Two-phase structure of micro ultrasonic powder molding ultra-high molecular weight polyethylene parts

Through micro ultrasonic powder molding (micro-UPM), micro semi-crystalline polymer parts were formed. By the proposed method, semi-crystalline polymer powder was rapidly heated and plasticized by ultrasonic vibration, after which the microcavity was filled with the melt under sonotrode pressure (p_u) to form a variety of micro polymer parts. Differential scanning calorimetry, scanning electron microscopy, and nanoindentation tests demonstrate that micro-UPM ultra-high molecular weight polyethylene (UHMWPE) parts consist of nascent and melt-recrystallized phases. Different t_u (0.5~1.5 s) values are utilized to form UHMWPE parts at p_u of 16 MPa and the holding time of 5.0 s. As t_u increases, a rise in the melting peak of the melt-recrystallized phase and a drop in the melting peak of the nascent phrase as well as crystallinity are further observed. When t_u is only 1.5 s, the crystallinity of the micro plastic part reaches the minimum value of 54.3% and the melt-recrystallized phase fraction f_R reaches the maximum value of 98.3%. Powder particle interfaces almost disappear in this case, and optimum quality of the micro UPM UHMWPE part is achieved. Single-crystal XRD tests demonstrate that the crystallinity in the core region of the micro polymer part is higher than that in the skin region. Compared with conventional microinjection plastic parts, the crystal orientation of micro-UPM polymer parts is less obvious.