The term polyester is used to describe a number of polymers which contain an ester group in the polymer chain. This includes a wide range of materials, including natural and synthetic materials, and thermosetting and thermoplastic resins. For design engineers, there are two important thermoplastic polyester resins, polyethylene terephthalate (PET), and polybutylene terephthalate (PBT).

PET has a high melting temperature, and exhibits excellent chemical resistance. It can exist in an amorphous or semi-crystalline state, depending on how it is processed. The primary use of PET is in synthetic fibers for the textile industry (where it is usually described simply as polyester). The next largest use is in packaging applications, where it is used as a film or in rigid form for food containers, plastic jars and bottles, and beverage containers. PET packaging is easily recycled, and is frequently reprocessed into carpet fibers, fleece jackets, and insulating fill.

PET is also formulated as an engineering material for injection molding, usually with glass fiber reinforcement. First marketed by DuPont under the tradename Rynite®, glass reinforced PET offers exceptionally high strength and stifness, outstanding chemical resistance, and high temperature performance. PBT is similar to PET, with somewhat lower physical properties, a slightly lower in use temperature limit, but better impact performance. PBT was first marketed by GE Plastics under the tradename Valox®.

Both PET and PBT are demanding materials to mold. They require special attention to keep the material dry (both before and during the molding process), as well as special attention to the temperature profile of the melt from the barrel to the mold. The design of the injection mold itself is also important, as heat transfer in the mold is critical. The properties of the molded part are affected by the crystallization of the material, and this is crystallization is dependent on the heat transfer dynamics of the molding process.

In a final molded part PET & PBT have exceptionally low moisture absorption, a low coefficient of thermal expansion. This gives them excellent dimensional stability over a broad temperature range. When proper molding controls are implemented, PET & PBT parts can be molded with incredible precision. This makes them excellent candidates for miniature electronic components.

PET & PBT are frequently used as a replacement for die cast metal parts.  They are also used extensively in electronics applications – both as a replacement for epoxies and other thermoset materials, as well as housings and plugs and receptacles. They are also used as housings in high temperature applications – clothing irons, hot melt glue guns, etc.

NOTE: Polyethylene terephthalate is NOT related to polyethelene, and is more accurately described as poly(ethylene terephthalate).