Friday, January 4, 2013

Delrin – the machinist’s best friend

In a world of specialized plastics requiring immense tensile strength or high wear resistance or minimal coefficient of friction, Delrin holds its own against the more versatile polymers such as PEEK and PTFE.
Our own experience with Delrin began with the PTFE price increases in 2010-2011, as we scrambled to find substitutes for PTFE to offer clients, without compromising too much on properties. As we have already stated in earlier articles – finding a true substitute for PTFE was futile. However, despite our attempts to push UHMWPE and PA66 as replacements (materials we were more familiar with), it was ultimately Delrin which clients were most comfortable in adopting.
What is Delrin?
Delrin (brand name of DuPont) is also commonly referred to as POM (Polyoxymethylene), polyacetal, or simply acetal. The names all refer to a polymer that is characterized by a high tensile strength, high stiffness, low coefficient of friction and excellent dimensional stability. In addition to its properties – Delrin is a relatively inexpensive material compared to PTFE and even PA66. This makes it a sought after choice in machined component development – as the parts are dimensionally very stable and significant trials can be done without being too expensive.
Properties of Delrin
  • Delrin is characterized by its high strength, hardness and rigidity to ~40 °C
  • In its natural form, it is a white (opaque) plastic, although it is easily pigmented and often available in a variety of colors
  • Delrin has a specific gravity of 1.410-1.420 g/cm3
  • As a homopolymer it is 75-85% crystalline with a melting point of 175°C, while as a copolymer has a slightly lower melting point of 165–175°C
  • It has a relatively low coefficient of friction of 0.2 – much higher than PTFE, but still suitable for a wide number of engineering applications
  • Delrin is resistant to a wide variety of chemicals including alcohols, aldehydes, esters, ethers, hydrocarbons, agricultural chemicals, and many weak acids and bases. This ability is even more impressive when we consider that even under harsh chemical environments, Delrin does not lose its dimensional stability
  • Electrically, Delrin rates slightly below PTFE, but is nonetheless a very useful substitute. Its dielectric constant (~3.5) is only slightly higher than PTFE (~2)
Advantages of Delrin:
  • High mechanical strength and rigidity
  • Toughness and high resistance to repeated impacts
  • Long-term fatigue endurance
  • Excellent resistance to moisture, gasoline, solvents, and many other neutral chemicals
  • Excellent dimensional stability
  • Good resilience and resistance to creep
  • Natural lubricity
  • Wide end-use temperature range
  • Good electrical insulating characteristics
Due to its versatility, Delrin finds uses in a number of applications including:
  • Automotives
  • Industrial equipments
  • Consumer goods
  • Medical equipments
  • Electrical equipments
Machining Delrin
As a machined item, Delrin is particularly easy to work with. With PTFE, we need to consider the softness of the material and also its sensitivity to temperatures, with nylons and UHMWPE, we need to be careful of the part melting during machining, with PEEK, the tool itself can break, if we do not control the RPM. However, Delrin is surprisingly accommodating as the part retains its stiffness, but is still soft enough that the tool is able to work through the plastic. In addition, the dimensional stability post machining is also excellent. While we have had instances of PTFE parts being under tolerance when shipped to colder climates, the same is not an issue with Delrin parts.

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