Meaning
Fluid resistance occurs when a polymer melt undergoes a sudden reduction in cross sectional area at the transition between a runner and a gate. The entrance pressure represents the specific energy loss attributable to the elastic deformation and acceleration of the melt as it enters a narrow channel. It is a discrete component of the total injection pressure, distinct from the viscous pressure drop that occurs during steady flow through the length of the channel.
Energy Loss
High molecular weight resins exhibit greater elastic effects than standard grades. This behavior results in a substantial entrance pressure that may limit the maximum achievable flow length. This phenomenon is often quantified using a Bagley plot, which separates the excess pressure drop at the entry from the linear pressure gradient within the capillary.
Moulding Impact
Excessive values of this variable contribute to melt fracture and aesthetic defects such as jetting or splay. Controlling the entrance pressure involves optimizing gate lead in angles and maintaining melt temperatures within the recommended processing window of the resin.
Tooling Design
Streamlined transitions reduce the magnitude of this energy loss. A tapered design lowers the entrance pressure and prevents material stagnation at the gate.
