A molded dimension is the result of tooling, material shrinkage, process conditions, measurement method and geometry. A tolerance should describe what the function needs—not what CAD can display.
Separate functional from noncritical dimensions
Start by identifying dimensions that control sealing, fit, alignment, optics, motion or assembly. Apply tighter tolerances only where the product actually needs them. Over-tolerancing increases tooling effort, inspection burden and scrap risk.
Shrinkage is not one number
Datasheets often provide shrinkage ranges, but actual shrinkage can vary with flow direction, wall thickness, gate location, pressure, packing, fiber orientation and mold temperature. Tool dimensions may therefore require iteration after first shots.
Geometry affects repeatability
Dimensions across a parting line, across separate slides or across long flexible walls can be harder to hold than dimensions formed within one rigid tool component. Tolerance analysis should follow the tooling architecture.
Use GD&T for relationships
When function depends on hole patterns, perpendicularity, flatness or coaxiality, geometric tolerances can communicate intent better than stacks of ± dimensions. Datum selection should reflect how the part is assembled and inspected.
Measurement is part of the specification
Specify a measurement method when flexible plastic can distort under probe force or temperature. Conditioning, fixture strategy and time after molding can all matter for precision parts.
Build a tolerance budget
Combine molded-part variation with mating-part variation in a stack-up. If the stack has no margin, tightening every plastic dimension is rarely the best first solution; redesigning the interface often produces a more robust assembly.
Injection-molding verification
Confirm resin-grade data, mold construction, texture/draft requirements, shrinkage assumptions and the selected molder's capability.