Melting and Solidification

In the plastication step of the process, melting is critical in controlling cycle time. Also, during thermoforming, the heating of the sheet is the most time consuming step of the whole process. For example, if we consider the melting of an infinite slab, at an initial temperature of To, as presented in Fig. 6.61, the heat supplied by thehot wall, set at a heater temperature of Th, will create a layer of molten polymer of thickness, X (t). 

Dimensionless pressure flow velocity profile for various dimensionless temperature 

For such a case, where the temperature can be computed as a function of time using 

The equation clearly demonstrates that the molten layer, with temperatures above Tm for semi-crystalline polymers and above Tg for amorphous polymers, will follow the following relation 

In either case, the growth rate of the molten layer during conduction melting follows the relation 

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Little subcooling => to assure that melting and solidification proceed at the same temperature. 

Applications of PCM cover many diverse fields. As mentioned before, the most important selection criterion is the phase change temperature. Only an appropriate selection ensures repeated melting and solidification. Connected to the melting and solidification process is the heat flux. The range of heat flux in different applications covers a wide range from several kW for space heating with water or air, domestic hot water and power plants to the order of several W for temperature protection and transport boxes (Figure 124). 

As it turns out, there are pharmaceutical implications associated with the polymorphism of glycerol esters, since phase transformation reactions caused by the melting and solidification of these compounds during formulation can have profound effects on the quality of products. For instance, during the development of an oil-in-water cream formulation, syneresis of the aqueous phase was observed upon using certain sources of glyceryl monostearate [13]. Primarily through the use of variable temperature X-ray diffraction, it was learned that... 

To pp. 41-43. The criticism of the methods based on Eq. (65) was expanded by J. J. Biker-man in a paper submitted to a magazine. These methods are not justified also because the quantities pj and pj in Eq. (64) refer to two different systems, in each of which a uniform pressure (Pl or pj) acts. In the experiments of Fig. 17 and the analogous tests, two different pressures are supposed to act in one system. A detailed consideration of such systems shows that in them no reversible melting and solidification, fully depending on the local curvature, can take place. Moreover, the actual pressures in the containers used depended on the flexing of the container walls, mentioned in Ref. 

PE, PP and PA are semi-crystalline polymers melting and solidification go accompanied by a (though gradual) volume jump. PS, PVC and PC are amorphous thermoplastics upon solidification they show no volume jump, but only a bend in the V-T relation. 

Bailey, A. E. Melting and solidification of fats. New York, London Interscience Publ. 1950. 

Although all polymer processes involve complex phenomena that are non-isothermal, non-Newtonian and often viscoelastic, most of them can be simplified sufficiently to allow the construction of analytical models. These analytical models involve one or more of the simple flows derived in the previous chapter. These back of the envelope models allow us to predict pressures, velocity fields, temperature fields, melting and solidification times, cycle times, etc. The models that are derived will aid the student or engineer to better understand the process under consideration, allowing for optimization of processing conditions, and even geometries and part performance. 

Heat, momentum, mass, entropy balances at finite domain structure levels of solids and liquids, during deformation, melting, and solidification ... 

Most real cases of polymer melting (and solidification) involve complex geometries and shapes, temperature-dependent properties, and a phase change. The rigorous treatment for such problems involve numerical solutions (12-15) using finite difference (FDM) or FEMs. Figure 5.9 presents calculated temperature profiles using the Crank-Nicolson FDM (16) for the solidification of a HDPE melt inside a flat-sheet injection-mold cavity. The HDPE melt that has filled the cavity is considered to be initially isothermal at 300°F, and the mold wall temperature is 100°F. 

Determinations of milk fat melting and solidification points are attempts to quantify the upper end of the milk fat melting point range. The result obtained from a measurement depends on the method used a melting point or a solidification point is defined by the technique and conditions of measurement, and the reported value is specific for these (Kaylegian and Lindsay 1994 Firestone, 1998). 

Figure 15. Typical milkfat melting and solidification curves obtained by differential scanning calorimetry (107).

The difference between the solidification and melting temperatures in cylindrical pores is due to the fact that the shapes of the interfaces present during these transitions are different. In spherical shaped pores however, there is no difference and the same thermodynamic equation can be used to describe both solid —> liquid and liquid -> solid transitions. Consequently by analysing both the melting and solidification curves, one determines a pore shape factor. In thermoporometry the shape factor for a porous material [68,69] can vary generally between 1 (spherical pores) and 2 (cylindrical pores). 

Busse, P. Deuerler, F. Poetschke, J. Stability of the ODS alloy CMSX6-AI2O3 during melting and solidification under low gravity. J. Cryst. Growth. 1998, 193, 413 25. 

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