You are here

Thermoplastic composites for mass transit applications

News International-French

24 Mar 2011

Weight and cost reduction in mass-transit buses, light rail and light/heavy trucks has a direct benefit in the form of reduced fuel consumption, less road wear, lower maintenance and increased durability. Several states in the United States have imposed stringent weight requirements on mass-transit buses and trucks. Millions of dollars are spent in maintenance and parts replacement due to corrosion and cracking. Noise and vibration reduction is even more important now, due to heightened environmental and noise pollution concerns. A number of components on existing mass transit vehicles need to be replaced or retrofitted. Reducing tooling costs by making bus parts more interchangeable and modular is also gaining more importance.

(Published on October - November 2007 – JEC Magazine #36)




The University of Alabama at Birmingham (UAB) has been working with partners National Composite Center (NCC), North American Bus Industries (NABI) and Ticona Inc. to develop advanced thermoplastic composite materials and manufacturing technologies for applications in mass transit buses, light rail and trucks. A significant amount of the developmental work has focused on thermoplastic materials that offer enhanced durability, impact resistance, damage tolerance, enhanced sound and vibration damping, low weight, and corrosion resistance. The design, analysis, process modelling, manufacturing concepts and prototypes that provide form-fit function and performance validation have now all been demonstrated for mass transit components. Innovative hot-melt impregnation technologies and process models developed by the UAB team provide the capability to combine various high-strain-energy fibres with a wide variety of tough thermoplastic polymers to produce low-cost composite products. In the hot-melt impregnation process, continuous reinforced tows are pultruded through a heated die while the individual filaments are wet out with the thermoplastic resin, e.g. polypropylene (PP), polycarbonate (PC), or polyamide (PA). This process can be used to produce impregnated rods, rovings and tapes. The rods are used to produce long-fibre thermoplastic (LFT) pellets; roving and tapes can be woven into fabrics. The thermoplastic material forms can be fabricated into continuous- and long-fibre composite structures, using low-cost fabrication techniques that include extrusion/compression moulding, thermo-stamping and vacuum thermoforming. The sound- and vibration-damping characteristics of these materials are several orders of magnitude higher than for the traditional metals used in present mass-transit systems. LFTs have high fibre-aspect ratio and can be moulded into complex geometries featuring ribs and knock-outs. They can also accommodate thickness variations and provide enhanced strength and stiffness to the part, without adding weight.


Over a five-year period (2002-2007), the UAB research team developed thermoplastic composite materials and manufacturing technologies for various mass-transit bus components. These include a two-passenger bus seat, a floor segment, a side body panel and structural frame segment, a battery-box access door, and an airconditioning roof-cover door that replaces similar components in conventional steel articulated buses used in city mass transit. For these components, weight savings of about 40% and projected lifecycle cost savings of about 45-50% were demonstrated using thermoplastic composites in replacement of traditional materials such as steel, plywood, and aluminium. An integrated process product development (IPPD) approach has been taken to conduct detailed design, finite element analysis, advanced process modelling, manufacturing, and component verification steps. These applications are briefly discussed below.


LFT bus seat

The E-glass/polypropylene LFT two-passenger bus seat weighed only 10 kg, as opposed to 21 kg for the existing steel/sheet moulding compound shell design used on most city mass-transit systems. The seat also met all guidelines imposed by the American Public Transportation Association (APTA). The thermoplastic structure required fewer assembly points than with the metal/sheet moulding compound (SMC) design. The LFT seat was manufactured using extrusion-compression moulding in a single-step operation. Process modelling was conducted to aid in the placement of the E-glassfibre-filled extruded PP charge in the mould cavity. Thermomechanical stresses, warpage, shrinkage, fibre distribution and flow fronts were predicted. The design cycle for the LFT bus seat is shown in figure 1.



Glass/PP tape consolidated floor segment

The conventional floor on mass transit systems is made from steel and plywood. During the lifetime of a standard bus, the floor has to be replaced a number of times, because steel corrodes and plywood rots due to acid washes and moisture. For the thermoplastic floor segment, glass/PP woven tape material was consolidated on a double belt-press to form a face along with a vacuum-thermoformed, ribstiffened core. The woven fabric E-glass/PP floor segment was modular, and 40% lighter than the conventional metal/plywood floor.



Figure 2 illustrates the conventional floor as compared to a modular thermoplastic glass/PP floor segment. The thermoplastic composite floor was significantly more damage tolerant, easy to replace and install, and had an order of magnitude improved vibration and sound damping characteristics.


Side body panel and structural frame segments

Conventional buses possess a welded tubular steel frame, over which cosmetic body panels are mounted. In the thermoplastic design, the structural load bearing was shared across the body panel and the structural frames. A structural frame made of powderimpregnated carbon-fibre-reinforced polyphenylene sulphide (C/PPS) was developed, for assembly to a body panel. The body panel featured a load-bearing sandwich panel made of random chopped E-glass/PP face sheets and a hexagonal PP core. The frame was 60% lighter than the corresponding steel solution, and the body panel assembled to the frame was 40% lighter for equivalent structural performance. The conventional steel frame bus and the corresponding thermoplastic composite sub-elements of the body panel and structural frame are shown in figure 3.



LFT battery-box access door

An LFT glass/PP battery-box door (figure 4) was designed and produced to replace a steel frame welded to sheet steel. The steel corrodes under corrosive battery-fluid environment. The LFT door featured ribs to support the structural loads, eliminating the steel frame entirely. The battery door is approximately 1 m long by 0.6 m wide, and the current design weighs about 12 kg. Adopting the thermoplastic composite technology can give weight savings of about 50 to 60% compared to the existing design. In order to reduce labour and therefore cost, the design approach for the LFTbased battery-box access door featured integrated stiffness to eliminate the steel frame as shown in figure 4. The ribs provide extra dimensional stability to the part by controlling out-of-plane



displacements caused by shrinkage and warpage of the parts as they are withdrawn from the tool and cooled to ambient temperature. The LFT door was assembled on an articulated bus and is currently undergoing durability and road tests.


LFT battery-box access door

A thermoplastic polyolefin (TPO) bonded to a vacuumthermoformed AZDEL SuperLite glass mat ribbed interior replaced the aluminium-skin door that gets welded to a steel stringer featured on current buses (figure 5). This reduced the AC door weight from



27 kg to 12 kg. Each bus has 12 such doors, so adopting thermoplastic composites technologies can reduce the roof weight by approximately 250 kg. The free-standing deflection of the thermoplastic composite door was 30% lower than the aluminium baseline. The glass mat was thermoformed on a ribbed tool and the TPO formed on a smooth curved tool. The glass-mat ribbed core was bonded to the TPO and the surface was easily painted to match the bus colour.