Multi Cavity Injection Molding Rheological Runner Balancing Principles
Rheological runner balancing uses melt rotation inserts to equalize thermal shear layers across runner branches, preventing filling imbalances across cavities.
Packing Pressure Transmission
Meaning
Efficiency with which hydraulic or electric force applied at the screw tip is converted into actual cavity pressure before the gate freezes. Packing pressure transmission determines the final density and dimensions of a molded component by compensating for the volumetric shrinkage of the cooling melt. Low transmission levels lead to sink marks and undersized parts.
Pressure Gradient
Resistance to flow increases as the polymer cools and its viscosity rises, creating a drop in pressure between the nozzle and the far end of the cavity. Packing pressure transmission is highest when the melt is hot and the flow paths are thick. As the plastic enters the packing phase, the material near the mold walls solidifies, narrowing the effective channel for pressure to move through.
If the gate is too small, it freezes before the cavity is fully packed, cutting off the transmission of force. This results in a part with a hollow or porous core and poor dimensional stability. Processors measure this effect by comparing the peak nozzle pressure with the readings from an in-mold transducer located at the end of fill.
Shrinkage Compensation
Volumetric reduction of the polymer is offset by forcing more material into the mold during the holding stage. This reliance on packing pressure transmission prevents the formation of internal voids.
Quality Variable
Strong transmission ensures that the part replicates the texture and dimensions of the tool. Inconsistent packing pressure transmission leads to warp and weight fluctuations between cavities. Maintaining a warm mold can improve the duration of the packing window.