Meaning
Hydrodynamic resistance loss occurs at the point where molten polymer transitions from a wide runner or gate into the restricted geometry of a mould cavity. An entrance pressure drop results from the sudden contraction of flow channels and the associated extensional deformation of polymer chains as they align to enter the gate. This phenomenon defines the minimum energy required to overcome inertia and viscous shear at the nozzle or gate interface before the resin reaches the extremities of the part.
Processing Dynamics
Injection moulding requires sufficient hydraulic force to move high-viscosity material through these narrow restrictions without inducing excessive shear heating. Entrance pressure drop increases significantly when the cross-section of a gate decreases relative to the preceding delivery channel. Excessive resistance at this entry point restricts the effective packing pressure, creating a risk of internal voids or incomplete filling in sections far from the gate.
High pressure spikes caused by restrictive geometries force machines to operate near their maximum hydraulic limit, which often accelerates wear on screw tips and check valves.
Tooling Geometry
Sharp edges at the entrance of a gate cause severe flow disruption compared to rounded or tapered transitions. Designers reduce the magnitude of an entrance pressure drop by incorporating generous radii at the runner-to-gate junction. A smooth transition allows the melt front to develop a stable velocity profile before reaching the cavity walls.
This modification stabilizes the filling rate and reduces the likelihood of jetting or melt fracture during high-speed injection cycles.
Material Behaviour
Molecular weight distribution and branching influence the magnitude of the pressure loss observed during resin transition. High-flow resins experience lower resistance at these narrow junctions, while heavily filled compounds require higher pressures to maintain constant flow rates. Regrind materials frequently show inconsistent viscosity profiles that make an entrance pressure drop difficult to predict throughout a continuous production run.
Accurate identification of this variable distinguishes between an insufficient clamping force and an inadequate delivery system design.
