Blow molded vehicle bumper beams.
Manufacturers of contemporary bumper systems must satisfy a complicated universe of regulatory, safety, aesthetic, performance, and cost criteria. Many of these factors are outside the control of the industry, which will probably face several new safety, fuel economy, and environmental challenges in the near future. Advances in materials and processes have allowed manufacturers to overcome current obstacles.
National Highway Traffic Safety Administration no longer requires bumpers for passenger motor vehicles to withstand impacts of 5 mph without visible damage to face bars. However, U.S. and Canadian federal regulations require that manufacturers still provide reasonably protective systems.
U.S. standards require bumper systems for passenger motor vehicles to withstand--with damage limited to bumper face bars--2.5 mph impact tests consisting of two corner pendulum impacts, two longitudinal impacts, and one barrier impact. Canadian regulations call for withstanding an impact of 5 mph without damage to safety systems, such as lights, hood latches, radiators, and horns.
While impact regulations are currently "softer," CAFE schedules remain in place. The difficulties of the motor vehicle industry, and related national economic considerations, will probably forestall higher-speed federal impact requirements for the near future. However, increased environmental pressure and resulting competition to market fuel-efficient vehicles will eventually result in even greater demands for lightweight, recyclable bumper systems.
Cost and Manufacturing Concerns
Bumper designers face constant demands for lower manufacturing costs and greater production simplicity. Thus, decisions about processes and materials are carefully evaluated.
Systems must also accommodate the increasing complexity of front-end vehicle design: rounded surfaces, lower front ends, integrated air ducts for engines, mounting points for auxiliary lighting, and attachment points for fascia.
Bumper Beam Design
In-depth study of materials is beyond the scope of this article, but examination of bumper beam design is not.
The most common bumper beam design, shown in Fig. 1, is a rolled or stamped steel bar with an energy absorption unit (EAU). The steel bar may take any shape, but extreme curvature drastically increases manufacturing costs.
Optimum cross section shape for an energy-absorbing bumper beam is the box section, which can incorporate special design features such as cooling ducts or lights, yet retain strength without needing an EAU.
Most common EAUs are steel hydraulic shock units, expanded polypropylene beads, polyester sleeve-steel tubes (such as those used on Chrysler products), and units made of polyethylene or polyolefin cells with "egg-crate" or honeycomb shapes.
Comparison of Typical Fascia/Impact Beam Systems
The following comparison assumes 100,000 parts each, at 6|ft.sup.2~:
Steel beam-hydraulic EAU. The steel roll-formed or stamped beam is attached to a hydraulic energy absorber. It has several advantages over other beams, including cost and interchangeability. A simple beam section may be more cost effective in rolled steel; a complex beam section with other functions integrated may be more cost effective in plastic. Figure 3 shows a simple bumper beam system.
Steel beams, if designed correctly, can accept low-speed impacts without deforming. Most plastic beams can be designed to take an equivalent number of impacts, thus meeting or exceeding Canadian legal requirements. Because beams are usually hidden behind fascia, the same system can be used for several models if the models do not incorporate different and inordinately complex shapes. Figure 4 shows a metal bumper beam section and several plastic composite bumper beam sections molded by various processes.
In comparison to systems made of engineered plastics, steel beam systems also have several disadvantages relative to tooling costs, susceptibility to corrosion, and weight. Depending on complexity of shape, a steel beam may require tooling more than twice as costly as that required for engineered plastics. Unless carefully (and expensively) protected, steel beams can quickly corrode from oxidation and a reaction to salts. Also, steel beams of equivalent strength are two times heavier than engineered plastic beams.
Transition to Engineered Plastics
Clearly, interest in bumper systems incorporating engineered plastics arose from the need to balance systems cost against weight, ease of assembly, simplified vehicle production, and styling considerations. Although systems using plastics may have greater unit costs, they have a 50% weight advantage and can reduce vehicle production line time, provide simplified assembly, and allow designers greater freedom. These advantages are especially important in applications requiring fewer than 100,000 units, as the trend toward niche marketing continues.
Within the realm of engineered plastics, several systems are currently in use.
Reinforced Reaction Injection Molded (RRIM) Beams
In this process, a urethane resin is injected into a glass-fiber reinforced preform to achieve desired shape and strength. The system is attractive because of its weight and materials cost advantages. Parts can be relatively light compared to their equivalents in steel (13 lbs vs. 20 lbs for a 6-|ft.sup.2~ beam). However, other plastics systems can produce even lighter parts.
Resins may be cheaper on a per-pound basis than rival engineered thermoset resins. However, RRIM parts costs are typically higher because parts must be made thicker to offer comparable performance. Figure 5 shows a bumper beam formed by RRIM.
Disadvantages include complexity, process sensitivity, distortion, and nonrecyclability. Relative to machine tooling, RRIM is one of the most expensive of the more commonly used engineered plastics processes used for bumper beams.
Achievement of optimum box-section shape requires the bonding together of two parts; otherwise, the system is limited to producing C-section beams, which require use of a separate EAU. Mixing of chemicals takes place at the press. Unless mixtures are carefully monitored, parts of widely varying and inadequate composition may result.
Glass/urethane, like all composites, can lose its strength characteristics when molded into highly complex shapes. Neither scrap from the manufacturing process nor post-consumer products can be recycled from these resins.
Compression Molded Bumper Beams
In compression molding of bumper beams, glass-reinforced polypropylene sheet is heated and pressed into the shape of the mold. Advantages of the system, which has received considerable attention from General Motors, Saturn, and Honda U.S., include light weight (equivalent steel beams are twice as heavy), tooling costs (just 20% of steel beam tooling costs, but probably more expensive than rival engineered plastics), and quick tooling changes. A compression molded bumper beam is shown in Fig. 6.
A disadvantage of the system is its complexity: It requires two pieces to achieve optimum box shape. Some systems use a bonded steel front plate; C-section beams require EUAs for vehicles weighing more than 3000 lbs.
Distortion is also a drawback. Because the system employs another composite, strength of compression molded beams can be inconsistent when the goal is to form complex shapes. Multicurved parts display unacceptable resin concentration around openings such as cooling ducts. Unidirectional fibers can solve the problem, but at relatively high cost. Still another disadvantage, generally true of composites, is nonrecyclability.
Injection Molded Bumper Beams
In injection molding of bumper beams, a compound such as polycarbonate--desirable because of its strength--is injected into a mold. The process has several advantages relating to materials variety, weight, tooling, labor, and recyclability.
Beams can be precisely molded in a wide range of materials and in highly complex shapes, offering considerable design freedom to integrate functional pieces. Injection molding can produce the desirable box beam, which requires no EAU. It can also produce the lightest parts of any process, while retaining required strength. Compared to an equivalent steel beam, an injection molded beam requires appreciably lower tooling costs. The process also has a moderate labor advantage over other processes, and compounds typically used in the system can be recycled. Figure 7 shows an injection molded bumper beam.
Disadvantages include complexity and cost. Achieving optimum box beam requires bonding two pieces; C-sections may require an EAU. Because of slight disadvantages in mold, press, and utility costs, injection molding of bumper beams is the most expensive of the engineered plastics processes.
It should be noted that none of the aforementioned systems using engineered plastics has a marked cost advantage over the others. Indeed, total costs per beam are roughly comparable for the three processes. Given such small differences, more weight should probably be accorded such factors as the wide variety of materials and design freedom available with injection molding, which allows engineers to solve problems related to integrating such functional parts as lights, cooling ducts, and fascia attachment points. The two systems using glass fiber reinforced plastics appear to have limited use in complex designs with such features as multiple curves, openings, integrated lamps, and attachment points.
Blow Molded Bumper Beam Systems
In this system, a parison or tube of a high-molecular-weight, high-density thermoplastic such as polycarbonate is extruded into a hollow mold. Air pressure pushes the material to mold walls to achieve the desired shape. Figure 8 shows a blow molded bumper beam, and Fig. 9, the blow molding process.
Recent applications include bumper beams on the Mazda Miata, using high-density polyethylene (HDPE); Hyundai Sonata and Scoupe, using polycarbonate; and 1993 Ford Probe, Mazda MX-6, Mazda 626, and Mazda MX-3, all using polypropylene. Blow molded aftermarket truck bumpers filled with foam are also available.
Systems designers were attracted to several advantages of blow molded systems. The advantages include weight, cost, simplicity, and flexibility and complexity of design.
Blow molded beams can, for instance, be cheaper than units made with the use of other processes and engineered plastics. In comparison to the steel-EAU alternative, blow molded beams can also reduce vehicle assembly costs by eliminating several parts.
Because blow molding can produce the optimum bumper beam shape--the box beam--in one step, it eliminates costly bonding and assembly steps that are required to produce box beams under other systems. Further, because inner ribs can easily be included in mold design, engineers can more readily specify box beams and thereby eliminate the additional complexity of the EAU. Also, mold walls can be easily adjusted for various thicknesses and applications, an advantage that can be extremely useful in reducing product development time to a minimum.
Blow molding, like injection molding, can readily accommodate complex frontal shapes. Its singular advantage, however, is that blow molded beams can handle such complexity while still providing--in one piece--the desirable box section for strength.
Limited materials, and the need to carefully monitor thickness, are among the disadvantages of blow molded bumper beams. Use of relatively inexpensive compounds to reduce costs requires an increase in material thickness to achieve equivalent strength, thereby offsetting cost advantages of less expensive materials. Because thickness tolerances cannot be set with the accuracy attainable in injection molding, the parison must be constantly controlled to maintain uniformity.
Future Applications and Developments
In light of current trends in passenger vehicle applications, blow molded bumper systems appear to have a bright future. Cab-forward automobile designs that push weight toward the front will place a premium on achieving lighter weights in front bumpers to avoid extreme weight bias. Because it can achieve desired strength at a relatively light weight, the blow molded beam must, along with its injection molded counterpart, be seriously considered in such applications.
It is also likely that lighter vehicles will be required to meet more stringent CAFE standards. Moreover, stiffer international competition will require motor vehicle manufacturers to increase their emphasis on finding ways to reduce time and money spent in building products. The one-piece, simple-to-assemble bumper beam available through blow molding should find acceptance under such conditions.
The van and light truck markets offer similar opportunities. Because the vehicles are used primarily to haul passengers, pressure from consumers and regulators will force manufacturers to achieve safety standards now met by automobiles. Eventually, complex front end designs will be commonplace for vans and light trucks; blow molded systems should make their mark there as well. Indeed, slightly texturized blow molded bumper systems could eliminate the need for fascias in such applications.
Recent development of coextrusion machinery used in other industries (such as bottle manufacturing) could permit production by blow molding, of Class A surfaces for passenger vehicles.
Coextruding two different materials would enable a blow molded box beam to have a paintable surface, eliminating need for expensive bonding to fascias.
Because it can produce one-piece bumper systems, blow molding will be an attractive alternative when recyclable vehicles become common through legislation or market pressure. Recyclables must be easily recoverable, with fewer parts of differing materials. One-piece bumper beams thus become more attractive because they can reduce disassembly costs and simplify sorting.
1. J. Greene and J. McFadden, "Future Requirements for Automotive Composite Bumper Beams," SAE Technical Paper Series 910692.
2. S. Mizunaga, N. Saeki, and H. Watanabe, "Development of Blow-Molded Bumper Beam," SAE Technical Paper Series 900834.
3. J. Best and L. Best, Automotive Plastics Report 91, Market Search Inc., Toledo, Ohio.
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|Title Annotation:||Blow Molding|
|Author:||Jula, John; Butterfield, Larry|
|Date:||Dec 1, 1992|
|Previous Article:||Structural composites.|
|Next Article:||Analyzing composite properties by rheological testing.|