Meaning
Spatial differences in hydraulic force distribution within a cavity during the densification phase affect the final dimensions of a molded component. Density variations occur because the viscosity of the cooling melt resists the transmission of force, creating a packing pressure gradient from the gate to the end of the flow path. Uniform density is difficult to achieve in long or thin walled parts where the material freezes off before the pressure is equalized.
Dimensional Stability
Variations in shrinkage lead to warped parts and inconsistent tolerances across a production run. A steep packing pressure gradient causes the areas near the gate to be overpacked while the distal areas remain underpacked.
Weight Variation
Mass consistency is a primary indicator of process stability in precision molding. Fluctuations in the packing pressure gradient result in part weight changes that signal a shift in melt temperature or valve gate timing.
Flow Path
Gating strategy determines how well the pressure is distributed throughout the mold. Using multiple gates reduces the packing pressure gradient by shortening the distance the melt must travel under high pressure. Large parts like automotive bumpers require sequential valve gating to manage the pressure drop.
Sequential control allows the moulder to open gates as the melt front passes, maintaining a more even pressure profile.
