Furniture maker Steelcase Inc. in Caledonia, Mich., loved the idea of a simple stacking chair with a seat and back that can move independently to fit the person sitting on it. The chair would be too expensive to make in aluminum and too heavy with Steelcase’s traditional bent-steel tubes. So Steelcase set out on a two-year R&D program to design the chair in plastic. It arrived at a novel combination of gas-assist molding with a long-glass composite.
Steelcase’s injection molding subsidiary, Attwood Corp. in Grand Rapids, Mich., helped develop the “gas-and-glass” process to make the chair. Attwood makes primarily boat and leisure seating and had some prior experience molding long-glass and nylon for Steelcase’s “Leap” swivel chair.
The design team first looked at using cored-out parts with ribs to achieve the required stiffness and weight. Unfortunately, “a cored-out part always has an ugly side,” says Steelcase engineering team leader Robert Battey.
That’s when Steelcase considered the possibility of gas assist to make hollow rigid parts. Steelcase had licensed gas-assist technology two years before from Cinpres (now Cinpres Gas Injection Inc.), Ann Arbor, Mich. According to Cinpres, gas assist has been used with short-glass compounds to make parts for snowmobiles and wheelchairs. These had only 1- to 3-mm fibers and were black, non-cosmetic parts with fibers showing on the surface. Cinpres knew of no applications of gas assist with long glass and wasn’t sure it would work.
Gas and glass
Attwood’s senior production engineer, Michael Kemen, was convinced it could be done. He spent a year of intensive Design of Experiments (DOE) to learn where the fibers went during molding of developmental parts. Eventually, his team came up with a modified screw and the know-how to use Cinpres’ new Plastic Expulsion Process (PEP) without breaking the long glass. PEP fully pressurizes the mold before expelling some melt into an overflow cavity (see PT, June ’02, p. 37).
The stack chair consists of two types of parts: the tubular frame and dual-material seat. Steelcase believes both are world “firsts.” The tubular frame, which also becomes the legs, is believed to be the first combination of gas-assist with long glass. It uses 11-mm-long fiberglass in a Verton nylon 6 compound from LNP Engineering Plastics Inc., Exton, Pa. Most of the frame is 35% glass, but 50% glass is used in the legs for extra support.
The tubular parts are also believed to be the first long-glass parts that achieve a cosmetic surface without painting or finishing. They meet a Class B1 matte finish. This was achieved by CNC machining of the molds, according to Steelcase sources. Another breakthrough, LNP says, is the decision to show off the resin-rich surface in five custom colors, whereas long-glass parts are usually black or natural.
The real surprise is that these tubular parts aren’t hollow. They have 1/8-in. to 1/4-in. walls and a center channel filled with a low-density network of glass fibers plus a little nylon and a lot of air. “This fibrous matrix takes up 30-40% of the space. It works like bones. In fact, we call it bone marrow,” says Battey. “It adds stiffness and cuts down on deflection. A part made with this low-density core is actually stronger than a solid part and much stronger than a hollow part.”
The chair seat and back are made with a two-shot process plus gas assist. The first shot is soft, unfilled PP to form the soft-touch surface of the slotted seat or back. Then the platen rotates 180° and closes a second time against a new mold half to form a rigid rim of 30% short-glass-filled PP. This second shot is hollowed out with PEP gas-assist.
Steelcase decided not to mold the chair in-house but to use custom molders with gas-assist experience that also mold a lot of reinforced materials, and thus have leverage on prices. Morton Custom Plastics in Lebanon, Ky., molds the tubular composite frame, while GAC Plastics LLC in Petoskey, Mich., molds the dual-shot seat and back.
Steelcase intends to expand the Cachet line into a suite of office furniture. “Our knowledge is astounding compared to two years ago,” Steelcase’s Battey says. “All that’s now in our tool kit for future projects. We will keep pushing the envelope.”