パルプ射出成形の研究 第１報 －バーフロー金型による流動特性の計測と成形品特性の評価－

抄録

Pulp injection molding was developed in recent years as a method for injection molding three-dimensional molded products using paper material made up mainly of pulp and starch (PIM material). However, no detailed investigations have been conducted on the characteristics of the PIM molding material and molded products. In this study, to evaluate material flow characteristics, the authors developed a new bar-flow mold which is inserted with temperature and pressure sensors at various points to investigate the effects of various molding conditions on flow length. In addition, the internal structure of PIM molded products was observed, and experiments were conducted to evaluate the effects of mechanical characteristics and environmental humidity, and water/moisture absorption characteristics. The following results were obtained;
(1) As molding conditions affecting flow, it was found that mold temperature and moisture contained in materials contributed more significantly than differences in material and gate shape. It was also clarified that flow length of 800 mm at maximum can be realized with a bar-flow cavity measuring 1 mm in thickness and 10 mm in width.
(2) Inside the molded products, many voids were formed due to the generation of high water vapor pressure and the evaporating/drying process following mold open and close operations. The shape and forming state of these voids were confirmed to be significantly related to the dimensional accuracy and shrinkage rate of the molded products.
(3) The expansion ratio following moisture absorption in the no-load state was found to have a difference of 1.7 to 2.5 times in the directions perpendicular and parallel to the fiber orientation. Contact angle water absorption tests revealed that PIM molded products can keep a high water repellency for several minutes.
(4) Results of tensile strength test with varied environmental humidity showed that strength and elasticity drop and breaking elongation of the PIM material increases with increasing humidity, and though breaking elongation was small in all cases (0.05 to 0.13), tensile strength of the PIM material was the same or higher than PP.