Anisotropic Shrinkage Variance in Glass-Filled Polyamide Secondary Tooling Nests
Secondary tooling nests for glass-filled polyamide accommodate anisotropic shrinkage through floating datum pins aligned with transverse contraction vectors.
Locator Clearance
Meaning
Fixture design parameters define the intentional gap or fit tolerance allowed between a workpiece feature and a locating pin or block. Locator clearance ensures that parts can be loaded and unloaded from inspection and machining fixtures without sticking or deforming. This gap must be tightly controlled when inspecting injection molded plastic parts to balance ease of use with measurement accuracy.
Positional Clearance
The gap size must account for the natural dimensional variation of the molded hole or slot that receives the locating pin. If the gap is too tight, the operator will struggle to load the part onto the fixture, which can bend the part or wear down the pin. Conversely, an excessive gap allows the part to shift, leading to inconsistent inspection measurements.
Fixture Assembly
Machinists specify the dimensional fit between the locating pin and its mounting hole to ensure alignment accuracy. Standard practices utilize press fits to secure the pin firmly into the fixture base.
Metrology Accuracy
Allowing too much movement on a fixture ruins the repeatability of coordinate measuring machine scans by introducing random offsets. In high-volume automotive molding, locator clearance is kept to the minimum value that still allows manual loading of the parts. If mold shrinkage fluctuates during a run, the clearance must be re-evaluated to prevent the locating pins from binding or allowing too much play.