Packaging strap coil and method for producing the same, packaging strap coil unit and packaging machine equipped with strap coil reel

Abstract: A strap coil (11) comprises a number of layers of a resin packaging strap (11a) wound helically and corelessly around a hollow (11b) defining the access of the strap coil (11). The strap (11a) is made of a thermoplastic resin at least on its external surface. In the strap coil (11) a plurality of perforations (11d) are pierced in the vicinity of an innermost end of the strap (11a) through to a plurality of laminated layers. Each perforation (11d) provides a melt-bonded area (11e) along its periphery to bond the laminated layers of the strap (11a) together. The innermost end of the strap is, thus, securely fixed to prevent unwinding thereof and the strap can be smoothly drawn out to the innermost end for strap-packaging of an item. There is also provided a packaging strap coil unit which provides deformation of the strap coil and a packaging machine.

Description: BACKGROUND OF THE INVENTION The present invention relates to a packaging strap coil and a method for producing a packaging strap coil comprising a number of layers of a packaging strap wound helically and corelessly around a hollow defining an axis of the strap coil, the strap being made of a thermoplastic resin at least on an external surface thereof. A tape-like thermoplastic resin packaging strap is employed for strapping a corrugated cardboard box or the like. The resin strap is usually manufactured by extrusion-moulding an olefinic thermoplastic resin (i.e. polypropylene, polyethylene terephthalate) into a required flat tape shape, in which case drawing and stretching of the resin is carried out at a rate exceeding the extrusion rate. The stretched resin strap is helically wound on a tubular paper core to form a strap coil. The strap coil is loaded on an automatic packaging machine, together with the tubular paper core. A strap therein is successively drawn out to strap a corrugated cardboard box or like item. When the strap coil runs out of the strap, it consists of the tubular paper core. The paper core has been marred by the strap and may not be reused as the core of the packaging strap coil. If the paper core is stained with a synthetic resin-based adhesive, it even fails to make a material for recycled paper. The tubular paper core, which is no longer available as a paper core or a material for recycled paper, is only to be discarded. Disposal of the paper core, however, still requires transportation and waste treating expenses. In view of the above problems, Japanese Patent Application Laid-open No. 315690/1995 (JP-A-7-325690) discloses a coreless packaging strap coil and a manufacturing method thereof, wherein a packaging strap is helically wound in the absence of a paper or like core. As shown in Figs. 19 and 20, the coreless strap coil 205 has a cylindrical hollow 206 at its axis. The innermost end of the strap 204 is melt-bonded at a melt-bonded area 241 with another strap layer overlaid on the top side thereof. To produce this strap coil, a strap 204 is wound on a winding roller 201 comprising a pair of separable right/left roller components 203, 213. Firstly, the start of the strap 204 is wound in one turn along the middle of the axial length of the winding roller 201 where the right/left roller components 203, 213 are joined. Then, the strap 204 provides another layer thereon. The laminated strap layers are melted and bonded to each other by applying a heated iron, thereby forming a melt-bonded area 241. Once the innermost end of the strap 204 is fixed, a predetermined length of the strap 204 is wound on the entire surface of the winding roller 201 to form a strap coil. Finally, the right/left roller components 203, 213 of the winding roller 201 are disjoined and removed from the strap coil. Thus obtained is a coreless packaging strap coil 205. In this packaging strap coil, the fixed innermost end of the strap is not drawn out into the hollow at the axis. Therefore, the strap coil remains firm and solid even during transportation. When this strap coil is loaded on an automatic packaging machine, the strap is drawn out from the outermost end and used for strapping a corrugated cardboard box or like items. As mentioned above, a heated iron is used to heat a predetermined area on the laminated portion of the strap wound on the winding roller, so that the laminated strap portion is melted and bonded together. However, as the iron fails to give a stable heating temperature, the bonding strength of the laminated strap layers may vary from batch to batch. Provided the innermost end of the strap is bonded too strongly, namely, provided the melt-bonded area possesses too high a peeling strength, the bonded area may not peel readily when used in the packaging machine. In some cases, the strap may be partially torn off at the rigidly bonded area. Actually, when a strap coil is loaded on the automatic packaging machine, the strap is drawn out along with the rotation of the feed rollers. If the laminated layers of the strap are bonded too strongly, the feed rollers may fail to peel off the strap layers, and worse, destroy themselves, depending on the rotation power. The strap coil of the prior art manifests more defects. As a strap coil is prepared by laminating a strap in a spiral form, it is somewhat extended in the axial direction. Although the innermost end of the strap is bonded with a layer or layers laminated thereon, the rest of the innermost rows of the strap coil which axially neighbour the bonded strap end are left unbonded with respect to the layers laminated thereon. Such a strap coil may unwind or collapse at the non-bonded area. US 4097 A represents the closest prior art opposite the present invention and constituting the features in the preamble of the independent claims. US 3 788 933 A discloses to pierce overlapping end portions and to melt-band them together at the periphery of each perforation. SUMMARY OF THE INVENTION In accordance with the invention there is provided a packaging strap coil characterised in that a plurality of perforations are pierced in the vicinity of an innermost strap end through to a plurality of strap layers laminated thereon, and wherein the laminated strap layers are bonded to each other at a melt-bonded area formed along the periphery of each perforation. Thus the invention may provide a packaging strap coil and a method for producing the same, wherein the innermost end of the strap is firmly fixed to prevent unwinding or disintegration of the strap coil, and further, the strap may be smoothly drawn out to the ultimate end when employed for strapping an item. Preferably there are provided a plurality of perforations pierced in the vicinity of an outermost strap end through to a plurality of strap layers laminated thereunder. It should be understood that a thermoplastic resin packaging strap employed in the present invention comprises at least a thermoplastic resin. This means that the strap may comprise a thermoplastic resin alone. But, on the other hand, the strap may be made of a fibre, paper, etc. whose surface is coated with thermoplastic resin. Moreover, in the packaging strap coil, it is preferable to prescribe the perforation as to the circumferential length, configuration and position thereof as well as the number of laminated strap layers through which the perforation should extend. These parameters are determined in view of the material, thickness and width of the packaging strap. Thereby, the laminated strap layers exhibit a balanced bonding strength and peeling strength. As described above, the strap coil of the above structure is characterised in that the laminated layers in the vicinity of the innermost strap end are bonded together at the melt-bonded area. The melt-bonded area provides a stable coil. As a result, the strap coil does not unwind or collapse on the inner cylindrical surface. Besides, the strap can be stably laminated on the bonded layers to form a solid strap coil. When perforations are provided in the vicinity of an outermost strap end through to a plurality of underlying strap layers, the strap will not unwind from the outermost end. This arrangement thus saves the trouble of fixing the outermost strap end by taping or thermal melt-bonding. The perforations are judiciously designed to hold the balance between the bonding strength and the peeling strength at the bonded layers of the strap. Therefore, on the one hand, the laminated portion of the strap is mutually bonded with a stable bonding strength at the melt-bonded area formed along the periphery of the perforation. On the other hand, in use of the strap coil, the bonded strap layers peel off properly at the melt-bonded area, instead of being forcibly torn away. In a further aspect the invention provides a method for producing a packaging strap coil which comprises the steps of winding an innermost strap end on a winding roller and laminating a plurality of a layers on the innermost strap end, characterised by piercing the laminated strap layers with a perforator heated to a predetermined temperature, thereby to form a perforation and melt its periphery; withdrawing the perforator from the laminated strap layers to melt-bond the periphery of the perforation; winding a required length of the strap helically on the winding roller to form a strap coil; and removing the winding roller from the strap coil. In the production method, the winding roller may expand or contract diametrically. The diameter of the winding roller is expanded while the strap is wound thereon to form a strap coil. Thereafter, the winding roller may be diametrically contracted and removed from the strap coil. Incidentally, the perforator may be in the form of a needle or a plate. In any of the above production methods, it is important to prescribe or predetermine the circumferential length, configuration and position of the perforation as well as the number of laminated layers of the strap through which the perforation should extend. These parameters are determined in view of the material, thickness, and width of the packaging strap so that the laminated strap layers exhibit a balanced bonding strength and peeling strength. The invention will now be described in detail, by way of example, by reference to the drawings. BRIEF DESCRIPTION OF DRAWINGS Fig. 1 is a perspective view of the appearance of a packaging strap coil of the present invention. Fig. 2(a) is a section of the main part of the above packaging strap coil; and Fig. 2(b) is a section of the main part of another packaging strap coil. Fig. 3(a) is a side view of a perforator for forming a perforation in the packaging strap coil of the invention; Fig. 3(b) is a plan view thereof; and Fig. 3(c) is a plan view of the perforation formed by the perforator. Fig. 4(a) is a side view of another perforator for forming a perforation in the packaging strap coil of the invention; Fig. 4(b) is a plan view thereof; and Fig. 4(c) is a plan view of the perforation formed by the perforator. Fig. 5(a) is a side view of still another perforator for forming a perforation in the packaging strap coil of the invention; Fig. 5(b) is a plan view thereof; and Fig. 5(c) is a plan view of the perforation formed by the perforator. Fig. 6(a) is a side view of yet another perforator for forming a perforation in the packaging strap coil of the invention; Fig. 6(b) is a plan view thereof; and Fig. 6(c) is a plan view of the perforation formed by the perforator. Fig. 7(a) is a side view of a further perforator for forming a perforation in the packaging strap coil of the invention; Fig. 7(b) is a plan view thereof; and Fig. 7(c) is a plan view of the perforation formed by the perforator. Fig. 8(a) is a side view of a still further perforator for forming a perforation in the packaging strap coil of the invention; Fig. 8(b) is a plan view thereof; and Fig. 8(c) is a plan view of the perforation formed by the perforator. Fig. 9(a) is a side view of a yet further perforator for forming a perforation in the packaging strap coil of the invention; Fig. 9(b) is a plan view thereof; and Fig. 9(c) is a plan view of the perforation formed by the perforator. Fig. 10(a) is a development showing the inner cylindrical surface surrounding the hollow 11b in a packaging strap coil; and Fig. 10(b)-(g) are developments each showing the inner cylindrical surface surrounding the hollow 11b in the packaging strap coil 11 of the present invention. Fig. 11(a)-(e) relate to comparative embodiments. Fig. 11(a) is a development showing the inner cylindrical surface surrounding a hollow in a conventional paper-core strap coil, with the tubular paper core removed; and Fig. 11(b)-(e) are developments each showing the inner cylindrical surface surrounding a hollow in a conventional coreless strap coil. Fig. 12 is a table showing the test results regarding to the embodiments of the present invention and a conventional technology described in Fig. 10 and the comparative embodiments described in Fig. 11. Fig. 13 shows various bonding patterns at the end of a thermoplastic resin strap as well as the peeling strength measured at the bonded areas, each pattern being different in the number and position of the perforation(s), etc. Fig. 14 is a graph showing the relationship between the diameter of the perforation and the peeling strength at the melt-bonded area in the packaging strap coil 11 of the present invention. Fig. 15 is a graph showing the relationship between the widthwise tensile strength of the packaging strap 11a and the peeling strength at the melt-bonded area in the packaging strap coil 11 of the present invention. Fig. 16(a)-(f) schematically describe a series of production steps of the strap coil 11 in one embodiment of the present invention. Fig. 17(a)-(g) schematically describe a series of production steps of the strap coil 11 in another embodiment of the present invention. Fig. 18(a)-(g) schematically describe a series of production steps of the strap coil 11 in still another embodiment of the present invention. Fig. 19 is a perspective view of a conventional strap coil. Fig. 20 is a perspective view showing a step of winding the strap end according to a method of producing the strap coil of Fig. 19. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The preferred embodiments of the present invention are hereinafter described with reference to the drawings. 1. PACKAGING STRAP COIL AND ITS PRODUCTION METHOD Fig. 1 is a perspective view of the appearance of a packaging strap coil of the present invention. The strap coil 11 comprises a plurality of layers of a packaging strap 11a helically wound around a hollow 11b which defines the axis thereof. The strap 11a is made of thermoplastic olefinic resins including polypropylenes, polyamides and polyethylene terephthalate which have been drawn out about five to ten times in the longitudinal direction to remarkably enhance their tensile strength. The strap 11a has a width of about 10 to 20 mm and a thickness of about 0.3 to 1 mm. The strap 11a can be made of any thermoplastic resin which improves its tensile strength by'drawing/stretching. A strap 11a of about 1,000 to 2,500 m long is wound helically to give a strap coil 11 having an axial length of about 100 to 250 mm, an inner diameter of about 200 to 410 mm (as the diameter of the hollow 11b defining the axis thereof), an outer diameter of about 300 to 650 mm and a weight of about 10 to 25 kg. The innermost end of the strap 11a locates at the middle of the axial length of the hollow 11b. Fig. 2 (a) is a section of the strap 11a, showing a portion in the neighbourhood of the hollow 11b in the strap coil 11. A perforation 11d is pierced in the innermost end of the strap 11a surrounding the hollow 11b, through to a plurality of layers of the strap 11a laminated on the top side of the innermost end. The thus laminated layers of the strap 11a are bonded together at a melt-bonded area 11e formed along the periphery of the perforation 11d. The perforation 11d and the melt-bonded area 11e are formed by piercing the laminated layers of the strap 11a with a heated perforator from the hollow 11b side or from the top side of the laminated portion of the strap 11a, or by irradiating a laser beam. Practically, the only requirement for the perforation 11d is to pierce through the innermost end of the strap 11a and at least one laminated layer of the strap 11a so as to bond the laminated layers together at the melt-bonded area 11e. Therefore, as shown in Fig. 2 (a), the perforation 11d may pierce through the entire thickness of two layers laminated on the innermost end of the strap 11a. Alternatively, as shown in Fig. 2(b), the perforation 11b may terminate in the middle of the second layer laminated on the innermost end of the strap 11a. The perforation 11d is provided with the use of a perforator 38. A circular-section pin-type perforator 38 having a sharp point (Fig. 3(a)) is held perpendicularly on a holder 39 (Fig. 3(b)) and heated above the melting point of the strap 11a. This perforator 38 is inserted from the inside of the hollow 11b into the widthwise centre of the innermost end of the strap 11a, thereby forming a perforation 11d in the widthwise centre of the strap 11a (Fig. 3(c)). On application of the heated perforator 38 into the innermost end of the strap 11a and the layer(s) laminated thereon, the periphery of the perforator 38 melts to provide a melt-bonded area 11e for bonding the laminated layers of the strap 11a. Where the innermost end and the overlapping layer(s) of the strap 11a are bonded by the melt-bonded area 11e formed along the periphery of the perforation 11d, the annular melt-bonded area 11e effectively serves as a core for winding the strap 11a in the form of a coil. Due to the bonding, the innermost end of the strap 11a is neither drawn out nor unwound. As the perforator 38, there may be also used a circular-section pin-type perforator having a hemispheric (round) point (Fig. 4(a)) which is held perpendicularly on a holder 39 (Fig. 4(b)). This perforator 38 is likewise heated above the melting point of the strap 11a and inserted from the inside of the hollow 11b into the widthwise centre of the innermost end of the strap 11a, thereby forming a perforation 11d in the widthwise centre of the strap 11a (Fig. 4(c)). The section of the perforators 38 is not limited to the circular section as mentioned above. Figs. 5 to 9 illustrate flat plate-type perforators 38 which provide slot-like perforations. The flat plate-type perforator 38 of Fig. 5 (a) and (b) has an oblong section and comprises rounded sides in the thickness direction. This perforator 38 is applied into the widthwise centre of the innermost end of the strap 11a, with the thickness direction of the perforator 38 being aligned with the width direction of the strap 11a. As shown in Fig. 5(c), the thus formed perforation 11d extends along the longitudinal direction of the strap 11a and has an oblong section with two rounded ends. The flat plate-type perforator 38 of Fig. 6(a) and (b) has a triangular section and a thickness gradually diminishing from one side to the other. This perforator 38 gives a triangular-section perforation 11d along the length of the strap 11a (Fig. 6(c)). The flat plate-type perforator 38 of Fig. 7(a) and (b) has a section comprising one square end and an opposite triangular end. The resulting perforation 11d extends along the length of the strap 11a, one end being shaped in a triangle (Fig. 7(c)). The flat plate-type perforator 38 of Fig. 8(a) and (b) comprises rounded sides in the thickness direction as well as an edged piercing head. This perforator 38 can easily form an oblong-section perforation 11d with rounded ends along the length of the strap 11a (Fig. 8(c)). The flat plate-type perforator 38 of Fig. 9(a) and (b) has a rectangular section and comprises an edged piercing head sloping from one side to the other. The resulting perforation 11d has a rectangular section extending along the length of the strap 11a (Fig. 9(c)). The perforator 38 for forming the perforation 11d is made of a material of high thermal conductivity (e.g. pre-shaped plated copper). Due to its high thermal conductivity, the whole part of the perforator 38 heats up quickly and uniformly to a prescribed temperature. As a result, it efficiently forms the perforation 11d and the melt-bonded area 11e in a short time. Fig. 10(a) is a development showing the inner cylindrical surface of the strap coil 11 surrounding the hollow 11b. The innermost end of the strap 11a locates at the middle of the axial length of the hollow 11b. At the innermost strap end, a single perforation 11d is formed in the widthwise centre of the strap 11a, whereby the innermost end of the strap 11a and the layers laminated thereon are mutually bonded at the melt-bonded area 11e. On the other hand, in the strap coil 11 of the present invention, a plurality of perforations 11d are provided in the inner cylindrical surface of the strap coil 11 surrounding the hollow 11b, together with the melt-bonded areas 11e formed along the periphery thereof. This structure avoids unwinding or disintegration of the strap 11a helically wound around the hollow 11b. By way of illustration, the strap coil 11 of Fig. 10(b) includes three perforations 11d, one formed in the innermost strap end and the others in the edgewise rows along the axial ends of the hollow 11b. Each perforation 11d is aligned with the others in the axial direction of the hollow 11b. The melt-bonded areas 11e are provided along the periphery of the perforations 11d. The strap coil 11 of Fig. 10(c) includes five perforations 11d, one formed in the innermost strap end and the others formed in pairs in the edgewise rows along the axial ends of the hollow 11b. The perforations 11d in the edgewise rows are circumferentially dislocated from the perforation 11d in the innermost strap end. The melt-bonded areas 11e are likewise provided along the periphery of the perforations 11d. Additionally, Fig. 10 (d) shows a combination of the structures of Fig. 10(b) and (c). Further, the strap coil 11 of Fig. 10 (e) includes a plurality of perforations 11d which are circumferentially equally spaced in the edgewise rows along the axial ends of the hollow 11b. The strap coil 11 of Fig. 10(f) provides a plurality of (e.g. three) perforations 11d in every axially adjacent row of the strap 11a around the hollow 11b, each being aligned with the others in the axial direction of the hollow 11b. Furthermore, the strap coil 11 of Fig. 10(g) interposes perforations 11d between the axially aligned perforations 11d of Fig. 10(f), in the central row and edgewise rows in the axial direction of the hollow 11b. In order to achieve a desired peeling strength (melt-bonding strength) and a widthwise tensile strength at the melt-bonded area 11e where bonding is effected between the laminated layers of the strap 11a, the perforations 11d are prescribed in terms of the configuration, dimension, number, position, etc., depending on the material, thickness, width, etc. of the strap 11a. The strap coils 11 of Fig. 10(a)-(g) based on different patterns of the perforation(s) 11d and the melt-bonded area(s) 11e were tested for vibration resistance, transportation resistance, loadability on a conventional strap coil reel in an automatic packaging machine (drum-set test), and packageability in an automatic packaging machine as loaded on the strap coil reel (machine-packageability test). The test results were compiled in Fig. 12, wherein Examples 1-7 correspond to the strap coil 11 of Fig. 10(a)-(g), respectively. The comparative examples were conducted using packaging strap coils 211 of Fig. 11 (a)-(e), each showing the development of the inner cylindrical surface surrounding the hollow 11b. The strap coils illustrated in Figs. 10 and 11 were made of a polypropylene strap. In Comparative Examples, it should be understood that the innermost strap ends 200 of all strap coils 211 were positioned at the middle of its axial length, where the strap ends 200 were left unsecured or secured in various manners. In Comparative Example 1 (Fig. 11(a)), the innermost strap end 200 was fixed on a tubular paper core (not shown) by means of a stapler 201 (staple width 10 mm). On the other hand, Comparative Examples 2 to 5 employed coreless strap coils 211. In Comparative Example 2 (Fig. 11(b)), the innermost strap end 200 was left unsecured. In Comparative Example 3 (Fig. 11(c)), the innermost strap end 200 was left unsecured, but annealed, before wound into a strap coil, for 10 minutes at 80°C to prevent its contraction and deformation. In Comparative Example 4 (Fig. 11(d)), the innermost strap end 200 was secured with the adjacent rows of the strap coil 211 by a kraft tape 202. In Comparative Example 5 (Fig. 11(e)), the innermost strap end 200 was secured with the all widthwise rows of the strap coil 211 by a craft tape 202. [TEST PROCEDURES] Vibration Test Each sample strap coil was horizontally laid on a 250-mm-diameter cylindrical mount placed on a vibration plate. The vibration plate was vertically vibrated 100 times/min. in the range of 15 mm. The grading was based on the following criteria. The samples graded ○ did not unwind or collapse on the inner cylindrical surface after 1 hour of vibration; the samples graded Δ had the inner cylindrical surface unwound after 30 minutes to 1 hour of vibration; and the samples graded X had the inner cylindrical surface unwound within 30 minutes of vibration. Transportation Test Each sample was packed in a corrugated.cardboard box (length x width x depth: 465 x 465 x 210(mm)) and transported by a lorry over a distance of 500 km. Each Example employed five samples in this test. The results in Fig. 12 indicate the number of samples whose inner surface had unwounded or collapsed during transportation. Drum-Set Test The drum-set test examined the handling property of the strap coil during a loading operation on a drum of an automatic packaging machine. The automatic packaging machines used were: NAIGAI Fll (hereinafter referred to "Machine A"), NICHIRO SX-500 ("Machine B") and STRAPACK RQ-8 ("Machine C"). Each Example employed two samples in this test. The results in Fig. 12 indicate the number of samples whose inner surface had unwounded or collapsed before completion of the loading. Machine-packageability Test The machine-packageability test examined the incidence of troubles during normal packaging operation, such as tape-blocking in the pre-feed rollers or the like. Each example employed two samples in Machines A, B and C, respectively. The resul.ts in Fig. 12 represent the number of samples which caused any such trouble. As apparent from Fig. 12, the strap coils 11 of the present invention as illustrated in Fig. 10(b)-(g) gave remarkable results. None of them had the inner cylindrical surface unwound. Besides, when used in an automatic packaging machine, they caused no trouble like blocking. In addition, the peeling strength of the thermoplastic strap end was measured using the test pieces 311 as shown in Fig. 13(a)-(f) made of a polypropylene strap. The test pieces 311 had their strap ends secured in different manners, for example, by varying the number and position of the perforation(s). The polypropylene strap as used in the above-mentioned tests such as the vibration test was wound around a 200-mm-diameter core, wherein the strap end was melt-bonded by the perforation(s) 11d at 20 mm inside from the innermost edge of the strap, according to the patterns shown in Fig. 13(a)-(e). For comparison, the strap end of the test piece 311 shown in Fig. 13(f) was fixed on a paper core 312 by means of a stapler 313. The test pieces 311 were stretched downwards at a rate of 50 mm/min. until the bonded area peeled off. The thus measured values are shown in the right column of Fig. 13. From the viewpoint of the appearance, the perforation 11d preferably has a small circular section, which does not stand out in the strap 11a. Hence, the finished strap coil looks almost like conventional coreless strap coils. In the circular-section perforations 11d as shown in Figs. 3 and 4, the peeling strength at the melt-bonded area 11e depends on the circumferential length of the perforation 11d, which is prescribed in accordance with-the material and thickness of the strap 11a. Therefore, a required peeling strength can be achieved simply by calculating the diameter of the perforation 11d from the prescribed circumferential length thereof and then forming a perforation 11d of such diameter, which gives the required melt-bonded area 11e along its periphery. Fig. 14 is a graph showing the relationship between the diameter of the perforation 11d and the peeling strength at the melt-bonded area 11e. The strap coil used herein was prepared by helically winding a polypropylene strap 11a having a thickness of 0.65 mm and a width of 15.5 mm. In this strap coil, the circular-section perforation 11d and the melt-bonded area 11e were formed by piercing a pin-type perforator 38 of Fig. 3 (a) and (b) which was preheated above the melting temperature of the strap 11a. The peeling strength at the melt-bonded area 11e was measured at a stretching rate of 200 mm/min. As shown in Fig. 14, a 0.4-mm-diameter perforation 11d showed a peeling strength P 1 of about 300 gf at the melt-bonded area 11e. A 0.6-mm-diameter perforation 11d showed a peeling strength P 2 of about 380 gf. In these cases, the bonded layers of the strap 11a were smoothly peeled off and not torn in the longitudinal direction. Further, the peeling strength at the melt-bonded area 11e can be increased by enlarging the diameter of the perforation 11d. A 0.8-mm-diameter perforation 11d showed a peeling strength P 3 of about 550 gf, and a 1.0-mm-diameter perforation 11d showed a peeling strength P 4 of about 700 gf. In these cases, however, the bonded layers could not be peeled off from each other, and one of the layers was torn-away in the longitudinal direction. In a thermoplastic resin strap 11a (for example, made of-polypropylene), a thinner strap 11a is more vulnerable to the longitudinal tear. Therefore, in proportion to the decrease of the thickness of the strap 11a, the diameter of the perforation 11d should be diminished to reduce the peeling strength at the melt-bonded area 11e. Thereby, the bonded layers of a thinner strap can be safely peeled off from each other at the melt-bonded area 11e without being torn away in the longitudinal direction. According to Fig. 14, a 0.65-mm-thick polypropylene strap 11a keeps a peeling strength of 400 gf or less at the melt-bonded area 11e, when the diameter of the perforation 11d is 0.6 mm or less. To ensure the same peeling strength, a 0.5-mm-thick PET (polyethylene terephthalate) strap 11a should have a perforation 11d with a diameter of 1.2 mm or less. As hereinbefore described, by reducing the diameter of the perforation 11d, the melt-bonded area 11e formed along its periphery becomes smaller and exhibits a lower peeling strength. In such circumstances, a desired peeling strength can be obtained by forming a plurality of perforations 11d, which, as a whole, provide the peeling strength at the melt-bonded area 11e. Although the above description has been focused on the circular-section perforation 11d, the perforations 11d shown in Figs. 5 to 9 can also achieve a desired peeling strength at the melt-bonded area 11e and widthwise tensile strength of the strap 11a, at which the bonded layers of the strap 11a are safely peeled off but not torn away. According to the same principles, the circumferential length, configuration, number, position, etc. of the perforation 11d are designed in view of the material, thickness and the width of the strap 11a. The perforations 11d of Figs. 5 to 9 have a section elongated in one direction, as described above. In order to prevent the strap 11a from being torn away, the longitudinal direction of the perforation 11d should be aligned with the longitudinal direction of the strap 11a. Fig. 15 is a graph showing the relationship between the widthwise tensile strength of the strap 11a and the peeling strength at the melt-bonded area 11e, with respect to the perforations 11d of circular section and those of rectangular section. The values of the circular-section perforations 11d are indicated by A 1 (diameter: 0.55 mm) and A 2 (diameter: 1.00 mm). The values of the rectangular-section perforations 11d are indicated by B 1 (0.55 x 5 mm) and B 2 (0.55 x 8 mm). The rectangular-section perforations 11d have their longitudinal sides aligned with the longitudinal direction of the strap 11a. As for the circular-section perforations 11d, expansion of the diameter results in increasing the peeling strength at the melt-bonded area 11e as well as the widthwise tensile strength of the strap 11a. In contrast, as for the rectangular-section perforations 11d, elongation of the longitudinal sides hardly affects the peeling strength at the melt-bonded area 11e (the value substantially remains at about 400 gf). On the other hand, the widthwise tensile strength of the strap 11a increases in proportion to the increase of the longitudinal length. Incidentally, it is desirable for a 0.65-mm-thick polypropylene strap 11a to have a peeling strength at the melt-bonded area 11e of 400 gf or less and a widthwise tensile strength of the strap 11a of about 2,000 gf. In case of the rectangular-section perforation 11d, these values are suitably achieved by adjusting the length of its longitudinal sides. Proper combination of the peeling strength and the widthwise tensile strength not only ensures safe peeling and tear prevention between the bonded layers of the strap 11a, but also prohibits unwanted unwinding of the strap coil 11. Besides, in the rectangular-section perforation 11d, the widthwise tensile strength of the strap 11a can be increased by extending its longitudinal length, instead of adding more perforations. Additionally, the strap coil 11 of the present invention may include a perforation 11d in the outer cylindrical surface thereof. The perforation 11d is pierced in the neighbourhood of the outermost end through to the underlying layers of the strap 11a, thereby bonding these layers at the melt-bonded area 11e formed along its periphery. This arrangement prevents the outermost strap end from being drawn out unnecessarily. Hence, a conventional strap end securing process such as taping or separate thermofusing is no longer required. Fig. 16(a)-(f) is a series of schematic views showing production steps of the strap coil 11. These steps are carried out, for example, by using a winding roller 31 shown in Fig. 16(a). The winding roller 31 has a hollow cylindrical shape composed of four equivalent cylinder frames 31a obtained by circumferentially quartering a cylinder of suitable axial length. Each cylinder frame 31a is spaced along a single circumference with a circumferential gap 31c to constitute a cylinder of circular section. The four cylinder frames 31a arranged along a single circumference can contract the diameter of the circular-section cylinder, when allowed to slide toward the axis of the cylinder. On the other hand, the contracted cylinder is diametrically expanded by sliding the cylinder frames 31a outwardly. An aperture 31d is formed beforehand in the middle of each cylinder frame 31a. The winding roller 31 accommodates four holders 39, each of which faces the cylinder frame 31a and holds a circular-section perforator 38 in place. Similar to the cylinder frames 31a, the four holders 39 have a curved configuration and spaced along a single circumference with a circumferential gap to constitute a cylinder of circular section. The holders 39 are also slidable in the radial directions. The perforator 38 is mounted on the outer surface of each holder 39 in such a manner that it can project radially through the aperture 31d formed in the cylinder frame 31a. Although not shown in the figure, a heater is provided inside each holder 39 for the purpose of heating the perforator 38, and the outer surface of the holder 39 is covered by a heat insulating material. In order to wind the strap 11a on the winding roller 31, a fixture 33 is attached to the start of the strap 11a (i.e. innermost end of the strap coil 11) guided by a strap guide 34. Then, as shown in Fig. 16(b), the cylinder frames 31a are allowed to slide outwardly along a single circumference to form a circular-section cylinder. Inside the winding roller 31, the holders 39 maintain a properly spaced relationship with respect to the cylinder frames 31a, thereby keeping the perforators 38 away from the cylinder frames 31a. The fixture 33 attached at the start of the strap 11a is fitted into the gap 31c created between a pair of neighbouring cylinder frames 31a, and thus secures the start of the strap 11a in the middle of the axial length of the winding roller 31. With these provisions being made, the winding roller 31 may start to rotate in the direction of the arrow T in Fig. 16 (b) and wind the strap 11a thereon. After the strap 11a has made a plurality of layers around the middle of the axial length of the winding roller 31 (Fig. 16(c)), the winding roller 31 is stopped at such a position that the apertures 31d in the cylinder frames 31a locate face to face with the perforators 38 projecting from the holders 39. In the next step shown in Fig. 16(d), the holders 39 which hold heated perforators 38 are allowed to slide in the radial direction towards the cylinder frames 31a. The perforators 38 are preheated above the melting point of a thermoplastic resin constituting the strap 11a. For example, for a polypropylene strap 11a, the perforators 38 are heated to about 200°C, which is higher than the melting point of polypropylene. Together with the outward displacement of the holders 39, the perforators 38 advance through the apertures 31d of the cylinder frames 31a and stick into the plural laminated layers of the strap 11a wound on the winding roller 31. In consequence, four circumferentially equally spaced perforations 11d (see Fig. 2) are formed simultaneously in the laminated layers of the strap 11a, leaving the periphery thereof in the melted state. Next, as shown in Fig. 16(e), the holders 39 are allowed to slide away from the cylinder frames 31a, with retracting the perforators 38 from the apertures 31d. Following the withdrawal of the perforators 38, the melted areas along the periphery of the perforations 11d solidify to form melt-bonded areas 11e (see Fig. 2), which bond the laminated layers of the strap 11a. Thereafter, the winding roller 31 is rotated until a predetermined length of the strap 11a is wound thereon. While the winding of the strap 11a proceeds, the strap 11a is axially reciprocated along the winding roller 31, so that the strap 11a is helically wound on the cylindrical surface of the winding roller 31 in a prescribed axial length thereof. After a predetermined length of the strap 11a is wound on the winding roller 31, the diameter of the winding roller 31 is reduced by sliding the cylinder frames 31a toward the axis thereof (Fig. 16(f)). The diametrically contracted winding roller 31 is removed from the axis of the helically wound strap 11a. Thus obtained is a coreless strap coil 11 whose axis is defined by a hollow 11b. In forming the perforations 11d in the laminated layers of the strap 11a, the configuration, number, position, etc. of the perforators 38 are suitably adjusted so as to hold a balance between the peeling strength (or widthwise tensile strength of the strap 11a) and the bonding strength at the melt-bonded area lie. Hence, use can be also made of one or more perforators 38 of Figs. 5 to 9. As an alternative embodiment, the holders 39 holding the perforators 38 may be rotated in synchronisation with the winding roller 31, thereby eliminating the step of aligning the cylinder frames 31a with the holders 39. This arrangement enhances the operation efficiency, because the perforations 11d are formed while the winding roller 31 is in rotation. The perforator 38 for forming the perforation 11d may be applied either at the bottom side of the innermost strap 11a as described in Fig. 16(a)-(f) (i.e. from the axis side of the winding roller 31), or at the top side of the laminated layers of the strap 11a as described in Fig. 17(a)-(g). The production steps shown in Fig. 17 (a) - (g) are carried out by using a winding roller 31 which comprises four cylinder frames 31a, as shown in Fig. 17(a). The cylinder frames 31a can slide outwardly along a single circumference to constitute the winding roller 31 of circular section (Fig. 17(b)). In the diametrically expanded winding roller 31, the start of the strap 11a guided by the strap guide 34 is fixed in the middle of the axial length of the winding roller 31 by fitting the fixture 33 attached to the start of the strap into the gap 31c between the neighbouring cylinder frames 31a. The perforator 38, held in the vicinity of the winding roller 31 by a suitable holder, is able to advance and recede in the radial directions of the winding roller 31. After the start of the strap 11a is fixed on the outer cylindrical surface of the winding roller 31, the winding roller 31 is allowed to rotate in the direction of the arrow T until the strap 11a makes a plurality of layers around the middle of the axial length of the winding roller 31. Then, the winding roller 31 is stopped at the position shown in Fig. 17 (b). The perforator 38, which is heated by a heater to a predetermined temperature, is allowed to approach the winding roller 31 from the external side thereof and to pierce in the laminated layers through to the start of the strap 11a. In consequence, a perforation 11d is formed in the laminated layers of the strap 11a, leaving the periphery thereof in the melted state. Then, the perforator 38 is allowed to recede from the winding roller 31 and pulled out of the laminated layers of the strap 11a, whereby the melted area around the perforation 11d solidifies into a melt-bonded area 11e. Following the formation of the first perforation 11d, the winding roller 31 effects a quarter of rotation (i.e. rotation of 90°) in the direction of the arrow T (Fig. 17(c)), and stops again. At this position, the preheated perforator 38 is allowed to approach the winding roller 31 and pierce another perforation 11d into the laminated layer of the strap 11a, and then the perforator 38 is allowed to recede.in the opposite direction and pulled out of the strap 11a. The formation of the perforation 11d is repeated, as shown in Fig. 17 (d) and (e), according to the bonding cycle which comprises rotating the winding roller 31 by 90 degrees (i.e. a quarter of rotation), piercing the perforator 38 into the laminated layers of the strap 11a to form a perforation 11d (see Fig. 2), and withdrawing the perforator 38 from the strap 11a to solidify a melt-bonded area 11e (see Fig. 2). In the end, there are provided a total of four circumferentially equally spaced perforations 11d and melt-bonded areas 11e. Following the formation of four perforations 11d and melt-bonded areas 11e, the winding roller 31 is continuously rotated to wind a predetermined length of the strap 11a into a coil (Fig. 17(f)). When the winding roller 31 ceases to rotate, the winding roller 31 is diametrically contracted by sliding the cylinder frames 31a toward the axis of the winding roller 31 (Fig. 17(g)). The contracted winding roller 31 is removed from the axis of the helically wound strap 11a. Thus obtained is a coreless strap coil 11 whose axis is defined by a hollow 11b. Instead of the heated perforator 38, a laser beam may be applied for the formation of the perforation 11d and the melt-bonded area 11e, as illustrated in Fig. 18(a)-(g). This embodiment likewise utilises the winding roller 31 comprising four cylinder frames 31a (Fig. 18(a)). Each cylinder frame 31a is operated to slide in the radial directions by an air cylinder 36 accommodated in the winding roller 31. While the winding roller 31 is diametrically contracted by sliding the cylinder frames 31a toward the axis of the winding roller 31, the fixture 33 is attached to the start of the strap 11a guided by the strap guide 34. Thereafter, as shown in Fig. 18(b), the cylinder frames 31a are outwardly displaced along a single circumference by the air cylinder 36 to constitute a circular-section winding roller 31 having an expanded diameter. The fixture 33 is fitted into the gap 31c created between the neighbouring cylinder frames 31a to fix the start of the strap 11a in the middle of the axial length of the winding roller 31. Referring to Fig. 18(c), a laser oscillator unit 35 is disposed in the vicinity of the winding roller 31, with orienting the direction of its laser beam irradiation along the radial direction of the winding roller 31. Further, the laser oscillator unit 35 is able to advance and recede with respect to the winding roller 31. With securely holding the start of the strap 11a on the outer cylindrical surface of the winding roller 31, the winding roller 31 is allowed to rotate in the direction of the arrow T until the strap 11a makes about three turns around the middle of the axial length thereof. After the winding roller 31 ceases to rotate, the laser oscillator unit 35 is allowed to approach the winding roller 31 and to irradiate a laser beam on the laminated layers through to the innermost end of the strap 11a. The laser beam melts a portion of the laminated layers and the innermost end of the strap 11a and provides a perforation 11d and a melt-bonded area 11e. The laminated layers are thus bonded together at the melt-bonded area 11e (see Fig. 2) along the periphery of the perforation 11d (see Fig. 2). After the formation of the first perforation 11d, the winding roller 31 effects a quarter of rotation (i.e. rotation of 90°) in the direction of the arrow T (Fig. 18(d)), and stops again. At this position, the laser oscillator unit 35 irradiates a laser beam on the laminated layers of the strap 11a to form another perforation 11d and a melt-bonded area 11e along its periphery. The formation of the perforation 11d is repeated according to the bonding cycle which comprises rotating the winding roller 31 by 90 degrees (i.e. a quarter of rotation) and irradiating a laser beam by the laser oscillator unit 35, thereby to form a perforation 11d and a melt-bonded area 11e along its periphery through the laminated layers of the strap 11a. In the end, - there are provided a total of four circumferentially equally spaced perforations 11d and melt-bonded areas 11e. Thereafter, the laser oscillator unit 35 is allowed to recede from the winding roller 31 (Fig. 18(e)), and the winding roller 31 is rotated continuously to wind a predetermined length the strap 11a into a coil (Fig. 18(f)). After the winding roller 31 ceases to rotate, the winding roller 31 is diametrically contracted by sliding the cylinder frames 31a toward its axis (Fig. 18(g)). The contracted winding roller 31 is removed from the axis of the helically wound strap 11a. Thus obtained is a coreless strap coil 11 whose axis is defined by a hollow 11b. As has been described, a laser beam irradiated from the laser oscillator unit 35 can form a smaller perforation 11d in a more stable and efficient manner. In addition, the laser oscillator unit 35 is composed of less expendable parts and thus easy to maintain. As the laser oscillator unit 35, there may be used a CO 2 laser oscillator unit which employs CO 2 gas as the laser medium. The CO 2 laser oscillator unit irradiates a laser beam of 27W output power at a wavelength of 10.5-10.7 µm as operated by a current of 6A. The irradiation time of the laser beam is in the range of about 0.1 to 5 seconds, preferably about 0.5 second, at which time the laser irradiation hardly affects the winding rate of the strap 11a. In addition to the CO 2 laser oscillator unit, the examples of the laser oscillator unit 35 include a helium-neon laser oscillator unit, a semiconductor laser oscillator unit and the like. It should be understood that the mode of the present invention should not be regarded as being strictly limited to the embodiments hereinbefore described but is defined by the scope of the claims. For example, the strap 11a of the strap coil 11, which is made of an olefinic resin or other thermoplastic resins in the above embodiments, may be prepared by coating a thermoplastic resin on the external surface of a base strap made of paper or fibres.

Claims: A packaging strap coil (11) comprising a number of layers of a packaging strap (11a) wound helically and corelessly around a hollow (11b) defining an axis of the strap coil (11), the strap being made of a thermoplastic resin at least on an external surface thereof, wherein a plurality of perforations (11d) are pierced in the vicinity of an innermost strap end through to a plurality of strap layers laminated thereon, characterised in that the laminated strap layers are bonded to each other at a melt-bonded area (11e) formed along the periphery of each perforation (11d). A packaging strap coil according to claim 1, wherein a plurality of perforations (11d) are pierced in the vicinity of an outermost strap end through to the plurality of strap layers laminated thereunder. A packaging strap coil (11) according to claim 1, wherein there is predetermined for the perforation a circumferential length, a configuration and a position thereof as well as the number of the laminated strap layers through which the perforation should extend, said parameters being based on material, thickness and width of the packaging strap, thereby satisfying a balance between a bonding strength and a peeling strength of the laminated strap layers. A method for producing a packaging strap coil of claim 1, which comprises the steps of: winding an innermost strap end on a winding roller and laminating a plurality of layers on the innermost strap end; characterised by piercing the laminated strap layers with a perforator heated to a predetermined temperature, thereby to form a perforation and melt its periphery; withdrawing the perforator from the laminated strap layers to melt-bond the periphery of the perforation; winding a required length of the strap helically on the winding roller to form a strap coil; and removing the winding roller from the strap coil. A method for producing a packaging strap coil according to claim 4, wherein the winding roller can expand or contract diametrically, the winding roller being diametrically expanded while the strap is wound thereon to form a strap coil, and thereafter, being diametrically contracted and removed from the strap coil. A method for producing a packaging strap coil of claim 1, which comprises the steps of: winding an innermost strap end on a winding roller and laminating a plurality of layers on the innermost strap end; irradiating a laser beam on the laminated strap layers, thereby to form a perforation and melt-bond its periphery; winding a required length of the strap helically on the winding roller to form a strap coil; and removing the winding roller from the strap coil. A method for producing a packaging strap coil according to claim 6, wherein the winding roller can expand or contract diametrically, the winding roller being diametrically expanded while the strap is wound thereon to form a strap coil, and thereafter, being diametrically contracted and removed from the strap coil.

Inventors: FUJITA KUNIAKI, KAWAMOTO IZURU, MARUTANI TETSUYA, OBA KAZUYUKI, TAJIKA SATORU, USUDA YUKOU, FUJITA, KUNIAKI, KAWAMOTO, IZURU, MARUTANI, TETSUYA, OBA, KAZUYUKI, TAJIKA, SATORU, USUDA, YUKOU
Applicants: SEKISUI JUSHI KK, SEKISUI JUSHI KABUSHIKI KAISHA
Application year: 1999
Main classification: B65H65
IPC codes: B29C65, B65D3, B65H65, B32B7, B65D85, B32B1, B65H75, B65H54, B65B13, B65D63, B29C53, B32B29