Determining Shut off Land Relief and Contact Stress in Multi Cavity Tooling
Balancing shut-off land relief area against clamp force prevents steel hobbing while holding plastic melt seal across multi-cavity injection tooling.
Cavity Separation Pressure
Meaning
Internal forces generated by the molten polymer during the injection and packing phases exert a counter-pressure against the mould halves. This cavity separation pressure works directly against the clamping force of the moulding machine. When internal force exceeds the tonnage holding the tool shut, the halves move apart.
This movement allows material to escape into the gaps, resulting in flash.
Pressure Distribution
Clamping requirements are calculated by multiplying the projected area of the part by the maximum internal pressure. High-viscosity resins require higher injection pressures, which increases the potential for separation at the parting line.
Process Stability
Consistent part weights depend on the tool remaining perfectly closed throughout the cycle. Fluctuations in melt temperature or injection speed change the cavity separation pressure from one shot to the next. Such variations lead to dimensional instability in the finished parts.
When the machine tonnage is near its limit, even small spikes in pressure cause the tool to breathe.
Hydraulic Response
Precision control of the clamp pressure ensures that the tool resists the internal forces without damaging the parting surfaces. Excessive clamping force to counter separation pressure can crush the venting channels or cause permanent tool deformation. Proper settings maintain a thin margin of safety between the machine capacity and the expected internal force.
Precise pressure regulation prevents damage to the tool and the machine.