Meaning
Mathematical distributions of relaxation times derived from dynamic mechanical analysis describe the molecular mobility of polymer chains. Within polymer processing, a continuous relaxation time spectrum provides a detailed profile of how a polymer melt releases stored elastic energy after deformation. This parameter determines the rate at which stress decays during the injection and cooling stages of moulding.
Rheological Analysis
Molecular structure and chain length dictate the shape of the spectrum. Long chains and highly branched architectures require more time to disentangle and relax. In contrast, short chains relax almost instantly.
A moulder uses these profiles to evaluate the difference between virgin resins and recycled blends, as the latter often exhibit a truncated spectrum due to chain scission during prior heat cycles.
Injection Stress
Residual stress in moulded parts causes warpage and dimensional instability. When the melt experiences high shear during cavity filling, the chains orient themselves along the flow lines. If the cooling rate exceeds the time required for these chains to relax, the orientation is frozen into the part.
This molded-in stress compromises the impact strength of the finished product.
Process Control
Adjusting processing variables such as melt temperature and hold pressure allows processors to manage stress relaxation. Datasheets typically report single-point viscosity values, which cannot predict the relaxation dynamics under real moulding conditions. By analyzing the complete spectrum, engineers can optimize cycle times without risking premature ejection.
This balance minimizes part warpage while maintaining high throughput. Long-chain polymer formulations benefit from extended cooling cycles, whereas simple linear resins can be processed rapidly. Processors rely on these calculations to design high-efficiency cooling layouts for their tooling.
