Method of fusion welding large plastic parts

Abstract

A large tubular plastic tank is formed by fusion welding together half-shell parts at lengthwise flange joints to form a tubular body. Then, end caps are fusion welded onto the outermost ends of the tubular body and an assembly of two or more bodies. Electric or electromagnetic energy is used to melt in situ the fusion weld elements which are captured between the joining surfaces of the parts. A fusion weld element is secured beforehand to linear a joining surface of at least one of two mated tank parts. The weld element exits the joint between two lengthwise-mated parts at the location of mating tabs extending from the mated tank parts; the tabs are subsequently cut away. A circumferential flange joint is made by securing a fusion weld element to at least one joining surface of the mated tank parts, where the ends of the element run through holes in the faying surface of the flange.

Claims

1. A method of making a tubular plastic tank or a portion thereof, the tank having a length and a length axis, comprising: (a) molding a first part-shell and a second part-shell, each part-shell having at least one lengthwise flange with a lengthwise joining surface thereon, and each part-shell having a tab extending from said at least one lengthwise flange, the tab having a tab surface parallel to the lengthwise joining surface of said at least one lengthwise flange; (b) securing a first fusion weld element to the part-shell lengthwise joining surface of at least one of the first part-shell or the second part-shell, wherein said first fusion weld element has at least one terminal end for connection to a source of electric energy; (c) forming a first assembly by mating the first part-shell with the second part-shell so the lengthwise joining surfaces of mated lengthwise flanges thereof form a lengthwise joint region with said first fusion weld element captured within the joint region and the at least one terminal end of the first fusion weld element captured between mating tab surfaces; (d) forming a first welded assembly by melting said first fusion weld element by applying electrical or electromagnetic energy thereto, thereby to weld together said mating flanges and said mating tabs; and, (e) then severing the tabs from the welded assembly.

2. The method of claim 1 wherein each part-shell is a half-shell, and wherein in step (d) the first welded assembly which is formed comprises a tubular body portion.

3. The method of claim 2 wherein said tubular body portion has at least one end comprising a circumferential flange with a circumferential joining surface thereon, further comprising: (f) providing a further part having a tubular body portion and at least one end comprising a circumferential flange with a circumferential joining surface thereon; (g) securing at least one second fusion weld element in the form of a loop to at least one of said circumferential joining surfaces, the second fusion weld element having free terminal ends and a loop end; (h) forming a second assembly by mating said tubular body portion of the first assembly and the tubular body portion of the further part, the second assembly having a circumferential joint between mated circumferential joining surfaces, wherein the at least one second fusion weld element is captured in said circumferential joint; and, (i) forming a second welded assembly by melting said at least one second fusion weld element by applying electrical or electromagnetic energy thereto; thereby to form a weld at said circumferential joint; wherein in step (h) the at least one second fusion weld element runs around the circumferential joint so the free terminal ends of said second weld element circumferentially overlap the loop end of said second weld element.

4. The method of claim 3 wherein at least one of the free terminal ends of the second weld element runs through a hole in one of the circumferential flanges.

5. The method of claim 3 wherein said further part is an end cap.

6. The method of claim 2 wherein said tubular body portion has at least one end comprising a circumferential flange with a circumferential joining surface thereon, further comprising: (f) providing a further part having a tubular body portion and at least one end comprising a circumferential flange with a circumferential joining surface thereon; (g) securing two second fusion weld elements, each in the form of a loop, to at least one of said circumferential joining surfaces, each second fusion weld element having free terminal ends and a loop end; (h) forming a second assembly by mating said tubular body portion of the first welded assembly and the tubular body portion of the further part, the second assembly having a circumferential joint between mated circumferential joining surfaces, wherein both at least one second fusion weld elements are captured in said circumferential joint; and, (i) forming a second welded assembly by melting each second fusion weld element by applying electrical or electromagnetic energy thereto; thereby to join the first part to said further part at said circumferential joint; wherein in step (h) one of the second fusion weld elements runs around a first portion of the circumferential joint and the other of the second fusion weld elements runs around a second portion of the circumferential joint, wherein the free terminal ends of one said second weld element circumferentially overlap the loop end of the other said second weld element.

7. The method of claim 6 wherein at least one of the free terminal ends of each of second weld element runs through a hole in one of the circumferential flanges.

8. The method of claim 6 wherein said further part is an end cap.

9. The method of claim 1 wherein each part-shell has two spaced apart lengthwise flanges.

10. The method of claim 1 wherein at least one of the first fusion weld elements is secured to said lengthwise joining surface by either tack welding or placement in a groove.

11. A method of making a tubular plastic tank or a portion thereof, the tank having a length and a length axis, comprising: (a) forming a first part having a tubular portion comprising a circumferential flange with a circumferential joining surface thereon; (b) forming a second part having a tubular portion comprising a circumferential flange with a circumferential joining surface thereon; (c) securing at least one fusion weld element in the form of a loop to at least one of said circumferential joining surfaces, the at least one fusion weld element having free terminal ends and a loop end; (d) forming an assembly by mating the first part tubular portion and the second part tubular portion, to form a circumferential joint between said circumferential joining surfaces of the mated circumferential flanges, wherein the at least one fusion weld element is captured in said circumferential joint; and, (e) forming a welded assembly by melting said at least one fusion weld element by applying electrical or electromagnetic energy thereto; thereby to join the first part to the second part at said circumferential joint; wherein in step (d) the at least one fusion weld element runs around the circumferential joint so the free terminal ends of said fusion weld element circumferentially overlap the loop end of said fusion weld element.

12. The method of claim 11 wherein at least one of the free terminal ends of said fusion weld element runs through a hole in one of the circumferential flanges.

13. The method of claim 11 wherein at least one of the fusion weld element is secured to said circumferential joining surface by either tack welding or placement in a groove.

14. The method of claim 11 wherein the second part is an end cap.

15. A method of making a tubular plastic tank or a portion thereof, the tank having a length and a length axis, comprising: (a) forming a first part having a tubular portion comprising a circumferential flange with a circumferential joining surface thereon; (b) forming a second part having a tubular portion comprising a circumferential flange with a circumferential joining surface thereon; (c) securing two fusion weld elements to at least one of said circumferential joining surfaces, each fusion weld element having free terminal ends and a loop end; (d) forming an assembly by mating the first part tubular portion and the second part tubular portion, to form a circumferential joint between said circumferential joining surfaces, wherein the two fusion weld elements are captured in said circumferential joint; and, (e) forming a welded assembly by melting each fusion weld element by applying electrical or electromagnetic energy thereto; thereby to join the first part to the second part at said circumferential joint; wherein in step (d) one of the fusion weld elements runs around a first portion of the circumferential joint and the other fusion weld element runs around a second portion of the circumferential joint, wherein the free terminal ends of one said weld element circumferentially overlap the loop end of the other said weld element.

16. The method of claim 15 wherein at least one of the free ends of at least one said fusion weld element runs through a hole in one of the circumferential flanges.

17. The method of claim 15 wherein at least one of the fusion weld elements is secured to at least one of said circumferential joining surfaces by either tack welding or placement in a groove.

18. The method of claim 15 wherein the second part is an end cap.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 is a perspective schematic view of a molded plastic tank comprised of multiple segments.

(2) FIG. 2 shows the tank of FIG. 1 in exploded view.

(3) FIG. 3 is an end view of body segments of the tank of FIG. 1 as they are stacked for storage or shipment.

(4) FIG. 4 is an exploded of the parts of a molded plastic septic tank having two tubular body pieces.

(5) FIG. 5 shows the parts of the tank of FIG. 4 partially assembled as two tubular bodies.

(6) FIG. 6 is a perspective view the tank of FIG. 4.

(7) FIG. 7 is a perspective and partial cutaway view of the joint between two half-shells of a tank, where a fusion weld element is embedded in one half-shell near the joint.

(8) FIG. 8 is like FIG. 7 and shows the parts after the fusion weld element has been energized sufficiently to melt and fuse the joints.

(9) FIG. 9 is a perspective view of portions of two tank half-shells ready to be mated, showing the placement of fusion weld elements on the joining surfaces.

(10) FIG. 9A is an end view shown the half-shells in FIG. 9 after they are mated.

(11) FIG. 10 is a partial end view of the joint a subassembly comprised of two fused half-shells, showing how loop type fusion weld elements run on a circumferential joining surface, and how the ends of one fusion weld element overlaps the loop end of the other.

(12) FIG. 10A is a fragmentary view like FIG. 10, showing an alternate embodiment of fusion weld element arrangement.

(13) FIG. 11 is a fragmentary lengthwise cross section showing how the lengthwise end of a tank tubular body mates with a like tubular body, and how a fusion weld element is captured in the joint.

(14) FIG. 12 is a fragmentary perspective end view two mated half-shells, showing how a fusion weld element is captured in the joint and between tabs which are extensions of the joint.

(15) FIG. 13 shows one of the half-shells of FIG. 12, and the fusion weld element tacked to the faying surface.

(16) FIG. 14 shows an end cap with a single loop type fusion weld element tacked to the faying surface.

(17) FIG. 15 is a partial view of a tank body showing splices/bands on the joints between the mating parts.

(18) FIG. 16 is a cross section through a portion of the tank body and band shown in FIG. 150.

DESCRIPTION

(19) FIG. 1 shows a plastic tank 20 comprised of a multiplicity of joined together parts; FIG. 2 shows the tank in exploded view; and FIG. 3 shows nested semi-cylindrical body segments in end view. The illustrations in FIG. 1-3 are largely schematic and they are prelude for description of FIG. 4-6. Tank 20 is comprised of a multiplicity of parts: Two semi-cylindrical half-shells 22 mate with each other to form the cylindrical (tubular) body of the tank, and a pair of end caps 24 close off the ends of the body. FIG. 3 shows how the half-shells 22 nest for convenient storage or shipment. Similarly, the end caps will nest.

(20) In other embodiments of the invention the half-shells, which are connected to make a tubular body portion, may be smaller than half-tanks. For instance, quarter-cylinder segments may be used. While the remainder of this exemplary description is about half-tanks, it applies to third-tanks, quarter-tanks, etc. While exemplary tanks of FIG. 1-3 have a circular cross section, in the generality of the invention a tank may have some other cross section, such as oval or rectangular. A reference herein to the circumference of a tank, and variations on it and the root word, is a reference to a circumscribing dimension of such a tank in a plane transverse to the length, even when the tank is not round.

(21) To fabricate a tank 20, the several parts are first molded. Then the half-shells 22 are connected to each other at lengthwise joints 26 to form a tubular body; then, the end caps 24 are connected to the ends of the body at circumferential joints 37. By fabricating a two or more tubular bodies and joining them end-to-end, an assembler-user can select a desired length and volume capacity of tank.

(22) One or more hatchways 23, shown in phantom in FIG. 1, may be attached to the tubular body where part of the wall is cut away or omitted. Optionally, bosses for cutting of ports may be formed as an integral part of the tank, as described below for tank 120.

(23) Different means may be used to join the tank parts 22, 24 to each other, including either or more than one of mechanical fastening or clamping, adhesive bonding, and welding. Joining by mechanical fastening (i.e., bolts) and clamping is generally described in the prior art referred to in the Background.

(24) The joints between the connected parts of a tank in the present invention are fused. While conventional plastic welding (e.g., hot air with filler rod or hot plate welding) might be used, a better welding means is described below. It is especially useful when assembly is anticipated at a field site where there may be limited skilled workers, equipment, and fixturing.

(25) FIG. 7 and FIG. 8 illustrate a basic approach to obtaining a fused joint between the joining surfaces (also called faying surfaces) of representative half-shells 22D, 22E where they meet at joint region 26E. In FIG. 7, a fusion weld element 44 is embedded in the plastic of half-shell 22D, prior to fusing of the joint 26E. In the present invention a fusion weld element is a component which can be heated by electrical or electromagnetic energy, sufficient to cause localized melting and fusion of both the element and the local plastic material of the parts being joined. Exemplary fusion weld elements comprise plastic with embedded metal pieces that are heated by electric resistance or electromagnetic induction are described further below. Commercial products are described below.

(26) Placement of a fusion weld element may be accomplished in different ways. For example, the fusion weld element 44 may be molded into the material of the half-shell 22D when it is formed, as suggested by the illustration of FIG. 7. Alternately, the element may be placed partly or wholly within in a groove on one of the faying surfaces; alternately, the element may be simply captured between the faying faces as they are pressed toward each other. Preferably, in the present invention, the fusion weld elements are tack welded with plastic to a joining (faying) surface. In FIG. 8 the parts 22D, 22E are shown after the fusion weld element 44 has been raised in temperature sufficient to melt the mating plastic surfaces, and then cooled. The weld joint 27E (where previously was located joint region 26E) is fused plastic which weld-joins the two parts. Some metal artifact from the metal portion of the fusion weld element may remain within the fused weld material.

(27) Fusion weld element 44 (which sometimes may be called fusion element or weld element herein) is a component which may be heated, such as by electric resistance heating or electromagnetic induction heating to induce melting of the element and local plastic environment. As an example, the fusion weld element may be a commercial product known as PowerCore Welding Rod (PowerCore International Ltd., Ottawa, Ontario, Canada). See also U.S. Pat. Nos. 5,407,514 and 5,407,520, the disclosures of which are hereby incorporated by reference. An exemplary PowerCore brand rod is an about 3/16 inch diameter thermoplastic rod having integrated electric resistance wires of very fine diameter. The ends of the rod run out of the joint/part at selected end points; and when the wires comprising the rod are connected to an electric power source they rise in temperature and heat the rod and surrounding plastic causing melting and fusion. The fine wires remain a part of the finished joint. That portion of the fusion weld element which sticks from the end of the joint is severed, as by chiseling, and discarded.

(28) As another example, the fusion weld element may be the preform which is part of the commercial Emabond electromagnetic welding system (Emabond Solutions Co., Norwood, N.J., U.S.) As described in Lamarca U.S. Pat. No. 7,984,738 (the disclosure of which is hereby incorporated by reference) the fusion weld element preform may be a structure comprised of plastic and magnetic particles. When an energized high frequency induction coil is placed in proximity to the joint, the particles act as susceptors of electromagnetic radiation and resultant induced eddy currents cause the element to become heated sufficiently to melt the preform and adjacent plastic, thereby fusing the joint. The metal particles remain within the fused plastic part.

(29) FIG. 4-6 show respectively, exploded, partially assembled, and assembled views of a plastic septic tank 120, which views generally correspond with FIG. 1-3. They are now discussed. FIG. 6 shows a large corrugated plastic septic tank 120, an embodiment of the present invention. FIG. 4 shows the tank in exploded view, and FIG. 5 shows the tank in partially assembled view, where two tubular body portions have been formed from half-shells. Tank 120 is comprised of separately injection-molded parts, preferably made of polyethylene or polypropylene. Other molding means may be used, although generally they are less precise in forming parts. Tank 120 has corrugations which are similar in character to those of a half tank described in U.S. Pat. No. 8,740,005. Tank 120 preferably has a nearly round cross section (compared to a tank having a nearly rectangular cross section, as described in the foregoing patent). Within the present invention, a tank may haveand likely will havea non-round cross section. For convenience of description, the terms cylindrical and circumferential are used, but will be understood to be approximations, and the description and invention will apply to non-round tanks.

(30) With reference to FIG. 5 and FIG. 6, tank 120 is comprised of two generally cylindrical tubular subassemblies 123A, 123B that are attached to each other along longitudinal axis L at tank-center joint 134. As shown by FIG. 4, each tubular body subassembly 123 is constructed by welding a pair of semi-cylindrical tank half-shells 122 to each other at lengthwise faying surfaces 128 to form a weld joint 126. The term faying surface is used interchangeably with joining surface herein. Each part of tank 120 has a flange where there is a weld joint to be made, for both stiffness and convenience, to increase the size of the faying surface and weld joint area and impart stiffness to the structure comprising the joint region.

(31) Preferably, every tank half-shell 122 is identical. Each half-shell is molded with a plurality of bosses 141. FIG. 4-6 illustrate how the several bosses which face upwardly on tank 120 may be cut out to provide access ports 131. The access ports are desirable when a tank is used for septic tank purpose. Covers or optional risers may be attached to the tank at or adjacent to the openings 131 during use of the tank. In septic tank application, the bosses 141 at the bottom of the tank are not cut out; they help provide a stable base for supporting tank 120 on a flat surface.

(32) When the body is formed by to create the tubular body parts 123A, 123B shown in FIG. 5, and the body parts 123A, 123B are joined to each other at joint 134, with any weld flash clean up as is needed, the resultant tubular body has ends which are circumferential, more particularly in some instances, circular. Each end has an associated planar joining surface. Referring to FIG. 6, each end cap 124 is then fusion welded so it is attached by a weld joint 137 to one end of the tubular body pair at the surface 138.

(33) Preferably, tank 120 is fabricated by permanently welding the parts 122, 124 to each other by using the kind of fusion weld elements mentioned above, most preferably a PowerCore commercial product. To make a joint, the electrical conductor ends of a fusion weld element are connected to a source of electric energy (emf), which causes current flow to flow through the fusion weld element. The resistance to passage of the current causes heating of the metal parts of fusion weld element which melts the plastic part of the fusion weld element which causes local melting of the molded plastic parts which are mated. Each element therefore has two terminal ends, that is, there are electric lead portions which are suited for connection to a power source or to lines from a power source. A fusion weld element may be run within a joint as one or more single strands; or it may be doubled back on itself in the form or a loop. In a circular joint a single strand fusion weld element may be run around the circumference more than one time, i.e., it may be a spiral. Preferably, the fusion weld elements are secured by plastic tack welding of the element to a faying surface. Optionally, the alternate means for holding a fusion weld element in place, described above in connection with FIG. 7-8, may be used.

(34) FIG. 9-14 show ways in which fusion weld elements may be placed on the tank parts. (In these Figures, the corrugations that characterize the parts of tank 120 are not shown for simplicity of illustration.)

(35) FIG. 9 shows two typical semi-circumferential half-shells 122, 222 ready to be mated. The half-shells are identical, that is, they are like the parts 122 referred to above. (For this part of the description, the parts of the upper half-shell in FIG. 9 and those which derive from it have the prefix 2, to enable distinction to be made between the parts of the lower half-shell which have the prefix 1)

(36) Half-shells 122, 222 have respective walls 127, 227. Each half-shell has a pair of lengthwise flanges 132, 232 which have faying surfaces 128A, 128B, 228A, 228B. Each half-shell lengthwise end comprises a flange 143, 243 upon which is the semi-circumferential faying surface 138, 238.

(37) In this aspect of the invention, a fusion weld element is placed on only one of the lengthwise faying surfaces of a half-shell, namely on one of the surfaces and not on the other lengthwise faying surface of the same half shell, i.e. on surface 128A or surface 128B, and on surface 228A or surface 228B. And a fusion weld element is placed on only one of the semi-circumferential faying surfaces 138, 238 at the ends of the half-shell, and not on the other.

(38) In FIG. 9, fusion weld elements 144, 244 have been secured to only one lengthwise joining surface on each tank, preferably by tacking, so that when the half-shells are mated, as shown by the arrows in FIG. 9, each fusion weld element will be captured within the joint region between the half-shells.

(39) To enable forming a weld joint between an end cap 124 and the body 123 that is created when the half-shells 122, 222 are welded to each other, a second fusion weld element 146, 246 is tacked onto the joining surface 138, 238 at the end of each half-shell 122, 222. Each second fusion weld element runs on a semi-circular arc, preferably as a loop, described below. There is no fusion weld element attached to the surface which is at the opposing end of each half-shell 122.

(40) When the half-shells 122, 222 are mated, the two fusion weld elements 146, 246, which are tacked onto the joining surfaces 138, 238 of the end flanges, cooperate to run around the entirety of circular joining surface at the end of a tubular body assembly 123. Thus surface which is the summation of surfaces 138 and 238 is ready to mate with and be welded to an end cap 124 having a plain faying surface, i.e., a surface which has no tacked-on fusion weld element, or with the like no-element faying surface of a second tubular body 123.

(41) FIG. 12 is a partial perspective view showing portions of half-shells 122, 222 in mated disposition, preparatory to fusion welding. Fusion weld element 144 is captured between the joining surface 128 and the mating joining surface of half-shell 222. When fusion weld element 144 is heated by being connected to a power supply, force is applied in directions indicated by vector A to push the flanges 132, 232 together. FIG. 13 shows only the bottom half-shell 122 of the set in FIG. 12. FIG. 13 illustrates how preferred fusion weld element 144 runs in a loop along the faying surface 128 so that both terminal/power-connection ends are at the same end of the assembly. Alternatively, the fusion weld element may comprise one strand, or two or more parallel separate strands and power connections may be made by energizing leads connected to the fusion welding element at each end of the half-shell.

(42) FIG. 12 also shows two tabs 160, 260 which are surplus with respect to the final tubular body 123. Tabs 160, 260 extend from the joining surfaces (faces) 138, 238 of the ends of the half-shells 122, 222, a the location where the ends of loop type fusion weld element 146 exit the joint between the half-shells. When the fusion weld element melts, the tabs are also partially melted. After the weld cools, the tabs and portions of fusion weld elements which are captured therebetween are removed, as by means of a sharp chisel or the like, leaving a smooth end surface on the now-formed tubular body 123. In the absence of the tabs, there may be a depression in the end surface where the fusion weld element remains are cut off, and that could upset subsequent welding of the end.

(43) When fabricating a tank 120 like that shown in FIG. 6, two like tubular bodies 123 are fabricated. The one is then welded to the other at flanges 138, to form circular joint 134. Alternatively, the second tubular body is not used, and the end cap is attached to the first body 123 instead. That process of making end-joints is now described.

(44) FIG. 9A is an end view of the tubular body 123 that is formed when the two half-shells 122, 222 shown in FIG. 9 are welded to each other at joint 126. FIG. 10 is a partial more detailed view of the end of body 123 which is shown in FIG. 9A. FIG. 11 is a partial lengthwise cross section through tubular body shown in FIG. 10, with the addition of a second tubular body 123A having a flange 143A which is configured and positioned for welding to the end of tubular body 123.

(45) With reference to FIGS. 9, 9A, 10 and 11 the fusion weld element 246 is in the form of a loop; the element is secured to the faying surface 238 of half-shell 222. The free ends, also called terminal ends, run across the joint 126 between the two half-shells and then through spaced apart holes 150 in the circumferential flange 143 of part 122, to exit from the flange face opposite the faying surface. The loop end 170 of the fusion weld element 146 that is secured to the faying surface 138 of flange 143 of half-shell 122 is positioned with respect to the holes 150 so that there is circumferential overlap between the free ends of fusion weld element 246 within the joint and the loop end 170 of fusion weld element 146. Similarly, as shown in FIG. 9A, there is like overlap between the loop end of fusion weld element 246 and the terminal ends of loop 146, on the diametrically opposite side of the tubular body.

(46) In an alternate way of carrying out the invention, the holes 150, in a flange may be omitted and, as shown in FIG. 10A the terminal ends 247 of fusion weld element 246 may be run on an overlapping but diverging path, with one end exiting from the joint region radially inward and the other exiting radially outward.

(47) The foregoing method of using two separate semi-circumferential fusion weld elements provides good melting of the faying surfaces at the locations where the fusion weld elements meet each other. And using two separate fusion weld elements on the end of a tubular body 123 enables a manufacturer to pre-place the fusion weld elements on the half-shells prior to shipment of the half-shells to storage or to a remote site where the body 123 may be formed from the half-shells without required skilled placement of the fusion weld elements at the remote site.

(48) A principle within the foregoing methodology is that each fusion weld element has an arc length on the faying surface of the end of tubular body 123 that is greater than the arc length of the path on the faying surface of a half-shell where the fusion weld element is tacked. Stated again, the fusion weld element length is greater than the length of a semi-circumference arc, in the particular case where the tubular body is comprised of half-shells. The foregoing arc on the end faying surface is that which runs between the points where the circumferential end faying surface meets the opposing side lengthwise faying surfaces of a half-shell.

(49) As illustrated by side cross section view of FIG. 11 and vector A, the parts are desirably urged toward each other as the fusion weld element melts. Upon melting, the fusion weld element merges with the plastic of the flanges and the gap between the joining surfaces closes up and a tight fused joint 134 is formed. If desired one or more additional tubular bodies may be formed and attached to the just-made assembly in like manner.

(50) Next, end caps 124 are attached to each end of the partially formed tank (the grander assembly) comprised of two tubular bodies 123. From the foregoing description, it will be appreciated that one end of the grander assembly will be characterized by a circular surface having fusion weld elements arranged as described in FIG. 9, and the process for making the cap-to-body joint will be as just described. The other end will be free of fusion weld elements.

(51) To attach a cap to the end of the grander assembly having no fusion weld elements, in accord with the exemplary embodiment described here, there is a choice of approaches. First, to make the tubular body 123 which is the last of a multiplicity of welded bodies 123, special half-shells may be used: each will have a semi-circumference loop running on both ends of the half-shells, rather than on one end only. Second, with reference to FIG. 14, an end cap 324 may be provided that has a fusion weld element loop 346 running around the circumference of the faying surface 342 of the end cap, preferably with holes 350 like the holes 150 and the same arrangement of loop end and terminal ends as described in connection with FIG. 10. Alternatively, the fusion weld element may be run on the end cap as two separate loop segments, as described. Alternatively, a single strand of fusion weld element may be used, or a spiraling-upon-itself fusion weld element may be used.

(52) In other embodiments of the invention, for any of the joints, a single strand, or multiple strands, or loops of fusion weld elements may be used interchangeably, although the preference is for the loop type fusion weld element configurations which have been pictured here. While tack welding of a fusion weld element to joining surfaces as described is preferred, other ways of pre-placing or holding a fusion weld element in position may be used. For instance, a fixture may be used. In other embodiments of the invention, a different sequence of joining the parts together may be used than has been described by example.

(53) FIG. 15 and FIG. 16 show another way in which exemplary parts 22 may be attached to each other, and mutually joined to parts 22F. A splice 40, also called a lap band 40, lies on top of the parts 22 at the location of joint 26 between mating parts. In the FIG. 16 cross section view, before welding, band 40 includes captured-opposing fusion weld elements 44. When the fusion weld elements 44 are raised in temperature the band is fused to the surfaces of the half-shells 22 at locations spaced apart from the joint 26. As shown in FIG. 15 one band 40 may be crossed by another right-angled band 40A when the tank has intersecting joints. Preferably, the first band would be fused to the tank parts before the second right-angled band is fused. A lap band may lessen the need for having precise joint fit. Optionally, a seal (not shown) may be placed within joint 26 when using a lap band.

(54) The mated parts may be urged together or held in intimate contact during the fusion welding process by means such as weights, fixtures, clamps or other temporary or permanent mechanical fastening means. A preferred means is to use a clamp which provides a sustained force so parts move together when the fusion weld element melts. For example spring loaded clamps may be used; alternately, pneumatically or hydraulically actuated clamps may be used.

(55) Preferably, as mentioned, the tank parts (half-shells and end caps) are injection molded or otherwise molded at a factory; and, the fusion weld elements are secured to faying surfaces in the ways described below. Then the tank parts are nested and shipped to a remote site which is a distribution site or near the point of use. The parts are taken from the nested stacks, mated, clamped or fixtured, and welded to each other. They are then transported to the point of use, and as desired, buried or set on the ground.

(56) When an end cap is attached to the both ends of the tank, a whole tank having good strength and integrity is constructed. While tooling in the form of clamps or fixtures is preferred to hold the parts together during joining, within the scope of the invention clamps or fasteners which are left in place may be used in addition or in alternative.

(57) While the invention is described in terms of tubular bodies which are made of half-shells, within the generality of the invention, as mentioned, the tubular bodies may be formed of more pieces, for instance 3 or 4 pieces. Optionally, for enhanced load carrying ability, a number of vertical braces may be placed within the interior of a tank adjacent the port openings, or bosses adapted to receive ports. Preferably, a pair of braces runs from the bottom of the tubular body up to the top of the tank. See commonly owned U.S. Pat. No. 9,260,854. In the generality of the invention, a tank can have one or more mechanical seal joints in combination with fusion weld joints.

(58) The invention, with explicit and implicit variations and advantages, has been described and illustrated with respect to several embodiments. Those embodiments should be considered illustrative and not restrictive. Any uses of words which relate to the orientation of an article pictured in space are for facilitating comprehension and should not be limiting should an article be oriented differently. Any use of words such as preferred and variations thereof suggest a feature or combination which is desirable but which is not necessarily mandatory. Thus embodiments lacking any such preferred feature or combination may be within the scope of the claims which follow. Persons skilled in the art may make various changes in form and detail of the invention embodiments which are described, without departing from the spirit and scope of the claimed invention.