April 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 moulding.

There are two types of acetal. Homopolymer, first 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.

Depending on the application and properties required, acetals can be further enhanced by the addition of various additives, and specialty grades such as Static dissipative, PTFE or glass fibre reinforcement filled materials are manufactured. Firstcut currently stocks five acetal resins: black and white copolymer acetal, black and white homopolymer acetal and beige static dissipative copolymer. In choosing between the various acetals, 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 moulded, the material's mouldability 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 mould it. The following are general guidelines for moulding acetal. (If parts are to be moulded by Protomold's rapid injection moulding process, our Design Guidelines should be used.)

  • Ideal moulded wall thickness of acetals is between 0.762 - 3.048mm. Your odds of success are greatly improved if you stay within these guidelines.  Thin walls may prevent proper mould filling, and overly thick walls can result in internal stress or voids.
  • Variations in wall thickness should be no greater 15% of the nominal wall thickness, and where wall thickness changes the transition should be smooth, not sudden. Acetal's shrink behaviour 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 moulded 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.  Adding a matching radius on corresponding outside corner prevents creation of a thick area.
  • Because acetals are self-lubricating, parts can occasionally be moulded 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 moulded, is dimensionally stable; however the material shrinks significantly as it cools. This can affect size tolerances achievable in moulding.

Whether you are machining a finished part or a prototype that will eventually be moulded, 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 moulded, 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.