By Ektinai Jansri
A two-layered product can be molded through various processes. Rotational molding is a useful process used to produce a two-layered thermoplastic product. However, the rotational molding requires a complicated technique as well as special and expensive equipment. The particle size and size distribution of plastic are one of the process criteria. In this research we studied the possibility of using the one-step rotational molding technique to produce a two-layered molding of polypropylene (PP) and polyethylene (PE) by focusing on the effects of different particle shapes, sizes of PP and the rotational speed.
The axial powder flow apparatus was used for molding study. The particle sizes of PP were determined as equal as, 2 and 4 times as large than those of PE. PP particles were determined to be a spherical and non-spherical shape, and the rotational speed was compared for 7 and 60 rpm. The two materials were mixed in a ratio of 50:50 (wt%) by a dry blending method. The phenomenon happening during the molding was observed and recorded in real time by a high-resolution camera. After molding, the specimens were investigated in terms of their layer separation, thermal property, surface quality, thickness distribution and mechanical property.
The results of the experiment indicated that the two-layered molding of PP and PE using an uncomplicated technique for rotational molding had a high probability of success. Particularly, in the case of PP with a spherical shape, it was 4 times as large as that of PE at the rotational speed of 7 rpm. It was also found that an increase in rotational speed resulted in a more chance of two-layered separation.