Meaning
State of balance in a solid body occurs when all applied forces and internal stress fields sum to zero with no resulting acceleration. Analyzing elastostatic equilibrium in polymer moulding determines how mould plates and cores deform under the high pressure of injected melt. This assessment ensures that the clamp force and mechanical strength of the mould can prevent excessive deflection.
The calculation governs the structural integrity of both the tool and the cooling part before ejection.
Mechanical Balance
Internal stress tensors must balance the external boundary pressures exerted by the pressurized polymer. During the pack and hold phases of injection moulding, melt pressures of up to one hundred megapascals act on the cavity walls. The steel plates must reach a stable state of rest where their elastic recovery forces match these injection forces, preventing flash from forming at the parting line.
Tooling Stability
Minimizing mould plate deformation ensures that the moulded parts maintain their target thickness and weight. If the mould does not maintain this balance within tight limits, the core can shift, leading to uneven wall thickness in the plastic component. This deflection also accelerates wear on the guide pins and parting surfaces, which increases tooling maintenance costs.
Dimensional Consequence
Residual stress distribution within the cooled polymer must reach a balanced state after the part is ejected from the tool. When the part is still in the cavity, the rigid mould walls prevent deformation, but once ejected, the internal stresses redistribute to achieve a new balanced state. This redistribution is the primary driver of part warpage and dimensional shrinkage, which moulders must compensate for by optimizing the cooling rate and hold pressure.
