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Effective Selection & Design Techniques

Plastics are increasingly being used to replace other materials like bronze, stainless steel, aluminum, and ceramics. The most popular reasons for switching to plastics include:

  • Longer part life
  • Elimination of lubrication
  • Reduced wear on mating parts
  • Faster operation of equipment / line speeds
  • Less power needed to run equipment
  • Corrosion resistance and inertness
  • Weight Reduction
With the many plastic materials available today, selecting the best one can be an intimidating proposition. Here are guidelines to assist those less familiar with these plastics.

Step One
Determine whether the component is a: Bearing and Wear application (i.e., frictional forces) or structural (static or dynamic) application

Determining the primary function of the finished component will direct you to a group of materials. For example, crystalline materials (i.e., nylon, acetal) outperform amorphous materials (i.e., polysulfone, polyphenylene oxide, or polycarbonate) in bearing and wear applications. Within the material groups, you can further reduce your choices by knowing what additives are best suited to your application.

Wear properties are enhanced by MoS2, graphite, carbon fiber and polymeric lubricants (i.e., PTFE, waxes).
Structural properties are enhanced by glass fiber and carbon fiber.

Once you have determined the nature of the application (wear or structural), you can further reduce your material choices by determining the application's mechanical property requirements. For bearing and wear applications, the first consideration is wear performance expressed in PV and "k"-factor. Calculate the PV (pressure (psi) x velocity (fpm)) required. Select materials whose limiting PV's are above the PV you have calculated for the application. Further selection can be made by noting the "k" wear factor of your material choices. The lower the "k" factor, the longer the material is expected to last.

Structural components are commonly designed for maximum continuous operating stresses equal to 25% of their ultimate strength at a specific temperature. This guideline compensates for the viscoelastic behavior of plastics that result in creep.

Step Two
Consider the thermal requirements of your application using both typical and extreme conditions.

A material's heat resistance is characterized by both its heat deflection temperature (HDT) and continuous service temperature. HDT is an indication of a material's softening temperature and is generally accepted as a maximum temperature limit for moderately to highly stressed, unconstrained components. Continuous service temperature is generally reported as the temperature above which significant, permanent physical property degradation occurs after long term exposure. This guideline is not to be confused with continuous operation or use temperature reported by regulatory agencies such as Underwriters Laboratories (UL).

The melting point of crystalline materials and glass transition temperature of amorphous materials are the short-term temperature extremes to which form stability is maintained. For most engineering plastic materials, using them at or above these temperatures should be avoided.

Step Three
Consider chemical exposure during use and cleaning.

Chemical compatibility can be difficult to predict since concentration, temperature, time and stress each have a role in defining suitability for use. Nylon, acetal and Ertalyte PET-P are generally suitable for industrial environments. Crystalline high performance materials such as Fluorosint filled PTFE, Techtron PPS and Ketron PEEK are more suitable for aggressive chemical environments. We strongly recommend that you test under end-use conditions. An overview of specific chemical resistance can be found here

Step Four
Before proceeding to steps 5-6, it may be appropriate to consider additional material characteristics.

  • Relative impact Resistance/Toughness
  • Dimensional stability
  • Regulatory/agency Compliance
Materials with higher tensile elongation, Izod impact and tensile impact strengths are generally tougher and less notch sensitive for applications involving shock loading.

Engineering plastics can expand and contract with temperature changes 10 to 15 times more than many metals including steel. The coefficient of linear thermal expansion (CLTE) is used to estimate the expansion rate for engineering plastic materials. CLTE is reported both as a function of temperature and as an average value.

Modulus of elasticity and water absorption also contribute to the dimensional stability of a material. Be sure to consider the effects of humidity and steam.

Agencies such as the Food and Drug Administration (FDA), U.S. Department of Agriculture (USDA), Underwriters Laboratory (UL), 3A-Dairy Association and American Bureau of Shipping (ABS) commonly approve or set specific guidelines for material usage within their industrial segments.

Step Five
Select the most cost-effective shape for your part.

Untitled Document We offer designers the broadest size and configuration availability. Be sure to investigate all of the shape possibilities--you can reduce your fabrication costs by obtaining the most economical shape. Consider Quadrant's many processing alternatives.

Note: From process to process, many material choices remain the same. However, there are physical property differences based upon the processing technique used to make the shape. For example:

  • Injection molded parts exhibit the greatest anisotropy (properties are directionally dependent).
  • Extruded products exhibit slightly anisotropic behavior.
  • Compression molded products are isotropic--they exhibit equal properties in all directions.
Choose Extrusion for:
  • Long lengths
  • Small diameters
  • Rod, plate, tubular bar, bushing stock
Choose Casting for:
  • Large stock shapes
  • Near net shapes
  • Rod, plate, tubular bar, custom cast parts
Choose Compression Molding for:
  • Various shapes in advanced engineering materials
  • Rod, disc, plate, tubular bar
Choose Injection Molding for:
  • Small shapes in advanced engineering materials
  • High Volumes (>10,000 Parts)

Step Six
Determine the machinability of your material options.

Machinability can also be a material selection criterion. In general, glass and carbon reinforced grades are considerably more abrasive on tooling and are more notch sensitive during machining than unfilled grades. Reinforced grades are commonly more stable during machining.

Because of their extreme hardness, imidized materials (i.e., Duratron PAI and Duratron CU60 PBI) can be challenging to fabricate. Carbide and polycrystalline diamond tools should be used during machining of these materials.

Customer Service Agent  Need help selecting the right plastic for your specific requirement? Call us today at (800) 326-3376.