Meaning
Dimensional variation allowed in the diameter of a cylindrical cavity or recess machined into mould plates is defined by this engineering limit. A precise bore tolerance dictates the acceptable deviation for guide pin bushings and ejector pin channels in injection moulding tooling. When machining drifts outside this specification, components may either bind during thermal expansion or allow molten polymer to leak into the gaps.
The boundary of this metric is restricted to the physical dimensions of the tool, distinguishing it from the shrinkage tolerances of the moulded plastic parts themselves.
Machining Precision
Manufacturing protocols for polymer tooling require high-repeatability boring or grinding operations to achieve the necessary dimensional control. The execution of a tight bore tolerance relies on high-grade tool steel and stable machining temperatures to prevent micro-scale drift. Tooling suppliers often quote datasheet values under controlled ambient environments.
Moulders find that maintaining these clearances across a long production run requires active coolant regulation within the mould plates.
Assembly Clearance
Mechanical movement within a mould requires a sliding fit that prevents binding while maintaining alignment. The application of bore tolerance governs the clearance between moving cores, ejector pins, and their respective housings. Insufficient clearance blocks the travel of these components.
Lubricants can fail under the resulting friction, leading to galling and unscheduled production halts.
Flash Control
High injection pressures force molten resin into any available micro-gap if the machined cavities do not fit tightly. Achieving a tight bore tolerance prevents the formation of plastic flash along ejector lines or sensor housings. Regrind material containing degraded polymer chains or inconsistent fillers exacerbates this leakage due to unpredictable viscosity drops.
When flash occurs, secondary trimming operations increase the overall unit cost of the moulded part.
