Tooling Surface Polish Impact on First Article Inspection Accuracy
Uncalibrated optical scans on mirror-polished plastic introduce up to fifteen micrometers of false elevations, ruining First Article Inspection accuracy.
Ra Roughness
Meaning
Surface texture evaluation relies on the arithmetic average of the absolute values of the profile height deviations from the mean line within a evaluation length. In injection moulding, Ra roughness defines the micro-scale finish of both the mould steel and the resulting plastic part. This parameter provides a general baseline for surface smoothness but does not distinguish between peaks and valleys.
It helps quality engineers assess whether a component meets cosmetic or functional requirements.
Surface Measurement
Profilometers use either a physical stylus or an optical sensor to track the microscopic surface variations of a moulded part. Measuring the Ra roughness of a polymer component requires a clear understanding of the material’s elasticity, as a soft resin can deform under the weight of a physical stylus. Optical measurement techniques avoid this issue by using light reflections to compile the profile.
The resulting value represents the average deviation, smoothing out individual high peaks or deep scratches that might exist on the sample.
Molding Quality
Replicating a specific surface texture requires careful control over the moulding parameters, as the finish on the plastic part does not always match the tool surface. High cavity pressures and elevated mould temperatures are necessary to force the polymer melt into the micro-cavities of the tool, ensuring the correct Ra roughness is achieved. If the mould temperature is too low, the polymer freezes prematurely, resulting in a part with a higher roughness value than the tool design specified.
This drift can cause aesthetic mismatches, especially in automotive interior parts where a consistent matte finish is required.
Specification Limit
Design drawings specify surface finishes to ensure proper assembly, seal performance, or coating adhesion. In high-precision applications, a material specification might demand a maximum Ra roughness of 0.8 micrometres to prevent friction and wear between moving plastic parts. Moulders must run regular tests across each production batch to ensure the parts remain within this limit, as tool wear can gradually degrade the finish over time.
If the roughness drifts beyond the specified range, the moulder must schedule tool maintenance or adjust the injection parameters to restore the required finish.