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.
Stylus Skidding
Meaning
Measurement errors in surface metrology occur when a physical probe fails to maintain continuous contact with the material surface during a scan. In the quality control of plastic parts, stylus skidding leads to distorted profile measurements and incorrect surface roughness calculations. This phenomenon happens when the stylus jumps or slides over high peaks or deep valleys due to high traverse speeds or insufficient measuring force.
It results in a surface finish that appears smoother than its actual physical state.
Dynamic Error
Mechanical surface profiling relies on the stylus following the precise contours of the micro-texture as it moves across the part. When stylus skidding occurs, the probe becomes airborne or slips sideways, failing to register the actual depth of valleys or the height of peaks. This loss of contact creates a flattened, inaccurate representation of the surface profile.
To avoid this dynamic error, technicians must adjust the traverse speed and ensure the contact force is high enough to keep the probe grounded without gouging the material.
Surface Interaction
Plastic parts present unique challenges for stylus-based metrology because polymers are relatively soft and flexible. While increasing the contact force can prevent stylus skidding, it can also cause the probe to scratch or deform the plastic surface, creating a permanent track on the part. Moulders must find a balance between a high contact force that maintains tracking and a low force that prevents surface damage.
If a polymer has a high coefficient of friction, the stylus can experience stick-slip behavior, which further degrades the measurement accuracy and can lead to false passes in quality control checks. This problem is especially pronounced when testing thermoplastic elastomers or soft polyolefins that yield under very low mechanical stresses.
Measurement Correction
Preventing profile distortion is critical when verifying that a mould cavity or a plastic part meets the required specifications. When stylus skidding is suspected, technicians can reduce the traverse speed of the profilometer or switch to non-contact optical measurement systems. Optical profilers eliminate the risk of surface damage and probe jumping by using light to map the surface texture.
This alternative approach is particularly useful for soft elastomers or very rough surfaces where a physical stylus cannot track the profile reliably.