Meaning
Elastic deformation of the steel plates and support structures of an injection mold under the influence of high internal cavity pressures. This mold base deflection can lead to flash formation, dimensional drift, parting line erosion and premature wear. The magnitude of the movement depends on the clamp force, bolster plate thickness, support pillar arrangement and the modulus of the tool steel.
Heavy bolsters resist this bending better than thin ones.
Pressure Load
Pressure from the injected melt pushes against the cavity walls with forces that can reach several thousand bars. When mold base deflection occurs, the two halves of the tool separate slightly. This opening creates a path for the molten plastic to escape.
Support pillars are often added to the ejector housing to minimize this bending.
Critical Tolerance
Dimensions of the molded part are compromised when the tool is not rigid. Wall thickness increases in the center of the part where the deflection is usually greatest. Such variations make it difficult to maintain the tight tolerances required for assembly.
Mold base deflection is a common cause of rejects in large-format parts where the projected area is high.
Economic Balance
Over-engineering the mold base to prevent movement increases the initial capital expenditure. However, the cost of maintenance and scrap from mold base deflection often outweighs the price of thicker steel plates. Using high-modulus tool steel can help reduce the physical footprint of the mold while maintaining rigidity.
Proper simulation of the tool under load during the design phase prevents these issues from appearing on the production floor. This analysis must account for the peak pressure which occurs during the transition from filling to packing. A tool that seems stable at lower pressures may fail when the full force of the machine is applied to a high-viscosity resin.
