By Design: Designing with polyesters

The unsaturated polyesters introduced in 1939 account for the bulk of the impressive work being done in the reinforced thermosetting or composites industry. A small amount of these materials are injection molded, but this article covers the larger-volume thermoplastic polyesters.

Fully saturated thermoplastic polyesters were patented in 1941. The two primary types of polyesters are polyethyl?ene terephthalate (PET) and polybu?tyl?ene terephthalate (PBT).

In this recurring 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 reach him at glennbeall [email protected].

PET materials were commercialized in 1953 for oriented film and fiber. This unique material has the ability to be fully amorphous or highly crystalline. With crystallinity rates of up to 60%, this material was at first not suitable for injection molding. The moldable grade that became available in 1966 led to the injection molding of billions of preforms that were stretch-blowmolded into oriented carbonated beverage bottles. Many PET products are now injection molded, but bottle preforms are still a major market.

In the late 1960s, Celanese (now Ticona) introduced a melt-processable, semicrystalline PBT polyester. Neat PBT was unimpressive, but the addition of glass-fiber reinforcement produced a useful injection moldable material that captured many applications. Today, fiber content varies from 15-50%, with 30% being the most common.

Defining characteristics

Up until the early 1990s, PBT was the polyester of choice for injection molding. Injection molding of bottle preforms was an exception. That large preform market prompted the continuing development of PET materials that were better suited to the injection molding of other products. At about the same time, manufacturers, who were trying to improve productivity, noticed that PET cost less than PBT. This was especially true for recycled bottle material, which became available at costs as low as $0.39https://www.plasticstoday.com/lb.

As molders gained experience, it became obvious that PBT was not as strong as PET. Like PBT, the unimpressive properties of neat PET limited its use. And as with PBT, the addition of glass fibers produced a PET that could compete with other engineering materials.

Referring to the physical properties chart on p. 30, the heat deflection temperature and tensile, flexural, and impact strength of reinforced PET surpasses that of PBT. Under normal circumstances, PET’s cost and better properties would have crowded PBT out of the market. By the time moldable PETs became available, PBT was well established. In spite of its easy flow, PET was more difficult to injection mold. In order to avoid a loss in molecular weight and a reduction in physical properties, PET had to be dried to a moisture content of 0.005%. Many custom injection molding operations did not have dryers that were suitable for PET.

All of the polyesters exhibit good chemical resistance. However, they should not be used in water at temperatures above 125ºF. A resistance to gasoline, oil, glycols, and alcohols accounts for under-the-hood automotive applications. Amorphous, unreinforced PET is the only transparent polyester.

Polyesters are the best of the reinforced thermoplastic materials for producing good-looking, resin-rich surfaces. Fire-retardant grades of PET and PBT are available with a UL 94 rating of V-0.

Part design tips

Wall thickness. Recommended thickness ranges from 0.030-0.125 inch. Thicker walls make it difficult to control crystallinity. Flow lengths are in the range of 2 inches with a 0.030-inch-thick wall and 16 inches with a 0.125-inch-thick wall. Thick walls cool slowly and that increases crystallinity. As a result, changes in wall thickness must be blended and kept to a minimum.

Radii. Radii distribute stresses over a broader area, increasing a part’s strength. Polyesters are notch-sensitive materials and should have large radii. Some recommend an inside radius of 0.020 inch; in my opinion, radii should be a minimum of 25% and preferably 50% of the part’s wall thickness.

Molding draft angles. Unreinforced polyesters can normally be molded with a 0.5ºhttps://www.plasticstoday.com/side draft angle. More abrasive reinforced polyesters require a draft of 1ºhttps://www.plasticstoday.com/side. Textured surfaces require a draft of 1ºhttps://www.plasticstoday.com/side plus 1º for each 0.001 inch of texture depth.

Projections. Many special features such as stiffening ribs, solid bosses, and snapfit latches are projections off of a part’s nominal wall. Projections should be limited to a thickness of 50% of the part’s wall thickness. Cavity filling will be improved by providing radii of at least 0.020 inch at the junction of a projection and the part’s nominal wall thickness.

Depressions and holes. Weldline strength and undesirable appearance are the main problems associated with molding holes and depressions. As with all fiber-reinforced materials, there is a significant loss of strength at weldlines. Part geometry and gate location must be carefully considered in order to avoid locating weldlines in heavily loaded areas.

The appearance of weldlines can be improved with proper molding conditions. Hotter cavity temperatures are especially helpful. The high-pressure flow of the melt through a cavity can bend the core pins that form small holes. Limiting the depth of holes to two to three times the diameter of the core pin will minimize core deflection.

All inside corners on holes should have the standard radii. As the material shrinks, it grips the core pins. Providing molding draft and a smooth polish on these core pins reduces ejection force, which allows shorter molding cycles.

Tolerances. One of the most important factors affecting attainable tolerances is the material’s mold shrinkage rate. For a 0.125-inch-thick unreinforced part, the average shrinkage rate will be 0.020 inhttps://www.plasticstoday.com/in. The shrinkage will be slightly greater in the perpendicular-to-flow direction. Thirty-percent fiber-reinforced parts will shrink 0.004-0.006 inhttps://www.plasticstoday.com/in in the direction of flow and 0.006-0.008 inhttps://www.plasticstoday.com/in perpendicular to flow.

An unreinforced 1-inch-long, 0.125-inch-thick polyester part can have a fine tolerance of ±0.0030 inch and a commercial tolerance of ±0.0043 inch. The tolerance on the same 30% fiber-reinforced part would be 0.0018 and 0.0037 inch, respectively. The fine tolerances should only be specified when the part’s dimension is more important than its cost.