Note: Descriptions are shown in the official language in which they were submitted.
1 325092
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ELECTRIC FUSION ~ELDING OF THERMOPLASTIC
This application is a division of Canadian Patent Application
Serial no: 529,474 filed February 11, 1987.
BACKGROUND OF THE INVENTION
The present invention relates to an improved
method of forming a weld between members of thermoplastic
materials. Such materials include polyethylene and poly-vinyl
chloride, polytetrafluoroethylene and the like, and specifically
includes linear high density polyethylene.
Numerous techniques have been developed for the joining
of thermoplastic materials including the use of adhesive and
welding. Certain of these materials, however, ar~ not adapted to
the use of adhesives, such as polyethylene. Such material has in
the past been welded by a technique known as extrusion welding
using a tool which applies heat to overlapped sheets of the
thermoplastic material and extrudes a bead of the same
thermoplastic material at the edge of the lap, which is intended to
fuse with the preheated sheets of the thermoplastic to be ~oined to
form a liquid fusion of the two sheets and the extrudate, thus
welding the overlapped sheets. This technique has not proven
entirely satisfactory in practise because of a lack of uniformity
in the welding process especially where hand held extrusion welding
tools are used on large sheets of material.
Another technique known in the prior art involves the use
of a so-called "speed tip" for welding thin sheets of material in
which a welding rod of the thermoplastic material is fed through
the nozzle of a hot air gun and a stream of hot air together with
25 the melted thermoplastic are applied to the area to be welded. Once
again problems of uniformity and completeness of weld are evident.
Frequently sush welds include pinholes or unwelded portions and can
also include portions in which excess heat has been applied and the
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DESCRIPTION OF THE PRIOR ART
The present invention i8 directed to an electrically
heated technique for forming either lap or butt welds between two
members of a thermoplastic material. Such techniques have been
attempted in the past as disclosed in the teachings of the
following U.S. patents.
:'
2,243,506 granted May 27, 1941, B.V. Mitchell
2,647,072 granted July 28, 1953, W.K. Smith
2,742,390 granted April 17, 1956, Beck
- 2,974,566 granted March 14, 1961, A.B. Hurley
3,049,465 granted August 14, 1962, P.E. Wilkins
3,061,503 granted October 30, 1962, G.E. Gould et al
3,348,640 granted October 24, 1967, R.R.Thompson et al
4,416,713 granted November 22,1983, Brooks -
In each of the above patents a technique for ~oining or
vulcanizing thermoplastic or rubber is disclosed in which an
electrical heating element is provided either to soften an adhesive
as in Brooks, or to cure a thermo setting adhesive bonding agent as
in Thompson, or to preheat plastic members which are then pressed
into intimate contact as taught by Gould, Wilkins, Hurley, Beck and
Smith. Mitchell teaches the vulcanizing of a patched rubber boot
using an electrical heater to achieve the vulcanization
temeperature.
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SUMM~RY OF THE INVENT I ON
In none of the known prior art is there taught an
electric fusion welding technique in which an electrical element is
provided surrounding a core of the thermoplastic material forming
the two members which are to be joined. In contrast the present
invention provides a method of welding adjacent mem~ers of a
thermoplastic material which comprises forming a welding rod of
this thermoplastic material, and wrapping the rod with an electric
resistance element. This welding rod is positioned between the
thermoplastic members to be welded and an electrical current is
applied to the resistance element. Simultaneously, the members to
be welded are pressed together. The current is applied for a time
and at an intensity to cause the welding rod and portions of the
thermoplastic material of the adjacent members to be welded to
soften and fuse thereby welding the two members together. The
resistive wire forms a mechanical reinforcement in the weld thereby
rendering a superior weld between the thermoplastic members. This
welding technique has been successfully used with materials
traditionally considered impossible to weld, such as high density
linear polyethylene, polyvinyl chloride, a thermoplastic material
sold under the trade-mark DELRIN, a thermoplastic material sold
under the trade-mark NYLON, and the like. Preferred forms of
electric re~istive element include windings of nichrome or
stainless steel wire, although it i8 within the concept of the
present invention to utilize other forms of resistive elements such
as coatings of suitable resistivity in place of the helical coil of
wire, provided such coating is compatible with the thermopla tic
material.
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BRIEF DESCRIPTION OF THE DRAWINGS
In the accompanying drawings:
Figure 1 is a perspective view of a welding rod in
accordance with the present invention, ~-
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Figure 2 is a cross-section through a welding rod of
Figure 1,
Flgure 3 is a cross-~ection through the welding rod
having a rectangular core,
Figure 4 illustrates a lap joint between two sheets
of thermoplastic u~ing the welding rod of Figure 1,
Figure 5 illustrates the completed weld of the
materials being joined in Figure 4,
Figure 6 is an enlargement of the weld area of
Figure 5,
Figure 7 illustrates the slectrical connections for
making a weld,
, : .
Figure 8 illustrates a butt weld between curved
members ~uch as for example a longitudinal butt weld on a pipe,
:
Figure 9 illu~trates the weld of Figure 8 on
~0 completion, ~ .
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Figure 10 is perspective illustrating the use of the
invention in performing a transverse butt weld on piping,
Figure 11 is a perspective view of a lap weld in .:
accordance with the present invention, . -
Figure 12 illustrates a continuous weld lap welding
technique utilizing the welding rod of the present invention.
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DESCRIPTION OF THE PREFERRED EMBODIMENT
Referring to Figure 1 of the drawing there is shown in
parspective a welding rod 9 in accordance with the present
invention. A rod 10 of thermoplastic material is wrapped with a
helical winding of nichrome or stainless steel wire 11. The size of
wire to be used in relation to the size of the core and the spacing
of the turns of the helix depends upon the thickness of the
material to be welded and the required current to bring about the
appropriate heating of the weld area.
In Figure 2 a cross-section of the rod 9 of Figure l is
shown in which the core 10 with its winding 11 are clearly
illustrated. Figure 3 illustrates an alternative form of core 10 of
rectangular cross-section with a similar winding 11 wrapped in
helical fashion about the rectangular core.
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It is also within the scope of the present inventions to
simultaneously wind a plurality of wires around the core which
wires may for example be connected together in parallel to reduce
the total resistance of the winding, for electrical rea~ons. ;~
Figure 4 illu~trates the procedure to be followed in
assembling the components for a weld. A pair of sheets of
thermoplastic material 12 and 13 are positioned on either side of
a welding rod having a core 10 and winding 11. An electrical
current is passed through the winding 11 and simultaneously
pressure is applied above and below on the sheets 12 and 13,
25 resulting in the melting and fusion of the core and the adjacent -
portions of the sheets 12 and 13 to form a unified weld. The
electrical resistance member remains in thP weld and reinforces or
"stitche" the weld mechanically.
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In Figure 6 there is an expanded cross-section through
the weld formed by the process illustrated in Figures 4 and 5. As
can be seen from this cross-sectional view, the material of the rod
10 is now indistinguishable from the welded material o~ the shests
12 and 13, and the weld is reinforced by the electrical resistance
member embedded therein.
Figure 7 illuætrates the electrical circuit required to
perform the welding operation. In its simplest form a source of
electrical power here represented as a variable voltage transformer
14 is connected to a source of power at 15 and is connected via a
suitable switch 16 to the winding 11. Current from the variable
voltage transformer is passed through the winding 11 for a
predetermined period of time causing the rod 10 and the adjacent
portions of the thermoplastic members 12 and 13 to be heated and
fused together by the application of pressure at the same time that
electric current is applied to the coil 11.
Figure B illustrates a butt weld between curved members
for example segments of a longitudinally slit thick walled
thermoplastic pipe. As before a welding rod consisting of a core 10
and a winding 11 is positioned between the thermoplastic members 17
and 18. Electric current is applied and pressure is subsequently
applied to urge the edges of the members 17 and 18 into contact.
The weld is completed when the welding rod is fused and embedded in
the weld. The structure of the weld is such that the welding rod
2~ becomes part of the body of material of the two thermoplastic
members that are joined and the weld is simultaneously reinforced
by the presence of the windinq 11. This latter completed
configuration of the butt weld is illustrated in Figure 9.
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Figure 10 is a perspective view illustrating the manner
in which a butt weld end to end of circular members such as
thermoplastic piping can be achieved utilizing a ring shaped
welding rod with a helical winding wound thereon. The ring shaped
welding rod is positioned between the members to be butt welded,
the members are pressed together to complete the weld.
Figure 11 illustrates in perspective a lap weld of the
present invention in which may be seen in the welding rod 10, the
winding 11, the thermoplastic members 12 and 13 overlapped and
welded.
Such welds may be made continuously aæ i8 illustrated in
Figure 12. In Figure 12 a bottom sheet 20 is fed by uitable means
includin~ rolls 21 and 22 into a welding region. Simultaneously a
welding rod 23 with a helical winding 24 thereon is supplied to the
continuous welding process. The welding rod is heated by the
application of an electric current between shoes 25 and 26, which
shoes are intended to contact several turns of the helix 24 causing
heating of the welding rod 23 which is then partially embedded in
the surface of sheet 20. Pressure is applied between the shoe 26
and the roll 21. The welding rod is thus embedded in the surface of
the sheet 20 with its upper edge exposed. Further heating of the
coil 24 occur~ between the shoe 26 and the shoe 27 due to a f~rther
electrical current passed through the coil 24. The heated strip 23
is then contacted by the upper sheet 28 and the weld between the
sheets 20 and 28 is completed by passing between rolls 29 and 22.
Clearly various modifications to the processes of the
present invention are possible, and all such modifications are
intended to be included within the scope of the appended claims.
For example it is within the scope of the present invention that
the welding rod with resistive wire could further be coated with an
additional layer of thermoplastic material where a second layer of
thermoplastic material in the weld would be desirable. -~
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