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    1. Archive

    Bonded films take the "bite"

    Nov. 4, 2004
    Glass fibers poking out of plastic surfaces can abrade mating parts. Here's a way to stop it.

    Steve Saxion
    Dimension Bond
    Chicago, Ill.

    A bonded film in cross section reveals the layered structure that forms the insulating barrier for GFTP parts. The substrate ( bluegray) is covered with nanosized reinforcement particles (pink) in a binder material. The black and dark gray layers above the reinforcement particles are films that reduce friction and wear.


    Glass-filled engineering thermoplastics (GFTPs) can replace metals in many applications. GFTPs are lightweight, stiff, strong, and inert to most process fluids and corrosives. And they can be molded to accurate, finished dimensions without secondary operations. But parts made from the materials can act like sandpaper when they rub against mating components and counterfaces. The culprit: glass fibers protruding from the plastic surfaces.

    Previous approaches to improve surface properties of GFTP parts have had mixed results. Conventional dry-film lubricants, for example, lower surface friction but are sheared by glass fibers protruding above the plastic. Coatings that rely on thermal curing are typically processed at elevated temperatures that degrade most plastics. Further, coatings don't adhere well to the glass portion of the composite.

    But a process of direct-bonding wearresistant films to both plastic and glass from Dimension Bond, Chicago, Ill., solves the abrasion problem. It directly bonds one of several bearing-grade films to such glass-filled GFTP thermoplastics as nylon, PBT, PET, PPS, and PPO. Film-to-GFTP bonding takes place without the elevated temperatures of conventional coatings.

    The films are made of multiple, laminar layers of matrix resinous bonding materials and nanocomposites. One formulation has the wear properties of bronze with a top layer of pure PTFE. The film cross section is similar to that of a bronzefilled, DU-style bearing with a PTFE top layer. Other formulations exceed the wear rate of bronze but with lower inherent dry friction. Wear rate is three to four-times that of bronze-PTFE bearing materials.

    Film thickness depends on the application but ranges from 0.0005 to 0.009 in. To some extent

    the percent of glass filler determines the amount of glass protruding through the surface. Films must cover the glass projections and extend to some additional "insurance" depth. An engineering rule of thumb says to cover the surface with a first layer 1.5 3 taller than the glass projections. To this first layer are bonded top layers that lower friction, boost wear resistance, or both. Final bonded-film thickness is typically 0.0011 to 0.009 in.
    WHERE BONDED FILMS MAKE SENSE
    Bonded
    Total fil
    Relative wear
    film
    thickness
    life versus
    Applications
    designation
    (in.)
    bronze/PTFE
    L36
    0.0007 to 0.003
    12
    Rack-and-pinion rod support
     
    Shaft-end bearings and guides
     
    Wheels
     
    Hubs
     
    Supports for heavy loads against
     
    smooth, ground counterfaces and O-rings
     
    Elastomer-seal counterfaces
     
    and mating surfaces
    L23
    0.0007 to 0.0045
    7
    Electrical switches
     
    Pump components
     
    Thrust faces
     
    Printer parts with high
     
    cycle and surface speeds
    L9
    0.0005 to 0.007
    14
    Automotive and apliance components
     
    Water-conditioner parts
     
    Solenoid bobbins with lower quality,
     
    rough counterface surfaces
    Can be applied to glass-filled engineering plastics such as PBT, PPS, PET, PPO, PEEK, acetal, nylon, polyester, polycarbonate, and others.

    Steering toward plastic bearings

    Yoke bearings (bearing cradles) in rack-and-pinion steering gear support the steering shaft opposite the pinion gear, letting the rod maintain precise contact with the pinion gear. The yoke bearings in this case were originally made of powder metal and needed a separately attached insert of steel-backed bronze/PTFE. The costly, multipart design required assembly and had the potential to come apart and jam the steering gear.

    The yoke bearings are now molded from PBT polymer, plus 20 to 30% glass fiber. Lifetime lubrication for the steering shaft comes from a 0.0025-in.-thick film bonded to the curved yoke section (wear interface) of the bearing. Wear tests show the bonded film has one-quarter to one-third the wear of the original bearing insert over 200,000 cycles on a simulated steering test rig run at 200 m/sec. The GFTP yoke also withstood the " curbing" simulation of shock loads.

    A powder-metal yoke bearing (right) uses a staked or riveted shoe as a wear surface. The GFTP replacement (left) uses an L26 bonded surface film that prevents glass fibers from abrading the mating shaft and provides lifetime lubrication.

    Smoothing valve moves

    The use of composite bearing films in a water-conditioning valve let engineers switch valve materials from brass to 20%-glass-filled, modified PPO resin. The change lowered material costs and cut the number of secondary manufacturing operations.

    Glass-filled PPO maintains dimensional tolerance of the valve body and rotator valve. But initial tests showed that plastic-to-plastic friction caused excessive wear and " slipstick," a condition of erratic opening and closing motion.

    The fix: a nanocomposite material applied to the rotating valve exterior (after a proprietary pretreatment process) to a thickness of 0.0007 to 0.0011 in. per side. The composite is cured at a low temperature that doesn't distort intricate valve surfaces. It withstands the harsh water environment, lowers valve-actuation torque, and lubricates the rotator valve for life.

    MAKE CONTACT
    Dimension Bond, 773-282-9900, www.dimensionbond.com

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