Meaning
Polymer melt deformation behavior combines both viscous liquid and elastic solid characteristics when subjected to shear forces during injection. This dual behavior is visco-elastic melt flow, which causes the material to store elastic energy as it is forced through the runner system. It leads to flow defects like die swell and jetting when the melt exits the gate.
Proper nozzle and gate geometry can minimize these effects by reducing sudden changes in flow velocity.
Elastic Recovery
When the polymer melt exits the gate or die, the stored elastic energy is released, causing the stream to expand. This expansion is driven by the visco-elastic melt flow and must be accounted for in runner and die designs. If the relaxation time of the melt is long, the part will exhibit higher residual stress and warpage.
Flow Instabilities
High shear rates during injection can exceed the elastic limit of the polymer, leading to melt fracture. Melt fracture occurs during visco-elastic melt flow and results in a rough, wavy surface finish on the moulded part. Reducing injection speed or increasing melt temperature helps to prevent these cosmetic defects by reducing shear stress.
Pressure Drop
The elastic component of the melt flow requires additional pressure to force the polymer through restrictions in the mould. Understanding visco-elastic melt flow is necessary for accurately predicting the pressure drop across the runner system in mould flow simulations. Accurate simulation ensures that the machine has sufficient injection pressure capability.
