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.
Angle Shut off Galling
Meaning
Metal transfer occurs when high contact pressure and sliding movement cause localized welding between the steel surfaces of an angled shut off feature in a mould. Angle shut off galling creates surface degradation on the sliding faces, which leads to dimensional inaccuracy in the moulded component. This degradation happens when frictional heat exceeds the thermal stability of the protective oxide layer on the steel.
Once this layer breaks down, atomic bonding takes place between the mating metal surfaces.
Tooling Geometry
High velocity injection stages increase the likelihood of this wear mechanism during the final phase of cavity filling. Engineers avoid this condition by specifying a clearance angle of at least three degrees between the mating faces. Proper application of high pressure lubricants or surface nitriding treatments reduces the coefficient of friction and prevents metal pickup.
Precise alignment of the guide pins keeps these angled faces from dragging against each other during the closing cycle.
Thermal Dynamics
Elevated melt temperatures in thermoplastic processing accelerate the rate at which steel faces lose their lubricity. Frequent cycling of the mould generates significant internal heat that softens the surface matrix of the metal. If the cooling rate of the insert lags behind the cycle time, the resulting thermal expansion creates excessive force at the shut off interface.
Operators monitor these variables to identify when the structural integrity of the tool components nears the threshold of failure.
Economic Consequences
Unplanned maintenance caused by metal adhesion demands an immediate halt to production to allow for polishing or insert replacement. Scrapped parts often result from the burrs and flash generated by the compromised shut off surface. This wear pattern represents a permanent loss of precision that cannot be corrected by adjusting machine parameters.
The cost of tool repair increases linearly with the depth of the damaged metal surface.