Managing Anisotropic Shrinkage and Restraint Fixturing in Polymer Metrology
Polymer metrology requires force-controlled restraint fixturing and thermal conditioning to isolate mold geometry from anisotropic shrinkage and sag.
Free State Modifier
Meaning
Geometric symbols indicate that a part requirement applies while the component is in its unconstrained condition. Designers use the free state modifier for non rigid parts that might deform under their own weight or due to internal stresses. This symbol tells the inspector not to use clamps or fixtures during the measurement process.
Tolerance Application
Definition of a circularity or flatness limit often requires this designation for thin walled plastic or rubber items. Without a free state modifier, an inspector might assume the part should be held in a way that simulates its final assembly. The modifier ensures that the base manufacturing quality is assessed independently of assembly forces.
Measurement Condition
Inspection reports must note if the part was measured on a surface plate without any external pressure. When the free state modifier is applied, the allowable variation is typically larger than it would be for a constrained part. This accounts for the natural flexibility of the material.
Gravity Distortion
Deflection caused by the mass of the object itself can influence the results of a metrology check. If a large plastic panel is checked under the rules of the free state modifier, the support points used during the test must be clearly defined to ensure repeatable data. Engineers analyze these results to distinguish between a defect in the mold and the normal behavior of a flexible component.