Designing With Acetal

By Glenn Beall

Related links: Features, Properties & Processing information for Acetal

How to work with this stiff, creep-resistant, self-lubricating, dimensionally stable engineering material.

1959 was an eventful year. Xerox introduced the first commercial copier. American Airlines launched commercial jet flights. Fidel Castro installed the first communist regime in the West. Alaska and Hawaii became states. The United States and Canada completed the St. Lawrence Seaway linking the Great Lakes to the Atlantic Ocean. And the DuPont Co. started producing Delrin.

In 1859 the Russian chemist Butlerov produced a brittle, white solid from formaldehyde. This was the precursor for polyoxymethylene or what we know as acetal. Nearly a century would pass before the industry produced the chemically pure formaldehyde required for high-molecular-weight polymers. In 1952 DuPont put Robert Macdonald in charge of a project to develop new plastic materials based on formaldehyde. A major technical problem was the material’s tendency to decompose to gaseous formaldehyde at molding temperatures. Thermal stability was improved with the development of techniques for end-capping the molecules. DuPont made acetal homopolymer commercially available in 1959 under the trade name of Delrin.

In 1962 Celanese Corp. (now Ticona) introduced an acetal copolymer under the Celcon trademark. In 1990 DuPont added a copolymer to its offering. The average cost of these two acetals in the early 1960s was $.95/lb. Both materials were well received. Production reached 10 million lb in 1970 and the price dropped to $.65/lb. Today sales exceed 400 million lb and the price for injection molding grades of acetal averages $1.39/lb and $.07/in3 in truckload quantities.

Defining characteristics
Acetals are opaque, semicrystalline thermoplastic engineering materials that are available as homopolymers or copolymers. The copolymers have slightly better melt flow and heat aging characteristics. All acetals are strong, tough, and stiff materials with a low coefficient of friction against metal and many other plastic materials. They are not hygroscopic and are resistant to a wide range of chemical reagents.

Acetal’s chemical resistance allows it to be used as packaging and dispensing components for many solvents, fuel, and cosmetic products. It is, however, affected by strong acids or strong oxidizing agents. The Bic disposable cigarette lighter used acetal for its chemical resistance, dimensional stability, and pressure-containing stiffness. In fact, through the mid-1970s, Bic was the world’s largest user of acetal.

Acetal is most frequently thought of for its impressive creep resistance. Its very low level of cold flow, coupled with excellent fatigue resistance, combine to make it the best available plastic material for springs. (The average values for general-purpose homopolymers and copolymers are shown in the table.)

Many different acetals have been developed for special applications. PTFE-filled acetals became available in 1964, followed by glass-fiber-reinforced grades in 1965. All of the properties in the table can be enhanced by adding fillers and reinforcing fibers. The one exception is impact strength. Acetals are notch-sensitive but tough materials that give a misleading low notched Izod impact value.

Special additives can produce toughened, less notch-sensitive grades. Acetals can be attacked by long-term exposure to ultraviolet light. Outdoor weatherability can be improved with additives. Other additives produce flame-retardant grades with a UL 94-HB rating.

Applications
Acetal’s low friction, wear, and creep, coupled with its self-lubricating characteristics and long-term dimensional stability, make it the first choice for plastic gears, bearings, cams, and pulleys. These products find wide usage in many markets.

Additional applications include:

Transportation: window cranks, door latches, shift levers, speaker grilles, mirror housings, and controls; fuel level sensors, pump housings, and gas caps; cooling fans, brackets, and trim strip clips; carburetor venturi tubes.

Electrical: coil forms and connectors; telephone terminal strips and fuse holders; switch and relay components; pushbuttons and key caps; videotape hubs and guides.

Plumbing: water faucet parts, stool flushing valves, filter bodies, and pressure regulator components; water meter housings and internal parts; pop-up lawn sprinkler and spray nozzles; water softener pumps, valves, and impellers; threaded fittings.

Consumer: dishwasher soap dispensers and spray nozzles; mixer bowls and blades; wear strips and instrument bodies in laundry washers and dryers; refrigerator shelf mounting brackets and door latches; mascara wands and vials; aerosol valves, nozzles, and sprayer pumps; springs.

Industrial: chain and conveyor links; gears and bearings; hose connectors; glue applicators, furniture casters, window and drapery hardware, cabinet hinges and latches; clock parts; small portable tool housings.

Part design tips
Wall thickness. Acetals are relatively easy-flowing materials and have certain processing information. The recommended ideal wall thickness ranges from .030-.125 inch. Small parts have been molded with .015-inch-thick walls. With special molding procedures, large parts have been successfully molded with .750-inch-thick walls. The maximum allowable wall thickness for acetal should, however, be .375 inch. Thicker wall sections contain high levels of residual stress and are susceptible to internal voids. Variations in thickness should be limited to 10-15% of the part’s nominal wall thickness and be smoothly blended from thick to thin.

Corner radiuses. An inside corner radius equal to 75% of the part’s wall thickness will minimize acetal’s notch sensitivity. The minimum allowable inside radius is 25% of the part’s wall thickness. This will also provide maximum strength, minimum molded-in stress, and improved melt flow.

Molding draft angle. Acetal is self-lubricating and some parts, such as bearings and gears, are molded straight without any draft angle. A 1/2-1° draft angle per side is recommended.

Projections. Many functional features such as stiffening ribs, solid bosses, and snapfit latches can be molded as projections off of a part’s nominal wall. Their thickness at the junction with the part should be limited to 50% of the part’s wall thickness. In cases where appearance and the absence of sink marks is critical, projections can be reduced to 40% of the part’s wall thickness.

Depressions and holes. The main problems associated with depressions is that they create weldlines and the core pins that form small holes are difficult to cool and are susceptible to bending. Weldlines can weaken a part while creating appearance problems. The weldline on a properly molded part can retain 80-90% or more of the material’s original tensile strength.

Acetal’s ease of flow allows the use of low injection pressures. These lower pressures reduce the bending forces on the core pins that form small holes. All inside corners on holes should have the standard radiuses. As the material shrinks, it grips the core pins. Providing molding draft on these core pins reduces ejection force, which allows shorter molding cycles.

Tolerances. Acetals are dimensionally stable materials in spite of the fact that they have a high mold shrinkage factor. Acetals are widely specified for precision parts, such as bearings and gears, that have to retain their size for extended periods of time. The attainable tolerance is determined by the material’s shrinkage factor, the part’s thickness, and the molding conditions.

A commercial tolerance on a 1-inch-long, .125-inch-thick molded acetal part is ±.0056 inch. A more costly fine tolerance on that part is ±.0030 inch.

August, 2006 - Reprinted with permission from Injection Molding Magazine. Copyright © Canon Communications LLC.