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.
Clamp Force
Meaning
Mechanical pressure holds two halves of an injection mould together to counteract the internal gas and polymer expansion during the filling cycle. This clamp force is expressed in tonnes or kilonewtons depending on the press manufacturer specifications. The threshold for this value depends on the projected area of the part multiplied by the cavity pressure required to ensure full density.
If this value remains too low, material escapes at the parting line and creates flash that ruins dimensional integrity.
Injection Requirement
Precise settings of this metric balance the need to seal the tool against the need to vent air trapped within the mould cavities. Machines provide a fixed tonnage rating that limits the size of parts a processor can produce on a given frame. Virgin resin often demands higher pressures due to its viscosity profiles compared to materials containing regrind or recycled content which possess different flow characteristics.
A moulder sets this value on the controller to match the calculated requirements of the tool design rather than the maximum capacity of the machine.
Operating Constraint
Tool damage occurs when an operator sets the machine tonnage above the structural limits of the steel plates. Excessive pressure flattens the parting lines or crushes the vents until the mould ceases to function. Consistent control of this variable ensures that the internal stresses stay within defined limits so that warping or sink marks remain absent from the final product.
Moulding Economic
Low margins force processors to optimize the tonnage settings to minimize electrical consumption and extend the life of the machine components. Unnecessary reliance on maximum settings increases the wear on tie bars and hydraulic systems. High internal pressure serves as a primary driver of tool wear over millions of cycles.