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 extentthe 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
3taller 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|
|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.
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.
Dimension Bond, 773-282-9900, www.dimensionbond.com