ELASTIC STRAP FASTENER ASSEMBLY

Abstract: A respirator harnes of polymeric components (10,41,42) and elastic straps (12,43) is made in a 2-part injection mold in a single step. The elastic straps (12,43) are secured to each polymeric component (10,41,42) by a cage that is integral with the polymeric component (10,41,42), and the cage has at least one transverse rib (18) that is fused to the elastic strap (12,43) and around which the strap (12,43) is stretched.

Description: Background of the Invention Field of the Invention The invention is concerned with an elastic strap fastener assembly, particularly in the form of a respirator harness that employs polymeric components in conjunction with elastic straps. Description of the Related Art A typical respirator harness is made of polymeric components and elastic straps which are sewn, mechanically secured, or ultrasonically welded to the polymeric components. Doing so requires a first step of molding the polymeric components and then a separate step of attaching each strap to one or more of the polymeric components. Visual inspection of Respirator Model 1482 of Pro-Tech Respirators, Inc., Buchanan, MI, indicates that part or all of the harness has been made in a single step. That is, an elastic strap has been laid at the parting line of a 2-part injection mold and so becomes part of the harness when a polymeric component is formed around an end of the strap. Such inspection also indicates that a series of vanes at the face of each part of the injection mold abutted each other at the elastic strap when the mold was closed and that the injected composition formed a polymeric cage encompassing an end of the elastic strap. The cage has a peripheral frame interconnected by a central bridgework consisting of longitudinal stringers that were formed in the valleys between the vanes and are fused to opposite faces of the strap. The stringers bridge the transverse members of the frame and keep the elastic strap at the approximate center of the cage. Summary of The Invention The present invention provides an elastic strap fastener assembly such as a respirator harness which, like the harness of the Pro-Tech Model 1482 respirator, can be made in a single step. As compared to the Pro-Tech respirator, the strap(s) of the elastic strap fastener assembly of the invention have significantly greater pull-out resistance. Hence, the novel strap fastener assembly provides a more robust and reliable respirator harness than the Pro-Tech harness. Briefly, the elastic strap fastener assembly of the invention, like that of the Pro-Tech fastener assembly, has at least one elastic strap, a polymeric cage encompassing a portion of the elastic strap, which cage has a peripheral frame and central bridgework that interconnects the peripheral frame and is fused to both faces of the strap. The polymeric cage of the elastic strap fastener assembly of the invention differs from the Pro-Tech strap assembly in that at least one protion of the central bridgework stretches the strap away from the approximate center of the cage. Preferably, the elastic strap is permeable, and any portion of the bridgework that stretches the strap also penetrates into interstices of the elastic strap, thereby enhancing the pull-out resistance of the elastic strap. An elastic strap of more open construction enhances penetration. Generally, better penetration occurs where the strap is stretched than where it is not. Higher strap porosity produces greater penetration and less deflection or stretching of the strap. When the elastic strap is impermeable, the materials of the elastic strap and the polymeric cage preferably are selected to form strong bonds to each other. For example, the elastic strap may be a block copolymer of polystyrene end blocks and an ethylene/butylene copolymer midblock (e.g., "Kraton" G from Shell Chemical), and the polymeric cage may be polypropylene. Bonding between those materials is enhanced by blending a small proportion (e.g., 5-10%) of polypropylene with the block copolymer prior to forming the strap. Preferably, the bridgework of the polymeric cage includes at least one transverse rib around which the strap is stretched. More preferably, there are at least three such transverse ribs, and the elastic strap undulates around opposite sides of adjacent of said transverse ribs. Usually the polymeric cage encompasses one end of the elastic strap, but it could encompass a portion of the elastic strap intermediate of its ends. Preferably the peripheral frame of the polymeric cage is substantially rectangular and continuous, and the strap emerges from a slot formed in the end of the frame. For esthetics, where the strap emerges from a rectangular frame, the transverse element of the frame should have a width not exceeding the nominal thickness of the elastic strap, thus better assuring good centering of the strap. When the elastic strap fastener of the invention is a respirator harness, it may include a 2-piece polymeric head cradle, two polymeric connectors, and four elastic straps. Integral with each piece of the head cradle and with each of the polymeric connectors is a polymeric cage that secures one end of one elastic strap. The other end of each strap can be connected to the respirator by adjustment buckles. All of the polymeric components of a respirator harness can be formed and secured to the elastic straps simultaneously using a novel 2-part injection mold that is formed at its parting line to receive each elastic strap. Where each elastic strap is received, each mold part can be formed with at least one open cavity that is elongated in the transverse direction, opposed to a like cavity of the other mold part, and closed by the received strap when the mold is closed. A first of said mold parts is formed with a secondary gate that opens into the interior of said elongated cavity to allow an injected polymeric composition to stretch the elastic strap into the opposing elongated cavity of the second of said mold parts, thus causing the strap to become partially wrapped around the transverse polymeric rib that forms in said pair of opposed elongated cavities. Using the novel 2-part injection mold, an elastic strap fastener assembly of the invention can be made by laying an elastic strap between the two mold parts to close said elongated transverse cavities, closing the mold, injecting a polymeric composition into the mold and through said secondary gate under a pressure sufficient to stretch the elastic strap into the opposing elongated transverse cavity, and hardening the polymeric composition to freeze the elastic strap in said stretched condition. The transverse polymeric rib that is formed in each pair of said opposing elongated cavities sometimes is completely obscured by the strap when the resulting polymeric cage is viewed from the side toward which the strap is stretched. On the other hand, when the elastic strap is quite porous or molding pressures are high, the polymer may flow through the strap, thereby completely encapsulating a portion of the elastic strap and enhancing the pull-out resistance of the strap. In such event, the degree to which the strap is stretched may be almost indiscernible. Like many respirator harnesses of the prior art, an elastic strap fastener assembly of the invention preferably employs an elastic strap that has a plurality of rubber strands knit together in parallel configuration with cotton or synthetic threads. Other useful elastic strap are strips of woven or nonwoven elastic fibers. Also useful are strips of elastomer. Brief Description of The Drawing The invention may be more easily understood in reference to the drawing, all figures of which are schematic. In the drawing: Fig. 1 is an isometric view, partly cut away to show details, of a polymeric component of a first respirator harness that incorporates an elastic strap fastener assembly of the invention; Fig. 2 is a plan view of the face of one part of a 2-part injection mold for making a second respirator harness of the invention; Fig. 3 is an enlarged detail of a portion of the mold part of Fig. 2 at which a polymeric cage is produced; and Fig. 4 is a cross section through an elastic strap fastener assembly that has been produced using the mold, one part of which is shown in Figs. 2 and 3, which section is taken at line 4--4 of Fig. 3. Detailed Description In Fig. 1, a polymeric cage 10 is integral with a connector loop 11 of a respirator harness. The polymeric cage, which encompasses one end of an elastic strap 12, has a rectangular peripheral frame 13 including longitudinal elements 14 extending along the sides of the elastic strap 12 and transverse elements 15 at each end of the cage. Bridging the longitudinal elements 14 are five transverse ribs 18 around which the strap undulates and is stretched. In Fig. 1, the elastic strap is cut away to show the shape of the transverse ribs 18, the ends 19 of which have been formed by secondary gates and are of somewhat reduced size. In Fig. 2, one part 20 of a 2-part injection mold includes a sprue 21 and a pair of runners 22 that feed three networks through four primary gates 24. The first network 25 forms one piece of a 2-piece head cradle and an integral polymeric cage. The second and third networks 26 and 27 form loop and hook connectors respectively, each having an integral polymeric cage. The cage-forming portions 30 of the networks 25, 26 and 27 are identical. Recesses receive elastic straps (not shown) to overlie each of the cage-forming portions of the networks. Channel 29 of each recess does not compress the straps, while shutoffs 28 compress the strap to prevent polymer leakage. Details of the cage-forming portion 30 of one of the networks 25, 26 and 27 are shown in Fig. 3, and indirectly by Fig. 4. In Fig. 3, a pair of longitudinal cavities 31 and a pair of transverse cavities 32 cooperate with identical cavities of the other part of the injection mold to form a continuous rectangular frame at the periphery of the polymeric cage. The cage-forming portion 30 includes longitudinal cavities 31 which are interconnected by a first set of elongated transverse cavities 33, at the edges of which are a plurality of vanes 34. The vanes extend beyond the edges of a strap (not shown) that is to be laid in the channel 29 and shutoff 28 to overlie the cage-forming portion 30. Abutting vanes 34 compress the strap to the same degree as do the shutoffs 28, except at secondary gates 35 which interconnect the first elongated transverse cavities 33 with three elongated transverse cavities 36 of a second set. The second part (not seen) of the injection mold is substantially identical to the first part 20 except that its secondary gates connect only to two elongated transverse cavities that are opposed to two elongated cavities 37 of said second set that alternate with said three elongated cavities 36. The injection of polymer into the mold produces an elastic strap fastener assembly 40 as shown in Fig. 4. In doing so the polymer progressively fills the pair of transverse cavities 32 and the pair of longitudinal cavities 31 to form a rectangular peripheral frame 41, while also filling the first elongated transverse cavities 33 to form a series of first transverse ribs 42. The peripheral frame 41 and the series of first transverse ribs positions a permeable elastic strap 43 essentially at the transverse center of the cage. Polymer flowing through the secondary gates 35 directly into the elongated transverse cavities 36 of the second set stretches the elastic strap 43 into the opposed cavities 37 to form a second series of transverse ribs 44. At each of the second series ribs 44, the polymer penetrates into interstices of the elastic strap and fuses to hold the strap in undulations around alternate sides of the transverse ribs of the second series. Where that second series of ribs 44 forms at the gates 35, there are polymeric bridges 46 between the first and second series of transverse ribs. These bridges 46 have been found to enhance the resistance of the second series of transverse ribs 44 to rotation when the fastener assembly 40 is subjected to pull-out forces. Comparative Example The harness assembly was removed from each of two Model 1482 half-mask respirators available from Pro-Tech Respirators, Inc. The force needed to pull the elastic strap (18 mm width) free from the molded plastic part was measured using a Chatillon force gauge, Model DRC100, in which tension was applied by attaching the free end of the strap to a clamp moving at 9.7 mm/second away from the force gauge such that the direction of applied tension was on the measurement axis of the force gauge. In each test there was a 90 mm length of the unstretched elastic strap between the force gauge clamp and the moving clamp at the start of the test. In testing each of the respirators, an elastic strap was pulled out of six molded plastic parts. Test results are reported in Table I. The pull-out force is considered to be directly proportional to strap width and the results are reported based on a unit width of 10 mm. Example 1 An injection mold for the elastic strap fastener assembly shown in Fig. 1 (except having smaller secondary gates) was installed in a 75-ton horizontal Van Dorn injection molding press, Model 75-RS-3F, equipped with a 148 cm³ (5 ounce) barrel and standard screw. Molding conditions used were: 1) barrel rear temperature 243°C (470°F); 2) barrel front temperature 254°C (490°F); 3) nozzle temperature 266°C (510°F); 4) mold temperature nozzle side 21°C (70°F), ejector side 38°C (100°F); 5) injection velocity 0.9 cm (0.35 inches) per second; 6) hydraulic pressure 81.6 bar (1200 psig); 7) first-stage injection pressure, 782 bar (11,500 psig); 8) second-stage injection pressure, 748 bar (11,000 psig); 9) boost time 2.6 seconds; 10: injection forward time 0.2 seconds; 11) hold time 2.0 seconds; 12) zero back pressure and no screw pullback; 13) shot size 2 cm (0.80 inches); 14) cushion 0.6 cm (0.25 inches). Injection molding grade polypropylene ("Huntsman" 5B56N) was combined with 3% dark gray colorant CBE 70882P available from C. B. Edwards and Company, Inc., Minneapolis, MN and was used to produce elastic strap fastener assemblies of the invention using two different 12.5 mm wide elastic straps. Each of the elastic straps had a plurality of parallel rubber strands which were knit together (Nos. 4422 and 6192 available from Providence Braid Co., Inc. Pawtucket, R.I.). The assemblies were tested after storage for 5 days at room temperature and according to the procedure of the Comparative Example. Results are reported in Table II. Examination of failed parts indicated that individual transverse ribs had rotated during pull-out of the elastic strap. This was attributed to the small size of the secondary gates, suggesting that an even higher pull-out force would be attained by designing the transverse ribs to resist rotation (as do the transverse ribs of polymeric cages shown in Fig. 1 of the drawing). Examples 2-4 The elastic strap fastener assembly of Fig. 4 was molded and tested according to the conditions of Example 1 except for changes in the polymer and the elastic straps as indicated in Table III. Improved rib stabilization resulted in higher pull-out resistance as reported in Table III. In testing the elastic strap fastener assemblies of Example 3, the elastic strap (SK531) did not pull out of the polymeric cages but failed by breaking and badly fraying. Examination of the fastener assemblies of Example 3 indicated that the polymer completely encapsulated the strap at each of the intermediate transverse ribs, the penetration being attributed to the highly porous nature of the SK 531 elastic strap. In Example 4, the polymer penetrated completely through the elastic strap, but some threads of the strap were visible. This degree of penetration was much greater than was achieved in Example 2 and was attributed to the lower melt viscosity of the "HiMont" polymer. In Example 2, there was some penetration, but the elastic strap nearly obscured each transverse rib around which it was stretched. When the polymer freely penetrates through the elastic strap from one side, the strap may not be appreciably deflected, but even if there were no deflection, complete penetration would confer high resistance to pull-out.

Claims: An elastic strap fastener assembly comprising an elastic strap (12) and a polymeric cage (10) encompassing a length of the elastic strap (12), which cage (10) has a peripheral frame (13) interconnected by central bridgework (18) that is fused to both faces of the strap (12) and characterised in that it positions the strap (12) essentially at the transverse centre of the cage (10), at least one portion of the bridgework (18) stretching the strap (12) away from the transverse centre of the cage (10). An elastic strap fastener assembly as defined in claim 1 wherein the said bridgework (18) comprises at least one transverse rib, and the strap (12) is streched around and fused to the transverse rib. An elastic strap fastener assembly as defined in claim 2 wherein the polymeric cage (10) has at least three of said transverse ribs (18), and the elastic strap (12) undulates around opposite sides of adjacent of said transverse ribs (18). An elastic strap fastener assembly as defined in claim 1 wherein the elastic strap (12) is permeable, and the polymer of the cage (10) penetrates into interstices of the elastic strap (12) where the bridgework (18) stretches the strap (12). An elastic strap fastener assembly as defined in claim 1 wherein the polymeric cage (10) encompasses an end of the elastic strap (12). An elastic strap fastener assembly as defined in claim 5 and further comprising a respirator harness including a polymeric head cradle, said polymeric cage (18) being integral with said polymeric head cradle. An elastic strap fastener assembly as defined in claim 1 wherein the peripheral frame (13) is continuous and is formed with a slot through which said elastic strap (12) emerges. An elastic strap fastener assembly as defined in claim 11 wherein the width of the frame (13) where the strap (12) emerges does not substantially exceed the nominal thickness of the elastic strap (12). A 2-part injection mold that is formed at its parting line to receive an elastic strap (43), each mold part being formed, where a strap (43) is received, with at least one open cavity (31,32) that is opposed to an open cavity (31,32) of the other mold part, both of said cavities (31,32) being closed by the received strap (43) when the mold is closed, a first of said mold parts being formed with a secondary gate (35) opening into the interior of said open cavity (31,32), thus permitting an injected polymeric composition to flow into said open cavity (31,32) of the first of said mold parts to stretch the elastic strap (43) into said opposing open cavity (31,32) of the second of said mold parts.

Inventors: BRAUN, DAVID L., BRAUN, DAVID L., C/O MINNESOTA MINING AND
Applicants: MINNESOTA MINING AND MANUFACTURING COMPANY
Application year: 1990
Main classification: A62B18
IPC codes: B29C45, A62B18, A01D34, A44B11, B29L31, B29D99