Meaning
Specification of the allowable variation in the location of a feature relative to a defined datum system represents a fundamental requirement in geometric dimensioning and tolerancing. Defining the positional tolerance of holes, ribs, and inserts is crucial in multi-component molded assemblies where parts must fit together during assembly. For injection molded parts, this variable is more difficult to control than for machined parts due to the non-linear shrinkage of polymer resins.
This value is established in the part drawing and is governed by the mold design and cooling conditions.
Moulding Control
Controlling the thermal history of the polymer inside the mold is essential for minimizing warpage and ensuring the part falls within specification. To achieve a tight positional tolerance, the mold cooling channels must be designed to cool the part uniformly across its entire surface. Uneven cooling rates create internal stresses that cause the part to twist after ejection, shifting critical holes out of their required locations.
Measurement Verification
Inspection of the part locations is typically performed using coordinate measuring machines or optical scanners. If a feature’s positional tolerance is very narrow, the measuring fixture must hold the part securely without deforming the plastic. This deformation can be minimized by using kinematic registration to locate the part on its functional datum surfaces during measurement.
Economic Consequence
Over-specifying this value can lead to a high scrap rate and require expensive modifications to the mold core and cavity. Selecting a wider positional tolerance that still ensures functional assembly is preferred to minimize production costs and ensure a robust molding process.
