Strength

Strengths calculated for plastic parts are often over estimated. This is typically due to the non-linear nature of plastic materials. When designing a part to withstand a given load, strength calculations are typically made considering part geometry and material properties. Engineers are generally educated for such analyses using linear elastic, orthotropic (same properties in all directions) material models. These material assumptions, while ideal, are good models for materials such as steel, aluminum, and some plastics experiencing limited loading. However, as a rule, the behavior of plastics is more complex and requires nonlinear analysis when attempting to accurately predict ultimate loads at failure. That is not to say that classical linear analysis cannot be used effectively as long as the designer is aware of its limitations.

In addition to non-intuitive behavior exhibited by non-linear materials, load capacity of a plastic part may be unexpectedly affected by the following:

• Plastic injection point
• Molding machine configuration
• Raw material handling
• Part thickness transitions
• Speed and duration of applied load
  

Whereas metal parts machined from a single lot of material will perform similarly if dimensionally equivalent, a loosely controlled molding process can produce dimensionally identical parts with significant variations in strength.