Note: Descriptions are shown in the official language in which they were submitted.
~3007~
METHOD AND APPARATUS FOR BONDING
THERMOPLASTIC MATERIALS
BACXGROUND OF THE INVENTION
This invention relates to an improved method
and apparatus for bonding thermoplastic materials to
one another.
Techniques for bonding thermoplastic materials
to one another have been known for quite some time.
Examples of such bonding techniques are described in
Welding of Plastics, Neumann and Bockhoff, ~einhold
~, Publishing Co., 1959, and include hot plate and friction
i 10 welding. By either of these techniques, the edges of
the plastic materials to be bonded are heated to bring
the plastic at the edges to its fusion temperature.
As soon as the edges are sufficiently heat-softened,
they are quickly joined together under pressure until
15 the melted or softened edges have cooled sufficiently
! to form a strong joint. During the welding operation,
the pressure between the two softened edges of the
thermoplastic materials should be sufficient to force
out air bubbles and to bring the entire edge surfaces
20 into intimate contact. The resulting pressure on the
softened edges as they are joined together results in
the formation of a rounded bead along the junction of
the two thermoplastic materials. In the past after the
bonded or welded edges cooled, the rounded bead was
` 25 removed by sanding in an area about the juncture of the
bonded edges or by cutting away the bead. This was
followed by a polishing step.
In many applications, however, the integrity,
reliability and durability of the weld or bond is of
30 critical importance. As one example, when thermoplastic
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pipes are bonded to one another by means of a hot plate
weld, it is very important that the weld have the
require strength and durability in order to serve the
purpose of conveying fluids under varying temperatures
5 and pressures in an environment which may be subject
to substantial vibrations. As a second example, some
battery jars are formed by hot plate welding techniques.
These battery jars contain a liquid electrolyte and
support a series of heavy electrodes. ~hen place in
10 situ, the battery jars are subjected to vibration and
occasional shock impulse forces, and accordingly, the
~ welds must be of substantial strength and durability to
I remain functional over a long period of time.
In order to test the integrity and reliability
15 of these welds a number of techniques have been develop-
ed. One method is to establish a very high electromag-
netic field across the weld to determine whether
dielectric breakdown occurs. If there are minute pores
and/or cracks in the weld, the dielectric strength of
20 the weld will be reduced and upon establishing the
electromagnetic field across the weld, a spark will be
generated.
Another technique for testing the integrity
and reliability of welds is to generate a mechanical
25 impulse force against the weld to determine its
` resistance to fracture. In the battery jar industry
this is accomplished by dropping a weighted dart from
a preset distance onto the weld to generate a very high
point pressure differential across the weld. Of course,
30 other impact techniques can be used depending upon the
design requirements of the finished product. These
techniques for measuring the reliability and strength
of welds have proven useful in many applications where
the integrity of a weld joint is of critical importance.
~3~071
Using these and other known testing techniques, it has
been found that the formation of hot plate welds by the simple
heating of the edges of the thermoplastic materials to be
joined and then forcing the edges against one another to form
the weld results in decreasing tensile strength of the material '
at the weld junction; that is the tensile strength of the mater-
ial at the weld junction can be 85 percent of the tensile
strength of parent material and lower. In addition, the dielec-
tric test failure rate resulting from generating a large elec-
tromagnetic field across the weld increases as much as 100 times
over the dielectric test failure rate of the parent material.
Further, the impact strength of such welds when tested by drop-
ping a dart onto the weld was found to be reduced substantially
over that of the parent material and in addition varied sub-
stantially at different points along the welds and from one weld
to the next to thereby reduce the overall reliability of the
weld. Further, the bending strength, particularly the flexural
deflection, of the weld about the axis of the weld was found
.,
to be reduced substantially.
It is therefore an object of this invention to provide
an improved method of bonding thermoplastic materials to one
another to improve the strength and reliability of the bond.
~ ::
~ It is another object of this invention to provide
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an improved apparatus for bonding thermoplastic materials to
one another.
Aacordingly, this invention relates to an improved
method and apparatus for bonding thermoplastic materials to
one another.
According to the present invention, then, there is
provided an apparatus for improving a heat welded junction in a
battery jar which is formed of thermoplastic material and is
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formed of two elongated members, one of the members being the
mirror image of the other member, each of which members open at
the top thereof and has an elongated open side and three elong-
ated closed sides which are heat welded together along the
edges of the open side, the apparatus comprising: means for
heating the thermoplastic material at the welded junction and
in the vicinity thereof to at least about the fusion temperature
of the thermoplastic material; and means for quickly reducing
the temperature of the thermoplastic material to at least below
its fusion temperature, the temperature reducing means including
means for drawing ambient air in and about the heated thermo-
plastic material.
According to a fuxther aspect of the present invention,
there is provided an apparatus for improving a heat welded junc-
tion formed between two pieces of thermoplastic material com-
prising: means for heating the heat welded junction of the
thermoplastic materials to at least about the fusion temperature
thereof;
means supporting the heating means for quickly re-
ducing the temperature of the thermoplastic material at thewelded junction to below the fusion temperature thereof, the
temperature reducing means including means for drawing ambient
air in and about the welded junction and past the heating means;
and means for actuating the means for heating and the means for
reducing the temperature to cause them to contact the weld junc-
tion.
According to yet another aspect of the present inven-
tion, there is provided a process for improving the bonding at
a heat welded junction in a battery jar which is formed of ther-
moplastic material and is formed of two elongated members, eachof the members being the mirror image of the other of the two
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~3QV7~
members, each of which members is open at the top thereof and
has an elongated open side and three enlongated closed sides,
which members are welded by heating the edges of at least one of
the elongated open sides and joining the edges under pressure to
form a welded junction, the pressure causing a bead to be formed
on at least one side of the welded junction, comprising: heating
the thermoplastic material at the welded junction and in the
vicinity thereof to at least about the fusion temperature there-
of under pressure sufficient to mash the bead; and drawing
ambient air over the heat welded junction to quickly cool the
thermoplastic material to below the fusion temperature thereof.
According to yet another aspect of the present inven-
tion, there is provided a process for heat welding two pieces
of thermoplastic materia} to each other comprising the steps of:
heating at least one edge of each of the two pieces to at least
about the fusion temperature of the thermoplastic material;
` joining the heated edges to one another under pressure to form
a welded junction, the pressure causing a bead of plastic to be
formed on at least one side of the welded junction; and heating
and quenching the welded junction, the thermoplastic and bead by:
(a) providing an electrical}y conductive strip of a titanium-
containing material of relatively high resistivity and passing a
current therethrough to generate heat sufficient to fuse the
thermoplastic; (b) disposing the heating strip on the bead under
a pressure so that the combined effect of heat and pressure
mashes the bead until it is substantially flat; and (c) rapidly
; cooling the area defined by the substantially flattened bead,
by drawing ambient air under reduced pressure over the area.
A preferred embodiment of the improved apparatus of
the present invention includes a strip of material which can be
heated and cooled relatively rapldly. The strip which is pre-
- 5a -
~.~.3007i
ferably the shape of the weld junction is supported by an in-
sulating material which has a grooved network throughout the
surface thereof which supports the strip. The strip may be
heated, for example, by an electric current and is cooled by
drawing air from the area surrounding the strip through the
groove network and out through a vacuum pump.
In operation, after forming the welded junction, the
strip of material is forced against the welded junction that
has been formed during the welding step and is heated to
approximately the fusion temperature of the plastic material.
The heated weld junction area and the strip are then rapidly
cooled by drawing ambient air past the junction area and the
strip through the groove network. When the plastic material
has cooled sufficiently, the strip is removed from the plastic
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material.
BRIEF DESCRIPTION OF THE DRAWINGS
Other objects, features and advantages of the
present invention will become fully apparent from the
5 following detailed description of the preferred embodi-
ments, the appended claims and the accompanying drawings
in which:
FIGURE l is a simplified illustration of a
welded joint having a rounded bead formed on each side
10 of the weld;
FIGURE 2 is a simplified perspective view of
: one embodiment of the apparatus for forming an improved
hot plate weld;
j FIGURE 3 is a perspective view of the preferredL 15 embodiment of the apparatus for forming an improved hot
plate weld;
FIGURE 4 is a cutaway side section view,
illustrated in an enlarged scale, of the apparatus of
. FIGURE 3;
FIGURE 5 is an enlarged section view of a
weld made in accordance with the process of the present
: invention;
FIGURE 6 is a cross-section view of a
: simplified apparatus using the embodiment of FIGURE
3;
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FIGURE 7 represents two end sections which
can be welded together to form a battery jar;
FIGURE 8 is a side elevation view of the
battery jar formed by welding the two end sections
depicted in FIGURE 7; and
FIGURE 9 is a plan view of the battery jar
of E'IGURE 8.
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DETAILED DESCRIPTI02~ OF THE INVENTION
_
In FIGURE 1 there is a cross-sectional view
of a weld joint formed by heating the respective edges
lO and 12 of two pieces of thermoplastic material.
5 After the respective edges have been heated to their
fusion temperature or until they become plastic, the
i edges are forced against one another to form a welded
! junction. The pressure on the molten plastic edges of
i the thermoplastic material resulting from forcing the
lO edges against one another creates a rounded bead ll on
each side of the weld. The dotted lines 13 and 13'
near each edge lO and 12 illustrate, in a simplified
manner, that portion of the thermoplastic material
which was reheated during the welding process. Weld
15 failure resulting from the aforementioned dart impact
; test procedure, frequently occurs between and along the
respective boundary lines 15 between the reheated
portions 13 and the non-heat;ed portions 17 of the
thermoplastic material.
Weld failure, as measured by the dart impact
test procedure, indicates that the thermoplastic
material at the welded junction is more brittle and less
ductile, than the parent material. While decrease in
~ tensile strength of the material at the welded junction
-~ 25 has been noted, brittleness and loss of ductility of
the material at the weld junction, compared to the
parent material, are more serious side effects of the
welding process. Moreover, the material at the junc-
tion is characterized by a much higher rate of dielec-
30 tric test failure, according to dielectric re~uirements
of the industry, compared to the parent material.
Various reasons for the variations in weld
impact strength at the welded junctions were proposed:
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(1) The molecular weight distribution of the thermo-
plastic materill might influence weld impact strength
and might account for the variations; (2) the material
was becoming oxidized during welding and was, therefore,
more brittle; and (3) the crystalline structure of the
material in and adjacent to the weld line was coarsened
due to the welding heat. However, it could not be
established that any one or combination of these
reasons resulted in the decrease of impact strength of
the thermoplastic material at the weld junction.
In accordance with the invention, it was dis-
covered that impact strengths of the material at the
welded junction could be increased and that dielectric
test failure of the material at the welded junction
could be substantially eliminated, (1) by heating the
15 material at the weld junction to a temperature at least
about the fusion temperature of the thermoplastic
material, optionally at elevated pressure, and (2) by
quickly quenching the heated junction to a temperature
at least below the fusion temperature. As stated
20 above, when welding two pieces of thermoplastic material,
a bead occurs along at least one side of the welded
junction. In accordance with the process of the
invention, the bead can be removed prior to the steps
of heating and quenching, but preferably it is not
25 removed.
The exact temperature of heating will depend
on the exact thermoplastic materials which have been
welded, and, for instance, can be as low as 300F fox
branched polyethylene and can be up to 900F when .he
30 thermoplastic is high density polyethylene thermoplastic.
That is, the exact temperature of heating will depend
on the fusion temperature of the thermoplas~i~ material,
i.e., that temperature at which it becomes molten. As
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a practical guideline, the exact temperature of heating
can be determined for a specific thermoplastic by
selecting that temperature at which sufficient fusion
occurs within a period of time up to about 25 seconds.
Pressure is applied to ths weld junction
during the step of heating or after the step af heating.
The pressure must be sufficient to cause the material
I at the weld junction to become substantially flat. In
practice, the pressure can vary widely depending on the
10 apparatus and temperatures used and can range from 20
to 90 lb./square inch. If the bead formed at the weld
junction is not removed, the pressure must be sufficient
to mash the bead against the welded junction until it
is substantially flat. Preferably, in the preferred
15 embodiment pressure is applied during the heating step
and the combined effect of the conditions of heat and
pressure is sufficient to mash the bead until it is
substantially flat.
After the pressure treatment, the heated,
20 welded junction is immediately quenched. Quenching
comprises rapidly cooling the heated, welded junction
to a temperature at least below the fusion point of the
thermoplastic material and preferably to a temperature
at which the thermoplastic lacks adhesive properties.
The quenching step is undertaken to resolidify the
material at the welded junction.
Quenching must immediately follow pressure or
heat-pressure treatment of the welded junction. That
is, quenching in accordance with the invention does not
include allowing the pressure or heat-pressure treated
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welded junction to cool at ambient conditions. Su~pri-
i singly, guenching after the steps of fusing the two
edges of thermoplastic and joining those edges under
pressure, i.e., i~mediately after formation of the
, 35 welded junction, does not ~n practice result in improved
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properties of the weld junction, witn respect to
dielectric properties, impact strength and flexural
deflection about the axis of the weld. Quenching may
be undertaken, for instance, by immersing the treated
junction into water.
In order to overcome the problem of decreased
tensile strength, low impact strength, high dielectric
test failure rates, and the problems resulting from
having a rounded bead extending along the longitudinal
10 length of the weld, an apparatus has been developed
which in its simplest form is illustrated in FIGURF. 2.
ln FIGURE 2, there is illustrated an insulating strip
19 which, in the preferred embodiment, is a ceramic
material or a high temperature plastic such as Torlon,*
a polyamide. A strip ~1, preferably of Titanium Alloy
6AL4B having a thickness of 0.30 millimeter and a width
of 25 millimeters, is positioned over the insulating
strip 19. As will be seen, the insulating strip 19
serves the dual function of an electrical and heat
insulator and as a mechanism for rapidly cooling, among
other things, the strip 21.
In the annealed condition, the strip 21 has
an electrical resistivity of approximately 180 micro-
ohms-centimeter, excellent corrosion resistance, and a
tensile yield strength of 130,000 lbs per s~uare inch
at room temperature. The high strength of the allGy i9
useful in resisting the local pressure forces generated
when first contacting the rounded weld bead. The high
strength is also useful due to the rorces imposed upon
~0 the Titanium strip when subjected to high temperatures.
As an example, when the Titanium strip is heated to
450-500F it inceases in length due to thermal expansion.
On the other hand, the plastic material outside of the
` weld zone, i.e., the area 13, is substantially able to
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keep the surface area of the bead about the heated
strip 21 from expanding or contracting along the weld
junction during the process of mashing the bead and
cooling the resulting mashed bead.
Consequently, the longitudinal expansion and
contraction of the heated strip 21 results in a shear
stress between the strip 21 and the weld bead material.
This could lead to flaws in the surface of the plastic
material and possible warpage of the plastic device
i 10 being formed by the welding step. Accordingly, the
Titanium Alloy strip 21 is placed under a longitudinal
tensile strain at room temperature which is slightly
greater than the maximum thermal strain which occurs
¦ during heating and cooling. The strain i9 maintained
j 15 constant during the heating of the strip 21 by tech-
niques known in the art. For instance, this strain
can be effected by screws 55 and 56 in FIGURE 6. In
j this manner each point along the alloy strip 21 remains
I in substantially the same location with respect to the
20 bead during heating and cooling, and accordingly the
length of the heated section of the strip 21 remains
substantially constant. Since a room temperature
stress of approximately 35,000 pounds per square inch
is necessary to provide the necessary strain on the
25 strip, which stress is reduced substantially under high
temperature, the strength of the strip 21 must be quite
~ high. It can be clearly seen that the 130,000 lbs/
s square inch tensile yie}d strength is more than adequate
for the stress levels induced into the strip 21.
As illustrated in FIGURE 1, the~heated strip
21 together with its insulating support 19 i9 pressed
against the bead 11 to cause it to fuse and become
plastic. The bead is pressed and flattened against the
weld area. During this operation, the fused thermo-
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plastic material will adhere to this strip 21. After
the strip is cooled below the melting point of the
thermoplastic material, the adhesion of the strip to
the thermoplastic material ceases and the strip can be
5 removed from the material.
It has been discovered that the welded joint
which has been heated in accordance with the invention
must be rapidly quenched in order to realize the
advantage of improved tensile strengths~ impact strengths
I 10 and dielectric properties of the material at the welded
¦ junction. Accordingly, a plurality of holes 23 are
formed in the strip 21, each of the holes being in
¦ communication with one another through a trough 25
! formed in the insulating support 19. In one embodiment
15 cool air is blown through the trough 25 and out through
the holes 23 about the heated thermoplastic material as
illustrated by the arrows in FIGURE 2. This quickly
! cools the thermoplastic material and the strip to
thereby provide the desired crystalline structure,
20 i.e., the mashed thermoplastic material illustrates a
smooth, closed surface having a very low dielectric
1 failure rate. The remolded weld bead forms and additional
J flat layer of material which becomes laminated to the
l~ parent material. Thus, the possibility of a minute
.! 25 flaw in the weld causing undesirable leakage is sub-
l stantially reduced.
j FIGURE 3 and 4 represent the preferred embodi-
i ment of the apparatus of the present invention. As
``, illustrated, an insulating strip 29, formed, for
30 instance, of ceramic or high temperature resistant
j plastic, has a groove 35 formed through the center
thereof with a plurality of transverse grooves 33 of
relatively small size being formed along the length
~ of the insulating strip 29. Positioned over the
i 35 insulating strip 29 is a heater band or strip 31 which
~3(~?07'1
-14-
preferably is formed of Titanium Alloy 6AL4V ha~7ing a
thickness of 0.30 mm and a width of 25 mm. This strjp~
as aforementioned .in connection with discussion of the
embodiment of FIGURE 2, initially is sirained at room
temperature to a level g;ea~2r ~n~n the maximum strair.
due to heat in order to maintai.n the posi_ion o~ the
strip in the same location with respect to the thermo-
plastic bead during the hot mashing operation.
In the embodiment of FIGUR~ 3, ambient air is
sucked in through the grooves 33 by means of vacuum
pump (not shown) which establishes a reduced air
pressure level of 0.2 atmospheres. By suc~ing cool
amabient air in through the grooves 33, a more uniform
distribution of air about the strip 31 and the mashed
bead is created, and hence a more uniform cooling of
the strip 31 and the mashed thermoplastic material is
achieved.
The groove 35 should havs a relatively small
width in order to provide support for the strip 31, and
accordingly the groove must be deep .n order to channel
the sucked in air from each of the grooves 33 to the
vacumm pump. In addition the grooves 33 should be
sufficiently wide to present a large cooling area to
the strip 31 but should not ~ SQ wide that the strip
31 is not given adequate support.
` In the preferred embodiment the grooves 33 are
~ 1.7 mm wide and only 0.17 mm deep, with each groove
: separated by a 0.5 mm land. This groove structure is
-~ designed to keep tha ben2ing stress in the ctrip 31 small
and at the same time to the present a relatively large
; area of the strip to the cooling air. At the same time,
.~ during the heating cycle, the grooves act as insulators
preventing a large heat transfer to the insulators 29.
; The central. trough 25 is deep and narrow so that it
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~'~ 3(~071
-15-
presents very little surface area to the strip 31 which
mighl: induca transverse bending stresses while at the
same time has a suf ficiently large cross-sectional area
to conduct the air from grooves 33 to the vacuum pump.
~sing the embodiment of FIGURE 2 and FIGURES
3 and 4, when electricity is conducted through the strip
21 and 31 it becomes sufficiently hot to bring the bead
11 illustrated in FIGURE 1 to about or above its fusion
or melting temperature. The support 20 for the insulator
10 19 or 29 and the strip ~1 or 31 forces the heated strip
against the bead to mash the bead against the weld area
until substantially flat. The reheated bead material
then becomes bonded to the plastic of the weld area as
illustrated in FIGURE 5 to form an improved weld joint.
15 The joint of FIGURE 5 is shown out of scale in order to
clearly illustrate how the mashed bead forms a thin
extra layer of bonded plastic material at the weld
junctions.
Turn now to FIGUR~ 6 which is a simplified
20 cross-sectional view of an apparatus for making battery
jars which use the embodiment of FIGURE 3. The strip
31 is disposed over insulator material 29 which, in
~ turn is supported on a steel frame 60 which defines an
,~ enclosed space S. The space S is in communication with
25 vacuum pump P and opens to trough 3S, which in turn
- communicates with grooves 33. When the vacuum pump is
actuated, it draws air under reduced pressure over the
~ strip 31 and the welded junction area and acts to cool
;~ ~ both. A copper coating 57 (shown in exaggerated form
~' 30 for clarity) is disposed on strip 31 on those areas
o~ strip 31 which do not contact the plastic material
to pxevent the Rtrip from overheating in these areas.
The strip 31 must be maintained under strain
as indicated during the aforementioned discussion of
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FIGURE 2. Screws 55 and 56 schsmatically depict one s~t
of means for effecting this s~rain; but obviously ~her_
are many recognized equivalents which can be used
instead. On turning the screw 56, an end of strip 31
is wound, thereby to provide the necessary strain on
strip 31. As shown in FIGURE 6, the insulator material
29 on which strip 31 is supported is disposed on a flat
surface. However, the surface which supports the
insulator material need not be flat but may have a sur-
face which conforms to the surface of thermoplasticworkpiece at the welded junction. Thus, if two thermo-
plastic pipes are welded together the surface will be
annular or cylindrical conformation. A piston and
cylinder arrangement actuates the framework 60 to
provide contact between the weld junction 41.
A second apparatus 58 is illustrated in
schematic form on the opposite side of the junction
41 of the plastic material 14 and serves to heat and
mash the bead formed at the other side of the junc~ion
' 20 41.
`I EXAMPLE
In operation, the embodiment OLC FIGU-~ES 3 and
' 4 using the apparatus of FIGURE 6 was applied to making
;, a battery jar of a propylene-ethylene copolymer blend,
of the type represented by FIGURES 7-9.
The elongated battery jar of FIGURES 8 and 9
~, comprises two end members 37 and 39, as illustrated
in FIGURE 7, each of which is open at the top thereo~
and has an elongated open side and three elongated
closed sides. The distance from the surface defining
the elongated open side of each member to its opposed
closed side is at least several times smaller than
the distance from the top to the bottom thereof.
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113(~071
Typically, the distance between the open side and the opposed
side is l/4 to l/10 the distance from the top to the bottom of
members 37 and 39. Each end member 37 and 39 has a wall thick-
ness which is substantially the same from the top to the bottom
thereof; i.e., there is no taper or draft from top to bottom and
each of the end members 37 and 39 is a mirror image of the othe~.
The end members 37 and 39 are heat welded at the respective
elongated ends to form the battery jar - illustrated in FIGURES
8 and 9.
The manner and method of making the battery jar end
members 37 and 39 are disclosed in U.S. Patent 4,118,265 to
James S. Hardigg.
The primary requirements for a battery jarare that it
be resistant to the battery acid, have no leaks, have substan-
tial dimensional accuracy, be resistant to shrinkage when the
battery is overheated, have high impact strength to withstand
accidents during battery manufacture and use, have uniform width
and length from top to bottom, that is, no draft, have straight
sides which are not bowed out or in, and have a capacity to
bend and/or deform during handling in order to prevent the
f~acture thereof.
As aforementioned as the respective edges of end
section members 37 and 39, illustrated in FIGURE 7, are heated
to the fusion temperature and then joined to one another to form
; a weld, the fused plastic material forms beads ll on the inside
and outside of the jar of the type illustrated in FIGURE 1 and
FIGURES 8 and 9. After the welded junction including the beads
has cooled, the insulator 29 and band 21 illustrated in FIGURE
3 are positioned along both the inside and outside weld area
against the beads formed during the initial welding step by
~13(~071
using an apparatus of the type illustrated in FIGURE 6.
The Titanium Alloy strip 21 is then heated over a -
time interval ranging from 2.5 seconds to over 20 seconds while
in pressing engagement with the beads~
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The beads thereby fuse and become flattened against tne
heated weld area 13 illustrated in FIGURE 1 to thereby
form a flattened weld joint as illustrated in FI&URE 5.
The strip 21 and the mashed bead are then cooled by
S drawing air at room temperature through the grooves
and the trough formed in the insulating strip 29. ~fter
the mashed bead has cooled sufficiently to no longer
adhere to the strip 21, the strip and insulating support
! were removed to form the final welded battery jar.
I 10 It has been discovered that when longer
heating and cooling times are used, the dart-impact
strength of the welded joints increase~. However, it
has also been discovered that as longer heating cycle
times are used, the battery jars warp, particularly at
the upper end adjacent to the open end of the jar, i.e.,
the jars bow inward or outward to an unacceptable
extent. The warpage resulting ~rom long heating times
apparently is due to the shrinkage which occurs in the
plastic material after heating it to the melting point.
Thus, the material in the area over which the bead is
mashed is brought to or near the melting point thereo~
and accordingly shrinks during cooling, whereas the
surrounding material which has not been reheated
does not shri~k.
~ne technique for overcoming the warpage
` problem is to preheat the welded battery jars to 180-
2~0F prior to the mashing process. This causes the
entire jar to shrink somewhat upon cooling, and accord-
ingly the differential in shrinkage betwPen the material
adjacent to the weld and the remainder o~ the battery
jar is substantially reduced. This technique, howeve;-,
i9 not desirable on a production line basis since the
lengthened cooling cycle required with preheated jaro
substantially increases the total manufacturing tlme of
the battery jars. It has therefore been discovered
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that by using a very short heating time in the range o.
3 to 4 seconds and heating the Titanium Alloy strips
to a higher temperature, an improved weld having high
dart-impact strength with substantially no warpage
results. The extent of warpage was further reduced by
utilizing a technique of drawing relatively cool ambient
air in under the strip 31 which has the effect of
cooling the battery jar material adjacent to the strips.
This results in a narrower zone of heated plastic
10 material subject to shrinkage which in turn reduces
the distortion in the walls of the battery jar due to
shrinkage. Thus, by using a relatively short heating
time cycle and drawing air in from the area surrounding
the heated plastic material, the overall cycle time
15 for treating the welded junction falls below 30 seconds.
,While the present invention has been disclosed
¦in connection with the preferred embodiments thereof, it
should be understood that there may be other modifica-
tion to the invention which fall within the spirit and
20 scope thereof as defined by the appended claims.
;
~'
:
`~ .
