ATLANTA - A unit of Motorola Inc. expects a two-year development project with Sajar Plastics Inc. soon to begin yielding Motorola's first cellular communications products made using gas-assisted injection molding, according to representatives from both companies. Other product conversions by Motorola from metal to plastics are likely to spring from the success of this project.
Plans call for Sajar in May to use Nitrojection Inc.'s Nitrojec-tion gas-assist process to start producing an estimated 25,000 components in the next year for a power amplifier used in base stations for cellular telephone networks, according to Scott Simpson, vice president of sales for the Middle-field, Ohio-based molder. The polycarbonate and PC/ABS parts will replace die-cast aluminum components that long have been the standard for such power amplifiers.
This project, which involved much convincing of skeptical Motorola corporate officials, has demonstrated that gas-assisted injection molding can produce ``a much cheaper and lighter-weight part that is just as robust as a die-cast part,'' said Motorola project manager Joyce M. Sprau.
Sprau, a senior mechanical project designer for Motorola's Cellular Infrastructure Group in Arlington Heights, Ill., noted that the process also offered advantages over structural foam, which she investigated first.
Even though Motorola's consumer product portfolio consists of millions of plastic parts for such items as pagers and cellular telephones, Sprau said she encountered initial resistance to the concept of substituting plastic for metal in an infrastructure product.
The power amplifiers each are about the size of a videocassette recorder, and consist of a housing with a molded-in handle, two cover panels and two seismic latches.
Each unit must have the strength to hold 35 pounds of equipment, while also passing stringent tests for ruggedness. As many as 24 of the units fit like drawers into a large frame within each cellular base station.
``My group makes products that nobody sees - large, structural parts,'' she said in an April 2 interview at the SPI Structural Plastics Division conference in Atlanta, where she gave a paper on her product's development. ``Nobody wanted to try plastics, until they put me on the project.''
Sprau said she first learned about gas-assisted injection molding from her resin supplier, Bayer Corp., which also suggested she discuss the design project with Sajar.
The early talks with Sajar started with very basic sketches, and developed from there, with Bayer providing the mold-flow and structural analysis, she said.
The key, Sprau told meeting delegates, was developing a reliable prototyping method compatible with the gas-assist process - to optimize gate sizes, locations and gas channels that would eliminate major changes and cost in the final tool.
The project team assessed various options - including selective laser sintering and liquid silicone rubber molds - but ended up using spray metal molding to produce its prototype tooling. With a delivery time of four weeks and being easily adaptable to gas-assist molding, the spray metal molds did the trick.
The only disappointment, Sprau said, was that the use of 10 percent glass-filled PC to make the large housings was hard on the molds and shortened their useful production life to just 60 pieces.
Still, it proved enough to allow the project to go forward.
Nevertheless, Motorola's management was so concerned that the unproven production molds would turn out bad parts that they wanted a backup prototype mold built, just in case.
``I talked them out of it,'' said Sprau, who was confident the tools would deliver quality parts. She said Diamond Tool and Die in Dayton, Ohio, built the production tools, handled late design changes and still delivered the final molds before deadline.
The early parts have been good, Sprau said.
``Hopefully,'' she added, ``in the future we won't have to do half the prototyping as we did [on this project]. There should be more confidence.''
The amplifiers will be put to their first real test in late April, according to Sprau, when an entire cellular base station will be fitted with them for a few weeks in Minneapolis, ``and we'll see if we can make phone calls.''
Sajar plans to use the Nitrojection process to make the glass-filled PC housings on a 500-ton injection press, and the PC/ABS covers on a 400-ton press. It will produce the PC latches using conventional, solid-wall injection molding on a 230-ton press, said Simpson. Sajar also is assembling and sonic welding the necessary metal inserts.
Sprau said the anticipated first-year's output will be both for original-equipment and replacement amplifiers, destined for use in U.S. and Japanese markets.
``Early on,'' Sprau said, ``we had to take baby steps, because management lacked confidence in the plastics conversion process.''
But this project has helped immensely - and ``absolutely'' will prompt Motorola to investigate further metal-replacement opportunities for infrastructure products. This could include, for example, the sheet-metal cages that currently hold the power-amplifers' frames in the base stations.
``I'd like to look at gas-assist for that,'' she said.