Meaning
Optical profiling discrepancies occur when the measured boundary height of a molded polymer part differs from its physical dimension due to local variations in material reflectivity or phase change. This phenomenon occurs when light interacts with polymer surface regions of varying density or crystallization, creating false phase shifts. Measurements obtained by optical means must therefore be calibrated to account for these localized optical properties, which do not correlate with physical contact stylus readings.
The error is bounded by the optical consistency of the molded material and the light source wavelength.
Interference Mechanism
Light reflecting from different regions of a polymer part can undergo unequal phase shifts. The resulting apparent step height error distorts the calculated height profile by several nanometers. Such distortion happens because the interferometer interprets the phase difference as a physical elevation change.
Moulders must calibrate for these phase changes to ensure the accuracy of micro-fluidic channel depth measurements.
Material Influence
Refractive index shifts between resins alter the optical path. Uncontrolled blending of regrind material introduces localized optical inhomogeneities that increase the apparent step height error during inspection. This makes consistency in the polymer feed necessary for reliable metrology.
Process Effect
High mold temperatures and high packing pressures can induce localized polymer crystallization near the cavity wall. This change in crystalline structure affects the local refractive index and increases the apparent step height error during post-moulding inspections. Tooling temperatures must be held within tight tolerances to prevent these localized variations across the production run.
When the process drifts, false rejection of acceptable parts can occur, increasing scrap rates.
