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In this bimonthly column, Glenn Beall of Glenn Beall Plastics Ltd. (Libertyville, IL) shares his special perspective on issues important to design engineers and the molding industry.You can plate it, leave it in water, and use it in electrical applications with no burning. If you need something better than ABS, go with PPO.

Glenn Beall

October 1, 2004

4 Min Read
By Design: Designing with PPO

In this bimonthly column, Glenn Beall of Glenn Beall Plastics Ltd. (Libertyville, IL) shares his special perspective on issues important to design engineers and the molding industry.

You can plate it, leave it in water, and use it in electrical applications with no burning. If you need something better than ABS, go with PPO.

The initials PPO are the easy way of saying polyphenylene oxide. This is a misnomer for polyphenylene ether, or PPE, which is the European designation. The original PPEs found limited applications due to their relatively high cost. The material was also susceptible to thermal degradation at processing temperatures.

In response to these limitations, General Electric’s (GE’s) John Hay discovered that PPE was completely miscible with polystyrene (PS). Alloying PS with PPE produced a lower-cost material with a broader processing window. In 1966, GE Plastics reintroduced this new material as a PPO under the now well-known trade name of Noryl. This polymer found many applications, but it was a single-source material. Some large potential users, such as the car and computer companies, were apprehensive about specifying a material that was only available from one supplier.

This marketplace opportunity was recognized by Borg Warner (BW), a highly respected supplier of Cycolac ABS materials. In 1982, BW introduced Prevex, a PS-modified PPE with similar properties. This material found a waiting market. Both Prevex and Noryl continued to expand their market share. Sales were 29 million lb in 1970 and expanded to an impressive 227 million lb in 1998. Sales in 2003 are estimated at 300 million lb. GE purchased BW’s plastics business in 1988.

Defining Characteristics

The styrene-modified PPOs are amorphous, opaque, engineering materials known for their stiffness and impact strength coupled with good temperature resistance. Varying the percentages of PPE and PS allows this material to be adapted to many different molding processes and market applications. Filled and fiber-reinforced grades are available, some with Underwriters Laboratories’ 94 flame retardancy rating of V0 and 5V. General purpose, injection molding grades are priced at $1.80/lb or $.068/cu in in truckload quantities. There are too many grades of PPO to be summarized here. One popular flame-retardant injection molding grade of Noryl (N 190) has a tensile strength of 7000 psi and a flexural modulus of 3.25 x 105 psi, with a notched Izod of 7 ft-lb/in and a heat deflection temperature of 205°F at 66 psi. Mineral-filled and glass-fiber-reinforced grades have much greater tensile and flexural strength with higher temperature resistance. Modified PPO is a bridge material that successfully fills the gap between polycarbonate and ABS.

Typical Applications

This versatile material finds uses as communication, medical, appliance, and business equipment and housings. Transportation applications include wheel covers, instrument panels, grilles, and electrical components. This material’s low hydrophilic characteristics allow its use in long-term contact with water for pumps, sprinklers, fittings, tanks, pipe, and water meters. Electroplated parts are used for EMI/RFI shielding, appearance-type plumbing applications, and as automotive and appliance trim. PPO’s excellent electrical properties and nonburning characteristics combine to allow its use as junction boxes and covers, coil bobbins, wiring terminal boards, fuse blocks, fire alarms, exit sign housings, and current-carrying switch gears.

In those instances when ABS or impact styrene is just not quite good enough, PPO is the next best choice.

Tips for designing with PPO

  • Wall thicknesses of .060 to .180 inch are ideal for PPO. Thicknesses of .030 to .375 and even .500 inch have been molded. Flow lengths of 10 inches can be molded with a .060-inch wall. Walls .125 inch thick can be up to 25 inches long. A tolerable wall thickness variation can be 25% with proper blending from thick to thin.

  • Radiuses improve melt flow and reduce stress. A good inside corner radius for PPO is one-half of the part’s wall thickness and never less than .015 inch.

  • Molding draft angles of ½° to 2° per side are recommended for PPO. Textured surfaces require a draft angle of 1½° plus 1°/side for each .001 inch of texture depth.

  • Projections such as ribs, bosses, and gussets on parts with walls less than .125 inch can be 50% to 60% of that wall thickness. These values should be reduced to 40% to 50% on parts with thicker walls.

  • Depressions, or holes, create weldlines. With proper molding procedures, these weldlines will not cause appearance or abnormal strength problems. Weldlines are rarely as strong as the surrounding material. They should be located in low-stress areas. Holes require standard molding draft and corner radius considerations. Limiting the depth of holes to two to three times the thickness of the core pin eliminates pin deflection.

  • Tolerances on PPO parts can be as fine as ±.001 inch on a .125-inch-thick, 1.000-inch-long part. A less costly commercial tolerance on the same part would be ±.002 inch. There are always exceptions and not all parts can maintain these tolerances all of the time. This is a low-mold-shrinkage material that shrinks uniformly in all directions. PPO is frequently chosen for large, precision parts that require a minimum of warpage.

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