Abstract:
An in-situ foam core component having first and second opposed thermoformed skins each having a thermal bond to an in-situ foam core disposed there between. At least one embossment In one of the skins joined to the other skin forming a pinch off junction. A through hole is formed in the pinch off junction sized to receive a fastener so that the embossment provides a sleeve structure which can transmit load between the first and second skins when the in-situ foam core component is attached to a member using a fastener extending through the hole.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims the benefit of U.S. provisional application Ser. No. 61/721,070 filed Nov. 1, 2012, the disclosure of which is hereby incorporated in its entirety by reference. 
     
    
     TECHNICAL FIELD 
       [0002]    The disclosed embodiments relate to an in-situ foam core stress mitigation component and method of manufacturing of same. 
       BACKGROUND 
       [0003]    Vehicle manufacturers attempt to reduce the weight of the vehicles in order to enhance the fuel economy of the vehicle. Often the reduction in weight compromises the strength of component parts. Recently, regulations, such as ECE17 and Federal Motor Vehicle Safety Standards (FMVSS), such as FMVSS202A, have mandated a stiffer component structure for vehicle seats. 
         [0004]    Recent components, meeting ECE17 regulation, such as seat frames comprising a plastic blend of polycarbonate and acrylonitrile butadiene styrene (PC/ABS), have increased the cost of seat frames as well as increased the weight of the seat frames relative to the blowmolded polyethylene seat frames that they replaced. 
         [0005]    Manufacturers need to connect seat frame components, including the PC/ABS seat frames, to fixed platforms in the vehicles. Current methods, such as bolting directly through a plastic component, result in areas of very high stress concentration. Very high stress concentration may result in premature fatigue failure of the plastic component or other structural damage to the plastic component during high acceleration and deceleration events of the vehicle. 
         [0006]    What is needed is a lightweight component that mitigates the stress concentration while remaining economically competitive. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0007]      FIG. 1  schematically illustrates a stress mitigation component according to at least one embodiment; 
           [0008]      FIG. 2A  schematically illustrates a stress mitigation component in use with a seat frame base according to at least one embodiment; 
           [0009]      FIG. 2B  schematically illustrates a seat frame base from prior art; 
           [0010]      FIG. 3A  schematically illustrates in a fragmentary cross-sectional view a stress mitigation component according to at least one embodiment; 
           [0011]      FIG. 3B  schematically illustrates in a fragmentary cross-sectional view of an alternative stress mitigation component embodiment; and 
           [0012]      FIG. 3C  schematically illustrates in a fragmentary cross-sectional view of another alternative stress mitigation component embodiment. 
       
    
    
     DETAILED DESCRIPTION 
       [0013]    Except where expressly indicated, all numerical quantities in the description and claims, indicated amounts of material or conditions of reaction and/or use are to be understood as modified by the word “about” in describing the broadest scope of the present invention. Practice within the numerical limits stated should be desired and independently embodied. Ranges of numerical limits may be independently selected from data provided in the tables and description. The description of the group or class of materials as suitable for the purpose in connection with the present invention implies that the mixtures of any two or more of the members of the group or classes are suitable. The description of constituents in chemical terms refers to the constituents at the time of addition to any combination specified in the description and does not necessarily preclude chemical interaction among constituents of the mixture once mixed. The first definition of an acronym or other abbreviation applies to all subsequent uses herein of the same abbreviation and applies mutatis mutandis to normal grammatical variations of the initially defined abbreviation. Unless expressly stated to the contrary, measurement of a property is determined by the same techniques previously or later referenced for the same property. Also, unless expressly stated to the contrary, percentage, “parts of,” and ratio values are by weight, and the term “polymer” includes “oligomer,” “co-polymer,” “terpolymer,” “pre-polymer,” and the like. 
         [0014]    It is also to be understood that the invention is not limited to specific embodiments and methods described below, as specific composite components and/or conditions to make, of course, vary. Furthermore, the terminology used herein is used only for the purpose of describing particular embodiments of the present invention and is not intended to be limiting in any way. 
         [0015]    It must also be noted that, as used in the specification and the pending claims, the singular form “a,” “an,” and “the” comprise plural reference unless the context clearly indicates otherwise. For example, the reference to a component in the singular is intended to comprise a plurality of components. 
         [0016]      FIG. 1  schematically illustrates a stress mitigation component  10  according to at least one embodiment. Stress mitigation component  10  includes a first skin  12  and a second skin  14  opposed to and separated from first skin  12 . First skin  12  may be continuously connected to second skin  14  in certain embodiments, such as in a blow molded part. In another embodiment, first skin  12  may be spaced apart from second skin  14  at essentially all points. Situated between first skin  12  and second skin  14  is an in-situ foam core  22 . In the illustrated embodiments, an embossment  16  is formed into first skin  12  either before or after the in-situ foam core  22  is formed. Embossment  16  includes a portion of first skin  12  drawn into a cupped shaped depression. Embossment  18  is formed into second skin  14  either before or after the in-situ foam core  22  is formed. Where embossment  16  and  18  meet a pinch off junction  20  is formed. Junction  20 , in at least one embodiment, is sufficiently thin that a man hole may be formed by removed by applying relatively low mechanical force, such as a cutting device  38 , or by manual pushing of an instrument, such as a fastener through the junction. 
         [0017]      FIG. 2A  schematically illustrates a fragmentary isometric view of a seat base frame foot  30  for use in mounting a seat base frame to a vehicle, according to at least one embodiment. Seat frame foot  30 , in at least one embodiment, includes stress mitigation component  10 . A portion of a blowmolded seat frame (not shown) has in-situ foam core  22  bonded to the blowmolded surfaces including those forming seat frame foot  30 , such as skins  12  and  14 . Seat frame foot  30  includes a base  32  into which embossments  16  and  18  are formed. Junction  20  is removed by puncturing with a fastener  34 . Fastener  34  is fitted inside a collar  36  formed in embossments  16  and  18  by first and second skins  12  and  14 . The stress concentration in the foot surrounding the fastener is unexpectedly reduced using collar  36  in combination with in-situ foam core  22  and the blow molded surfaces when compared to prior art configurations, such as in  FIG. 2B .  FIG. 2B  schematically illustrates a prior art seat frame foot  40  where a fastener  42  is driven through the blow molded surface and in-situ foam core  22  without the embossments  16  and  18  defining the collar  36 . The stress concentration is at least an order of magnitude greater than the embodiment illustrated in  FIG. 2A . The prior art blowmolded seat frame foot  40  failed to pass ECE 17 testing. In another embodiment, the peak stress concentration is reduced by a range of 20% to 500% relative to prior art blowmolded seat frame foot  40 , based on finite element analysis modeling. In another embodiment, the peak stress concentration is reduced by a range of 50% to 200% relative to prior art blowmolded seat frame foot  40 . 
         [0018]    Seat frame foot  30  having skin  12  and  14  comprising a polypropylene composition having a thickness ranging between 0.025 inches and 0.25 inches with in-situ foam core  22  density ranging from 1 lb/ft 3  to 5 lbs/ft 3  formed of expanded polypropylene (EPP) that is expanded using steam, passes ECE 17. The seat frame foot  30  is a portion of a seat frame that surprisingly reduces the weight by 5 to 15 pounds of a seat frame relative to a PC/ABS 60/40 composition equivalent. 
         [0019]    The seat component can alternatively be a seat back with a through hole embossment formed at the hinge or seat back latch attachment point to receive mounting fasteners. 
         [0020]    In at least one embodiment, skin  12  or  14  thickness may range from 0.02 inches to 0.5 inches. In another embodiment, skin  12  or  14  thickness may range from 0.125 inches to 0.25 inches. 
         [0021]    In at least one embodiment, in-situ core  22  thickness may range from 0.15 inches to 6 inches. In another embodiment, in-situ foam core  22  thickness may range from 0.2 inches to 4 inches. In another embodiment, in-situ foam core  22  thickness may range from 0.5 inches to 1 inch. 
         [0022]    Skins  12  and/or  14 , in at least one embodiment, are formed of a composition of any moldable composition. Non-limiting examples of the composition include, but are not limited to, a liquid silicone rubber, a synthetic rubber, a natural rubber, a liquid crystal polymer, a synthetic polymer resin, and a natural polymer resin. In another embodiment, skins  12  and/or  14  are formed of a composition of a thermoplastic polymer, a thermoset polymer, or blends thereof having a viscosity ranging from 0.1 grams/10 min. to 40 grams/10 min. The viscosity is measured according to ASTM D-1238 at 190° C. with a 2.16 kg weight. In yet another embodiment, skins  12  and/or  14  are formed of a composition of a polyolefin including a polypropylene and polyethylene having a viscosity ranging from 1 grams/10 min. to 30 grams/10 min. In yet another embodiment, skins  12  and/or  14  are formed of a composition of a vinyl-containing composition, including polystyrene and acrylonitrile-butadiene-styrene (ABS). 
         [0023]    In-situ foam core  22 , in at least one embodiment, is formed of a composition of any fluid-expandable material. Examples of fluid-expandable material include, but are not limited to, a polyolefin polymer composition, a biopolymer expandable bead, an alkenyl aromatic polymer or copolymer, a vinyl aromatic polymer resin composition, and a polystyrene polymer composition. In at least one embodiment, the polyolefin polymer composition includes polyolefin homopolymers, such as low-density, medium-density, and high-density polyethylenes, isotactic polypropylene, and polybutylene, and copolymers of ethylene or polypropylene with other polymerized bull monomers such as ethylene-propylene copolymer, and ethylene-vinyl acetate copolymer, and ethylene-acrylic acid copolymer, and ethylene-ethyl acrylate copolymer, polystyrene, and ethylene-vinyl chloride copolymer. These polyolefin resins may be used alone or in combination. Preferably, expanded polypropylene (EPP) particles, cross-linked expanded polyethylene (xEPE) particles, polyphenyloxide (PPO) particles, biomaterial particles, such as polyactic acid (PLA), and polystyrene particles are used. In at least one embodiment, the polyolefin polymer is a homopolymer providing increased strength relative to a copolymer. It is also understood that some of the particles may be unexpanded, also known as pre-puff, partially and/or wholly pre-expanded without exceeding the scope or spirit of the contemplated embodiments. 
         [0024]    Pre-expanded beads, in at least one embodiment, are the resultant bead after raw bead has undergone a first expansion step of a two-step expansion process for beads. During the first expansion step, raw bead is expanded to 2% to 95% of the fully expanded bead size. The fully expanded bead is the bead that forms in-situ foam core  22 . In another embodiment, pre-expanded bead is the result of the first expansion step where raw bead is expanded from 25% to 90% of the fully-expanded bead size. 
         [0025]    A fluid for the second expansion step of the two-step expansion process for beads causes the pre-expanded beads to expand completely to form the fully expanded beads. Examples of the fluid include, but are not limited to, steam and superheated steam. In certain embodiments, after expansion with the heated fluid, the beads in contact with the skins  12  and  14  are bonded to the skin forming a thermal bond with in-situ foam core  22  therebetween. 
         [0026]    Polyolefin beads and methods of manufacture of pre-expanded polyolefin beads suitable for making the illustrated embodiments are described in Japanese Patent Nos. JP60090744, JP59210954, JP59155443, JP58213028, and U.S. Pat. No. 4,840,973. Non-limiting examples of expanded polyolefins are ARPLANK®, and ARPRO®, available from JSP, Inc. (Madison Heights, Mich.). The expanded polypropylene, such as JSP ARPRO™ EPP, has no external wall. 
         [0027]    In at least one embodiment, in-situ foam core  22  density, after expansion by steam, ranges from 1 lb/ft 3  to 25 lbs/ft 3 . In at least one embodiment, in-situ foam core  22  density ranges from 1.5 lbs/ft 3  to 15 lbs/ft 3 . In at least one embodiment, in-situ foam core  22  density ranges from 2 lbs/ft 3  to 9 lbs/ft 3 . In at least one embodiment, in-situ foam core  22  density ranges from 3 lbs/ft 3  to 6 lbs/ft 3 . 
         [0028]    In at least one embodiment, skins  12  and/or  14 , with a range of 0.025 inch thickness to 0.1 inch thickness, are comprised of metallocene polypropylene. Such a combination is found to improve adhesion between skins  12  and/or  14  and in-situ foam core from  22  formed of EPP. 
         [0029]    Turning now to  FIG. 3A , stress mitigation component  10  is schematically illustrated in a fragmentary cross-sectional view according to at least one embodiment. Embossment  16  that draws skin  12  approximately half the way through stress mitigation component  10  meets embossment  18  that draws skin  14  to meet embossment skin  12  forms junction  20 . The combination of embossments  16  and  18  forms collar  36 . Junction  20 , in at least one embodiment is cut out with cutting device  38 . Preferably the embossments are circular in cross section and outwardly taper from the periphery of the junction to the skin surface at an angle of Θ, 1 to 5 degrees to provide die draft. 
         [0030]      FIG. 3B  is a variation  10 ′ of the stress mitigation component  10  of  FIG. 3A  with junction  20 ′ provided with an annular frangible zone  44  to facilitate knocking out central section of junction  20 ′ with a punch  46  or fastener to form a through hole. 
         [0031]      FIG. 3C  is another variation  10 ″ of the stress mitigation component  10  of  FIG. 3A  with junction  20 ″ formed between a single embossment in one of skins  12 ″ and the generally flat opposed skin  14 ″. A single embossment is sufficient in in-situ foam core component applications where the thickness of the foam core relatively small e.g. one inch or less. However a pair of embossments is typically preferred to minimize thinning of the embossment wall. 
         [0032]    In at least one embodiment of the method of forming the seat frame foot  30  is blow molding. Mold portions define a blow mold cavity into which a molten polypropylene parison is extruded from an extruder. The parison defines an internal parison cavity. 
         [0033]    The blow mold portions close upon the parison. Gas is injected into the parison cavity inflating the hot parison while still soft and deformable to conform to the walls of the blow mold cavity defining the skins  12  and  14  having a cavity which may be larger than the original parison cavity. Sliding mold portions push on the parison to form the embossments  16  and  18  and junction  20 . 
         [0034]    Feed apertures are cut through skins  12  and  14 . Staged filling begins as partially pre-expanded EPP beads are introduced to the cavity through an EPP introduction device fitted to the blow mold portions. At a first stage, EPP beads are introduced through the feed aperture. Filling at the next stages continues until the cavity is filled. 
         [0035]    The EPP introduction device is withdrawn from the apertures. The apertures are plugged. Steam injection needles are inserted through blow mold portion and skins  12  and  14  into the filled cavity. A fluid for the second expansion step of the two-step expansion process for beads causes the pre-expanded beads to expand completely to form the fully expanded beads. Examples of the fluid include, but are not limited to, steam and superheated steam. 
         [0036]    Steam is injected through steam injection needles into partially pre-expanded beads causing the beads to expand forming the in-situ foam core  22  in the area that was previously the cavity. The contacting layer of beads bond to skins  12  and  14  forming a thermal bond. 
         [0037]    In at least one embodiment, in-situ foamed core  22  is formed by expanding a pre-expanded bead inside a blow molded shell. In at least one embodiment, pre-expanded bead is re-compressed by 10 vol. % to 70 vol. % when being dispersed. Upon being dispersed, pre-expand bead re-expands within the cavity. 
         [0038]    While exemplary embodiments are described above, it is not intended that these embodiments describe all possible forms of the invention. Rather, the words used in the specification are words of description rather than limitation, and it is understood that various changes may be made without departing from the spirit and scope of the invention. Additionally, the features of various implementing embodiments may be combined to form further embodiments of the invention.