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.
Rz Roughness
Meaning
Surface metrology uses the average of the five largest individual peak-to-valley heights over a specified evaluation length to characterize the micro-geometry of a material. In polymer manufacturing and mould design, Rz roughness is used to measure the extreme profile deviations that could affect part ejection and cosmetic appearance. This metric captures localized surface flaws that would be averaged out by other surface measurements.
It allows engineers to ensure that the cavity surfaces are smooth enough to prevent polymer parts from sticking in the tool during ejection.
Profile Calculation
Calculating the metric requires dividing the total measurement length into five equal sampling lengths and identifying the highest peak and lowest valley in each section. The average of these five values yields the Rz roughness of the polymer surface. This measurement is highly sensitive to isolated surface defects, making it an excellent choice for evaluating critical seal surfaces.
By analyzing these localized extremes, manufacturers can predict whether a rubber or plastic part will leak or wear prematurely.
Molding Application
High-quality tooling requires specific surface treatments to achieve the desired release properties and finish on the moulded component. When machining a mould cavity, measuring the Rz roughness helps the toolmaker verify that the EDM (electrical discharge machining) or milling marks have been sufficiently polished out. If these deep micro-grooves remain, the molten plastic will flow into them under high pressure, mechanical locking the part in the cavity and causing drag marks or scuffing during ejection.
Keeping this value low is particularly important for polymers with high shrinkage rates or when moulding deep-draw parts with minimal draft angles. Ensuring the surface falls within the specified limits reduces cycle times by allowing the part to release cleanly and quickly.
Quality Control
Maintenance protocols for injection moulds should include regular measurements of the part finish to detect tooling wear and degradation. When checking a production run, measuring Rz roughness allows quality control technicians to spot the initial signs of mould surface erosion or deposit build-up from polymer additives. If the value begins to rise, it suggests that the mould steel requires cleaning or re-polishing to prevent defects.
This monitoring is essential when using glass-filled resins, which are highly abrasive and can quickly erode the polished surfaces of the mould cavity.