Melt orientation

The problem-solving approach that ties the processing variables to products properties includes considering melt orientation, polymer degradation, free volume/molecular packing and relaxation, cooling stresses, and other such factors. The most influential of these four conditions is melt orientation, which can be related to molded-in stress or strain. 

The application of an extending field to the melt simultaneously uncoils and orients the molecules. The extension of the molecules leads to a decrease in their effective flexibility (see Eq. (4)). The essential question is how high should the values of melt orientation and the changes in flexibility be to ensure formation of ECC in subsequent... 

The injection molding with rotation (MWR) is an example of processing at lower temperatures, pressure, etc. It is also called injection spin molding or injection stretched molding. This BM process combines injection molding and IBM, as performed in IBM reviewed, except it has the additional step of with melt orientation (Dow patent). The equipment used is what is commercially available for IM except the mold is modified so that either the core pin or outside cavity rotates. The rotated melt on its preform pin is transferred to a blow mold. The end product can come directly from the I MM mold or be a result of two-stage fabrication malting a parison and BM the parison.164... 

The question of why electrochemists have so seldom used a technique which is their own is of interest. First, only the faces of the lowest surface energies can be obtained [the (111) and (100) faces for the fee system]. Second, if an epitaxial growth is observable on some metals it is not on others. Up to the present, dl measurements were done on faces grown from the metal electrolyte solutions only in the case of silver." "" These silver faces were grown in glass or Teflon capillaries on a substrate which was a massive silver crystal grown from the melt (oriented, cut, and polished according the desired face). 

Spinning speed, spinline stress, melt orientation... 

Crystallization of an oriented mesophase> The crystallization process of oriented nematic melts can lead in certain cases to highly oriented crystalline fibers while, in other cases, the crystallite orientation is totally or partly disrupted by the process of crystalliza-tion% Thus, DDA-8, MAA-8 and its copolymers MAA-DDA-8 lead to strongly oriented crystalline fibers on crystallization from nematic melts oriented in a magnetic field of 12 Tesla On the other hand, DDA-9 and its copolymers are much less oriented under the same circumstances. 

The polymer melt flows through a die to the next process step flat, circular, or slot dies are preferred. The flow through the die must be uniform across the exit plane. However, this is complicated by the nonlinear dependence of melt viscosity on both temperature and shear rate in the die (23,24). The suitability of a material is determined by measuring the flow properties with a capillary rheometer in the temperature and shear-rate range expected. Melt elasticity can cause flow instabilities, which affect haze and thickness (27,28) or the operation of downstream equipment. Exit melt velocity, flow characteristics, and quenching rate may impart significant orientation to the polymer. In some instances, melt orientation is reduced in others, it is maintained by quenching. 

Blown-Film Processes. Bubble orientation technology permits simultaneous biaxial orientation directly from the melt (24) or by inflation of a quenched tube. With homogeneous films, the latter method may give films of slightly different properties than those of films produced on tenter frames. Melt orientation is less effective and properties are different because of lower orientation. 

The injection pressure in the mould cavity decreases considerably with distance from the gate. The location, shape and number of gates influence melt orientation. This explains the very large variation in the shrinkage which sometimes occurs between different parts of a moulding, particularly complex mouldings. 

Another area of current research deals with combining the earlier work on production of polymer blends in the elastic melt extruder and the observation of the maximm in melt elasticity as a fimction of composition as fo md in the orthogonal rheometer studies on blends. It would be expected that as the melt elasticity increased the forces required for melt orientation would also increase. This has been confirmed in current studies of melt tension in fiber spinning experiments. The important point is that the composition of the blend can be used to control the molecular orientation. 

The elastic modulus of samples crystallized from the melt increases with melt orientation, but even in the most favorable circumstances it is an order of magnitude lower than that predicted for a perfectly aligned sample. This is indicative of the large number of defects (principally entanglements and chain ends) trapped within the sample, which limit the overall level of ordering that can be developed. 

As melt orientation prior to crystallization increases, the entropy of the sample decreases due to better alignment of the molecules. One result of this improved alignment is a general increase in the degree of crystallinity as a function of increased orientation, which reflects the lower energy barrier to crystallization. This effect is most noticeable for linear polyethylene samples, especially those with high molecular weights. 

One of the characteristics of injection molding is that melt orientation imparted by rapid flow is frozen into the final product. Solidification occurs fastest next to the mold surface, with the result that highly oriented skins are often found overlaying a less oriented core. Orientation may be beneficial or detrimental. 

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