Weld Lines in Nylon 6 Melt-Blended Nanocomposites J. Tung, G.P. Simon, G.H. Edward Cooperative Research Center for Polymers, Department of Materials Engineering, Monash University, Clayton, Victoria 3800, Australia The effects of weld lines in injection moldings of nylon 6 and nylon 6 nanocomposite samples were investigated by comparing single-end-gated and double-end-gated tensile samples. The single-gated samples have no weld line, whereas the double-end-gated configuration pro- duces a weld line at the center of the gauge length. Nylon 6 shows little variation in tensile properties for samples with or without weld lines, remaining ductile and tough, even with weld lines present. However, nylon 6 nanocomposites containing organically modified montmorillonite (organoclay), produced by a melt blend- ing technique, exhibits rigid and brittle behavior for both single (no weld line) and double-end-gated (with weld line) samples. The organoclay increases the tensile strength but reduces the strain-to-failure significantly in both cases. A modified L 16 orthogonal array based on the Taguchi approach with three levels was designed to run injection-molding experiments to allow production of a modest number of samples to identify the most important process factors. The results were analyzed using the statistical tools signal-to-noise (S/N) ratio and analysis of variance (ANOVA), in particular showing that the principal process factors for the double-end-gated nylon 6 nanocomposite samples are mold and melt tem- peratures. POLYM. ENG. SCI., 45:1606 –1614, 2005. © 2005 Society of Plastics Engineers INTRODUCTION In recent years, layered silicate nanocomposites have attracted a great deal of interest from researchers, being a new class of composite that contain nanoparticles dispersed in a polymer matrix [1]. Nylon 6 nanocomposites are usu- ally comprised of montmorillonite-containing silicate layers that are 1 nm in thickness and are preferably fully del- aminated and uniformly dispersed within the polymer ma- trix [2]. Nylon 6 organoclay nanocomposites can be pro- duced by either melt blending, in situ polymerization [3], or solution [4] mixing techniques. Melt blending is the most common method for preparing nylon 6 nanocomposites, with the organoclay directly mixed with the molten poly- mer. The melt blending technique is economical and straightforward, and while maximum exfoliation of organo- clay is not usually achieved, such a combination with nylon 6 produces a relatively high level of delamination [5]. The injection molding process involves the injection of a polymer melt flow into a cavity mold where the melt cools and solidifies to form a plastic product and is a three-phase process comprising filling, packing, and cooling phases. Weld lines are a source of mechanical weakness and their formation is an unavoidable defect in injection molding due to the complexity of modern product design. The formation of weld lines inevitably occurs when there are cores, pins, and multiple gates, all of which divide the molten polymer flow pattern in the cavity mold. While weld lines can be deleterious in homopolymer moldings, the problem can be amplified in two-phase systems, such as glass-reinforced thermoplastics [6]. Weld lines also consist of v-notches at the weld line surface, which is often forgotten as one of the possible causes of weakening, potentially acting as stress concentra- tors by increasing the stress level at the bottom of a v-notch [7]. Chang and Faison [8] reported that v-notch appearance and weld line locations depend on the design of mold and product, and subsequently processing conditions. The ap- pearance of weld lines is found to be more visible on one surface than the other of the molding. Using optical micros- copy, fan-shaped v-notches are noticeable at the edges of the weld lines. It was reported that the fan-shaped appear- ance is caused by the polymer melt fronts impinging at the weld region [8]. The different appearance of both sides of the specimen is attributed to the geometry of the specimen and the location of the mold parting line, through which air escapes from the advancing melt fronts. A number of remedies for controlling processing condi- tions have been investigated in order to enhance the molec- ular diffusion at the weld line boundaries in injection- molded products [9]. The most significant processing parameters that have been identified in polymers such as polystyrene, polypropylene, polycarbonate, and polyoxym- ethylene (POM) are, in decreasing order of importance: melt temperature, mold temperature, holding pressure, holding time, and injection velocity. Temperature was found to be Correspondence to: J. Tung, e-mail: jason.tung@spme.monash.edu.au Contract grant sponsor: Cooperative Research Centre for Polymers (CRC-P). DOI 10.1002/pen.20455 Published online in Wiley InterScience (www.interscience.wiley. com). © 2005 Society of Plastics Engineers POLYMER ENGINEERING AND SCIENCE—2005