Optical CMM Gauge Capability Evaluation for Sub Millimeter Molded Features
Optical CMM capability on sub-millimeter molded features requires material-specific bias correction and telecentric backlighting to maintain GR&R under 20%.
Polymer Translucency Bias
Meaning
Erroneous readings occur when light penetrates the surface of a semi transparent material before reflecting back to the sensor. A polymer translucency bias causes a vision system to perceive the surface of a part as being deeper or further away than it actually is.
Material Property
Light interaction varies based on the resin type and the presence of pigments or fillers. In cases of high polymer translucency bias, the light scatters within the material and softens the appearance of edges. This effect makes it difficult for automated systems to find a sharp boundary for measurement.
Light Penetration
Depth of the photon travel depends on the wavelength of the illumination and the density of the part. Reducing polymer translucency bias often requires switching to a shorter wavelength light source or applying a temporary matte coating. These adjustments ensure that the reflection comes from the outer surface rather than the internal structure.
Gauge Error
Inaccuracy introduced by subsurface scattering can lead to the rejection of good parts or the acceptance of bad ones. When polymer translucency bias remains uncorrected, the resulting data fails to represent the physical dimensions of the component. Metrology labs must calibrate their instruments against known samples of the same material to account for this shift.