Design News for Mechanical and Design Engineers

In 1999, IBM announced the world's first personal computer to be made from 100% recycled resin for all major plastic parts. The IntelliStation E Pro, a Windows NT-based workstation, contains three and a half pounds of plastic. The prime engineering resin was converted to 100% recycled plastic at no extra cost. In fact, one of the eight recycled parts of the system unit is now 20% less expensive to manufacture.

For more than a decade, manufacturers have placed increasing emphasis on evaluating the recyclability of their products. Through corporate initiatives, usually referred to as "Design for the Environment" or "Design for Demanufacturing," engineers are encouraged to consider how a product's components may be reused at the end of its life.

Designing for recyclability may not be appropriate for every product. Current material recycling technology and market prices are such that complete disassembly may not be profitable because the costs far outweigh revenues from the recovered materials and parts. But for those parts where it is practical, not only will such a focus garner respect for a company, but it can save money and lead to an overall better product.

Manufacturers of automotive reflectors, for example, can safely incorporate up to 25% of direct metalized ULTEM^®polyetherimide (PEI) part scrap without sacrificing functional or aesthetic properties. In economics, this translates into about 20 to 25 cents a part. "Our largest customers mold on the order of just under 4 million parts a year," says James Wilson, commercial technology manager for GE Plastics, developers of ULTEM.

Until 1998, the material of choice for reflectors was bulk-molding compound (BMC), a thermoset similar to an epoxy.

A typical molding operation may scrap approximately 5 to 10% of its parts. BMC parts that do not meet spec are usually landfilled, says Wilson, as BMC has a limited recyclability. "People talk about grinding it up and using it for concrete and stuff like that, but it really has very limited uses," he says. "There is no direct use back into the product of concern."

ULTEM, on the other hand, is an amorphous thermoplastic. "It has an excellent melt stability. So a manufacturer can reprocess ULTEM several times without causing a significant reduction in mechanical properties," says Wilson.

The recyclability aspect of ULTEM is growing as the aesthetic requirements for reflectors become more stringent. "If you look at a car that is 5 to 10 years old, the outer portion of the lamp is faceted like a shower door," says Wilson. "You can't see what is behind it." This older reflector technology used straight parabolic reflectors and focused the light beam with molded prisms on the plastic lens in front.

About five years ago, a major design trend prompted pristine-clear optics-free lenses. Now the light is focused with a complex faceted reflector. "Compared to old technology, you can see every tiny flaw," says Wilson.

For the older style reflectors, it didn't matter if there was a little dust inclusion in the metalized part, says Wilson. "This didn't affect performance and you couldn't see it in the aesthetics. Today, with optics-free lenses, some companies have seen their scrap rates double because of the aesthetic issues."

With direct metalized ULTEM resin, both molding scrap and metalized scrap can be reintroduced into the process feed-stream. "The net result is ULTEM resin can be a 100% yield product," he continues.

In addition to recycling, OEMs who use ULTEM find that the process requires fewer stages.

Wilson acknowledges that the cost differential between BMC and ULTEM is significant. ULTEM costs about five times that of BMC on a price-per-pound basis. "But the system cost savings advantage is just enormous," he adds.

GE launched its first domestic ULTEM program in 1998 with the Lincoln Town Car. By the end of 2000, GE had 7 million reflectors on the road. "By the end of 2003, we will have 23 million on the road," says Wilson.

Superplasticity. Aerospace and automotive industries are always on the look out for recycling potentials. With superplastic forming (SPF), manufacturers can create parts that are extremely complex in shape, but are also environmentally correct.

Superplasticity describes the ability of a material to withstand large amounts of elongation without the occurrence of necking. Typical materials used in SPF applications include Ti-6Al-4V titanium alloy and certain aluminum alloys. Here, material breakage is prevented with the strain-rate effects by causing the material to harden and then progressively adjust into an even pattern. During the SPF process, the material is heated to the SPF temperature within a sealed die. Pressure is then applied, forcing the material to take the shape of the die pattern. Since the flow stress of the material during deformation increases rapidly with increasing strain rate, the ability to deform the material uniformly requires a precise control of strain rate and strain rate sensitivity.

"The superplastic forming process can be successfully simulated using the finite element method," says Reza Sadeghi, product manager for MSC.Marc, a simulation package from MSC.Software. "In fact, making complex SPF parts requires it." Variables such as strain-rate can be examined more closely than can be done during the actual processing. The method can also be used to predict thinning, forming time, and areas of void formation. Ultimately, it can be employed for shape optimization, thus reducing the number of prototypes and forming trials required to produce an acceptable part. The combination of rigid-plastic flow formulation, frictional contact, and rate-dependent and grain-sensitive material models has demonstrated that the superplastic behavior can be fully simulated.