Ageing of polymeric surfaces exhibiting flow marks—Effects on surface characteristics

Abstract

Surface defects associated with manufacturing processes can severely impair the appearance of polymeric products. These defects can be enhanced with ageing (time, UV-light, heat and weather conditions). In the present contribution, the interest is focused on the defects generated during injection molding of elastomermodified polymers that are often used in the automotive sector to produce interior and exterior components. In particular, defects denoted as ''flow marks'' or ''tiger stripes'' were investigated. Two different grades of elastomermodified polypropylene (PP)-containing mineral fillers were studied here before and after heat ageing for 9 weeks at a constant temperature of 95 C. Although injection moldings are one of the grades that did not exhibit any flow marks, moldings of other PP-grade showed such defects and they were enhanced after the ageing process. The properties and structure of the glossy and the dull bands (as well as the effect of the thermal ageing) have been assessed by several characteristics such as microhardness, nanohardness, crystallinity, and surface composition. An increase in surface hardness was noted after the ageing treatment as a consequence of increased crystallinity. Dull and glossy bands also exhibited different values of the nanohardness, glossy bands were harder, and the dull bands appeared to be less crystalline. The surface composition of glossy and dull regions was slightly different; the surface regions of the glossy areas contained higher amounts of filler particles. The effect of the ageing on the striped appearance is furthermore discussed in relation to possible depletion of the stabilizer system.

Effect of microstructure tuning by rapid heat cycle molding on the photodegradation behavior of isotactic polypropylene

“…From the previous DSC and XRD results, it is clear that the chemical crystallization caused by photodegradation leads to the changes in crystal structure of iPP, such as an increase in crystallinity, which increase its hardness. 56 As the duration of UV exposure increases, the photodegradation extends to the interior of iPP, leading to the degradation and breakage of its molecular chains, and the molecules no longer have a more regular and tight arrangement, so the hardness of iPP decreases. 57 Compared with CIM, RHCM maintains a higher hardness during photodegradation, and RHCM90 has the least change in hardness, indicating that photodegradation has less effect on it.…”

Neural network feature and architecture optimization for injection molding surface defect prediction of model polypropylene

“…[23,24] Previous studies conducted by Iannuuzzi et al, Chang et al, and Mathieu et al have discussed possible flow mechanisms in charge of imparting these surface defects. [25,26,27] As this occurrence is a multi-variable problem and nonlinear, a ML algorithm is an approximation method. This approximation method can be used to predict defects within a part, just as they have been used in the past to predict the amount of warpage in an injection molding process based on a combination of input parameters.…”

Development of Unstable Flow and Morphological Analysis of Flow Marks in Injection Molding of Polypropylene and Polyolefin Elastomer Blend

“…The flow marks have some common features. They all consist of visually glossy and cloudy regions that alternate on the surface of the injection molded products and usually appear downstream of the flow, which is almost perpendicular to the flow direction of the melt. – In many cases, the period of the flow marks is reversed on the opposite side. – That is, the cloudy and glossy regions on one side correspond to the glossy and cloudy regions on the opposite side. It is worth noting that the flow marks adversely affect not only the desired appearance of the injection molded products but also their required long-life mechanical properties. – Therefore, it is important and necessary to clarify the generation mechanism of the flow marks and reduce or eliminate them.…”