer according to the type of envelope
tape while still moist may be advantageous.
required, is cut off to reveal the edge of the mandrel and
As indicated in FIG. 2, the taped mandrel is placed
the sheath, having sufficient ?exibility, can be opened
between a pair of plane, chromium-plated, steel plates
and stripped from the mandrel without permanent defor 65 4 and 5 and pressed cold for a few seconds in a hydrau
mation.
lic press, applying, for example, 10 tons pressure. The
The polytetrafluoroethylene tape is of unsintered ma
taped mandrel between its plates 4 and 5 is now removed
terial, formed conveniently by extrusion. The pressing
from the press and, if the moist tape has been used,
and heating steps to which the taped mandrel is sub
allowed to dry or dried by moderate heating, to say
jected preferably comprise initial cold pressing, removal 70 100° C., before being baked in an oven to fusion tem
from the press of the taped mandrel held between cover
perature, for example, for one hour at 360° C. After
plates, heating in an oven to eifect sintering during which
baking, the taped mandrel between its plates 4 and 5 is
appears under the pressure and heat.
.
3,031,357
3
1%
quickly replaced in the cold press, immediately subjected
polytetra?uoroethylene and a volatile liquid, winding
said tape toroidally around a rigid plane ring mandrel,
to say 10 tons pressure and left to cool for approximate
ly ?ve minutes. After removal from the press a periph
eral edge of the polytetra?uoroethylene sheath now
formed on the mandrel, in the example illustrated the
outer edge 6, is cut o?, as indicated by the line X—X in
alternately in opposite directions in superposed layers,
pressing said taped mandrel between plane metal plates
for a few seconds without heating removing the volatile
liquid to dry the tape at any stage in the method thus
far effected, subjecting said mandrel with the tape in
dried condition thereon to heating to effect sintering of
FIG. 2, and the remaining major part of the sheath re
moved from the mandrel forms the ?nished envelope.
As shown in radial cross-section by FIG. 3, the enve
said tape, immediately after said sintering pressing said
lope is a ring 7 having an outwardly directed channel 8 10 taped mandrel between unheated plane metal plates until
said tape is cool and has become formed into a complete,
formed by a U-section wall 9 of even thickness.
smooth-surfaced, homogeneous sheath enclosing said man
An envelope formed in this way is found to have a
drel, cutting only one peripheral edge of said sheath to
smooth polished surface and, the polytetra?uoroethylene
reveal said mandrel and stripping said sheath from said
being translucent, although the lines of overlap of the
turns of the tape can be more or less faintly seen criss
15
mandrel, said sheath constituting the gasket envelope.
crossing at close intervals around the ring, the thickness
of the envelope material is uniform and presents no un
evenness which would impair surface sealing. In the
case of the example given above of an 8" ring, the ?n
ished thickness of the polytetra?uoroethylene is 0.015".
It will be appreciated that the method provided by the
present invention o?ers a substantial advantage over di
rect moulding or machining methods. For any size of
gasket only a plane steel mandrel ring of appropriate
size is required, as compared with the fully shaped mould
parts required for direct moulding.
We claim:
A method of making a gasket envelope which com
prises preparing a thin tape comprising. an unsintered
References Cited in the ?le of this patent
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