Heat-sealed container and cover

Abstract: Process for heat-sealing a thermoplastics cover (16) to a thermoplastics container (2) in which the cover (16) has a channel (18') formed thereon for receiving a sealing lip (14) on the container walls (6,8). The lip (14) and a floor (26') of the channel (18') are melted and then cooled to form beads (46,48) having a solidified skin (50,52) of thermoplastics material over a molten core (54,56). The lip (14) is mated with the channel (18') and the melted portions thereof coalesced with the melt from the floor (26'). Some of the coalesced melt (60) exudes between the lip (14) and the sides of the channel (18') and solidifies to provide additional bonding. The product produced by the process is also claimed.

Description: This invention relates to a method of producing hermetic heat-seals between a thermoplastics cover and a thermoplastics container, as specified in the preamble of claim 1. Thermoplastics (e.g., polyethylene, polypropylene, etc.) containers and covers are well-known in the art and have been used for many years in the battery industry to house cell elements of the battery. The covers have been sealed to the battery containers in a number of ways with the view towards preventing electrolyte leakage from the batteries, and, in some cases, maintaining super-ambient pressure in the container and preventing ambient air incursion into the battery. A popular technique for sealing a thermoplastics cover to a thermoplastics container is a butt-type heat-seal made by simultaneously pressing the top edge of the container walls and a corresponding mating surface on the cover against a heated aluminium platen to melt the plastics thereat and, after removing the heated platen, pressing the melted regions of the cover and container together to form a butt weld therebetween. US-A-3,686,056, US-A-3,441,448 and US-A-3,778,314 disclose variations of this process. This technique is simple and inexpensive but typically produces seals which reliably can only withstand internal pressures of less than about 20.68 kPa (3 psig). Another technique for sealing covers to containers is to provide a channel around the periphery of the cover, filling the channel with a suitable adhesive (e.g. epoxy resin), embedding the upper edge of the container walls in the sealant and allowing the sealant to cure or otherwise harden. This is a more complicated and costly technique than the aforesaid heat-seal process. Another technique, similar to the aforesaid epoxy resin technique, places a strip of metal particle-filled thermoplastics material in the cover channel along with the upper edge of the container wall. Thereafter the seal area is bombarded with microwaves to heat the metal particles, fuse the plastics matrix thereabout and thus bond the cover and the container. Such a technique is shown in US-A- 4,460,663 and "Welding Process Seals Deal in Battery Manufacture", Plastics Design Forum, Sept./Oct. 1988, pages 36 and 37. Like the aforesaid epoxy resin process, this latter technique is more complicated and more expensive than the simpler heat-seal technique. So-called "gas-recombination" batteries, operating on the oxygen cycle, consume the internally-generated gases within the battery itself rather than exhausting them to the atmosphere. Batteries of this type are sealed and contain pressure-relief valves for venting the gases to the atmosphere only after pressure within the container exceeds a predetermined value. Generally speaking, higher internal pressures result in more effective gas recombination with internal pressures of at least about 34.47 kPa (five (5) psig) being desirable. The operating pressure of batteries has heretofore been limited to some extent by the ability of the container-cover seal to withstand these higher internal pressures. It would be desirable therefore to have a heat-seal between the container and the cover which is both simple and economical to make, and which is capable of withstanding higher internal pressures than are possible with butt-type heat-seals alone. A method of heat-sealing a thermoplastics cover to a thermoplastics container according to the present invention is characterised by the features of claim 1. It is an object of the present invention to provide an improved process for making a heat-seal between a thermoplastics container and cover, which seal is capable of withstanding internal container pressures greater than about 34.47 kPa (5 psig) without the need for extraneous sealant or filler material at the joint between the container and cover. This and other objects and advantages of the present invention will become more readily apparent from the detailed description thereof which follows and which is given hereafter in conjunction with the accompanying drawings, in which: Figure 1 is a side-sectioned view through a thermoplastics multi-cell battery container and cover to be heat-sealed together in accordance with the present invention and prior to the heating step; Figure 2 is an enlarged, side-sectioned view of a portion of the container and cover of Figure 1 during the heating step; Figure 3 is an enlarged, side-sectioned view like that of Figure 2 after the heating tool has been removed; and Figure 4 is an enlarged view like that of Figures 2 and 3 after the container and cover have been pressed together in accordance with the present invention. The invention contemplates a method of heat-sealing a thermoplastics cover to a thermoplastics container. The invention is particularly applicable to the manufacture of single- and multi-cell electric storage batteries of the type that generate significant internal pressures during cycling and must remain hermetically-sealed to prevent escape of internal gases and electrolyte as well as the prevent the invasion of ambient air thereinto. Accordingly, the invention will be described in the context of a battery container, but, in its broadest aspects, is not limited or in any way restricted thereto. Similarly, whilst the invention will be described primarily in the context of a single-cell battery it is to be understood that it is equally applicable to multi-cell batteries wherein the container is divided into a plurality of individual cell compartments by intercell partition walls. Such intercell partition walls are sealed to the battery cover in the same manner as the outer peripheral walls of the container are sealed to the cover. In the present invention a thermoplastics container (e.g., polypropylene) has a set of walls defining at least one cavity in the container for receiving electrochemically-active components of the container (e.g., a plurality of alternately-spaced positive and negative polarity plates separated one from the other by a porous separator such as microporous polyethylene sheet or fibrous glass mat). Each of the walls has a sealing lip extending along at least one (e.g., upper) of its edges. The cover has a pair of parallel ribs defining a continuous channel inwards of the periphery of the cover. A channel floor lies at the bases/roots of the ribs. The sealing lip on the container mates with the channel on the cover in tongue-and-groove fashion such that the lip is bonded to the cover within the channel in a manner to be described hereafter. Just prior to mating the container wall lip with the cover channel, the floor of the channel and the lip are heated with a heat source and melted to provide one bead of melt along the channel floor and another bead of melt along the edge of the lip. After the channel floor and lip have been heated sufficiently to form the respective melt beads, the heat source (e.g., an A1 plate) is removed and the beads allowed to cool sufficiently to form a skin of solidified melt over the surface of the bead whilst leaving the core of the bead still melted. Thereafter, the container and cover are quickly mated, in tongue-in-groove fashion, and pressed together so that the respective skins on the beads are ruptured and allow the melted cores to coalesce before solidification thereof occurs. Continued pressing of the lip into the cover channel further causes the coalesced melt to exude down either side of unmelted portions of the lip and between the lip and the ribs defining the channel and thereat to solidify and provide an additional bond to that formed by the coalescence of the lip bead melt and the channel bead melt. The assembly is then allowed to cool into a substantially homogeneous weld where the core melts have coalesced as well as a good bonded joint between the lip and the channel via the melt exuded from the coalesced cores to between unmelted portions of the lip and the ribs. In accordance with a preferred embodiment of the present invention, the container and cover will be heated via a heated aluminium plate containing elongated ridged projections which fit up into the channels and locally heat the channel floor. Heating is preferably accomplished by positioning a heated aluminium plate between the container and the cover whilst they are aligned for mating and pressing the container and cover against the heated plate for a time sufficient to melt the thermoplastics material in contact with the plate and the immediate surrounding region. During heating the projections/ridges are embedded into the channel floor causing a flow of thermoplastics material from the floor down along the sides of the heated projections/ridges. Figure 1 illustrates a battery container 2 having end walls 4, side walls 6 and intercell partition walls 8 defining a plurality of cavities 10 for housing electrochemically-active components 12 (shown in phantom) of the battery. Sealing lips 14 on the upper ends of the walls 4, 6 and 8 mate with a channel formed in a cover 16 and are cohesively bonded to the cover therein. More specifically, the cover 16 includes a continuous channel 18 which extends around the periphery of the cover contiguously inwards of a peripheral edge 20 of the cover 16. The channel 18 is defined by parallel ribs 22 and 24 depending from the principal plane of the cover and by floor 26 at the base of the channel 18 (i.e., at the roots of the ribs 22-24). A thickness A of the cover 16 at the floor 26 is greater than the thickness B of the cover outside the channel 18 to provide additional thermoplastics material for exuding about the lips 14 of the container walls 4, 6 and 8. Similar channels 18' defined by parallel ribs 22' and 24' and floor 26' are provided transverse the cover 16 as determined by the number of intercell partition walls 8 in the container to be sealed. Several sets of wing-like guides 28 are provided periodically along the length of each channel 18' to guide the intercell partition walls 8 into position in the channels 18' during the final assembly. Similarly, a series of single guides 29 are provided along the length of the continuous channel 18 at the opposite ends of the cover for guiding the end walls 4 of the container into channel 18. The guides 28-29 serve further to guide the heating tool into place as will be discussed hereinafter. The lips 14 on top of the container walls 4, 6 and 8, as well as the floors 26 and 26' of the channels 18 and 18' respectively are preferably substantially concurrently heated by pressing them against a heated plate 32 (i.e., at ca 371°C ± 28°C (700°F ± 50° F)) positioned therebetween. The heated plate has a lower planar surface 34 for engaging the tops of the lips 14, and peripheral and transverse elongated projections/ridges 36 and 38 respectively which extend into the channels 18 and 18' to melt the floors 26 and 26' thereof. As best shown in Figure 2, an upper edge 40 of the projection/ridge 38 is pushed into the floor 26' of the channel 18' as the thermoplastics material thereat melts and exudes down around the upper edge 40 as illustrated at 42. At the same time, the sealing lip 14 on top of the walls 4, 6 and 8 and in contact with the plate 32 melts and mushrooms as illustrated at 44 in Figure 2. After the floors 26, 26' and the lips 14 have melted, the heated plate 32 is removed (see Figure 3) and beads of melt 46 and 48 formed respectively in the channels 18 and 18' and on the lips 14 are allowed to cool sufficiently to form a first skin 50 of solidified thermoplastics material on the cover bead 46 and a second skin 52 of solidified thermoplastics material on the lip bead 48 while leaving respective cores 54 and 56 of beads 46 and 48 still molten. Whilst the cores 54 and 56 are still molten, and as best shown in Figure 4 (only one joint shown), the container and cover are pressed together so that the lips 14 mate with the respective channels 18 and 18' (only 18' shown). The molten beads 46 and 48 are pressed together so as to rupture their respective skins 50 and 52 and cause their molten cores 54 and 56 to coalesce into a unified weld bead 58 and to exude down (as shown at 60) between the lip 14 and ribs 22' and 24' to also bond with the sides of the lip 14 and the faces of the ribs 22'-24' on either side of the lip 14. Six-cell (12V) starting-lighting-ignition (SLI) lead-acid storage battery containers made from polypropylene, heat-sealed in accordance with the present invention, have demonstrated a capability of withstanding internal pressures of up to 48.26 kPa (7 lbs/in²) and are ideally suited for use with gas-recombinant batteries or other sealed batteries which require significant super-ambient internal pressures for most effective operation. Whilst the invention has been described in terms of a specific embodiment thereof it is not intended to be limited solely thereto but rather only to the extent set forth hereafter in the scope of the claims which follow.

Claims: A method of heat-sealing a thermoplastics cover (16) to a thermoplastics container (2), wherein said container (2) has a set of walls (4,6,8) defining at least one cavity (10) and a sealing lip (14) extending along at least one edge of each of said walls (4,6,8), and said cover has a pair of parallel ribs (22,24;22',24') defining a substantially continuous channel (18,18') inwards of a periphery of the cover (16) and mating with said lip (14) in tongue-and-groove fashion, said channel (18,18') having a floor (26,26') at the base thereof, and wherein the method comprises heating said sealing lip (14) and said floor (26,26') to melt the thermoplastics material thereat, and then pressing the melted regions of said lip (14) and said floor (26,26') together to form a weld therebetween, characterised in that the method comprises the steps of: substantially concurrently heating said floor (26,26') and said lip (14) to provide a first bead of melt (46) in said channel (18,18') at the bases of said ribs (22,24;22',24') and a second bead of melt (48) along said lip (14); cooling each of said beads (46,48) sufficiently to solidify only the outermost surfaces thereof so as to form a skin (50,52) of said thermoplastics material over a molten core (54,56) of said beads (46,48); pressing said first and second beads (46,48) together in said channel (18,18') so as to rupture said skins (50,52) and cause said cores (54,56) to coalesce one with the other and to exude between said ribs (22,24;22',24') and unmelted portions of said lip (14); and cooling the coalesced cores (54,56) to weld said cover (16) to said container (2). A method of heat-sealing a thermoplastics cover (16) to a thermoplastics container (2) according to claim 1, in which said lip (14) and said floor (26,26') are substantially concurrently contacted with a heated tool (32) so as to provide said first bead (46) of melt in said channel (18,18') and said second bead (48) of melt along said lip (14), and then said tool (32) is removed from contact with said lip (14) and said floor (26,26') prior to cooling each of said beads (46,48). A method of heat-sealing a thermoplastics cover (16) to a thermoplastics container (2) according to claim 1, in which a heated tool (32) is embedded in said floor (26,26') along the length of said channel (18,18') for a time sufficient to partially melt said floor (26,26') and to provide said first bead (46) of melt in said channel (18,18'), said lip (14) is contacted with a heated tool (32) so as to provide said second bead (48) of melt along said lip (14), and then said tools (32) are removed from contact with said lip (14) and said floor (26,26') prior to cooling each of said beads (46,48). A method of heat-sealing a thermoplastics cover (16) to a thermoplastics container (2) according to claim 2, in which said floor (26,26') has a greater thickness of said thermoplastics material than the remainder of the cover (16) overlying said cavity (10).

Inventors: MEADOWS CLARENCE ALFRED, MEADOWS, CLARENCE ALFRED
Applicants: GEN MOTORS CORP, GENERAL MOTORS CORPORATION
Application year: 1992
Main classification: B29C65
IPC codes: B29C65, B29L22, H01M2