Meaning
Rate at which a material undergoes deformation relative to its original dimensions defines an important parameter in both testing and high-speed melt processing. For thermoplastic resins, the strain rate during moulding determines the flow resistance and shear heating within the injection nozzle and gates. This parameter also dictates the performance of the finished part under rapid impact loads during service.
Processing conditions must be controlled to prevent the material from experiencing excessive forces that exceed its limits.
Polymer Response
Viscoelastic properties of plastics mean that their mechanical behavior depends on how fast they are deformed. At a low strain rate, the polymer chains have time to untangle and slide past one another, resulting in ductile behavior. Under a high rate, the chains cannot respond quickly enough, making the material behave as a rigid and brittle solid.
Testing Specification
Standard tensile and impact tests must define this rate to ensure that the data on polymer datasheets is comparable. Variation in the strain rate during testing can lead to misleading values for tensile strength and elongation at break. Engineers use these standardized rates to select the appropriate polymer grade for parts that must survive impact.
Moulding Control
Gate design and injection speed are the primary variables that determine the deformation rate. High speed increases this rate, which can lead to melt fracture. Moulders must optimize these variables to balance speed and part quality.
