February 2010 | Focus: Nylon/Acetal
Nylon and acetal have a proven, successful performance record in a variety of diverse engineering applications, making these two materials the workhorses of the engineering plastics groups.
Machining offers many advantages to part designers. Machining nylon and acetal parts not only offer the economic advantages of providing parts in small or intermediate quantities but also allow design freedoms that can only be met by machining. However, nylon and acetal have unique characteristics that must be understood when machining. Read more to find out what you need to know.
The right plastics for design applications
Nylons and acetals are often chosen over other materials due to the advantages offered by their unique characteristics. Read more for two case studies where nylon was chosen for its precision, rigidity and durability; and where acetal copolymer was used for its lubricity, wear and fatigue resistance.
Evolution of the art of casting nylon
When first developed, cast nylon was natural in color. So, to differentiate it from extruded nylon, a blue dye was added. The material performed extremely well in most applications where it was determined to be suitable but had a tendency to melt under certain bearing conditions.
To reduce its coefficient of friction and thus improve its performance as a bearing material, methods were developed to allow the addition of lubricants directly into the raw material mix, some liquid in the form of oil and some solid in the form of wax. These additives provided a significant improvement by increasing the material’s PV capability, increasing it from 3,500 for a standard nylon to 16,000 for a lubrication filled nylon. This new product had significant impact in applications where external lubrication is undesirable or difficult to maintain. Read more on cast nylon and its strengths and weaknesses, as well as its growth in available sizes.
Introduction to acetals
Acetal resins (polyoxymethylene or POM) are engineering thermoplastics based on formaldehyde polymerization technology. These highly crystalline resins are strong, rigid and have a low coefficient of friction against metals, acetals and other plastics. They are creep resistant so they are good for parts where dimensional stability is important. There are two types of acetals, the homopolymer and copolymer.
Acetals do not absorb a large amount of moisture and are resistant to a wide range of solvents. Acetals are attacked by both strong acids and oxidizing agents. In addition, the homopolymer is not recommended for use in strong caustics. Acetal copolymer is more resistant to hot water and hot air than the homopolymer. Because of the chemical make-up differences, it is not recommended that you substitute one type of acetal for another without discussing it with your supplier. Read more in IAPD's Introduction to Plastics.
Nylons (polyamide or PA) were the first of the thermoplastic engineering resins. These crystalline plastics are available in many compositions, ranging from molding and extrusion materials to solution and fluidized bed coatings and casting resins. Nylon 6/6, the most widely used of the nylon plastics, is available in a number of formulations for molding and extrusion. Other general types are nylon 6/10 and nylon 6/12. These are higher priced materials, used where greater dimensional stability is required.
Nylon 6 is the second most widely used of the nylons. Its properties are similar to those of 6/6, but it absorbs moisture more rapidly and its melting point is 70°F/21°C lower. The lower processing temperature and the less crystalline nature of nylon 6 result in slightly lower mold shrinkage.
Nylons 11 and 12 have better dimensional stability and electrical properties than the other nylons because they absorb less moisture. These types, which are more expensive than the others, are available compounded with plasticizers to provide greater flexibility and ductility. Read more in IAPD's Introduction to Plastics.
Test Your Knowledge
What do you know about nylons and acetals? (Answers are at www.iapd.org/popquiz.html.)
1. What does the "6" refer to in nylon 6?
a. number of carbon atoms in the starting chain
b. number of additives added
c. trade name
d. 1/1,000 of its tensile strength
2. One of the major features of acetal is its ability to resist:
a. strong acids
b. oxidizing agents
d. strong caustics