Composite structural closures are not new to the automotive industry. There are many types of composites panels that have been utilized by various OEMs, including fiberglass, carbon fiber, resin transfer molding (RTM), sheet molding compound (SMC), and injection molding. Each process has its advantages and disadvantages for material cost, tooling cost, dimensional stability, recyclability, processing time, and surface finish. An OEM will decide one method over another depending on the vehicle and structure, according to researchers from Nissan Technical Center NA, who participated in the “Automotive Composites” technical session at the SAE 2014 World Congress.
Thermoset material systems are advantageous for low-volume applications due to low tooling cost. These are often the first choice for a new vehicle class or a specialty vehicle/trim based on an existing platform. Because these processes rely on extended cycle times for the resin curing process, the cost to apply to high-volume vehicles becomes prohibitive.
Recyclability is an ever increasing consideration for OEMs due to the need for in-process reclamation, end of vehicle life (ELV) regulations, and disposal cost. Composite structures have been at a disadvantage to metal structures due to the difficulty to identify and separate various resin families. There are many resin systems that can offer the structural, dimensional, and other performance characteristics required; however, achieving a unified material family to make reclamation possible is an added challenge.
Considering the benefits of thermoplastic injection molding for cycle time, recyclability, surface finish, and cost, Nissan has now decided to utilize injection molded panels for its composite liftgates. The company claims that the first North American production of a high-volume, fully recyclable automotive closure arrived with the launch of its 2014 Rogue. (This innovation was included in the “New technologies for 2014” section of the December 2013 issue of Automotive Engineering.) Extensive CAE optimization, material characterization, supplier co-development, and a robust bonding strategy made the development possible, according to the researchers.
Reinforcement and material optimization
Composites can offer up to and above a 50% weight reduction versus traditional steel structures; however, to achieve these levels while maintaining cost and performance, careful optimization must be applied. Past composite liftgates have required substantial metal reinforcements inside the structure to maintain the strength, durability, and dimensional accuracy of the part over time. This also diminished the system weight reduction that could be realized. By utilizing a hinge structure change, a significant amount of reinforcing metal was able to be reduced. An auxiliary benefit of this new design was increased third-row headroom.
Advancements in material characterization and processing have allowed for reduced material thickness without sacrificing strength or appearance. Nissan’s latest co-developed outer panel material is a high-flow, high-stiffness thermoplastic olefin (TPO), supplied by LyondellBasell. This allows for further thin walling of the outer panel due to improved flow for tool filling. Overall rigidity is maintained vs. the previous generation. This optimization contributes to further weight reduction and cycle time reduction for injection molding economics.
The inner panel is a mold-in-color 30% long glass fiber-reinforced polypropylene (PP-GF30). Long glass fiber is known to provide significant strength and stiffness improvements without sacrificing impact properties as with short glass composites. In addition to superior impact strength, another breakthrough for Class A applications is good fiber dispersion, the researchers claim. Through glass size optimization and pultrusion process control, the inner panel LGF PP can be color-matched to the rest of the TPO upper trim with no fiber bundling.
Nissan’s relationship with material suppliers has led to a high degree of cooperative development. Supplied by Hitachi Automotive Systems, the full liftgate assembly on the Rogue is 30% lighter than comparable stamped steel systems. Due to parts integration, low scrap, and reuse of offal possible with injection-molded thermoplastics, raw-material costs on the outer panel were reduced 35% compared to SMC. Advanced Composites, Inc. also supplied materials, and Magna-Decostar was the processor.
Panel bonding strategy, and other design considerations
One bottleneck of any composite process is the joining of the panels. Dissimilar materials are typically joined together by adhesives, riveting, mechanical fastening, or a combination of the three.
Due to appearance requirements, riveting is not possible for this Class A surface. Adhesive bonding is very robust, but tends to take a long time for the adhesive to cure sufficiently for handling.
The annual volume for Nissan’s latest liftgate is expected to be higher than the outgoing model (130k+ per year). To meet the volume demands, the adhesive bonding strategy is to “divide and conquer.” Multiple bonding cells were created and dimensionally validated against a master bonding cell. This allows for very repeatable dimensional accuracy even though multiple cells are used. It also allows for much greater throughput without sacrificing the necessary time for the adhesive to cure.
Despite the many advantages to a composite structure, there are some design considerations that must be studied, understood, and overcome prior to widespread adoption of composite liftgates.
The property of creep is inherent to any viscoelastic material, especially polymers. With the reduction of metal reinforcements along the pillar sections of the liftgate, panel creep was planned for and expected. Nissan built prototypes and subjected them to expected loads at elevated temperatures.
After quantitative analysis, the total amount of creep over time was calculated. The mitigation plan was to build the initial panel with the creep taken out, effectively making the fresh panel smaller than the long-term expected outcome. After time, the panels will creep to be within the normal expectation of styling, which will maintain appropriate panel parting gaps between the composite liftgate, bumper fascia, and steel body panels.
Liftgate rigidity is another important consideration. The PP-GF30 and TPO structure is not as rigid as the outgoing mild steel structure in the cross-car or torsional directions. Nor is it as rigid as the previous-generation liftgate structures that used significant internal metal reinforcements.
Due to the lessened lateral and torsional rigidity, the parting line gaps needed to be increased and auxiliary guidance parts added to ensure that no damage would come to the liftgate or surrounding body structure during door closing. It is an unfortunate side-effect of the lightweighting optimization process, but should not have any auxiliary detriments.
This article is based on SAE International technical paper 2014-01-1059 by Aaron C. Tenorio and David D. Lipka of Nissan Technical Center NA, with additional reporting by Ryan Gehm.