Defects matte

When a molten plastic is forced through a die it is found that under certain conditions there will be defects in the extrudate. In the worst case this will take the form of gross distortion of the extrudate but it can be as slight as a dullness of the surface. In most cases flow defects are to be avoided since they affect the quality of the output and the efficiency of the processing operation. However, in some cases if the flow anomaly can be controlled and reproduced, it can be used to advantage - for example, in the production of sheets with matt surface finish. Flow defects result from a combination of melt flow properties, die design and processing conditions but the exact causes and mechanisms are not completely understood. The two most common defects are... 

E.B. Tadmo et al Quasicontinuum analysis of defects in solids. Phil. Mag. A Phys. Cond. Matt. Defects Mech. Prop. 73, 1529 (1996)... 

Matt N, Ghyselinck NB, Wendling O, Chambon P, Mark M (2003) Retinoic acid-induced developmental defects are mediated by RARb/RXR heterodimers in the pharyngeal endoderm. Development, 130 2083-2093. 

In semi-automatic moulding the operator may inspect each unit for features including colour, dispersion and uniformity of colour, standard and uniformity of gloss or mattness, and the presence or otherwise of defects like silver streaks or stringing. However, it is preferable if assessments can be made in numerical terms, expressed in units that may be compared quantitatively and subjected easily to statistical treatment and analysis. It is often convenient in the course of... 

Tadmor, E.B., Ortiz, M. and Phillips, R. (1996a) Quasicontinuum Analysis of Defects in Solids. Philos. Mag. A — Phys. Cond. Matt. Struct. Defects, Mech. Prop., 73, 1529-1563. 

Mattes, J., Yang, M., Mahalingam, S., Kuehr, J., Webb, D.C. et al. (2002) Intrinsic defect in T cell production of interleukin (1L)-13 in the absence of both lL-5 and eotaxin precludes the development of eosinophilia and airways hyperreactivity in experimental asthma. The Journal of Experimental Medicine, 195, 1433-1444. 

Sharkskin is a surface defect that occins at high extrusion rates, resulting in a rough matt or orange peel finish. The problem occurs in the extrusion die when the shear stress or shear rate exceeds a critical value. 

Matt, N., N. B. Ghyselinck, O. Wendling, P. Chambon, and M. Mark. 2003. Retinoic Acid-Induced Developmental Defects Are Mediated by Rar 3/Rxr Heterodimers in the Pharyngeal Endoderm. Development 130, no 10 2083-93. 

Flow defects, especially as they affect the appearance of a product, play an important role in many processes. Flow defects are not always undesirable, as, for example, in producing a matt finish. Six important types of defects can be identified, which are applied here to extrusion because of its relative simplicity. These flow analyses can be related to other processes and even to the complex flow of injection molding. 

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