ISO 25178 Areal Surface Texture Parameters for Optical Inspection
ISO 25178 defines 3D areal surface parameters like Sa and Sdr, replacing 2D profiles to qualify optical non-contact inspection on textured tool steel and mouldings.
Surface Texture Drawing Callout
Meaning
Technical documentation defines the topography of a moulded polymer component through a set of symbols and numeric values designated as a surface texture drawing callout. Engineers define the required arithmetic average roughness or peak to valley height on technical prints to control the light diffusion and tactile feel of the finished product. This standard governs the validation of mould tooling, ensuring the texture engraved into the steel cavity matches the desired aesthetic or functional finish of the plastic part.
Tooling Variance
Mould makers achieve these specified parameters by applying processes such as chemical etching, sand blasting, or laser texturing directly onto the metal surface of the mould cavity. Each process leaves a distinct physical signature that a surface texture drawing callout quantifies to prevent deviations during mass production. Variations occur when the depth or density of the texture on the tool steel deviates from the print, altering the shrinkage rate of the molten resin and affecting the dimensional accuracy of the final moulded geometry.
Resin flow patterns change as the plastic encounters different levels of resistance from these textures, forcing a compromise between part geometry and the desired visual finish.
Material Relationship
High viscosity resins frequently fail to replicate the finer details of a complex texture, leaving the part with a glossy or inconsistent finish that deviates from the intent of the surface texture drawing callout. Amorphous polymers often provide better fidelity to the texturing on the tool steel compared to semi crystalline materials, which experience more significant differential shrinkage when encountering textured surfaces. Virgin resin behaves consistently across the moulding run, whereas batches containing regrind often show increased surface variability due to changes in molecular weight distribution and rheology.
Datasheet values for surface appearance remain theoretical, as the actual output depends on the pressure profiles, cooling times, and temperature settings the moulder maintains to fill the textured areas of the cavity.
Measurement Protocol
Profilometers and optical scanning instruments verify the height and spacing of surface irregularities against the specific tolerances provided by the surface texture drawing callout. These tools extract data from several random locations on the part to ensure the average roughness value falls within the acceptable range defined by the design team. Non contact laser scanners provide efficiency when measuring soft elastomeric parts, as physical contact during stylus based testing risks deforming the surface and yielding incorrect topography results.
Consistent calibration of measuring equipment remains the primary method for isolating the influence of the mould finish from the effects of the moulding process.