Fastener retention system

A fastener retention system, including for a filter housing, applies an axial retention force in a plastic molded structure having a bore in a standing tower extending along an axis. A fastener extends axially in the bore and has a first axial end retained in the bore in snap fit relation, and a second distally opposite axial end extending axially beyond the bore for mating with a retaining member holding a cover on a base in a filter application in assembled condition. The fastener is secured to the tower in permanent snap fit relation. The fastener is axially slid into the bore before crystallization and hardening of the plastic. The fastener is axially trapped in the bore between spaced facing abutments, and is anti-rotationally keyed thereto.

BACKGROUND AND SUMMARY OF THE INVENTION
 The invention relates to a fastener retention system for applying an axial
 retention force in a plastic molded structure, including axially clamping
 a filter element in a filter housing, and to methods for making same.
 It is known in the prior art to insert fasteners in plastic in various
 ways, including molding the fastener directly into the plastic, welding
 the fastener into bosses, e.g. by sonic welding or the like, clipping the
 fastener over molded-in features, snap-in past flexible tabs, etc.
 The present invention provides a system and method for retaining fasteners
 in plastic, and does not require the use of extra parts or complex
 assembly procedures. The invention allows both low cost standard fasteners
 and custom fasteners to be assembled into plastic without requiring the
 use of robots, expensive custom tools, welders, or additional parts. The
 present invention provides a sturdy, inexpensive part, with minimized
 content and manufacturing cost.
 The invention is particularly useful for a fastener applying an axial
 compression force holding a filter element between a cover and a base of a
 filter housing. In the latter application, the invention retains the high
 durability for pull-out, push-out, and torque-out, that inserted fasteners
 must maintain to prevent filter failure.

DETAILED DESCRIPTION OF THE INVENTION
 FIGS. 1 and 2 show a filter housing 20 for a filter element 22. The filter
 housing includes a base 24 having an axially extending standing tower 26,
 and a cover 28 mounted to the base by a fastener 30 applying an axially
 compression force holding filter element 22 between cover 28 and base 24.
 The fastener has a first axial end 32, FIGS. 3 and 4, secured to tower 26
 in snap fit relation, to be described, and a second distally opposite
 axial end 34 releasably securing cover 28 to base 24 at retaining member
 36, FIGS. 1 and 2.
 The filter element and filter housing are known in the prior art, including
 various fastening systems for applying the noted axial compression force.
 Filter element 22 has a perimeter sidewall 38, FIG. 2, surrounding a
 hollow interior. Tower 26 extends axially upwardly or forwardly within the
 hollow interior and is surrounded by sidewall 38. Sidewall 38 extends
 axially between base 24 and top wall 40 of cover 28 and is axially held
 therebetween by the axial compression force applied by the fastener. FIGS.
 1 and 2 show an air filter, with air flowing into the housing at air inlet
 42 and then flowing radially inwardly through the perimeter sidewall 38 of
 filter element 22 into the hollow interior thereof and then exiting at air
 outlet 44, all as is known. The present invention is directed to an
 improved fastener retention system.
 Tower 26 is provided with first and second axially spaced snap fit
 abutments 46 and 48, FIGS. 3 and 4. Axial end 32 of fastener 30 is axially
 slidable forwardly, as shown at arrow 50, FIGS. 8 and 4, past abutment 46,
 to be described, in snap fit relation, FIG. 3, and into engagement with
 abutment 48, and is axially trapped between abutments 46 and 48 in
 permanent snap fit relation. Retaining member 36 is releasably attachable
 to axial end 34 of fastener 30 and engages cover 28 at top wall 40 to hold
 cover 28 and base 24 in axially assembled condition. In preferred form,
 fastener 30 is a bolt having a bolt head providing the noted first axial
 end 32, and having a distally opposite threaded end providing the noted
 second axial end 34. Bolt head 32 is permanently axially trapped between
 first and second axially spaced abutments 46 and 48 in snap fit relation.
 Retaining member 36 is a plastic cap molded around hex nut 52 which is
 internally threaded complementally to threaded end 34 of the bolt, and
 engages the latter in threaded relation and is tightenable thereon to
 apply the noted axial compression force.
 Base 24 and cover 28 of the filter housing are plastic molded structures.
 Molded plastic base 24 has a bore 54, FIG. 4, extending axially through
 tower 26 along axis 56, FIG. 2. Fastener 30 extends axially in bore 54 and
 has the noted first axial end 32 retained in bore 54 in the noted snap fit
 relation, and the noted second distally opposite end 34 extending axially
 forwardly and upwardly beyond bore 54. Axial end 32 of the fastener at the
 noted bolt head has distally opposite axially facing first and second
 shoulders 58 and 60. Shoulder 58 faces axially rearwardly. Shoulder 60
 faces axially forwardly. Bore 54 has the noted axially spaced distally
 oppositely facing first and second abutments 46 and 48. Abutment 46 faces
 axially forwardly. Abutment 48 faces axially rearwardly. Shoulder 58
 axially faces abutment 46, FIG. 3. Shoulder 60 axially faces abutment 48.
 Axial end 32 of the fastener has a perimeter complementally shaped to bore
 54 and is received in the bore in keyed relation to prevent rotation of
 fastener 30 about axis 56. In the disclosed embodiment, the perimeter of
 bolt head 32 is hex shaped, as is the perimeter of bore 54. The axial
 spacing of abutments 46 and 48 is substantially equal to the axially
 spacing of shoulders 58 and 60, to substantially prevent axial movement of
 fastener 30 in bore 54.
 In preferred form, a plurality of first abutments 46, FIG. 7, are
 circumferentially spaced around bore 54, and a plurality of second
 abutments 48, FIG. 6, are circumferentially spaced around bore 54.
 Abutments 46 are interdigitated relative to abutments 48 in axial end
 view, FIG. 6. Each of first abutments 46 is axially nonaligned with a
 respective second abutment 48 and instead is axially aligned with and
 axially spaced from a respective circumferential gap 62 between respective
 second abutments 48 in interdigitated manner. In preferred form, three of
 the noted first abutments 46 are provided, and are triangulated about bore
 54, and have three respective compressible ramps 64, FIGS. 3 and 4,
 extending axially forwardly and radially inwardly from bore 54 to first
 abutments 46, and are radially compressed by bolt head 32 of the fastener
 sliding axially forwardly therepast as shown at dashed line in FIG. 8.
 Three of the noted second abutments 48 are also provided, and are
 triangulated about bore 54 in the noted interdigitated manner relative to
 triangulated abutments 46. Abutments 48 have respective ramps 66 leading
 thereto, which ramps extend axially rearwardly and radially inwardly from
 bore 54 to abutments 48. Ramps 66 additionally have a reinforcement rib
 68, FIG. 5, extending axially longitudinally therealong for strength
 reinforcement of the forward end section of tower 26.
 The invention further includes a method for making a fastener system for a
 plastic molded part such as 24 for an axial force retention system.
 Plastic molded part 24 is typically formed of molded plastic material
 including crystallizing hardeners such as nylon and the like. Plastic
 molded part 24 is formed by a heated molding operation, followed by
 cooling, during which the plastic of the part crystallizes and hardens.
 The present method involves the step of sliding fastener 30 axially
 forwardly as shown at arrow 50 through bore 54 while plastic molded part
 24 is still warm from the molding operation, and before the plastic
 crystallizes and hardens. It has been found that if such axial sliding
 step of fastener 30 is not done until after cooling and hardening
 crystallization of the plastic, then tower 26 is subject to significantly
 greater risk of breaking or cracking. By sliding fastener 30 axially
 through the bore while the plastic is still warm from the mold, abutments
 46 are still soft and compressible, and are radially compressed during the
 noted axial sliding step of fastener 30, including forward axial movement
 of bolt head 32 at arrow 50 past abutments 46, whereafter abutments 46
 snap in radially behind bolt head 32 at shoulder 58 and lock the fastener
 in place. Molding of plastic molded part 24 including tower 26 is enabled
 by a plurality of first mold draw surfaces such as shown schematically at
 70 on FIG. 9, extending axially through respective gaps between respective
 second abutments 48 and having an axially rearwardly facing mold surface
 72 forming a respective first abutment 46 axially aligned therewith, and
 by a plurality of second mold draw surfaces such as shown schematically at
 74 in FIG. 8, extending axially through respective gaps between respective
 first abutments 46 and having an axially forwardly facing mold surface 76
 forming a respective second abutment 48 axially aligned therewith. The
 first mold draw surfaces such as 70 are withdrawn axially forwardly as
 shown at arrow 78 relative to bore 54, and the second mold draw surfaces
 such as 74 are withdrawn axially rearwardly as shown at arrow 80 relative
 to bore 54.
 It is recognized that various equivalents, alternatives and modifications
 are possible within the scope of the appended claims.