A quantitative study of the appearance quality of weld-line which had been previously examined by human eyes was performed.
ABS (Acrylonitrile-Butadiene Styrene) dumbbell samples were molded with a weld-line in the middle and the weld-line gap depths were measured by a surface roughness measuring instrument. An automatic-search multiple-regression analysis for optimum factors was applied in order to generalize the test results.
The results are summarized as follows;
(1) It became clear that weld-line quality could be quantified by weld-line gap depth.
(2) A reliable equation relating weld-line gap depth, resin viscosity at the weld and cavity pressure was obtained.
(3) The allowable limits of variation in molding factors for producing invisible weld-lines (below 1.0μm of weld-line gap depth) were determined using the multiple regression analysis.

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The melting and crystallization behavior of new syndiotactic polystyrene (s-PS) with high tactisity were investigated and compared with those of isotactic polystyrene (i-PS). The weight average molecular weights (M_w) of the s-PS samples were 3.5×10⁵ (s-PS 35), 1.14×10⁶ (s-PS 114) and 2.00×10⁶ (s-PS 200), respectively, and that of i-PS was 2.6×10⁶. The melting temperatures of s-PS and i-PS with various mole-cular weights were measured by dilatometry and DSC. The melting temperature of s-PS 35 measured by dilatometry was 275°C, and that of i-PS was 233°C. The difference in temperature between the melting point and the crystallization point in the thermal hysteresis of s-PS 35 was 25°C to 30°C, which was the half of the i-PS value. The equilibrium melting temperature of s-PS 35 obtained after the Hoffman-Weeks method was 275°C, that of s-PS 114 was 279°C and that of s-PS 200 was 282°C. The spherulites of s-PS samples were obtained by isothermal crystallization from the melt with thin thickness. All these spherulites displayed positive birefringence. The number of spherulites was unchanged through out the observations and the spherulites were nucleated at the same place during every crystallization experiment. The Avrami exponent of the isothermal crystallization was 3 for each s-PS sample with various molecular weights. Thus the s-PS spherulites were believed to grew by heterogeneous nucleation with three dimensional growth, which was the same for the i-PS isothermal crystallization. The maximum crystallization rate constants of s-PS were at least 10⁵ times larger than those for a i-PS within the present experimental conditions. The crystallization rate constant of the s-PS samples at a given crystallization temperature decreased with increasing M_w, of the s-PS samples.

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