Data for the same materials generated at lower pressures and higher velocities provide significantly different results and can be found in Figure 2 below.
Stolarski compared the rolling contact performance of nylon 6/6 and acetal and found acetal to exhibit significantly higher rolling contact fatigue resistance . In a similar study looking at nylon 6 and acetal, however, Harrass et al. reported wear rate values of between 0.5 and 2 (x 10-6 mm3/Nm) for nylon 6 and values between 3 and 5 (x 10-6 mm3/Nm) for acetal .
Nylons and acetals are available in internally-lubricated grades designed to reduce friction and increase wear life. Nylon is commercially available in several proprietary formulations with limiting PVs extending beyond 15,000 psi-ft/min (with a 4:1 safety factor applied). The limiting PV of Delrin® AF (a popular PTFE-filled grade of acetal) is around 8,300 psi-ft/min. It is worth noting that the results of the aforementioned Harrass study suggest wear-enhancing additives like MoS2 and PTFE may actually result in accelerated wear in a rolling contact situation .
Continuous operating temperatures for acetals are often reported in the 180 – 190°F range, which is roughly 20 – 30°F below that of nylon 6/6.
Nylons and acetals are widely used in the food processing and automotive industries where they are exposed to a broad range of chemicals. Strong acids or bases could present compatibility issues for either material. Compared to acetal homopolymers (Delrin®), acetal copolymers tend to exhibit better resistance to alkaline solutions (and to a lesser extent acidic solutions). Because chemical compatibility is tied to environmental factors such as temperature, it is always advisable to test materials under realistic application conditions prior to service.
While nylon materials are readily machinable, acetal is widely regarded as the most machinable plastic. We have encountered multiple applications in which acetal was specified simply due to it being preferred by the machining team.
Both material groups are available in a wide range of rod diameters and plate thicknesses. Standard stock shape availability is provided in Table 3.
TABLE 3: STANDARD STOCK SHAPE AVAILABILITY
||1” – over 20” diameter
||0.187” – 8” diameter
||0.187” – 20” diameter
||0.25” – 8” thick
||0.030” – 4” thick
||0.030” – 6” thick
||1.75” OD – over 36” OD
Custom shapes, including large diameter rings and discs exceeding 80” in diameter, can also be manufactured from nylon 6. Metal and X-ray detectable grades of acetal and blue-colored versions of both materials are also readily available.
Unfilled acetals and nylon materials are FDA-compliant (although this is not always the case for filled grades of these materials). Additionally, nylons and acetals that meet European food standards are available.
Lastly, acetal shapes tend to be roughly 25% less expensive than nylon 6/6 shapes and 25% more expensive than nylon 6. These figures vary with shape dimensions because nylon 6 stock shapes are produced by casting (as opposed to extrusion for acetal and nylon 6/6), which results in different set-up and manufacturing costs.
Nylon and acetal are workhorse engineering plastic materials that compete for a wide variety of end uses. There are, however, some significant differences between their performance characteristics and size availability that can make one material more suitable for a particular application.
The preceding information is intended to be a quick comparison of key material properties for screening purposes during material selection procedures and not a comprehensive discussion of each material.
Have a question about selecting the right material for your application?
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ABOUT THE AUTHOR
Dave Seiler is a Product Specialist at Curbell Plastics who focuses on high-performance engineering plastics. He has literally been involved with plastics his whole life (his first word was “Kynar”) and spends much of his free time reading academic literature on a diverse base of polymer materials. Yeah, he’s that guy. Dave is ready to take your call, set up a webinar, or travel to your facility to discuss your unique application needs and help identify cost-effective engineered plastic candidate materials. Contact Dave.
1. Sepe, Michael. “Dimensional Stability After Molding: Part 4.” Plastics Technology. Plastics Technology, 25 Mar. 2013, https://www.ptonline.com/articles/dimensional-stability-after-molding-part-4.
2. Zytel®/Minlon® Design Guide – Module II. DuPont Engineering Polymers, p. 9.
3. Delrin® Design Guide – Module III. DuPont Engineering Polymers, pp. 6-9, 37.
4. McKeen, Laurence W. Fatigue and Tribological Properties of Plastics and Elastomers. 3rd ed., Elsevier, 2016, pp. 91, 221.
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8. Quadrant High Performance Products and Applications Guide. Quadrant, 2017, pp. 11, 17, 19.
9. Stolarski, T.A. “Rolling Contact Fatigue of Polymers and Polymer Composites.” Advances in Composite Tribology, by K. Friedrich, Elsevier Science Publishers B.V., 1993, p. 659.
10. Harrass, M., et al. Tribological Behavior of Selected Engineering Polymers Under Rolling Contact. Elsevier, 2009, p. 641.