Appearance characterisation of textured polymeric surfaces
Doctoral thesis, 2005
Appearance demands for industrial products are generally increasing since they are closely related to the overall quality impression. Consequently, there is a need to define appearance through its main descriptors, which are gloss, colour and surface texture, as well as the relations between them.
Surface textures imposed on injection-moulded plastic parts used in interior car components were characterised by means of spatial wavelength-dependent analysis (including fractional analysis, auto-correlation functions and power spectral densities) as an interesting alternative to the traditional roughness parameters, since this provides information in both amplitude and profile directions. Such information was used to relate the surface topography of the plastic component to its optical response in the sense of appearance (colour and gloss).
A modification of the general scalar Kirchhoff approximation, which accounts for geometrical features of the glossmeter and considers the surface descriptors determined by the spatial wavelength-dependent analysis, is used to describe the experimental gloss values of the textured surfaces. The agreement between the measured and the calculated values for incidence angles of 20° and 60° confirmed the general validity of the approach, although the textured surfaces used here are probably too rough for the Kirchhoff theory to be strictly valid. A poorer agreement was noted at a higher angle of incidence (85o in this case). For very smooth surfaces (without any visible texture), the classical Bennett-Porteus theory could in many cases describe the gloss level with sufficient accuracy.
Furthermore, the measured and perceived colour of a plastic object were shown to be determined not only by the optical properties of its constituents (polymer and colorants and their concentration) but also by the surface texture of the component. A psychometric study based on human perception revealed that the limits of acceptance of colour differences varied substantially with the surface texture. In some cases, the observers accepted a larger colour difference for the textured than for the smooth surface, whereas in other cases the reverse situation was noted.
Finally, attention was paid to the effects of processing conditions on the appearance descriptors. For all the surface textures examined, a better replication of the mould texture was obtained with a low melt viscosity at a high shear rate (high injection speed or short injection time) and a high mould temperature. This gave a higher gloss, a lower lightness and an increase in the yellow-blue coordinate for the (visibly) smooth surface regions, whereas the opposite tendency was observed for the rough textures. An increase in the holding pressure had an effect which was similar to but smaller than that observed when the filling rate or mould temperature was increased.