How to Set Up Your Sample Evaluation Criteria?
Setting up clear evaluation criteria is essential before samples arrive — define tolerances, surface finish, and material grades in a signed form. This is not a step you can wing.
Surface finish is typically specified using VDI 34003 standards (VDI 12 to VDI 45) or SPI standards (A-1 to D-3). A glossy cosmetic face needs an SPI A-2 or better, while a hidden structural rib might be fine at SPI C-1. Make sure your supplier knows which surfaces are cosmetic A-surfaces and which are functional B-surfaces. Material requirements should reference a specific grade from a named manufacturer — not just “ABS” or “PC.” For example, specify “SABIC Cycolac MG47” instead of generic ABS. This ensures the supplier cannot substitute a cheaper grade that may have different impact strength, melt flow index, or colorability. Document everything in a Sample Approval Form (SAF) that both parties sign before tooling begins.
The SAF should list: critical dimensions with tolerances, surface finish per surface zone, material grade and color (Pantone/RAL reference), required test reports (dimensional report, material certificate, color match report), and the number of sample pieces to be submitted.
What Does Dimensional Inspection Involve?
Dimensional inspection compares measured part dimensions against drawing tolerances using CMMs, calipers, and optical equipment. It tells you whether the molded part matches the design intent within acceptable tolerances. Here is how to do it right. First, ensure the supplier measures a statistically meaningful number of samples — we recommend at least 5 pieces from different cavities (for multi-cavity molds) and different cycles. Measuring only one piece tells you nothing about process consistency. The supplier should provide a dimensional inspection report (often called an FAI report) that lists every called-out dimension on the drawing, the nominal value, the tolerance band, and the actual measured values for each sample piece.
Look for dimensions that are trending toward the tolerance limit — even if they are technically “in spec,” a dimension consistently sitting at +0.24 mm on a ±0.25 mm tolerance is a red flag that the process is not centered. Common measurement tools include coordinate measuring machines (CMMs) for complex geometries, optical comparators for profile checks, micrometers and calipers for linear dimensions, and surface roughness testers for finish verification. At our facility, we use Zeiss CMM equipment for critical dimensions and project-specific go/no-go gauges for high-volume production checks. Pay special attention to features that are prone to shrinkage variation: wall thickness transitions, rib bases, and areas near gates.
These locations tend to show the most dimensional deviation between sample and production because they are sensitive to packing pressure and cooling uniformity.