Sponsored By

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.

Glenn Beall

September 26, 2002

5 Min Read
By Design: Polystyrene part design

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.

The first polystyrene (PS) was a natural material distilled from tree resin by the French chemist, Bonastre, in 1831. Another French chemist, Berthelot, is credited with producing the first man-made PS based on the synthesis of ethylbenzene in 1869. No commercial uses were found for this material until production started in England in the early 1930s. In 1937, Dow Chemical Co. introduced PS to the North American market, and today PS is a major injection molding material. Approximately 8 percent of the plastic produced in the U.S. is PS—the fourth largest in volume.

Polystyrene is a senior citizen member of the plastics industry. It is intellectually stimulating to work with the recently introduced materials such as Ultem, liquid crystal polymers, or the new alloys and blends. But well-established materials have the advantage that only comes with experience. These older materials are fully developed, well understood, and provide fewer surprises than the new materials.

The first polystyrenes were crystal clear, rigid, brittle, amorphous thermoplastics that became known as general purpose polystyrene (GPPS), or simply GPS. In volume, their published selling price is $.42/lb. With an average density of only 1.045 g/sq cm, the cost is $.0150/cu in. This is a low cost for a transparent material with a flexural modulus of 500,000 psi and a tensile strength of up to 8200 psi. However, this material’s Achilles’ heel is its low notched Izod impact strength of .25 to .45 ft-lb/in.

The brittleness of GPS limited its early use. This deficiency was overcome in the late 1940s by the addition of rubber to produce high-impact polystyrene, or HIPS. Notched Izod impact strength increased to .9 to 4.1 ft-lb/in, but there was a price to be paid. HIPS costs $.46/lb and $.0165/cu in. Flexural modulus and tensile strength declined to 260,000 to 370,000 psi and 2325 to 6000 psi, respectively. Worst of all, HIPS lost GPS’s transparency and lustrous surface finish.

Today, packaging accounts for approximately 30 percent of the PS used in the U.S. Consumer electronics and appliances, notably televisions, radios, air-conditioner housings, and cassettes, use 9 percent. Housewares and furniture use 8 percent and toys consume another 7 percent.

Designing With PS
In commercial use for 65 years, polystyrene’s stiffness, transparency, low cost, and especially its ease of processing make it a favorite for injection molding. It would be reasonable then, but a mistake, to assume that everyone knows how to design parts for good old PS. The impressive growth of the plastics industry results in new designers entering the field each year, and many of these newcomers have yet to have their first experience with PS. In this regard the following design guidelines will be helpful.

  • Wall thickness minimums can be as little as .010 inch for small parts and disposable packaging items. One supplier indicates an impressive flow-length-to-thickness ratio of 150:1.

    PS is a low-mold-shrinkage-factor, amorphous material. Theoretically there is no limit to the maximum thickness that can be produced. Thick-walled parts are, however, only practical when function is more important than cost, and when the mold is hot enough and the gate is large enough to allow continuous cavity packing during an extended cooling cycle.

    The preferred wall thickness for GPS and HIPS is a minimum of .030 inch and a maximum of .250 inch. Although not desirable, PS parts can tolerate wall thickness variation of up to 25 percent. As always, changes in thickness should be gradual.

  • Radiusing inside corners on GPS parts is critical, as this is a hard, brittle material. The minimum corner radius should be 25 percent of the part wall thickness. The higher elongation and impact strength of HIPS make it more tolerant of small inside radiuses. Maximum strength is achieved in both materials with a radius of 75 percent of the wall thickness.

    Outside corner radiuses should be equal to the inside radius plus the wall thickness. These proportions produce a uniform wall thickness around the corner. This, in turn, produces uniform cooling and mold shrinkage with a reduction in molded-in stress and warpage.

  • Molding draft angles and a smooth polish are mandatory with GPS due to this material’s hard, brittle surface as demolded. The inside surfaces of parts should have a draft angle of at least 1/2 degree  and preferably 1 degree per side. The low mold shrinkage of PS does not allow it to shrink very far away from the cavity during cooling. A 1 degree per side draft angle is recommended on outside surfaces. The softer nature of HIPS makes it more tolerant of minimum draft angles.

  • Projections such as stiffening ribs, bosses, gussets, and standing walls can have a thickness equal to 75 percent of the wall to which they are attached. Projections with greater thicknesses can result in sink marks and molded-in stress. In those instances where the avoidance of sink marks is mandatory, it is desirable to reduce the thickness of a projection to only 65 percent of the part’s wall thickness.

  • Depressions, or holes, are easy to produce in PS. This easy flowing, amorphous material is capable of producing good-looking, strong weldlines. GPS is more challenging in this regard, as the best of weldlines will deflect light as it passes through a transparent part.

    Draft angles are required along the depth of holes in order to facilitate easy part ejection. Irregularly shaped holes must avoid sharp inside corners that can produce molded-in stress.

  • Tolerances for GPS and HIPS are the same. A .125-inch-thick part that is 1 inch long can be held to a commercial length tolerance of ±.003 in/in. A fine tolerance would be ±.002 in/in.

    The low and uniform mold shrinkage of PS renders it a dimensionally stable material. While using materials of this type there is a natural tendency to take advantage of the situation and specify a fine instead of a commercial tolerance. This understandable urge must be resisted, as a fine tolerance can result in higher mold and molding costs. There are always exceptions to this rule, but the ideal tolerance is the largest tolerance that produces an acceptable part.

Sign up for the PlasticsToday NewsFeed newsletter.

You May Also Like