Solidification speed

In spite of this dominance of heat flow, the solidification speed of pure metals still obeys eqn. (6.15), and depends on temperature as shown in Fig. 6.6. But measurements of v(T) are almost impossible for metals. When the undercooling at the interface is big enough to measure easily (T, -T 1°C) then the velocity of the interface is so large (as much as 1 m s 0 that one does not have enough time to measure its temperature. However, as we shall see in a later case study, the kinetics of eqn. (6.15) have allowed the development of a whole new range of glassy metals with new and exciting properties. 

The solidification speed of salol is about 2.3 mm mim at 10°C. Using eqn. (6.15) estimate the energy barrier q that must be crossed by molecules moving from liquid sites to solid sites. The melting point of salol is 43°C and its latent heat of fusion is 3.2 x 10 ° J molecule F Assume that the molecular diameter is about 1 nm. 

The still unknown constant 7 is yielded from the condition (2.206) for the solidification speed. It follows from (2.207) that... 

K. Stephan and B. Holzknecht [2.50] have solved the solidification problems dealt with in 2.3.6.2 in this way Unfortunately the expressions yielded for terms with i > 1 were very complex and this made it very difficult to calculate the solidification-time explicitly K. Stephan and B. Holzknecht therefore derived simpler and rather accurate approximation equations for the solidification speed. 

LCP, on the other hand, maintain their rigid, rodlike structure even in the molten phase that entangling does not occur. LCPs have high flowability, and in practice, wall thickness of 2 mm will flow at least 1000 mm and thinness of 0.5 mm will flow 100 mm or more. Flash rarely is a problem even with this high flow characteristic. The flow length is 2 - 5 times longer than PPS and PA (polyamide), but the occurance of flash is less frequent. This can be explained by solidification speed and shear. Compared with other plastics, the solidification speed of LCP is more than 10 times faster that LCP solidifies before flash occurs. Under high shear rate, melt... 

Q Solidification speed - part thickness and coohng system Q Thickness of molded part - inejction pressure and temperatures Q Thin part - flow charateristics of LUPOY ... 

Fig. 6. Weld pool shape and resultant weld—metal solidification direction, (a) Slow welding speed, (b) Rapid welding speed.

The nature of the solidification process in these cements has received little attention. Rather, attention has focussed on the crystalline components that form in cements which have been allowed to equilibrate for some considerable time the nature of such phases is now quite well understood. Gelation is reasonably rapid for these cements and occurs within a significantly shorter time than does development of crystalline phases. The conclusion may be drawn that initial cementition is not the same as crystallization, but must occur with the development of an essentially amorphous phase. Reactions can continue in the amorphous gelled phase, but are presumably limited in speed by the low diffusion rates possible through such a structure. However, reactions are able to proceed substantially to completion, since in many cases X-ray diffraction has demonstrated almost quantitative conversion of the parent compounds to complex crystalline mixed salts, though several days or weeks of equilibration are required to bring this about. 

A stirrer with large blades operating at high speed is essential. Inadequate stirring results in solidification of the reaction mixture and makes proper washing of the product very difficult. 

Hofmeister et al. 4l() employed two high-speed thermal imaging systems to record spatial and temporal temperature distributions at the splat-substrate interface, and to observe droplet spreading during impact and solidification on a quartz plate. They observed... 

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