Meaning
Maximum height of a three dimensional surface area calculated as the sum of the highest peak and the deepest valley provides a measure of the total profile range. Sz parameter is an areal roughness metric defined in the iso 25178 standard and is the three dimensional equivalent of the rz parameter. It governs the detection of the most extreme features on a surface, which can be critical for both the performance and the safety of a polymer part.
The parameter stops being a reliable measure of the typical surface texture if there is a single isolated defect or piece of dust on the part during the measurement. Moulders use sz values to ensure that there are no features on the mould or the part that could lead to structural failure or aesthetic issues.
Peak Calculation
Computation of the sz value involves finding the highest point and the lowest point in the measurement field and adding their absolute distances from the mean plane. This makes the sz parameter extremely sensitive to outliers and local defects. In the production of high pressure plastic components, a single deep scratch or a high peak can act as a stress concentrator and lead to the premature failure of the part.
Measuring the sz value allows the quality control team to identify these dangerous features before the parts are shipped to the customer. Because it considers the entire range of heights, it provides a worst case scenario for the surface quality.
Defect Sensitivity
Sensitivity of the sz parameter to the most extreme features makes it an excellent tool for troubleshooting issues with the moulding process. If the sz value on a part is much higher than the value on the mould, it could indicate a problem with the material flow or the cooling process. For example, if the plastic is not completely filling the mould, it can create deep valleys or voids that will show up as a high sz value.
If the part is being damaged during the ejection process, this will also result in a significant increase in the maximum height. Moulders use this data to identify the root cause of the problem and to make the necessary adjustments to the machine or the tool.
Tolerance Safety
Establishing a maximum limit for the sz parameter in the design specification provides an extra layer of safety for the part. While the sa and sq parameters provide a good overall measure of the surface, they can hide the presence of a few deep defects that could be critical. Sourcing teams use sz measurements to ensure that the parts they are buying are free from any major surface flaws.
This is especially important for parts used in medical or aerospace applications where reliability is the most important factor. By including sz in the metrology report, the supplier provides proof that the parts have been thoroughly inspected and meet all the quality requirements. The sz parameter is the final check that ensures the surface is truly within the specified limits.
