The effect of the properties of the polymer materials, such as molecular weight, molten viscosity, crystallization rate and the particle size of the powder, on the quality of selective laser sintering (SLS) parts is researched. The results indicate that the molecular weight affects the quality of the SLS parts through the melting viscosity. SLS parts of higher density can be fabricated with polymer materials of lower melting viscosity. Crystallinity largely affects the precision of the SLS part-shrinkage is more serious with increasing crystallinity. SLS parts sintered with polymer powder materials, whose melting peak and crystalline peak differ greatly, have high dimensional precision. The particle size of the powder affects not only the precision but also the density of the SLS part. The appropriate particle size is about 75-100 mm.
ABSTRACT:The glass-transition temperatures and melting behaviors of poly(ethylene terephthalate)/poly(ethylene 2,6-naphthalate) (PET/PEN) blends were studied. Two blend systems were used for this work, with PET and PEN of different grades. It was found that T g increases almost linearly with blend composition. Both the GibbsDiMarzio equation and the Fox equation fit experimental data very well, indicating copolymer-like behavior of the blend systems. Multiple melting peaks were observed for all blend samples as well as for PET and PEN. The equilibrium melting point was obtained using the Hoffman-Weeks method. The melting points of PET and PEN were depressed as a result of the formation of miscible blends and copolymers. The FloryHuggins theory was used to study the melting-point depression for the blend system, and the Nishi-Wang equation was used to calculate the interaction parameter ( 12 ). The calculated 12 is a small negative number, indicating the formation of thermodynamically stable, miscible blends.
ABSTRACT:The transesterification reaction of poly(ethylene terephthalate)/poly(ethylene 2,6-naphthalate) blends during melt-mixing was studied as a function of blending temperature, blending time, blend composition, processing equipment, and different grades of poly(ethylene terephthalate) and poly(ethylene 2,6-naphthalate). Results show that the major factors controlling the reaction are the temperature and time of blending. Efficiency of mixing also plays an important role in transesterification. The reaction kinetics can be modeled using a second-order direct ester-ester interchange reaction. The rate constant (k) was found to have a minimum value at an intermediate PEN content and the activation energy of the rate constant was calculated to be 140 kJ/mol.
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