Meaning
A mechanical feature in injection mould tooling consists of a non-parallel interface between the cavity and core inserts where the mating surfaces meet at a pre-engineered draft angle. An angled shut off prevents flash by using the interference of the slanted steel faces to create a seal as the mould closes. This configuration directs the clamping force into a lateral compression of the contact area rather than a perpendicular crushing of the parting line.
Tooling Geometry
Precise clearance settings determine the success of this interface during the high-pressure stage of the injection cycle. The machinist maintains a deliberate gap in the cold state to allow for thermal expansion of the steel inserts as the hot resin flows into the tool. Should the gap close too early or fail to make contact due to deflection, molten plastic escapes the cavity and forms unwanted projections on the part.
Proper alignment relies on guide pins or heel blocks to center the inserts before the slant surfaces engage.
Resin Flow
High viscosity materials require tighter tolerances because the plastic resists compression better than low viscosity grades. A moulder adjusts the injection speed to control how quickly the pressure wave encounters the sealing surface. Rapid filling sometimes causes metal deformation if the contact angle is too shallow for the force applied.
Operators track the consistency of these shuts to identify wear patterns that appear on the steel after long production runs.
Material Performance
Virgin resin properties determine the susceptibility of the part to flash when the shut off function degrades. Recycled compounds with inconsistent melt flow rates put additional stress on the interface because they create unpredictable internal pressures inside the cavity. A specification for a moulded part defines the tolerance at the shut off as a functional dimension linked to the geometry of the tool.
Metal fatigue at the sealing point represents a permanent loss of dimensional control.
