By Design: Designing with polycarbonate

If you need a transparent engineering plastic with high heat and impact resistance, polycarbonate is a good choice.

If you handle a compact disk today you will be touching polycarbonate (PC). To date PC is the only material capable of filling the demanding requirements of that application.

In 1859 the Russian chemist Butlerov described a PC type of material. This discovery was repeated by Einhorn in the late 1800s. Fifty years passed before these discoveries were seriously pursued by General Electric in the U.S. and Farbenfabriken Bayer in Germany. Both announced pilot plant quantities in 1956. One hundred years after Butlerov’s discovery, Bayer was producing commercial quantities under the trade name Makrolon. GE followed with Lexan in 1960 and Dow introduced Calibre in 1984.

There was a waiting market for this unique plastic material. Worldwide consumption reached 40 million lb/year in 1970, and 218 million lb in 1980. This rapid growth is testimonial to the material’s usefulness. Today PC is second only to nylon in volume and is now the fastest-growing engineering polymer.

Designing Characteristics

Polycarbonate is an amorphous thermoplastic that combines transparency with high temperature and impact resistance. There is no other engineering plastic with this combination of properties. There are different types of PC, but a high-viscosity grade can be defined as follows.

Physical properties are around a tensile strength of 9000 psi; flexural modulus is 340,000 psi, with a heat deflection temperature of 270°F at a 264-psi loading. Strength and temperature resistance can be increased with the use of fillers and reinforcements.

Most polycarbonates have a notched Izod impact strength in the range of 12 to 17 ft-lb/in, which is retained at low temperatures. Polycarbonates are highly notch-sensitive materials. They actually have higher impact strength than indicated by the standard notched Izod impact test. For example, a .125-inch-thick, right-angled-shaped part with an inside corner radius of .010 inch had an impact strength of 2.5 ft-lb. Increasing that radius to .020 inch resulted in an impact strength of 20.2 ft-lb. In other words, doubling the size of the radius increased the impact strength by a factor of eight. This is the reason why designers make such a fetish of radiusing the corners on PC parts.

Light transmission is 86% to 89%. This is just below acrylic at 91% to 92% and general purpose polystyrene at 88% to 91%.

Flame-retardant grades are available with a UL 94 V-0 and 5V ratings.

Polycarbonate has an excellent balance of physical properties, but it lacks the chemical resistance of semicrystalline polymers. Specifying PC requires careful attention to the chemical environment of the application.

Polycarbonate is also available alloyed with ABS, acrylic, polyetherimide, polyurethane, PBT, and PET polyesters. The base polymer and all of these alloys can be tailored for specific applications with the addition of fillers and/or fiber reinforcement.

Polycarbonate fills the gap between ABS and PPO and the higher-temperature-resistant and more costly materials such as polysulfone, polyetherimide, polyphenylene sulfide, and liquid crystal polymers. The list price for injection molding grades of PC was $1.96/lb in 1988. Today the material costs $1.38 to $1.65/lb, or an average of $.065/cu in. These prices are, however, increasing.

Applications

GE is a major user of electrical insulating materials. The good electrical properties of PC are one of the reasons why GE originally pursued the development of this material. A favorable UL rating, plus low smoke and corrosive gas emissions, accounts for PC’s use in telephones, computers, printers, copiers, other business machines, and laboratory and diagnostic equipment.

The combination of transparency, coupled with weatherability and impact strength, allows PC to be used for bulletproof windows, machine guards, lighting applications, and as window panes, especially in those instances where vandalism is a problem. Other transparent applications include greenhouse glazing, optical safety lenses, solar collectors, and automobile head and taillight lenses. A large new application just now being commercialized is side and rear car windows.

This FDA-sanctioned material has a long history in the food handling industry as processing bowls, mugs and glasses, tableware, pitchers, storage containers, baby and water cooler bottles, and in some instances as microwave cookware. Many of these applications rely on PC’s transparency and temperature resistance.

The amount of PC used in the construction industry is second only to PVC. Other markets include medical products, toys, portable tools, and photographic and sporting equipment, especially in applications where low-temperature impact strength is important.

PC part design tips

 

  • Wall thicknesses of .012 to more than 1 inch have been molded with PC. However, a minimum of .050 inch and a maximum of .375 inch are recommended, with .125 inch being ideal. Flow lengths of 2 inches with a .030-inch thickness and a 16-inch flow can be achieved with a .125-inch wall. With proper blending, wall thickness variations can be as much as 25%. At thicknesses somewhere between .140 and .160 inch, PC’s room temperature notched Izod impact strength declines with a change from a ductile to a brittle type of failure.
  • Radiusing corners improves melt flow and allows notch-sensitive PC to develop its impressive impact strength. An inside corner radius of 60% of the part’s wall thickness is ideal. The absolute minimum is .020 inch.
  • Molding draft angles of ½° to 1° per side will suffice for most PC parts, but there are exceptions.
  • Projections of all types are molded with PC. In order to avoid sink marks and molded-in stress, the thickness of projections should be limited to a maximum of 60% of the part’s wall thickness.
  • Depressions, or holes, create weldlines. Acceptable appearance and tensile strength retention of 99% can be achieved with proper molding conditions. Glass-fiber-reinforced PC can suffer a loss of 35% to 45% of its tensile strength at weldlines. 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 PC parts can be as small as ±.0025 inch on a .125-inch-thick, 1-inch-long part. A standard, less-costly tolerance on the same part would be ±.004 inch. This amorphous material, which shrinks uniformly parallel and perpendicular to the direction of flow, is frequently specified for precision parts requiring a minimum allowable warpage.

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