Tamper evident closure

Abstract: A tamper evident cap (10) for a container having a plastic curled ring (16) at its lower end of reduced internal dimensions for engaging the underside of interfering means (40) on the container (36) and which has induced stresses therein that enhance and reinforce its engaging function. In addition, lines of weakness (25) are included as a peripheral line of weakness above the curled ring or vertical lines of weakness (90) in the curled ring (16) or both. In capping, the curled ring (16) is sufficiently resilient to pass about and below the container interfering means (40) and into interfering engagement therewith without rupturing the described lines of weaknesses (25,90). In the initial uncapping, either the peripheral line of weakness (25) will rupture leaving the ring (16) on the container (36) or the vertical lines of weakness (90) will rupture and separate the ring (16) into segments while allowing the entire cap to be removed. In either embodiment clear evidence of tampering or opening is provided. In forming the cap, the cap preferably is formed by conventional molding techniques such as injection molding with a plastic preform for forming the curled ring. The curled ring is subsequently formed by tools of the invention that curl the preform into desired cross sectional shapes such as an O, U, J or coil.

Description: This invention relates to tamper evident plastic caps for indicating the condition of containers and particularly to caps having separable or breakaway portions to provide a clear and unequivocal indicator of the condition of the containers, e.g., that they have been opened or tampered with. The invention also relates to containers comprising such caps. The present application is a divisional to application EP 87900478.6, now European Patent No. 0 254 729, which claims a method of forming a tamper evident cap, a tamper evident cap having a form as obtainable with said method, and a container comprising such a cap. While bottle caps evidencing tampering by means of depending breakaway rings and separable portions are in popular current use, they have many defects and limitations in their manufacture and performance. Malleable metals such as aluminum have been used to produce breakaway ring caps wherein the lower portion of the skirt contains a peripheral line of weakness having spaced bridges and a ring portion therebelow rolled inwardly onto and under a peripheral locking ring on the bottle neck during capping to engage it in an interference fit. When the container is uncapped, the bridges break, leaving the depending ring attached to the bottle neck. An option to such a cap design is the addition of vertical lines of weakness peripherally spaced, wherein the depending ring ruptures and the ring portions so produced remain affixed to the cap upon its removal from the container. A common defect in the performance of such a cap is that it is entirely dependent on the control maintained in the cap-rolling operation and since this is accomplished on the packaging line it is often poorly controlled with the result that such caps can appear untampered with in cases where they have been. Additional defects and limitations on the use of metal roll-on caps result from sometimes poorly formed and easily stripped threads; high frictional resistance with the bottle neck, especially with those of glass, making both capping and resealing difficult and erratic; frequent loss of seal when the caps are dented, especially noted in pressurized containers; difficulty in uncapping due to poorly defined exterior fluting needed to adequately grip the cap; and their rapidly rising cost. Some of these limitations can be reduced or eliminated by preforming the entire cap prior to capping including a peripheral line of weakness and a ring with an interfering bead therebelow. However, the non-resilient nature of such caps call for special container neck designs and cap skirt designs with their own set of limitations and defects as well as restrictions on the type of container materials which are suitable to coact with them. See, for example, U.S. Patent No. 3,460,703. As a result of the defects and limitations noted for metal tamper evident caps, a substantial interest has been displayed in the use of plastic caps for such purposes. Plastics caps present prospects for easier uncapping and more reliable resealing, resistance to denting and resultant loss of seal, well defined exterior fluting for good gripping and comparatively lower cost. However, plastic caps introduce a different set of limitations encountered in their manufacture, capping and subsequent performance. Early tamper evident plastic caps possessed depending rings which could be heat-shrunk to engage the bottle neck locking ring. However, this approach imposed on the packager the added bottling line operation and expense of heating and shrinking the ring after capping. In addition, difficulty in precisely controlling the operation also has led to erratic shrinkage with the result that caps appeared tampered with when they bed not been. As a result of objections to the heat-shrunk tamper evident plastic cap, interest has turned to producing the required ring undercut for an interference fit with the neck prior to the capping operation. To date this goal has been achieved only by making undesirable compromises in manufacturing complexity and cost to achieve the undercut as well as by loss of some effectiveness in performing the tamper evidence function as a result of the ring design and the inherent nature and properties of plastic materials. Ring undercuts which are solid, molded-in circumferential beads preclude the use of the more rigid, non strippable" polymers such as polystyrene and add to manufacturing complexity and cost by requiring molds with complicated part ejection systems which increase mold cost and lower molding productivity. A slitting operation subsequent to molding to provide the line of weakness may also be required. However, its tamper evidence may be marginal because the amount of undercut possible even with such compromises typically is also marginal. That is, the feature may grip the bottle neck sufficiently to perform adequately in normal use but not when confronted by a serious tamperer. The performance of such features is also impaired by the inherent ease of deformation and elongation of the plastic materials used which may allow them to stretch beyond their elastic limit during capping, increasing their diameter and thereby reducing the amount of ring interference with the container neck. Further, such breakaway rings are sometimes engaged with the container neck under a continuing stress which over a period of time produces plastic creep, thereby further reducing the tenacity of the gripping engagement and the integrity of the tamper evidence. Attempts to avoid the problems associated with producing plastic caps having a solid molded-in undercut on its breakaway ring have led to designs and manufactuing methods which produce a plurality of flaps on the ring interior which may fold out of the way on removal from the mold and which are subsequently folded into an interfering position prior to capping. See, for example, U.S. Patent No. 4,497,765, U.S. Patent No. 4,394,918 and U.S. Patent No. 4,470,513. Such approaches have their own manufacturing problems which include complex and costly molds, manufacturing operations subsequent to molding or because of the ready foldability of their interference-producing flaps, their tamper evidence can be readily foiled by a serious tamperer. EP-A-0 049 876 discloses a closure cap for bottles or containers which is secured to the bottle or container neck opening by means of some kind of locking piece or projection. Preferably, said projection comprises a plurality of peripherally spaced apart rigid lugs which are hooked onto peripherally projecting flanges of the bottle or container neck and are in an interfering engagement with said neck only at the tips of the lugs. GB-A-1 048 727 discloses a method of making a synthetic thermoplastic article and a synthetic thermoplastic article made by said method. The synthetic thermoplastic article includes a curled edge which is curled by means of die curling or by rotary curling. The thermoplastic articles usually are containers, such as cases or boxes, or closure caps or lids for such containers. There is, however, no mention that the curled edge is provided for effective sealing or can be used in a tamper evident closure cap. Another problem is associated with the fact that otherwise standard bottle neck designs often vary only because their locking ring dimensions vary and prior art plastic caps having preformed breakaway rings are specific to a single bottle neck design. As a result a cap from a given production mold can only be used by packagers employing very much the same bottle neck locking ring design and dimensions. This dictates the use of a larger number of smaller molds resulting in reduced productivity and higher cap manufacturing cost. Also, it has not generally been feasible to produce metal or plastic caps including depending rings which break into segments and which remain attached to the cap, except by using roll-on metal or heat shrink plastic rings, with the various deficiencies already noted. Plastic caps with molded-in ring undercuts and vertical lines of weakness present problems with premature ring breakage during mold removal or during capping. Thus, known tamper evident caps with depending breakaway ring features are beset with drawbacks and problems associated with the inherent characteristics of the materials selected; the need to perform difficult-to-control operations on packaging lines; complex and expensive manufacturing tools; low manufacturing productivity and high costs; the fact that the more rigid plastics are restricted in use; and the relative ease with which the feature may be overcome by a serious tamperer and other problems. The above objects are achieved by the cap and container according to the claims. In accordance with the present invention, there is provided a new and unique tamper evident cap for a container having a lower breakaway or separable ring portion, wherein the cap has a simplified design; low mold and production cost; readily controlled and uniform quality and performance; a wide selection of plastics which are suitable; an ability to coact with a greater variety of container neck designs and dimensions; tamper evidence which is difficult-to-defeat and imposed stresses which reinforce the function of the cap. Generally, the cap of the invention is used with containers having an opening surrounded by a lip or rim with a depending neck having an upper means for releasably engaging a cap and a lower means for interfering with a ring portion of the cap so that the ring, upon uncapping, either will break from the cap or separate into segments while remaining attached to the cap. The cap of the invention includes a lid for closing and opening the container, a depending skirt having means for releasably engaging the container engaging means in closing and opening the container, a plastic ring depending from the skirt having a curled portion at its lower end providing a reduced internal dimension which engages the underside of the container interfering means and which has induced stresses therein that enhance and reinforce its engaging functions. In addition, lines of weakness in the ring are included either as an intermediate peripheral line of weakness between the skirt and curled portion or vertical lines of weakness in the curled portion or both. In capping, the curled portion is sufficiently resilient to pass about and below the container interfering means and into interfering engagement therewith without apparent rupturing of the described lines of weaknesses. In the initial uncapping, either the peripheral line of weakness will rupture leaving the ring on the container or the vertical lines of weakness will rupture and separate the ring into segments while allowing the entire cap to be removed. In either embodiment clear evidence of tampering or opening is provided. In a preferred embodiment, the cap is molded and, as molded, includes a generally cylindrical tube or band depending from the cap skirt which is reformed by tools of the invention that define the lower and inner and/or outer portions of the curled ring portion into the desired curvilinear cross sectional shapes, such as an O or coil. Typically, the desired cross sectional shapes of the invention can provide reduced internal dimensions for a polypropylene band of at least about 5 per cent for a 23 mm cap and an arc or curved portion of at least about 50 degrees. In a preferred embodiment, the cylindrical band is reformed by curling into a hollow "0" ring shape wherein its outside diameter is typically close to that of its original outside diameter and its inside diameter is substantially less than the outside diameter of the engaging portion of the container neck. The reforming is achieved by means of a curling tool which compresses the lip of the cylindrical tube preform thereby turning it inwardly and then upwardly channeling and gradually altering the direction of such movement over its curved working surfaces. The initiation of the curling action may be facilitated by providing a taper to the lower free end of the preform. Such a curling action at this point produces a "J" shape or semi-circular radial cross section. After leaving the curved working surface of the tool as it further compresses the cylindrical preform, the lip takes an upward and outward direction resulting from the continuing compression and the stresses imposed by its plastic memory, thereby completing the formation of the hollow "0" ring shape. In another embodiment, the compression of the cylindrical portion can proceed beyond this point and produce a coiled ring. To facilitate the curling operation or to alter the dimensions, shape or character of the resultant curled ring, the curling tool can be heated, spun or rolled along the free end of the cylindrical portion during its shaping. The periphery of the free end of the cylindrical portion may be curled simultaneously or sequentially. In other embodiments, the ring can be exposed to elevated temperatures for short periods before capping to alter its dimensions, shape or character or the preform for the ring can be heat treated for the same purpose. A feature of the invention is that the use of the curling action to reform the cap ring lower end portion not only produces the desired "0" ring or other shape with reduced internal diameter, but also alters and improves the inherent properties of the plastic thereat to reinforce its neck engaging strength. That is, the curling operation during its deformation and reforming of the plastic in the lower and portion also produces built-in stresses therein which are distributed in such a way as to enhance its function. This prestressing places the outward portion of the "0" ring shape in a state of tension or extension and the inward, neck engaging portion in a balancing state of compression. Such a condition of compression increases the effective elastic limit and tensile strength of the plastic so that the ring will better resist temporary or permanent elongation and loss of interference with its coacting neck interference means. Thus, it is significant to note that the curling method of the invention used to produce the cap ring portion on the one hand produces the desired shape and interfering means by exceeding the elastic and tensile limits of the plastic, yet, on the other hand, yields an article having greatly enhanced elastic and tensile limits in the portion of the ring which performs the critical neck engaging function. Another feature of the invention is that the cap, as molded, has no interfering bead or other shape projecting from the interior of its cylindrical tube preform as in prior art cap designs which can interfere with ejection of the cap from its production mold. As a result, simple and low cost mold ejection mechanisms can be used. For instance when made from such semi-rigid plastics as polypropylene and poylethylene, the entire cap can be stripped from the mold, avoiding otherwise called for mold parts and operations such as unscrewing, side-acting or other mechanisms and permitting a closer spacing of the mold cavities. This lowers manufacturing cost through lower cost and longer lasting molds and higher production rates derived from producing larger numbers of caps per molding cycle and reductions in the length of the molding cycle. Another feature of the invention derived from the fact that the cylindrical tube portion of the cap, as molded, has no undercut to interfere with mold ejection, is that rigid plastics with low elongation characteristics such as polystyrene can be used with the only additional requirement being that suitable ejection means be used for other cap undercuts such as internal threads. Such requirements are readily met by forming the threads using unscrewing or collapsing core molds thereby allowing the cap manufacturer to avail itself of the potential merits of employing such rigid plastics to achieve lower manufacturing cost or improved cap performance. While such rigid plastics lack large amounts of elongation, they do possess a sufficient amount to be suitably curled for the successful practice of the invention. This results in large part from the imposition of significant stresses by the curling method of the invention which enhance the plastics properties by compressing the interior, working portion of the curled breakaway ring effectively increasing its elongation and strength thereat. Still another feature of the cap of the invention derived from producing the ring undercut subsequent to molding is that the degree of undercut and interference with the bottle neck is not related to or limited to the degree of difficulty in removing such undercuts from the mold. As a result, very much deeper undercuts can be made to achieve a greater gripping tenacity with the bottle neck, thereby presenting would be tamperers with a higher degree of difficulty in defeating the tamper evidence and assuring a more reliable performance in normal use. Yet another feature of the invention is that relative height and diameter of the curled interfering portion of the ring is determined in a separate reforming operation so that a single cap as molded, may be used to produce caps which will fit a variety of bottle neck designs where the height and diameter of the neck locking ring varies. The costs of the curling tools and the curling operation affect overall cap costs very little, enabling the cap manufacturer to take advantage of the economies of larger scale production for a number of customers or users from fewer and larger production molds. Another feature of the invention is that the curling operation of the invention may be performed in sequence with or as a part of other steps taken in the manufacture of the cap. That is, it may be performed during or consequent to removal from production molds or during or consequent to cap finishing operations such as liner insertion or lid decoration. It may also be performed simultaneously with or consequent to a curling operation to produce the linerless seal of WO 87/03560. Also the curling operation has been demonstrated to be non-critical in respect to unusual control or conditions so that automatic operation to produce uniform product can be readily used to achieve high quality at low cost. Another feature of the invention derived from producing the undercut of the ring subsequent to molding is that the peripheral or vertical lines of weakness can be readily molded into the cap, thus avoiding subsequent slitting operations. Since the cylindrical tube preform has no internal bead to resist stripping from the mold and since the tube itself is relatively thin so that it presents little resistance to being stripped, the bridges which attach it to the upper portion of the cap can be strong enough so that they do not break when being stripped from the mold and yet weak enough to readily break in use during removal of the cap from its bottle neck. For the same reasons the vertical lines of weakness can be readily molded into the tube preform without concern for premature breakage upon mold removal. Thus, the cap of the invention may be molded with the same ease and freedom from limitations as currently produced caps having heat shrink bands as their tamper evidence. Another feature derived from the fact that the line of weakness can be readily produced in the mold is that its presence, indicated by the distinctive molded slots and bridges, can be made quite obvious, calling the consumer's attention to the tamper evident feature prior to purchase. Thus, the caps of the invention have simple design; are less easily tampered with and are more reliable; have improved versatility and broader usefulness; can use more rigid plastics; have enhanced physical properties due to prestressing; and are low cost. The cap of the invention can be produced automatically and reliably without unusual controls or conditions. It is composed or simple operations yielding Low cost caps based on lower cost and longer lasting molds and tools; shorter molding cycles; greater density of and numbers of cavities per mold; the production of the lines of weakness in the production mold, and greater economy of scale derived from the versatility of use for the output of a single mold. The invention will now be described in detail in connection with the drawings, in which : FIGURE 1 is a bottom perspective view of one embodiment of the cap of the invention, partially broken away to show details thereof; FIGURE 2 is a bottom perspective view of the cap of FIGURE 1 after molding and which, at this stage, includes a lower preform portion for the ring; FIGURE 3 is a longitudinal sectional view of the cap of FIGURE 2, wherein the preform is about to be engaged by a curling tool of the invention; FIGURE 4 generally is the same as FIGURE 3, except that the curling tool has engaged the preform and initiated the inward curling of the free end thereof; FIGURE 5 is generally the same as FIGURE 4, except the curling tool has almost completed the curling of the ring; FIGURE 6 is a longitudinal sectional view of the cap of FIGURE 1 with a completed "O" ring ready for engaging the neck finish of a bottle; FIGURE 7 is the same as FIGURE 6, except that the bottle has been closed by the cap and the "O" ring is in engagement with the interfering means on the neck finish; FIGURE 8 is a longitudinal view of the cap and neck finish of FIGURE 7, wherein the cap has been removed from the neck finish leaving the ring portion thereon as evidence of opening or tampering with the container; FIGURE 9 is an enlarged sectional view of the ring portion of the cap shown in FIGURES 1 and 6, schematically illustrating the stresses induced therein in the hoop direction; FIGURE 10 is a plan view of the ring portion of FIGURE 9; FIGURE 11 is a stress-strain graph for the stresses in the previously illustrated ring portion; FIGURE 12 schematically illustrates what occurs to the "0" ring of FIGURE 11 upon release of the hoop stresses; FIGURE 13 is a longitudinal sectional view of another embodiment of a cap of the invention after molding and before the preformed lower end is curled to form a ring; FIGURE 14 is a longitudinal sectional view of the cap of FIGURE 13, wherein the preform has been curled outwardly to form a ring having a coiled cross sectional shape; FIGURE 15 is a longitudinal sectional view of the completed cap of FIGURE 14 engaging and initially closing the illustrated bottle; FIGURE 16 is a longitudinal sectional view of still another embodiment of a cap of the invention, wherein the preform portion has a larger diameter than the diameter of the skirt portion of the cap and is about to be engaged by a holder and curling tool of the invention to form an inwardly curled ring in the preform; FIGURE 17 is the same as FIGURE 16 except that the inwardly curled ring has been fully formed; FIGURE 18 is a longitudinal sectional view of the finished cap of FIGURE 17 fully engaging and closing the illustrated bottle; FIGURE 19 is a longitudinal sectional view of another embodiment of a cap of the invention after molding, wherein the preform includes vertical slits and notches in its free end; FIGURE 20 is the same as FIGURE 19, except that an inward curled "O" ring has been formed in the preform; FIGURE 21 is a longitudinal sectional view of the cap of FIGURE 20 engaging and initially closing the illustrated bottle with the assistance of a capper; FIGURE 22 is a longitudinal view of the cap of FIGURES 20 and 21 after the initial uncapping illustrating the ruptured radial portions in the "O" ring while the ring remains attached to the skirt of the cap; and FIGURE 23 is a bottom view of the cap of FIGURE 22. Referring to Figure 1 there is shown a semirigid threaded cap 10 of plastic having a lid 12, a depending peripheral internally threaded skirt 14 including an upper portion 13, an intermediate peripheral line of weakness 17 and a lower breakaway ring 16. The illustrated breakaway ring 16 includes a cylindrical upper end 20 and a free curled lower end 24. As shown, the end 24 of the breakaway ring 16 is curled inwardly and forms a hollow annular "O" or coil ring. The intermediate peripheral line of weakness 17 includes a plurality of bridges 23 connecting the upper skirt portion 13 with the upper end 20 of breakaway ring 16 and slots 25 between and separating the bridges 23. Figure 2 shows the cap 10 of Figure 1, as molded and before formation of ring 16, having a tubular vertical band 18 with a lower free end 22 having a lip 30 and an upper end 20 having spaced bumpers 19 which are integral with and which extend into the bridges 23. Referring to Figures 3-5, there is shown a preferred method of forming the curled portion 24 of ring 16. As shown in Figure 3, the cap 10 is held in holder 62 and already has been formed by conventional molding techniques, such as injection molding, with the vertical tubular band 18 having its upper end 20 integral with the bridges 23 and with its lower free end 22 ready for curling. The curled portion 24 of ring 16 is formed with a curling tool 26, which in Figure 3 has been positioned under the cap 10 ready to engage the lip 30 of the preformed band 18. The curling tool 26 includes an annular groove 28 of a concave cross section suitable for shaping and dimensioning the curled "O" ring portion 24. As shown in Figure 4, the forming operation is accomplished by pressing the groove 28 of the tool 26 against the lip 30 of the band 18. In this embodiment the deepest portion 33 of groove 28 representing the center of its concavity is located inwardly of the cylindrical plane of band 18. This is shown by the dotted lines 35 of Figure 3. Also, the groove 28 has a slanted portion 39 outwardly and tangent to its concavity to facilitate centering of the tool and cap. At the initial engagement of lip 30 with groove 28 the bridges 23 collapse and the tubular band 18 rotates slightly in response. This response is assured by dimensioning the bridges to be larger (and stronger) in the radial direction and smaller (and weaker) in the circumferential direction. When the bridges 23 collapse, upper end 20 of cylindrical band 18 presses against the upper face 27 of slots 25 and is restrained from further movement by the projecting restraining means 21 located on the inner edge of the upper face 27 of slots 25 and by the upper face 27 itself. As the movement of tool 26 relative to the wall 18 continues the cylindrical sides of the wall 18, beginning with the lip 30, are forced inwardly and upwardly to assume an interim "J" shape 29. As this relative movement continues the lip 30 is forced upwardly out of groove 28, and at the same time is forced outwardly in response to the stresses developed therein while being shaped by tool 26, thereby producing the desired "O" ring curl 24. After formation of the curl 24, the curling tool 26 is withdrawn from the cap 10 and the cap 10 is ready for capping. As shown in Figure 3 there is a taper in lower free end 22 extending from the rim 30 which facilitates the initiation of the curl 24 and assures development of a full round curve thereat. Also to facilitate the curling operation of the invention, in the case of polypropylene the tool 26 may be heated to a temperature of 33°C (100°F) to 149°C (300°F) for curling cycles of one to five seconds. Lower temperatures and cycles are preferred to maximize the desired stresses of the invention imposed by the curling operation on the curl 24 and discussed hereafter in Figures 9 to 13. Typical dimensions of the "O" ring curl 24 in a 28 mm polypropylene cap are about 1.27 to 3.05 mm (0.050 to 0.120 inches) for the curl height or width and about 0.38 to 1.02 mm (0.015 to 0.040 inches in wall thickness with the radial cross section curled about 270 to 540 degrees to reduce the internal diameter of tubular wall 13 about 5 to 15 percent. Referring to Figures 6-8, there is illustrated the use of the ring 16 of Figures 1-5 as a breakaway tamper evident feature of the cap 10 for a bottle 36 (partially shown) having an externally threaded neck 38, an upper thread 42 and a lower locking ring 40. As the cap 10 is threaded onto the bottle neck 38, the curled inside portion 32 of the ring 16, which is a smaller dimension than locking ring 40, is stretched or deflected and forced past the locking ring 40 into an interfering engagement therewith as shown in Figure 7. The ring 16 is restrained from upward movement by the locking ring 40 so that the unthreading action will rupture bridges 23 leaving the ring 16 on the bottle neck 38 as evidence that the bottle has been previously opened. This is illustrated by Figure 8. To maintain the axial alignment between the ring 16 and the upper portion 13 of the skirt 14 during cap removal, spaced bumpers 19 are located below each of the bridges 23. Without such bumpers 19, it is possible for the ring 16 to slide sideways under the locking ring 40 after some of the bridges 23 have ruptured thereby compromising the ability to rupture the remaining bridges 23 before losing thread engagement. With the bumpers 19 engaging the vertical surface 43 of locking ring 40, when one or several, but not all, of the bridges 23 break, the ring 16 is held in axial alignment until all the bridges 23 have ruptured. The curling method of the invention not only produces the hollow "O" ring 24 or other shapes of the invention creating the desired interference with the bottle neck, but it also alters the inherent physical properties of the plastic in such a way as to enhance its neck-engagement tenacity. That is, the plastic at the inwardly directed interfering portion 32 is made stronger, more resilient and creep resistant as a result of the stresses imposed on the plastic during the curling operation. This is explained by FIGURES 9 to 12. The curling operation, by imposing an alternative shape on the cylindrical vertical band 18, introduces stresses to the resultant shape of the ring 16. That portion of the ring 16 which is stretched is in extension or a state of tension and that part which is compressed is in a state of compression. The stresses vary with the degree of stretching or compression and, as in any static condition, the total amount and direction of each kind of stress balance one another. The stresses of primary concern are those imposed in the hoop direction since it is in this direction that the inside portion 32 of the cap ring 16 must be stretched to overcome its interference with the bottle locking ring 40. The hcop stresses introduced by the method of the invention result from the increase or decrease of the hoop diameter of the cylindrical band 13 when it is reformed into the "O" ring 24 or other shape ring 16 of the invention. If it is considered that the band 18 is composed of an integrated stack of hoops, eacn of which must be expanded radially (stretched) or compressed to produce the reformed shape, then a condition of tension or compression will be imposed on each of the hoops in the resultant shape as shown in FIGURES 9 and 10 (respectively a radial cross section and a horizontal plan view of the "O" ring 24). The level of stress is relative to the distance from the location of the original cylindrical band 18 and tension is highest at point B located in that portion of the "O" ring 24 which is furthest from the inside interfering portion 32 and, therefore, plays a secondary role in gripping locking ring 40; stresses are nil at the intermediate location of the original cylindrical preform 18; and compression is highest at point A located in that portion of the "O" ring 24 which plays the principal role in gripping the locking ring 40, the undercut 32 itself. FIGURE 11 shows a typical stress-strain curve for a semi-rigid plastic (e.g., polypropylene) suitable for the practice of the invention. The condition of the hoop stress at points B and A are shown on the curve at typical locations for the practice of the invention (5% elongation and beyond and the corresponding level of compression). From the location of point A in FIGURE 11 it can be seen that the curling action alters the properties of the inwardly directed undercut portion 32 of the "O" ring 24 significantly. Because this undercut 32 is highly compressed, its elastic modulus is maximized and the amount that it can be elongated before plastic or permanent deformation occurs is significantly increased (proportional to the ratio of AE to OE in FIGURE 11). The former increases the amount of stress required to obtain enough elongation to overcome the interference between the undercut 32 and the locking ring 40 and the latter prevents the undercut 32 from permanently stretching during or after capping thereby maintaining a relatively greater level of its engagement strength. These factors are highly beneficial towards maximizing the ability of the undercut portion 32 of the "O" ring 24 to generate the maximum amount of engagement with the locking ring 40 and to resist decay of such engagement over long time periods resulting from plastic creep. Thus it can be seen that the distribution of stresses imposed by the curling operation of the invention contributes to the enhanced performance of the "O" ring shape 24 to provide the required interference with the bottle neck. That the stresses described in the above analysis do in fact exist and are distributed in the manner described is demonstrated by annealing tests to observe the changes in shape which develop as sucn stresses are relieved. For such test 0.51 mm (0.020 inch) thick cylindrical bands 18 of polypropylene were curled in the manner of the invention to yield "O" ring shapes 24 having a ring outside diameter of 31.24 mm (1.230 inches), inside diameter of 26.16 mm (1.030 inches) and a curl outside diameter of 2.54 mm (0.100 inches). The "O" ring 24 was then split radially to facilitate the full release of imposed stresses and then exposed to a temperature of about 149°C (300°F), somewhat under the plastic's melting point, for five minutes. Under these conditions, the "O" rings 24 opened up in the hoop direction to a radius of 63.5 mm (2.5 inches), a four-fold increase, demonstrating the presence of very high levels of hoop stresses and hoop stress differentials. The dramatic nature of this result, illustrated in Figure 12, substantiates the preceding analysis wherein the contribution of the stresses imposed by the curling operation alters and enhances the physical properties of the plastic used to manufacture the cap 10 so as to develop higher levels of interference between the "O" ring 24 and the bottle neck locking ring 40 and less decay of such interference. In addition to an "O" ring, the ring 16 can be of different cross sectional shapes which have the described desirable properties, including coiled cross sectional shapes. Also, the ring 16 and its lower end 24 can have various circumferential configurations. For example, the cap 10 as molded can include a castellated band 18 or free end 22 having circumferential segments with spaces therebetween. Thereafter, the free end 22 comprising segments can be curled as described herein to produce a segmented curled lower end 24 with spaces therebetween. Alternatively, the unmodified band 18 can be curled by a curling tool 26 in which the groove 28 is modified to produce on curling an undulated or scallop shape to the inside portion 32 of the curl 24. In use, such caps 10 will function as described to provide a clear indication of the condition of the capped container. The caps of the present invention also can be used for a wide variety of containers and for a wide variety of products. Typically, cap sizes range from about 20mm to 120mm and bottle and/or jar sizes range from about 56.7 g (2 ounce) to 3630 g (128 ounce) capacity. Larger capacity containers such as drums or kegs are also suitable for the practice of the invention as are smaller vials and other containers. Further, the caps of the present invention can include other features including metal lids, seals, etc. Specifically, the caps of the present invention can include the linerless seals disclosed in WO 87/03560. Referring now to Figures 13-15, there is shown another embodiment of the ring 16 of the invention, wherein the lower end 22 of the tube 18 is curled in an opposite direction to the "0" ring curl 24 of Figures 1-8 to provide an outward curl. In addition, the line of weakness 17 is produced after molding. Figure 13 shows the cap 10 after molding and after a subsequent slitting operation to produce the line of weakness 17 wherein the upper end 20 of band 18 is slit by appropriate knives (not shown) completely through its wall but not through the spaced internal bumpers 19. This results in spaced internal bridges 23 connecting the band or tube 18 through the bumpers 19 to the upper skirt portion 13. Below the cap 10 the curling tool 26 is positioned with its forming grooves 28 ready to engage the lower end 22 of band 18 so as to shape it outwardly and upwardly. The outward direction of the curl 24 is achieved by locating the center of concavity 33 of groove 28 outwardly of the cylindrical plane of wall 18 as illustrated by the dotted lines 35 of Figure 16. In this embodiment, to develop the maximum amount of undercut 32 to engage the neck locking ring 40 the undercut 32 must be displaced inwardly from the upper end 20 of band 13 as much as possible. Therefore it is necessary to develop the maximum curvature to the undercut 32. This is accomplished by locating the center of concavity 33 of groove 28 close to the plane of wall 18 and by removing any restraints to the free inward displacement of undercut 32 resulting from the stresses imposed by the curling operation, by providing ample free space in the inward slanted portion 39 of groove 28 as shown in Figure 14. Figure 15 shows the engagement of undercut 32 of the cap 10 with the locking ring 40 of bottle neck 38 which results from the curling method of this embodiment of the invention. Figures 16-18 illustrate another means of forming the line of weakness 17 subsequent to molding and during the curling operation of the invention. In this case the cap 10 is molded to produce the tubular vertical band 18 having an inside diameter larger than the outside diameter of the upper skirt portion 13 and attached to skirt portion 13 by means of a radially extending flange 44. In spaced arrangement on the underside of flange 44 are a plurality of bridges 23 which also connect band 18 to skirt portion 13. The holder 46 for cap 18 during the curling operation is constructed with a peripheral cutting edge 48 located around the edge of its well 50. During the curling operation the cap 10 is placed in the well 50 of the cap holder 46 and the groove 23 of the tool 26 engages the lip 30 of the cap band 18. As the tool 26 proceeds to curl the band 18, pressure is applied to the upper surface of flange 44 by the holder cutting edge 48 which proceeds to sever flange 44 so that it no longer connects band 18 to the upper skirt portion 13 but cutting edge 43 does not proceed to sever the spaced bridges 23 located on the underside of flange 44. In this manner the desired line of weakness 17 is created. Figure 18 shows the cap of Figure 17 in engagement with the neck 38 of bottle 36. In this embodiment the neck 38 has a comparatively large diameter locking ring 40 and adjacent lower neck portion 41 so that the cap ring 16 undergoes a comparatively large expansion to snap over locking ring 40 and is also maintained in a state of significant expansion thereafter. However, because the curling operation has developed a state of significant compression in the inside portion 32 of ring 16 such expansion does not produce a comparatively high tension or elongation therein to weaken it as discussed in Figures 9-12. Referring now to Figures 19 to 23, still another embodiment of the invention is illustrated, wherein the ring 16 remains attached to the upper skirt portion 13 of cap 10 upon its removal from the bottle neck 38 as a result of strengthening the peripheral line of weakness 17 and providing one or more vertical lines of weakness 92 in the ring 16. Figure 19 shows the cap 10 as molded with crack initiating notches 89 leading into vertical grooves 90 ending in slots 25 which provide vertical lines of weakness 92 in the cylindrical band 13 from its bottom rim 30 to its upper end 20. Between the slots 25 are bridges 23 connecting the upper portion 20 of band 18 to the upper skirt portion 13. Figure 20 shows the result of employing the curling operation of the invention on band 18 to produce the ring 16 having an "O" ring shape 24 in which the notches 89 and the now radial/vertical grooves 90 . produce radial/vertical lines of weakness 92 throughout the full cross section of the ring 16. Figure 21 shows the cap 10 fully seated on the bottle neck 38 after capping wherein the inside portion 32 of the ring 16 has stretched to snap over the neck locking ring 40 with the result that the cracks 93 initiated by the notches 89 have progressed toward the bottom 37 of the ring 16 as a result of said stretching. However, since the ring 16 is hollow and the outer portion 34 is not contiguous with the inner portion 32 but is more remotely connected through the bottom portion 37, and because the outer portion 34 is restrained by the capper head 88, it is not subjected to significant stretching during the capping operation and crack 93 does not propogate beyond the bottom portion 37. Because the ring 16 is hollow the capper head 88 can be made to prevent the outer ring portion 34 from stretching without restraining the inner ring portion 32 from stretching, as required, and snapping over the neck locking ring 40 during capping. This provides assurance that the radial crack propogation does not occur in the outer portion 34 and that the ring 1G remains intact during and after the capping operation as shown in Figure 21. During uncapping the bridges 23 are of sufficient cross section and strength to resist rupture. As a result, the uncapping stresses developed in the ring 16 concentrate at the remaining portions of the radial/vertical grooves 90 in the lower and outer ring portions 37 and 34 which thereupon rupture in a tearing action allowing the entire cap 10 with the broken ring portions 95 to be removed intact from bottle neck 38. Figures 22 and 23 show the cap 10 after uncapping wherein, as a result of the hoop stresses imposed on the ring 16 by the curling method of the invention, the ends 94 of the broken ring portions 95 spring outwardly making the tamper evidence more obvious while at the same time facilitating the recapping of the bottle by displacing the ring portions outwardly from the cap lower opening 96. The degree to which the broken ring portions 95 and their ends 94 spring outwardly is governed by the lengths of slots 25 and the amount of hoop stress developed by the curling operation. The invention in its broader aspects is not limited to the specific described embodiments and departures may be made therefrom within the scope of the accompanying claims without departing from the principles of the invention and without sacrificing its chief advantages.

Claims: A tamper evident cap for a container, comprising: a top wall (12), and a plastic skirt (14) depending from said top wall (12) including a line of weakness (17) therein, and a lower resilient free end (24) curved away from said skirt (14) into a resilient curled free end of plastic which reduces the breadth of said skirt (14) prior to capping and which is adapted upon capping to provide an interference with a container (36) so that upon initial removal of the cap from the container the line of weakness separates to indicate the condition of the container, wherein said curled resilient interfering free end (24) of plastic has a cross sectional shape of a coil or O. The cap of claim 1, wherein said curled free end (24) is curved inwardly. The cap of claims 1 or 2, wherein said curled free end (24) is prestressed with an outer portion in a state of tension and an inner engaging and interfering portion in a state of compression. The cap of claims 1 to 3, wherein the cap (10) is a twist cap. The cap of claims 1 to 4, wherein said line of weakness (17) is above said curled free end (24). The cap of claims 1 to 5, wherein said line of weakness (17) comprises slots (25) peripherally spaced about said skirt above said free end, bridges (23) between said slots (25) connecting said lower free end (24) to said skirt (14) having peripheral dimensions which are less than the radial dimensions and which are adapted to collapse in a peripheral direction upon the initiation of curling said free end. The cap of claim 6, comprising a projection (21) depending from said line of weakness (17) adapted to engage said lower free end and limit axial movement between said skirt and free end at said line of weakness upon collapsing of said bridges (23) during curling. The cap of claims 1 to 4, wherein said line of weakness (17) includes a vertical line of weakness (92) in said lower curled free end. The cap of claim 8, wherein said vertical line of weakness (92) is internal and is not visible externally prior to separation. A container comprising the cap of claims 1 to 9. The container of claim 10 wherein said container (36) has an opening therein and a finish about the opening with upper means (42) for engaging the cap (10) and lower means (43) for interfering with the initial removal thereof, wherein said plastic skirt (14) of said cap (10) has internal means for coacting with said container engaging means (42) on said container finish for closing and opening said container, and wherein said curled free end (24) of said plastic skirt (14) engages the underside of said container interfering means (43) upon the initial capping of said cap (10) and provides interferences with said container interfering means (43) upon initial removal of said cap (10) to effect separation of said line of weakness (17) therein and thereby indicate the condition of said container (36).

Inventors: THOMPSON MORTIMER STAFFORD, THOMPSON, MORTIMER STAFFORD
Applicants: ZAPATA INNOVATIVE CLOSURES INC, ZAPATA INNOVATIVE CLOSURES, INC.
Application year: 1986
Main classification: B65D41
IPC codes: B65D41, B29C53