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.
Morphology of microinjection moulded polyoxymethylene
Abstract A microinjection moulding machine was used to obtain micromouldings of polyoxymethylene, in order to study morphology development during the process. The method of design of experiments was used to investigate statistically the effects of processing variables on the microstructural features of the mouldings. The morphological features were identified by microtoming the samples in both transverse and longitudinal (flow) directions and observing the microtomed sections under a polarised light microscope. Morphology evolution along the flow direction was followed by microtoming the specimens along the centre plane longitudinally and sequentially. A five-layer skin core structure was identified for micromoulded polyoxymethylene. The development of the structure was explained, based on mechanisms which were similar to those proposed for conventional injection moulding. Injection speed was found to be the most significant factor affecting morphological features of the final moulding. Moreover, the average plunger velocity, which is directly related to the cavity filling flow rate, was found to have good correlation with skin layer thickness. The distributions of crystalline polymorphs were observed and explained, in light of the distributions of the flow and thermal patterns in the mould. Morphology evolution along the flow direction reflected the distribution of pressure, temperature and velocity of the polymer melt during the microinjection moulding process. The results provided some indications regarding micromoulding mould design.
Neutron Residual Stress Measurements on Rail Sections for Different Production Conditions
Rail sectioning with subsequent neutron diffraction experiments has been used to assess residual stresses in the rails. In this study we present the results of neutron stress measurements performed at the NIST Center for Neutron Research (NCNR) on rail sections from rails that were produced under various conditions. Specifically, these are air-cooled, air-cooled and roller-straightened, head-hardened and head-hardened and roller-straightened. More significantly, a head-hardened and roller-straightened rail was also studied after service to elucidate evolution of the service-induced residual stresses. In the latter case both a transverse-cut slice and the central region of a 0.53 m long piece were studied. Measurements on this piece are the first in which triaxial stresses have been determined for an intact rail. Neutron strain measurements with 3×3×3 mm3 spatial resolution were successfully employed for transversally cut slices to verify the difference in the stress state depending on the production process. Although examination of slices allows determination of only two-dimensional stresses in the plane of the slice, additional measurements on obliquely-cut slices, which were also carried out, and utilization of FEM gives the possibility of reconstructing the full triaxial stress distribution. Together, these approaches provide a better understanding of rail fabrication and the possibility of improving the durability and safety of rails in the future.Copyright © 2004 by ASME