As noted in “Part I” of our SPE Automotive Composites Conference & Exhibition (ACCE) overview (November/December 2015, p. 40), many changes are occurring in the composites industry and exciting new technologies are near commercialization or just coming to market. The global composites supply chain is under intense pressure to drive down costs and cycle times on part production to help OEMs lower the weight and costs of components for land as well as air transport. The 2015 ACCE, held in September in Detroit, broke exhibition and attendance records and was a hotbed of discussion and networking about the challenges facing all transportation OEMs. What follows are two additional technologies this writer saw at the show that hold great promise.

Another Way to Turn Molders into Prepreggers

As noted previously, laminate composites (usually supplied as B-stage sheets of unidirectional (UD) carbon fiber rovings impregnated with epoxy resin) are of great interest to transportation OEMs, owing to their high mechanical performance at very-thin cross-sections and very-low part weight, low voids, and good drapeability. The downside is that these materials can’t produce the complex geometries of a process like resin-transfer molding (RTM), they have relatively high labor and scrap rates (increasing costs), and they have relatively slow production speeds when cured via conventional vacuum-bagging or autoclave systems (also increasing costs). Even the newer out-of-autoclave (OOA) systems are still slow versus a process like conventional thermoset compression molding, which can produce big parts in 2.5- to 3-min. button-to-button cycles. 

Much work has been done on so-called snap-cure resin systems (2 min. or shorter) and on further developing OOA processes and equipment to reduce cycle times, but the high labor and scrap rates of laminate composites still need to be addressed. Based on two interesting papers presented at the SPE ACCE, it seems that attention is now being focused on dealing with labor/scrap issues, which could help drive part costs down significantly and make these high-performance materials more affordable for higher-volume vehicles. If either approach proves viable, it could do for laminate composites what inline compounding (ILC) did for pelletized long-fiber thermoplastics (LFT) 15 years ago with the direct-LFT process.

One approach was developed by Fraunhofer Institute for Chemical Technology (F-ICT, Pfinztal, Germany) and involves what the group calls the InPreg (inline-prepreg) process. Since F-ICT helped develop the original D-LFT process in the late ‘90s and the direct-sheet-molding compound (D-SMC) process about five years ago, the organization has a proven track record of helping molders eliminate compounders and semi-finished goods manufacturers to produce their own material just prior to molding.

F-ICT’s approach this time involves using the same kind of four-part, B-stageable epoxy system that prepreg producers (prepreggers) use in industry. However, since the InPreg process assumes processors will mold parts physically close to and shortly after prepreg production, it eliminates the need to freeze and store semi-finished product prior to shipping to customers who must thaw the material before molding parts.

They’ve also combined a lower cost heavy (50K) tow grade of carbon fiber (with properties more than adequate for automotive) that has been formed into a unidirectional, non-crimp fabric (NCF). This dry NCF impregnates faster and more easily than unidirectional (UD) dry spread fiber tapes, which are the usual intermediate step between rovings and prepreg. Further, by producing and impregnating a single layer at a time, very precise fiber/resin ratios are achieved, and voids are eliminated, yielding a high-quality product. 

Instead of forming the part via vacuum-bag or autoclave cure, InPreg material is designed to be processed in a standard compression press widely available in the automotive industry. Both forming and curing are done in the press, eliminating the time, equipment, space, and cost of the preforming step that normally precedes laminate molding. Eliminating steps, labor, and scrap, helps lower costs.

A key aspect of the process is access to the four-component epoxy resin system supplied by Huntsman Advanced Materials (Basel, Switzerland) that consists of resin (Araldite LY1556), hardener paste (Aradur 1571), accelerator paste (Accelerator 1573), and a polyamine-based hardener (Hardener XB3471). Pre-curing to achieve B-stage is done at 90ºC for 4 min. and final/full cure is done at 150ºC and 7.5-8 bar pressure for 10 min. Pre-impregnation and cutting/stacking steps can be done parallel to molding; since InPreg prepreg will remain stable for several days after production, molders can build inventory to feed the slower molding process without needing to freeze/thaw material before molding. 

An interesting follow-on study at F-ICT compared performance of comparable conventional (purchased) prepreg vs. InPreg prepreg molded via autoclave and vacuum-bag (both single-sided tools), and compression molding (matched-metal dies), as well as a third laminate produced for/used in wet-compression molding. The InPreg/compression molded material achieved performance comparable to conventional/autoclave-cured laminates and superior to vac-bag parts. It also was the second fastest process (after wet-compression) but had better properties and lower voids than wet-compression molding.

Current F-ICT calculations predict comparable parts in InPreg/compression molding will cost 45% more than wet-compression molding but one-eighth the cost of conventional prepreg/autoclave. To advance the project further, F-ICT seeks industry partners with suitable applications of significant volume to justify further work. 

Lowering Costs for Carbon Composite Parts

An attention-grabbing multi-piece set of carbon composite chassis components on the 2015 Zenos E10 sports car (from Zenos Cars Ltd., Norfolk, UK), was both a winner of the ACCE’s 2015 “Best Part” competition in the Materials Innovation category and the subject of the event’s final keynote, presented by inventor Antony Dodworth, chief technology & manufacturing officer, Bright Lite Structures, LLC (BLS, Peterborough, UK).

The components—front and rear bulkheads, floor pan, and left and right side body-side panels bonded together as a single large module around a central aluminum spine—make use of a patent-pending honeycomb-sandwich composite. It’s said to meet or exceed the vehicle’s compression, stiffness, and torsional rigidity requirements and yield a chassis module that is stiffer, lighter (15-20%), and has more design versatility than conventional carbon composite materials.

Parts are produced from simple rolled goods via wet-compression molding, which can rapidly mold deep-draw 2.5-D and 3-D designs that wouldn’t be possible without complex and expensive preforming operations in most other composite molding processes. The dry fiber/core/fiber reinforcement stack is laid up on a table, then carried via robot to the spray booth where both sides are coated with resin before being moved to the tool and manually laid in place and attached to a clamp frame. The tool closes in three distinct stages, allowing forming and curing to be done in a single step.

Interestingly, for manufacturers seeking lower costs and/or “green” technology, the Zenos panels use significant recycled content in both the honeycomb cores (made by BLS from recycled polycarbonate (PC) “straws”) as well as the fiber-reinforcement package (a proprietary blend of recycled chopped carbon fiber mat plus a multilayer NCF layup of other materials of varying densities for sections requiring higher mechanical performance).

A unique 70/30 blend of Vitrox polyurethane (PUR) resin from Huntsman Polyurethanes (Auburn Hills, Michigan, USA) and an unnamed RTM epoxy are mixed just before spraying with an AutoRIM five-component metering unit from Hennecke Corp. (Lawrence, Pennsylvania, USA). The PUR provides a strong physical-property profile, excellent adhesion to the honeycomb core, and outstanding impact strength/toughness for a given resin T_g. The epoxy improves crash energy management and seals micro-porosity in the foamed urethane to prevent moisture transfer. The urethane uses a novel catalyst, said to provide stable low viscosity (for rapid fiber impregnation) and tunable/long working times until snap cure, making it ideal for these large parts. The PC core is very stiff and has a high T_g that is well matched to that of the PUR/epoxy blend. 

Owing to significant parts integration, the design saves assembly time and costs vs. metals and monolithic carbon composites. Wet-compression molding is equally applicable for prototyping and high-volume commercial production, helping reduce component costs further so carbon composites become affordable for a wider range of vehicles.

Further, owing to significant use of recycled materials, and part geometry and nesting efficiency on the cutting table, the material/process combo uses less fiber and resin and generates less scrap than conventional carbon composite production systems, yet reportedly produces parts of comparable performance at lower total material costs. Additionally, capital costs are significantly reduced vs. autoclave, OOA, or preformed RTM processes owing to the simplified manufacturing process, press, and tooling, which can be produced in aluminum or steel. 

BLS is currently molding with 6-bar molding pressures at a 12-min. button-to-button cycle, a figure that is expected to decrease towards 6 min. as process automation improves. The company says their manufacturing cells can be quickly set up adjacent to OEM assembly facilities, and they can go from concept to production in under 12 months’ time. 

Note: Read more about both of these technologies and view 15 years of ACCE proceedings free at speautomotive.com/aca.