Relationship Of Shear Histroy, Morphology – Microstructure And Mechanical Properties Of Micro Injection Molded Parts
The relationship of shear history, morphologymicrostructure and mechanical properties of the microscale parts was investigated based on the polypropylene parts with thickness 0.2mm and 0.5mm molded under varied injection speed. Shear rate was analyzed using Moldflow. 0.5 mm parts showed skin-core structure in the thickness direction with imperfect shish-kebab structure appeared in the transition layer between skin layer and core layer, however, the transition layer of 0.2 mm parts shows columnar crystal. The whole shear level in shear history increased with injection speed increasing for all the parts with two thicknesses. The ratio of skin layer of 0.5 mm parts decreased as the injection speed increased, which result in the decreasing of yield stress, modulus, breaking strength and elongation at break. The ratio of skin layer of 0.2 mm parts increased with injection speed increasing, and results in increasing of yield stress, modulus and breaking strength, and decreasing of elongation at break.
Morphological evolution of HDPE parts in the microinjection molding: Comparison with conventional injection molding
To compare the difference of morphological evolution of HDPE micropart and macropart, micropart with 200 μm thickness and macropart with 2000 μm thickness were prepared. The PLM images of micropart and macropart exhibited a similar “skin–core” structure, but the micropart showed a much larger fraction of orientation layer. The SEM observation of core layer of micropart featured an unoriented lamellae structure and shear layer of micropart showed a highly oriented shish-kebab structure. The 2D-WAXD patterns of shear layer of macropart indicated twisted oriented shish-kebab (KM-I) structures, however that of micropart indicated untwisted oriented shish-kebab (KM-II) structures which was firstly found in microinjection molding. The diffraction pattern of the micropart exhibited stronger azimuthal dependence than the shear layer of macropart, indicating the most pronounced orientation of HDPE chains within lamellae. © 2012 Wiley Periodicals, Inc. J Appl Polym Sci, 2012
Brittle-ductile transition and toughening mechanism in POM/TPU/CaCO3 ternary composites
Polyoxymethylene (POM)/elastomer/filler ternary composites were prepared, in which thermoplastic polyurethane (TPU) and an inorganic filler, CaCO 3 were used to achieve balanced machanical properties of POM. A two -step processing method, in which the elastomer and the filler were mixed to a masterbatch first and then the masterbatch was melt-blendend with pure POM, was used to obtain a coreshell microstructure with CaCO 3 covered by TPU. A brittleductile transition phenomenon was observed with increasing TPU content for this ternary system. To better understand the toughening mechanism, we investigated the fractured surface, interparticle distance, and the spherulite size of POM as function of the TPU and Ca CO 3 content. The critical TPU content depended on not only the content of CaCO 3 , but also the size of CaCO 3 particles. The observed brittle-ductile transition was discussed based on the crystallinity and spherulite size of POM as well as Wu's critical interparticle distance theory. The results showed that the impact strengh of POM/TPU/CaCO 3 ternary system depends on a critical, interparticle distance, which varies from one system to another. The dependence of the impact strengh on the spherulite size was considered for the first time, and a single curve was constructed. A critical spherulite size of 40 micron was found, at which brittle-ductile transition occurs, regardless of the TPU and CaCO 3 content or the size of CaCO 3 particles. Our results indicate that the spherulite size of POM indeed plays a role in determining the toughness, and must be considered when discussing the toughening mechanism.