Meaning
Physical properties describe the ability of a polymer melt to deform under stress and then recover part of that deformation when the stress is removed. Rheological compliance is the ratio of the strain to the applied stress in a viscoelastic material. It is a measure of the softness or flexibility of the polymer structure at a specific temperature and timescale.
Elastic Response
Internal storage of energy occurs when the polymer chains are stretched out of their random coil configuration. Rheological compliance captures this stored energy as a recoverable strain that acts like a spring within the melt. This property is responsible for phenomena like die swell, where the polymer expands as it exits a nozzle because the internal stresses are being released.
Deformation Recovery
Time dependent behaviour is a hallmark of the materials used in the plastics industry. After the load is removed, the rheological compliance shows how the material slowly returns toward its original shape. This recovery is never perfect in a melt because some of the energy is always lost as heat due to the viscous flow of the chains past each other.
Elastic Limit
Measurement of these variables allows a scientist to map the linear and non-linear regions of a material flow. By plotting the rheological compliance against the duration of the test, a moulder can determine the creep resistance of a part. This is required for components that will be under constant load during their service life, such as plastic clips or structural housings that must maintain their dimensions without sagging or loosening over time.
Every resin grade has a unique compliance profile that determines its suitability for precision engineering.
