Melt cooling rate

Modified ETEE is less dense, tougher, and stiffer and exhibits a higher tensile strength and creep resistance than PTEE, PEA, or EEP resins. It is ductile, and displays in various compositions the characteristic of a nonlinear stress—strain relationship. Typical physical properties of Tef2el products are shown in Table 1 (24,25). Properties such as elongation and flex life depend on crystallinity, which is affected by the rate of crysta11i2ation values depend on fabrication conditions and melt cooling rates. 

With falling temperature, both the solubility of the activator oxides (see Section 3.6.1.1) in magnetite and the rate of adjustment to the new phase equilibrium decline. Therefore, by rapid cooling of the melt the activator oxide distribution can be frozen in a condition corresponding to that of a higher temperature [348], According to [285], there may exist relationships between the melt cooling rate, the appearance of certain phases, and the reducibility of the catalysts. 

Spherulite size in un-nucleated material is very sensitive to melt cooling rates. It is therefore essential to have good control of cooling rates in such materials in order to achieve end product consistency. 

Sha Sharif, A., Mohammadi, N., Ghaffarian, S. R. Model prediction of the ESCR of semicrys-talline polyethylene Effects of melt cooling rate. J. Appl. Polym. Sci. 112 (2009) 3249-3256. 

Fig. 12. Differential scanning calorimetry scans of the PET/LC materials during the cooling experiment from the melt. Cooling rate = 20°/min.

A higher mold temperature favors a uniform melt cooling rate, minimizing residual stresses, and improves the surface finish, mold release, and product quality. The mold cooling rate affects finished product quality. Polyether type TPU can set up better and release better. 

The melting point of commercial Teflon PEA is 305°C, ie, between those of PTEE and EEP. Second-order transitions are at —100, —30, and 90°C, as determined by a torsion pendulum (21). The crystallinity of the virgin resin is 65—75%. Specific gravity and crystallinity increase as the cooling rate is reduced. An ice-quenched sample with 48% crystallinity has a specific gravity of 2.123, whereas the press-cooled sample has a crystallinity of 58% and a specific gravity of 2.157. 

Cooling rates can affect product properties in a number of ways. If the polymer melt is sheared into shape the molecules will be oriented. On release of shearing stresses the molecules will tend to re-coil or relax, a process which becomes slower as the temperature is reduced towards the Tg. If the mass solidifies before relaxation is complete (and this is commonly the case) frozen-in orientation will occur and the polymeric mass will be anisotropic with respect to mechanical properties. Sometimes such built-in orientation is deliberately introduced, such as... 

The time available for disorientation as the melt cools from Tp to T. This will depend on the value of Tp-T where is the temperature of the environment (the mould temperature in injection moulding) since this will with the specific heat determine the rate of cooling. The time will also depend on Tp-T since this will determine the extent of cooling. 

Number of cavities, layout and size of cavities/runners/gates/cooling lines/side actions/knockout pins/etc. Relate layout to maximize proper performance of melt and cooling flow patterns to meet part performance requirements preengineer design to minimize wear and deformation of mold (use proper steels) lay out cooling lines to meet temperature to time cooling rate of plastics (particularly crystalline types). 

Another interesting example is the melt crystallization of s-PS. For the case of rapid cooling from the melt, the hexagonal a form is obtained [7-9], while for low cooling rates or for isothermal crystallizations, the crystalline form which is... 

The polymorphism can be inconvenient also for the processing, when small variations of the processing conditions can produce samples in different crystalline forms. This is, for instance, the case of s-PS, for which the crystalline form obtained by cooling from the melt (a and/or p) is dependent not only on the cooling rate but also on the crystalline form of the starting material, on the maximum temperature to which the melt is heated as well as on the time for which the melt is held at this maximum temperature (Sect. 3.1). This requires, in order to get reproducible manufacts, an extremely accurate control of the processing conditions, which is, of course, undesirable in thermoplastic materials. 

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