Thermoplastics: The Long View on Cost and Weight Reduction

By Pritam Das

Long Fiber Thermoplastics (LFT) is one of the fastest growing materials technologies in the composite industry. Expected to grow 30 percent annually in the next five years, LFT is challenging and even replacing metals and thermoset composites in a number of applications.

After successfully demonstrating its capabilities to the Federal Transit Administration (FTA), the University of Alabama at Birmingham (UAB) and the National Composite Center (NCC) collaborated to make a LFT battery box access door for North American Bus Industries (NABI). The production ready composite part replaced the bus manufacturer’s steel door delivering a 60 percent weight reduction and an estimated cost savings of 44 percent.

Based in Anniston, Alabama, NABI designs, manufactures and assembles buses with some of the lowest life-cycle operation and maintenance costs available.

“NABI was looking for ways to save weight,” said George Husman, Associate Director for Engineering Research at UAB. “Lighter weight structures improve fuel economy and road impact. We wanted to team with NABI to identify bus components for weight reduction because we knew the company was progressive and eager to look at new technologies that could support its business and the industry.”

Managed by UAB

The project, performed under contract with the FTA, was managed by UAB, which provided component design and analysis, process modeling and testing. Historically, NABI used a metal door built with a steel tubular frame attached to a steel skin. The part required welding of multiple components. Two pneumatic struts were required to hold the door open. UAB redesigned the door as a rib stiffened single component structure requiring just one, lighter weight strut. A systematic study was conducted to determine the optimum level of fiber reinforcement needed to meet NABI’s strength and stiffness requirements. Polypropylene (PP)/E-glass LFT with 40 percent glass fiber, produced by Ticona, was selected.

Initial tests showed that a single plasticated charge of E-glass/PP molten material located in the center of the tool resulted in incomplete filling of the mold. Warpage and print through of the ribs also challenged the team. Simulations revealed that two equivalent charges of E-glass/PP molten material would fill the mold, but result in knit lines. Iterative process modeling helped the team modify the charge placement to a single charge with greater mold coverage to achieve complete filling of the mold. The processing parameters were optimized to overcome warpage and print-through of the ribs.

“We are working to further develop this iterative process between flow molding modeling and component design because it allows us to understand the orientation of fibers and then optimize the structural properties and weight benefits up front prior to the cost of fabricating tooling,” said Husman. “We have found that this approach ensures that the first part is a good part. It also reduces the risk of applying new technology to a commercial part.”

Tool design required the front face (exposed face of the bus) to be machined to a highly polished surface with the back face rough machined with no finishing. Computer-Aided Design (CAD) drawings were submitted to NCC for fabrication.

NCC’s In-House Capability

“NCC’s rapid prototyping capability, quick commercialization skills and LFT manufacturing cell made it the ideal place to manufacture a production ready component,” said Husman. “For a lot of manufacturers interested in technology like LFT, the capital equipment costs alone can be prohibitive. NCC is able to help offset those development costs because they have the processing tools in place and the technical expertise to develop cost effective manufacturing processes for larger scale LFT structures.”

NCC tapped Dayton-based Estee Mold & Die Inc. to build a mold for the bus door measuring 24.5 inches wide, 44 inches long and eight inches deep. In addition to building the mold, Estee was required to provide a mirror finish. Estee machined the mold from two rough pieces of steel. The tool’s front face, the side to be polished, was P20 prehardened mold steel.

“Generally, the finish requirements that we were given for the bus door mold would typically be done for consumer parts that have optical requirements for characteristics like clarity,” said Steve Ponscheck, Program Manager for Estee. “The size of the mold to be polished was also unusual.

Estee built the mold in just eight weeks, devoting two weeks to achieving the mirror finish. The nine-step polishing process began with an 80 grit disc. Final polishing was done with coarse hairbrushes and a diamond paste. The mold was then delivered to NCC.

The Center used a CA Lawton Plasticator to produce and extrude the E-glass/PP molten material into the mold. A 400-ton compression molding press completed the manufacturing step. Due to LFT’s high viscosity, it can be difficult to achieve proper flow through a mold’s thin areas.

“We were able to further refine processing parameters by adjusting the pressure and temperature during the molding of test parts,” said Harry Couch, Program Manager for Long Fiber Thermoplastics at NCC. We found we could achieve optimum flow by using a temperature of 350 degrees Fahrenheit for the first three minutes on the bottom tool (polished finish) and 180 degrees Fahrenheit on the top tool. After the first three minutes we brought the temperature of the bottom tool down to 180 degrees Fahrenheit in 10 to 12 minutes.”

Since the battery door had to be painted to match the remainder of the bus, NCC had to develop a paint system for the polypropylene based LFT. Historically it is very difficult to paint or coat components made of polypropylene due to its low surface energy and unreactive surface chemistry which hinders adhesion and bonding.

“We initially tried paint system designed for Polypropylene using a surface adhesion promoter,” adds Couch. “But we found that the paint treatment had a corrosive affect on the glass fiber content. After researching the problem, we identified a new surface adhesion promoter made by Eastman Chemical. It helped the paint adhere to the polypropylene but didn’t corrode the glass fibers reinforcements in the LFT substrate.”

NCC supplied the finished battery door and paint procedure to NABI for final painting of the component. Prior to painting, NABI conducted fitment trials. Hinges and door handles were secured to the door and the fitment conducted on a BRT bus at NABI’s Anniston plant. The door fit the aperture perfectly and when locked in position provided adequate sealing. The Center’s recommended paint procedure also made it possible for NABI to achieve a high quality finish.

The new LFT battery door exceeded customer expectations and delivered the additional advantage of being corrosion resistant. NABI is currently field testing the composite door.

Into Air Conditioning

In the final year of its contract with FTA, UAB and NCC are working with NABI to develop Air Conditioner roof door panels for the company’s BRT bus.

“It’s a larger part and may require a different manufacturing process,” said Husman. “We may also explore the possibility of molding in the color versus painting the component.”

Aside from performance benefits, resources like UAB and NCC help provide less costly design and manufacturing processes making it possible for manufacturers to mine new technology fields and capture near-term commercialization for part applications without having to invest in large capital equipment expenditures and training for new skill sets. The ability to partner with industry experts also promotes market acceptance of new technology developments more quickly – helping to advance and grow composite applications.

Pritam Das is an application development engineer of composites materials at NCC. He specializes in thermoplastic composites applications. He can be reached at pdas@compositecenter.org.