Materials Update 19934

Replacing metal with plastic in thousands of applications over the past several decades has provided opportunities for many products. Now, after several years of R&D, it appears that a replacement for lead has become a reality in the marketplace as heavy-weight composite materials—tradenamed Ecomass—are beginning to appear.

Ecomass compound consists of a polymeric binder such as nylon, PE, PPS, PEEK, or TPE with a filler—tungsten, copper, iron, or stainless steel. A technology transfer company, Ideas to Market (I2M, Austin, TX), in 1997 acquired the patent for the compound and subsequently granted PolyOne an exclusive worldwide license to manufacture and sell the compound through a joint venture.

The potential to provide a replacement for lead in a variety of applications promises some long-term gains for OEMs and molders that master the processing of this material. But it’s not without its challenges. Ecomass is 96 percent tungsten by weight and 50 percent plastic by volume. It is physically heavy—a 1-gal pail of Ecomass weighs 35 lb, with composite densities ranging from 6.0 to 11.0 g/cu cm, which makes many molders want to avoid it.

Upcoming Applications
Atlas Precision Inc. (Arden, NC) is one custom injection molder that’s not afraid of looking for materials and products that promise unique niches and growth potential. Ecomass is one of the materials Atlas is working with to help customers mold new products.

Two products, currently in clinical trials, include a syringe shield and carrying case invented by William Patton McDonald, chairman and chief scientific director for Molecular Diagnostics of America Inc. (Birmingham, AL). The syringes are used to inject patients with radioactive isotopes for use in positron emission topography (PET) imaging.

PET, a nonsurgical, computer-aided imaging technique, is used for diagnosing coronary artery disease, cancer, and other disease states by taking a 3-D picture of metabolism on a molecular level. The syringe shield, along with a carrying case for transporting the syringes, will reportedly offer protection to both the technician and the patient from the radioactive isotopes by eliminating gamma radiation leakage. It also negates needle stick injuries of health care workers.

Additionally, the newly designed syringe shield is more ergonomically friendly and cost efficient to produce than current syringes, which use clamps machined of tungsten or steel.

Such shielding has traditionally fallen in the realm of lead, but long-term exposure poses a health threat, and its use in paint and gasoline has been banned for many years. As a result, more industries are looking for replacement materials. A radiation shielding grade, Ecomass RS, is said to provide full and partial radiation shielding without the hazards associated with lead. It’s made with a combination of tungsten powder and barium sulfate.

Tooling, Molding Considerations
Tooling for Ecomass requires high precision for the entire mold—not just core and cavities, explains Robert Bulla, marketing and sales manager for Atlas. Tolerances are critical and wear is a concern, especially in gate areas. “A valve-gated hot runner system needs to be used because you’re trying to eliminate a lot of wear areas in the tool. Erosion can be an issue,” he says.

It’s recommended that any project considered for Ecomass be prototyped first. “Because they’ve not dealt with these densities before, customers might not know what they’re up against,” advises Bulla. “For example, there can be no sharp corners in the part design.”

However, Jack Turner, PolyOne’s market development manager for Ecomass, notes that the material has considerable design flexibility in the selection of metal powders and choices of substrates for increased functionality such as temperature range, application environment, impact, and post-molding finishing, including plating, painting, or powder coating.

In the molding process, the wear on the screw, barrel, and tips is reportedly comparable to other aggressive compounds such as glass-reinforced nylon and PP. Conventional metals and surface treatments for these materials are also effective with Ecomass.

Handling the material is another consideration from a safety standpoint. As the material is very heavy (bulk densities up to 350 lb/cu ft), care must be taken moving it from the pallet into the hopper, and then from the hopper to the feedthroat. “The average loader can’t pick up one pellet,” says Bulla.

Also, you can’t use a hopper that holds 600 lb of ABS and fill it with Ecomass. “It won’t hold up, obviously,” cautions Bulla, adding that drying the material is also a challenge.

Atlas has invested considerable time in the processing of Ecomass. “It processes fairly well, but you have to take into consideration that you’re dealing with a very heavily loaded material that has the ability to wick heat out of the barrel,” says Bulla.

Turner adds that the material is still basically a thermoplastic. “Even in its uniqueness, it still flows, reacts to shear, and shrinks like plastic,” he says.