Meaning
Variations in the filling characteristics of multiple mould sections describe a condition where different parts within the same shot exhibit inconsistent weights or dimensions. Cavity imbalance occurs when the melt does not reach every gate at the same moment or under the same pressure. This phenomenon often results from non-uniform cooling or shear heating within the runner system.
It creates a situation where one part might be flashed while another in the same cycle is short-moulded. The boundary of the term is reached when the variation is within the acceptable tolerance of the quality specification.
Rheological Divergence
Molten polymer behaves as a non-Newtonian fluid which means its viscosity changes based on the speed and pressure of the flow. When the plastic travels through a runner system, the friction against the walls generates heat that lowers the viscosity of the material. If the runner layout is not naturally balanced, the plastic entering one cavity might be hotter and less viscous than the plastic entering another.
This difference leads to uneven packing and varied shrinkage rates across the tool. A balanced runner design aims to give every flow path an identical length and history of shear to minimise these effects. This physical reality is why a simple H-pattern runner is often preferred over a ladder-style layout.
In a ladder layout, the resin reaches the first gates sooner and at a higher pressure, while the further gates receive material that has cooled and slowed down. Engineers must account for these fluid dynamics to prevent a situation where the moulder is fighting the physics of the delivery system. High-viscosity resins like polycarbonate are particularly sensitive to these imbalances because they require high pressures to fill the tool.
Dimensional Variance
Parts produced in an imbalanced tool often fail to meet the strict dimensional requirements of a high-precision assembly. A cavity that fills early and packs more densely will produce a part that is slightly larger and heavier than its counterparts. This variation causes problems during post-moulding operations like sonic welding or automated assembly.
When the operator adjusts the machine settings to fix a short shot in one cavity, they might inadvertently cause flashing or burning in another. The lack of uniformity forces a wider process window that can compromise the overall quality of the production run.
Tooling Correction
Engineers use flow simulation software to predict where these imbalances will occur before the steel is cut for the mould. Adjustments to the gate size or the runner diameter can compensate for the differences in flow resistance between the near and far cavities. In some cases, the cooling lines are modified to provide more aggressive heat removal in the areas that stay hot longer.
This iterative process ensures that the finished tool performs predictably across the entire shot. Proper maintenance of the gates and runners is also required to prevent the buildup of carbonised resin that would reintroduce the problem. Consistent monitoring of part weight remains the primary method for detecting this issue.
