Meaning
Geometric dimensioning and tolerancing of molded parts requires the establishment of specific points, lines, or areas to serve as references for inspection and measurement. Through datum targets, designers define exactly where the inspection fixtures must contact the uneven surfaces of a molded polymer body. This methodology ensures repeatable measurement setups despite the inherent warping and surface variation typical of plastic parts.
Fixturing Precision
Repeatable alignment of a flexible or warped plastic part during quality control requires rigid, localized contact points. In datum targets, these positions are designated on the drawing and replicated on the inspection fixture with high precision. Without these specified contact areas, different operators would hold the part differently, leading to inconsistent measurement values and false rejections.
This alignment consistency ensures that measured part deviations are representative of actual part quality.
Shrinkage Control
Non-uniform shrinkage and warpage in injection molding can distort the primary reference planes of a part. By using datum targets, metrologists can isolate the critical functional regions of the part from the overall warpage. This selective placement allows the molded component to be measured and accepted based on its interface with mating components rather than its global distortion.
Manufacturing Guidance
Tool designers utilize the designated reference points to align the cavities during the mold-making process. Placing datum targets on stable, non-cosmetic areas of the part prevents ejector pins from interfering with the measurements.
