Appearance of Textured and Pigmented Polymeric Surfaces
Doctoral thesis, 2011
The surface appearance is of great importance in the production of high-quality injection-moulded products since it is strongly connected to the quality impression. The surface appearance of any object may be described in terms of colour, gloss and texture. A deeper understanding of the relations between these attributes is required and also of the human visual perception of appearance. Visible surface defects have a direct effect on the quality impression of the product. Knowledge about the formation of such defects facilitates their elimination.
The relations between colour, gloss and surface texture of injection-moulded polymeric specimens was evaluated by means of a spectrophotometer, a glossmeter and surface topography characterization. An increase of the lightness of the specimens gave a higher measured gloss of textured specimens, which was attributed to a contribution from bulk scattering. Increased surface roughness altered the colour of the surface; it mainly became lighter and less saturated. The magnitude of the change was determined by the colour of the material and was in a satisfactory manner predicted using a model developed for xerographic printing paper. The effect of the surface texture on the measured gloss was evaluated for polymeric surfaces with very small differences in texture and gloss. In attempt to describe the relation, a modification of the general scalar Kirchhoff's theory was used. Surface topography parameters were obtained by means of optical laser profilometry and a SEM technique, the latter estimating the surface topography from micrographs constituting stereographic pairs. The variations in gloss were however in several cases not revealed by the differences in the topography measurements; thus the Kirchhoff's model could only partially be applied.
An observer panel was employed in order to study the visual perception of appearance. Specimens differing in gloss and texture were visually assessed. When assessing gloss, the observers were in many cases in fair agreement with the measured specular gloss. In some cases the observers were however able to discern differences between specimens not revealed by the measurements. An influence of the colour on the visually perceived gloss of textured specimens was also noticed, which did not correspond to the measured specular gloss values. Significantly improved correspondence between measured gloss and visually perceived gloss was achieved when measures of contrast gloss was obtained by means of a multi-angle spectrophotometer. The relation between visually perceived gloss and the reflectance properties of textured polymeric specimens was further studied by the determination of bi-angular reflectance distribution functions obtained from measurements of angle-resolved light scattering.
Finally, a particular aesthetic defect called ghost mark, which locally alters the gloss of the surface, was characterized. Small-scale deformations of the surface texture were observed in the defect area possibly imposed by the injection-moulding holding pressure or due to non-uniform thermal surface shrinkage during cooling.