March 2012 Machining Tip

Material Matters: Acetal

Acetal is a high-performance engineering polymer often used for parts that would otherwise be made of metal. Chosen for its distinct characteristics, it is widely used in both machining and injection molding.

There are two types of acetal. Homopolymer, produced by DuPont as Delrin®, consists of a chain of identical oxymethylene units. Copolymer, introduced by Celanese as Celcon®, consists of a chain of alternating oxymethylene and oxyethylene units. While the two acetals differ in some ways, they share basic characteristics.

All acetals are strong, tough, and stiff with very high creep resistance, making them ideal for mechanical parts like gears and chain links. They are highly abrasion resistant (though less so than nylons or polyethylenes) and have a low coefficient of friction against metal and other plastics, making them an excellent choice for bearings, bushings, and cams. They are affected by strong acids or oxidizing agents, but otherwise have high resistance to most chemicals and low water absorption and are widely used for packaging and dispensing components. In general, they can withstand a wide range of temperatures, but tend to degrade when exposed to ultraviolet light.

Acetal’s inherent strength can be further enhanced by the addition of glass fiber reinforcement. Firstcut stocks four acetal resins: white copolymer acetal, black and white homopolymer acetyl, and 20% glass filled homopolymer. In choosing between homopolymer and copolymer acetal, consider the following.

Homopolymers have:

  • higher flexural modulus than copolymers, i.e., are stiffer at both room temperature and elevated temperatures
  • higher impact strength making them somewhat less likely to fracture on impact; the advantage over copolymers becomes more significant at extreme temperatures
  • higher fatigue endurance, the ability to withstand repeated cycles of stress
  • greater elongation at yield
  • a continuous use temperature of 95°C vs. 90°C for copolymer
  • higher porosity (there may be a porosity line present in the extruded sheet stock that Firstcut uses)

Copolymers have:

  • better dimensional stability
  • better resistance to basic (high pH) solutions such as bleach
  • greater resistance to degradation by exposure to steam, hot water, or hot air
  • lower porosity due to shrinkage in extruded shapes

While none of these differences are large, they may guide choice of acetal when a particular characteristic is critical to the application.

Acetals are highly machinable, but if the machined parts are prototypes for parts that will be injection molded, the material’s moldability characteristics must also be considered in both part design and resin choice. In other words, the fact that you can machine it doesn’t guarantee that you can mold it. The following are general guidelines for molding acetal. (If parts are to be molded by Protomold’s rapid injection molding process, our Design Guidelines should be used.)

  • Ideal molded wall thickness of acetals is between .030 and .125 inches. Your odds of success are greatly improved if you stay within these guidelines.
  • Thin walls may prevent proper mold filling, and overly thick walls can result in internal stress or voids.
  • Variations in wall thickness should be no greater than 15% of the nominal wall thickness, and where wall thickness changes the transition should be smooth, not sudden. Acetal’s shrink behavior is particularly affected by wall thickness.
  • The thicker the walls, the greater the shrink rate, which can be problematic when tolerances are tight.
  • Acetals are somewhat “notch sensitive,” meaning that they can fracture where there is a sharp break in a surface, whether molded in or acquired after production. Radiusing inside corners helps reduce the possibility of fracture. Minimum inside corner radius for acetals is 25% of wall thickness; 75% is preferred.
  • Because acetals are self-lubricating, parts can occasionally be molded with little or no draft. However, ½° to 1° draft is recommended.
  • Projections from a part wall—ribs, bosses, and the like—should be no greater than 50% of wall thickness where they join the wall. If sink on the opposite side of the wall is a concern, projections should be limited to 40% of wall thickness.
  • Depressions—thin areas of a wall—can cause splitting of the resin flow and result in knit lines where separate flows meet. Reducing the size and depth of depressions helps minimize the impact of knit lines.
  • Holes formed by core pins are, in essence, 360° inside corners and should follow the guidelines above for radiusing of corners. Also, because acetal shrinks as it cools and tends to grip cores, core pins should be appropriately drafted to facilitate ejection.
  • Acetal, once it is molded, is dimensionally stable; however, the material shrinks significantly as it cools. This can affect size tolerances achievable in molding.

Whether you are machining a finished part or a prototype that will eventually be molded, the material you choose will impact both the design decisions you make and the performance of the finished piece. Because some of acetal’s characteristics—tendency to shrink, for example—affect molded, but not machined, parts, it must be treated differently depending on whether you are machining prototype or finished parts. This will allow you to take full advantage of the unique capabilities of the material.

Watch this video tip on Acetal.