Nylon fasteners such as screws, flat washers, hex head cap screws, hex nuts spacers and standoffs etc. are some of the most in demand fasteners in the world of plastics. Many people choose nylon fasteners because they are cheaper and more well-known than many other plastic fasteners. For many applications, nylon fasteners work just fine. However, nylon has several weak points which may cause nylon fasteners to fail in various extreme situations.
Nylon was first developed in the late 1930’s by Wallace Carothers at DuPont and introduced as a fabric at the 1939 New York World’s Fair. Much of the world’s production of nylon is still used in the textile industry for cloth. Since 1939, nylon has been incorporated into such diverse products as toothbrushes, carpets and truck tires.
Nylon 6/6 is a popular grade for nylon fasteners. It is a semi-crystalline polymer, one with crystalline regions within an amorphous matrix. It is a polyhexamethylene adiamide in which the amide group –(-CO-NH-)- provides hydrogen bonding between polyamide chains.
While much of the world’s production of nylon 6/6 is used for cloth, it is also an important thermoplastic engineering resin extruded into bar shapes or for injection molding and used to manufacture nylon fasteners including machine screws, nuts, washers, spacers, bearings, gears, and fittings. However, there are situations where Nylon 6/6 fasteners are definitely not the right choice for an engineering application. Here’s a list of the six top reasons NOT to use nylon fasteners in an application:
1. Moisture: Nylon is hygroscopic, meaning that it attracts and absorbs water from the surrounding environment. The composition of the plastic is changed as water molecules are suspended between the molecules of the material. Over time Nylon 6/6 will swell considerably and begin to degrade. Nylon fasteners are definitely not recommended for use in or around water.
2. UV Resistance: Nylon 6/6 is not UV resistant.* When placed in sunlight over time the physical and mechanical properties of nylon fasteners will degrade. They will often turn yellow and become brittle.
3. Chemical resistance: Nylon 6/6 is generally not chemically resistant. It performs poorly in acids and halogens such as fluorine and chlorine. It stands up slightly better to ketones (acetone, camphor) and halogenated hydrocarbons (carbon tetrachloride, dichlorobenzene.) It is not recommended for use with these chemicals.
4. Temperature: Nylon 6/6 has a continuous service temperature of 223⁰ F (121⁰ C.) Nylon fasteners are not recommended for use in ovens, nor in contact with boiling water. Teflon (PTFE) is a much better choice.
5. Flammability: Nylon 6/6 has a UL 94 V-2 rating.** 94V-2 is a UL test rating in which a specified test specimen stops burning within 60 seconds after two applications of ten seconds each of a flame with flaming drips allowed. So nylon fasteners melt rather than burn when in contact with an open flame. If an application requires a nonflammable material, Nylon 6/6 is not recommended. ***
6. High Strength: Plastics that are strong enough to replace metals are much in demand. The ultimate tensile strength of nylon 6/6 is 10,000 PSI . This is not unreasonably low, as other common engineering plastics such as PVC and polypropylene have an ultimate tensile strength of 9,580 and 5,800 PSI respectively. However, nylon fasteners are certainly not the strongest available nor are they as strong as metal fasteners. The ultimate tensile strength of FR4/G10 is 45,000 PSI. Cast iron has a UTI of 29,000 PSI and copper comes in at 31,900 PSI while aluminum has a UTI of 43,500 PSI. Nylon 6/6 is not recommended when higher strength is required and definitely not as a replacement for metal.
*There are grades with additives such as carbon black that improve the UV performance of Nylon 6/6.
**Grades with flame retardant additives are available on the market.
***Additives are used to improve the flame resistance of Nylon 6/6.
Have you used nylon fasteners in your applications? How did they perform for you? Let us know in the comments below.
Looking for more information on the many different grades of nylon? Download our complimentary guide!