The Stiffness of Plastics Explained

Stiffnes of plastics
Stiffness is one of the key mechanical properties of plastic, along with strength, hardness, and toughness. The stiffness of plastics is described by flexural modulus (the ability of a material to bend), which is measured in pounds per square inch (psi). The higher the flexural modulus, the stiffer the material; the lower the flexural modulus, the more flexible it is.

Stiffness vs. Strength

In addition to flexural modulus, the stiffness of plastics is sometimes referred to as flexural strength or tensile modulus. This mixing of terms can result in confusion as to the difference between stiffness and strength. The strength of a plastic (typically referred to as tensile strength or ultimate strength) is how much stress a plastic can withstand without breaking when it is stretched or pulled (physical failure). Stiffness of plastic is the ability of the material to distribute a load and resist deformation or deflection (functional failure). These properties are often needed in conjunction with one another in demanding applications.

5 Plastics with Excellent Stiffness

  1. Acetal – This stiff, high-strength, low-friction engineering plastic is easy to machine and can hold complex, tight tolerances. Homopolymer acetal (Delrin®) is extremely stiff, with a flexural modulus of 14,300 psi, and provides superior wear and abrasion properties as well as excellent chemical resistance.
  2. PAI (Torlon®) – This plastic offers both stiffness and strength, with certain grades of Torlon® plastic measuring a flexural strength of 24,000 psi and a tensile strength of 18,000 psi. PAI maintains its properties at extreme temperatures and is the highest-performing melt-processable plastic.
  3. PEEK – An all-around powerhouse, PEEK combines high stiffness (24,000 psi) and strength (14,000 psi) with outstanding chemical resistance and a continuous service temperature of 500ºF. PEEK also has superior creep resistance, excellent fatigue and stress-crack resistance, and very low moisture absorption.
  4. Ultem® – PEI resins such as Ultem® offer high stiffness and strength, superior elevated thermal resistance, and broad chemical resistance. With a flexural modulus of 22,000 psi, Ultem® offers predictable stiffness and strength up to 200ºC/392ºF. It also has great processability, is inherently flame resistant in most grades, and has excellent stress and cracking resistance when exposed to aircraft and automotive fluids, alcohols, acids, and more.
  5. PPS – PPS is most commonly known for having the broadest resistance to chemicals of any advanced engineering plastic. However, it also boasts an impressive flexural modulus of 21,000 psi. PPS maintains its stiffness and strength over wide temperature ranges and has excellent hydrolysis resistance, high dielectric strength, advanced electrical properties, and a very low coefficient of linear thermal expansion.

Need Help Choosing a Stiff Plastic?

At Reading Plastic, we’re experts at machining the most advanced plastics on the market, including those with superior stiffness. We not only help you select the appropriate material, we can also machine and fabricate your parts to hold tight tolerances. If you need help determining the right plastic for your application, reach out to us today at (610) 926-3245.

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