The use of microhardness (H ) to characterize the changes in microstructure, molecular orientation and micromechanical properties of injection-molded polymer materials has been the object of increasing interest [1–4]. In addition, it is known that process variables induce important changes in the microstructure and properties of the molded material [5, 6]. Hardness variations often occur in the surface and across the thickness of the molded samples. As a result, the mechanical properties can be controlled by processing variables such as melt and mold temperature, injection pressure, etc. [2]. Furthermore, it is known that microhardness can detect microstructural changes in glassy polymers, offering information about aging effects below the glass transition temperature (Tg) and conformational changes and local molecular rearrangements above Tg [7–10]. Earlier studies [11] have shown that microhardness may give useful information about the correlation between processing parameters near and at the weld line or knit line, i.e. the region where separated melt fronts reunite. In practice, it occurs in injection molding, i.e. after a flow obstacle or in case of multiple gating for melt streams of the same material or in case of two component injection molding for melt streams of different polymer materials [12]. The aim of the present note is to extend the above studies to the microhardness investigation of two glassy polymers (PC and PS) processed using a two-component injection molding system. Specifically, we wish to examine the H -value across the weld line arising when the two opposite flow fronts are filling the cavity of the mold. Polycarbonate (PC) (Makrolon 3200, Bayer) with Tg= 148 ◦C and high molecular weight polystyrene (PS-168 N BASF) (Tg= 100 ◦C) samples were used in this study. The moldings were prepared in the form of ASTM tensile bars (gauge length 170 mm, thickness 4 mm and width 10 mm) using a two-component injection molding machine in which both melt streams (of same material, though differently colored) can be controlled independently. The bars were molded using for PC a melt temperature Tm= 270 ◦C and a mold temperature Tw= 80 ◦C, respectively. The injection pressure for both melt fronts was 170× 105 Pa and the speed of the melt front was 200 m s−1. In the case of PS, values of Tm= 230 ◦C and Tw= 50 ◦C were used. The speed of the melt front and the injection pressure here were 0.2 m s−1 and 110× 105 Pa, respectively. Microhardness was measured across the boundary at the surface of each of the moldings using a microindentation tester with a Vickers square based pyramidal diamond indenter. The hardness value (in MPa) was derived from the residual projected diagonal impression using, H = k P/d2, where P is the applied force in N, k is a geometrical constant equal to 1.854, and d is the length of the projected indentation diagonal in m. A load of P = 0.25 N and a loading cycle of 6 s were used. Special care was taken to make indentations whose diagonals were parallel and perpendicular to the injection direction (z). Fig. 1 illustrates the variation of H on the surfaces of the molding along the injection direction, z, for the PC and PS samples, respectively. We denote the position of the boundary plane as z= 0. Results clearly show first the gradual decrease of H for both PC and PS samples along the z direction, until a minimum value at z= 0 is reached. Then one observes a further increase
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Aug 27, 2007
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