CA1130071A - Method and apparatus for bonding thermoplastic materials - Google Patents
Method and apparatus for bonding thermoplastic materialsInfo
- Publication number
- CA1130071A CA1130071A CA318,226A CA318226A CA1130071A CA 1130071 A CA1130071 A CA 1130071A CA 318226 A CA318226 A CA 318226A CA 1130071 A CA1130071 A CA 1130071A
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- Prior art keywords
- strip
- heating
- opening
- junction
- welded
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Abstract
318,226 METHOD AND APPARATUS FOR BONDING
THERMOPLASTIC MATERIALS
ABSTRACT OF THE DISCLOSURE
A method and apparatus is disclosed for bonding two pieces of thermoplastic material to one another. The method includes the steps of heating the edges of the plastic materials to be joined to at least their fusing temperatures and then forcing the heated edge portions against one another to thereby form a bonded junction. A bead is formed along at least one edge of the junction of the plastic materials as a result of the pressure of the two plastic materials bearing against one another. The welded junction is heated, optionally at elevated pressure, to at least its fusion temperature and is then rapidly cooled. The resulting weld has a high impact and dielectric strength and has a smooth overall appearance.
THERMOPLASTIC MATERIALS
ABSTRACT OF THE DISCLOSURE
A method and apparatus is disclosed for bonding two pieces of thermoplastic material to one another. The method includes the steps of heating the edges of the plastic materials to be joined to at least their fusing temperatures and then forcing the heated edge portions against one another to thereby form a bonded junction. A bead is formed along at least one edge of the junction of the plastic materials as a result of the pressure of the two plastic materials bearing against one another. The welded junction is heated, optionally at elevated pressure, to at least its fusion temperature and is then rapidly cooled. The resulting weld has a high impact and dielectric strength and has a smooth overall appearance.
Description
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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.
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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 , - ~3(~(~7i 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 ,........................... -- 5 ~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 -- 5b -- , ~
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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 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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g (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 :~i 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 , * A Registered Trade Mark ;:
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~13(~07i 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 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 .
~'~ 3(~071 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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~13(~71 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-
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.
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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 , - ~3(~(~7i 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 ,........................... -- 5 ~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 -- 5b -- , ~
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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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g (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 :~i 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 , * A Registered Trade Mark ;:
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~13(~07i 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 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 .
~'~ 3(~071 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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~13(~71 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.
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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.
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Claims (22)
1. An apparatus for improving a heat welded junction in a battery jar which is formed of thermoplastic material and is formed of two elongated members, one of said 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 elongated closed sides which are heat welded together along the edges of said open side, said apparatus comprising:
means for heating said 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, said temperature reducing means including means for drawing ambient air in and about said heated thermoplastic material.
means for heating said 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, said temperature reducing means including means for drawing ambient air in and about said heated thermoplastic material.
2. The apparatus of claim 1, which further includes actuating means for causing said heating means and said means for temperature reduction to contact said welded junction, where-by said actuating means applies pressure to said welded junction.
3. The apparatus of claim 2, which further includes support means for said means for heating.
4. The apparatus of claim 3, wherein said temperature reduction means is contained in said support means.
5. The apparatus of claim 4, wherein said means for heating comprises an electrically conductive metallic strip of high electrical resistivity and means for coupling electric current to said metallic strip.
6. A process for improving the bonding at a heat welded junction in a battery jar which is formed of thermo-plastic material and is formed of two elongated members, each of said members being the mirror image of the other of said two members, each of which members is open at the top thereof and has an elongated open side and three elongated closed sides, which members are welded by heating the edges of at least one of said elongated open sides and joining said edges under pressure to form a welded junction, said pressure causing a bead to be formed on at least one side of said welded junction, comprising:
heating the thermoplastic material at said welded junction and in the vicinity thereof to at least about the fusion temperature thereof under pressure sufficient to mash said bead;
and drawing ambient air over the heat welded junction to quickly cool the thermoplastic material to below the fusion temperature thereof.
heating the thermoplastic material at said welded junction and in the vicinity thereof to at least about the fusion temperature thereof under pressure sufficient to mash said bead;
and drawing ambient air over the heat welded junction to quickly cool the thermoplastic material to below the fusion temperature thereof.
7. An apparatus for bonding two pieces of thermo-plastic material to one another, said two pieces having been joined together to form a weld with a bead being formed along the length of said weld, said apparatus comprising:
means for heating said thermoplastic material at the welded junction and the vicinity thereof to at least about the fusion temperature of the thermoplastic;
means for quickly reducing the temperature of said heated thermoplastic material to at least below its fusion temperature, said temperature reducing means including means for drawing ambient air in and about said heated thermoplastic material and past said heating means; and means for actuating both of said means for heating and said means for reducing the temperature to cause them to con-tact said weld junction.
means for heating said thermoplastic material at the welded junction and the vicinity thereof to at least about the fusion temperature of the thermoplastic;
means for quickly reducing the temperature of said heated thermoplastic material to at least below its fusion temperature, said temperature reducing means including means for drawing ambient air in and about said heated thermoplastic material and past said heating means; and means for actuating both of said means for heating and said means for reducing the temperature to cause them to con-tact said weld junction.
8. An apparatus for improving a heat welded junction formed between two pieces of thermoplastic material comprising:
means for heating said heat welded junction of said thermoplastic materials to at least about the fusion temperature thereof;
means supporting said heating means for quickly re-ducing the temperature of said thermoplastic material at said welded junction to below the fusion temperature thereof, said temperature reducing means including means for drawing ambient air in and about said welded junction and past said heating means; and means for actuating said means for heating and said means for reducing the temperature to cause them to contact said weld junction.
means for heating said heat welded junction of said thermoplastic materials to at least about the fusion temperature thereof;
means supporting said heating means for quickly re-ducing the temperature of said thermoplastic material at said welded junction to below the fusion temperature thereof, said temperature reducing means including means for drawing ambient air in and about said welded junction and past said heating means; and means for actuating said means for heating and said means for reducing the temperature to cause them to contact said weld junction.
9. The apparatus of claim 8, wherein said heating means comprises an electrically conductive metallic strip of high electrical resistivity and means for coupling electric current to said metallic strip.
10. The apparatus of claim 9, wherein said strip comprises a titanium-contained material.
11. An apparatus for improving a heat welded junc-tion formed between two pieces of thermoplastic material com-prising:
means for heating said heat welded junction of said thermoplastic material to at least about the fusion temperature thereof;
means for quickly reducing the temperature of the thermoplastic material at and proximate to said welded junc-tion to at least below the fusion temperature thereof, said temperature reducing means including:
(a) means for supporting said heating means, said supporting means having an opening in the center thereof over which said heating means is disposed;
(b) means for drawing ambient air over said welded junction, past the heating means and through said opening; and (c) means for actuating said means for heating and said means (a) and (b) to cause them to contact said weld junction.
means for heating said heat welded junction of said thermoplastic material to at least about the fusion temperature thereof;
means for quickly reducing the temperature of the thermoplastic material at and proximate to said welded junc-tion to at least below the fusion temperature thereof, said temperature reducing means including:
(a) means for supporting said heating means, said supporting means having an opening in the center thereof over which said heating means is disposed;
(b) means for drawing ambient air over said welded junction, past the heating means and through said opening; and (c) means for actuating said means for heating and said means (a) and (b) to cause them to contact said weld junction.
12. The apparatus of claim 11, wherein said heating means comprises an electrically conductive strip of titanium-containing material and means for coupling a source of elec-tricity to said strip.
13. The apparatus of claim 11, wherein said support means comprises an insulator material and wherein said opening is a trough, said trough being in communication with grooves in said insulator material, formed by excision of material therefrom.
14. An apparatus for improving a heat welded junc-tion formed between two pieces of thermoplastic material com-prising:
a strip of electrically conductive material of high electrical resistivity;
means for coupling electrical energy to said strip;
means for supporting said strip having at least one opening in the center thereof, said strip being disposed over said opening;
a frame defining an enclosed space and having an opening in a surface thereof for communicating the outside of said frame with said enclosed space, said strip and said support means being supported by said frame so that the opening in said support communicates with the opening in said surface;
means, in communcation with said enclosed space, for drawing ambient air from the area about said strip through said opening in said support and through said opening in said surface of said frame into said enclosed space; and means for actuating said framework to cause said strip to contact said heat welded junction.
a strip of electrically conductive material of high electrical resistivity;
means for coupling electrical energy to said strip;
means for supporting said strip having at least one opening in the center thereof, said strip being disposed over said opening;
a frame defining an enclosed space and having an opening in a surface thereof for communicating the outside of said frame with said enclosed space, said strip and said support means being supported by said frame so that the opening in said support communicates with the opening in said surface;
means, in communcation with said enclosed space, for drawing ambient air from the area about said strip through said opening in said support and through said opening in said surface of said frame into said enclosed space; and means for actuating said framework to cause said strip to contact said heat welded junction.
15. The apparatus of claim 14, wherein the opening in the surface of said support means is a trough and wherein said trough communicates with grooves in the surface of said support means, said grooves conducting ambient air past and in contact with said strip to thereby cool said strip.
16. The apparatus of claim 14, wherein said strip is formed of an electrically conductive titanium-containing material.
17. The apparatus of claim 14, wherein the curvature of the surface of said frame corresponds to the conformation of the welded joint.
18. A process for heat welding two pieces of thermo-plastic material to each other comprising the steps of:
heating at least one edge of each of said two pieces to at least about the fusion temperature of said thermoplastic material;
joining said heated edges to one another under pressure to form a welded junction, said pressure causing a bead of plastic to be formed on at least one side of said welded junc-tion; and heating and quenching said welded junction, the thermo-plastic and bead by:
(a) providing an electrically conductive strip of a titanium-containing material of relatively high resistivity and passing a current therethrough to generate heat sufficient to fuse said thermoplastic;
(b) disposing said heating strip on said bead under a pressure so that the combined effect of heat and pressure mashes said bead until it is substantially flat; and (c) rapidly cooling the area defined by said sub-stantially flattened bead, by drawing ambient air under reduced pressure over said area.
heating at least one edge of each of said two pieces to at least about the fusion temperature of said thermoplastic material;
joining said heated edges to one another under pressure to form a welded junction, said pressure causing a bead of plastic to be formed on at least one side of said welded junc-tion; and heating and quenching said welded junction, the thermo-plastic and bead by:
(a) providing an electrically conductive strip of a titanium-containing material of relatively high resistivity and passing a current therethrough to generate heat sufficient to fuse said thermoplastic;
(b) disposing said heating strip on said bead under a pressure so that the combined effect of heat and pressure mashes said bead until it is substantially flat; and (c) rapidly cooling the area defined by said sub-stantially flattened bead, by drawing ambient air under reduced pressure over said area.
19. The process of claim 18, wherein said strip is disposed over an opening in a support means, formed of an in-sulative material and wherein said opening is in communication with means for drawing ambient air under reduced pressure.
20. The process of claim 18, wherein a heating strip is applied to each side of said welded junction.
21. The process according to claim 18, wherein said strip is an element in an apparatus comprising:
said strip, attached to a source of electricity;
means for supporting said strip, having an opening in the center thereof, wherein said strip is disposed over said opening and covers only up to a substantial part of the opening;
a frame defining an enclosed space and having an opening in a surface thereof which allows access from outside of the frame into said enclosed space, to which said strip and support means are attached, so that the opening in said support coincides with the opening in said surface; and means, in communication with said enclosed space, for drawing ambient air from the outside, through the opening of said support and, in turn, through the opening in said frame surface into said enclosed space.
said strip, attached to a source of electricity;
means for supporting said strip, having an opening in the center thereof, wherein said strip is disposed over said opening and covers only up to a substantial part of the opening;
a frame defining an enclosed space and having an opening in a surface thereof which allows access from outside of the frame into said enclosed space, to which said strip and support means are attached, so that the opening in said support coincides with the opening in said surface; and means, in communication with said enclosed space, for drawing ambient air from the outside, through the opening of said support and, in turn, through the opening in said frame surface into said enclosed space.
22. A system for welding two pieces of thermoplastic material to one another comprising:
means for heating at least one edge of each of said pieces to at least the fusion temperature of the thermoplastic;
means for joining said heated edges to one another under pressure to thereby form a welded junction, said pressure causing a bead of plastic material to be formed on at least one side of said welded junction; and means for quickly heating said bead of plastic material to about its fusion temperature and rapidly cooling said heated bead to a temperature below said fusion temperature, wherein said means for heating and cooling comprises;
a strip of electrically conducting material of high electrical resistivity, attached to a source of electricity;
means for supporting said strip, having an opening in the center thereof, wherein said strip is disposed over said opening;
a frame, defining an enclosed space and having an opening in a surface thereof which allows access from outside of the frame into said enclosed space, to which said strip and support means are attached, so that the opening in said support coincides with the opening in said surface;
means, in communication with said enclosed space, for drawing ambient air from the outside, through an opening of said support and, in turn, through the opening into said enclosed space; and whereby heat can be generated by passing an electric current through said strip and whereby cooling can be effected by actuating said means for drawing ambient air under reduced pressure to draw said air through said opening in said support means and, in turn, through the opening in the surface of said frame.
means for heating at least one edge of each of said pieces to at least the fusion temperature of the thermoplastic;
means for joining said heated edges to one another under pressure to thereby form a welded junction, said pressure causing a bead of plastic material to be formed on at least one side of said welded junction; and means for quickly heating said bead of plastic material to about its fusion temperature and rapidly cooling said heated bead to a temperature below said fusion temperature, wherein said means for heating and cooling comprises;
a strip of electrically conducting material of high electrical resistivity, attached to a source of electricity;
means for supporting said strip, having an opening in the center thereof, wherein said strip is disposed over said opening;
a frame, defining an enclosed space and having an opening in a surface thereof which allows access from outside of the frame into said enclosed space, to which said strip and support means are attached, so that the opening in said support coincides with the opening in said surface;
means, in communication with said enclosed space, for drawing ambient air from the outside, through an opening of said support and, in turn, through the opening into said enclosed space; and whereby heat can be generated by passing an electric current through said strip and whereby cooling can be effected by actuating said means for drawing ambient air under reduced pressure to draw said air through said opening in said support means and, in turn, through the opening in the surface of said frame.
Applications Claiming Priority (2)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| US86256077A | 1977-12-20 | 1977-12-20 | |
| US862,560 | 1977-12-20 |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA1130071A true CA1130071A (en) | 1982-08-24 |
Family
ID=25338768
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA318,226A Expired CA1130071A (en) | 1977-12-20 | 1978-12-19 | Method and apparatus for bonding thermoplastic materials |
Country Status (1)
| Country | Link |
|---|---|
| CA (1) | CA1130071A (en) |
-
1978
- 1978-12-19 CA CA318,226A patent/CA1130071A/en not_active Expired
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