Datum Target Placement Mechanics for Compliant Thermoplastic Parts
Datum target placement on compliant plastic parts requires locating pads on rigid structural features and using restrained-state inspection standards.
Non-Rigid Moldings
Meaning
Category of plastic components characterized by a lack of inherent structural stiffness that allows for measurable deformation under their own weight. Handling non-rigid moldings requires specialized support during measurement to ensure repeatable data. These parts often include thin-walled elastomeric seals or large, flexible automotive interior panels.
Measurement Challenge
Gravity distorts the geometry of flexible parts when they are placed on a standard coordinate measuring machine. Inspection of non-rigid moldings typically involves a restraining fixture to hold the component in its intended assembly position. This restraint ensures that the dimensions are checked against the design intent rather than the free-state distortion.
Assembly Condition
Functional performance depends on the fit achieved once the part is fastened to a rigid mating structure. Designers specify the state of non-rigid moldings using datum targets that define how the part should be held during verification. Without these definitions, repeatability between different inspection sites becomes impossible.
Material Elasticity
Low modulus resins such as thermoplastic elastomers provide the flexibility needed for these applications. High elasticity in non-rigid moldings makes them resilient to impact but susceptible to creep if stored under load. Extended cooling times prevent the formation of permanent sets during the ejection of these components.