Radial expansion ratio

Radial expansion ratio (RER) and unit density are important physical properties of the extruded foams. High RER and low unit density are ideal for foams because of the reduced cost. Because of the low elasticity of plain PLA, most of PLA foaming materials couldn t attain high RER. Scanning electron micrographs of the foam surfaces and cross-sections are shown in Fig. 5.31. The surfaces of... 

Radial inflow turboexpanders are the best equipment for this application because they handle condensing streams, provide a high expansion ratio in a single stage, and are custom-built to accommodate the plant s process conditions. 

According to Gunn (1968), the radial Peclet number in particulate fluidization (liquid-solid systems) ranges between 1 and 10 for values of Rep in the range 4—1000. Furthermore, the maximum mixing coefficient is found for sf = 0.7. Finally, the lateral (radial) mixing coefficients in gas-solid fluidized beds decrease constantly (for Rep > 10) from about 10 - 0.05 by increasing the expansion ratio from 0.01 - 0.2. 

Typically, the expansion ratio between die and blown tube of film would be 1.5 to 4 times the die diameter. The drawdown between the melt wall thickness and the cooled film thickness occurs in both radial and longitudinal directions and is easily controlled by changing the volume of air inside the bubble and by altering the haul off speed. This gives blown film a better balance of properties than traditional cast or extruded film, which is drawn down along the extrusion direction only. 

Starch degradation may reduce radial expansion, which is critically related to product texture. Chinnaswamy (1993) has summarized many factors that affect expansion of starchy materials. Extmsion pressure is a better predictor of expansion than is die nozzle length/diameter ratio. Starches with 50% amylose showed optimal expansion, and barrel temperatures close to 150°C and low feed moistures favor expansion. Irradiation and certain additives were thought to improve expansion, but only sodium chloride proved to be useful. Over a range of amylose contents, 1% NaCl consistently produced more expansion than did the starch alone. 

Fig. 7.14. Normalized radial residual stresses as a function of coating thickness, I/a, for varying coefficients of thermal expansion (CTE) of the coating, Oc = 10,70,130 x 10 /°C (a) Young s modulus ratio Ej/Em = 0.333 (b) Ei/En, = 1.0. After Kim and Mai (1996a, b).

With a ratio between storage pressure and ambient pressure of about 2 or greater (for air), the flow rate is limited to the sonic velocity of the fluid at the end of the flow restriction (choked flow). At this point, the fluid pressure can be greater than the ambient pressure. The remaining expansion occurs beyond the flow restriction, where the release accelerates both radially and axially. 

As in true strain, the expression above takes into account cross-sectional area changes in a certain cohesive structure (or cake) of powdered material. If the material is isotropic, another possible expression that includes the Poisson s ratio p (the ratio of transverse strain and axial strain resulting from uniformly distributed axial radial stress during static compression of the material in absolute value). The Poisson s ratio, or bulk modulus, permits prediction of the transverse contraction or expansion that occurs when a stress is applied longitudinally. 

Thermal radial [10- expansion axial 6k->] Young s modulus [GPa] Poisson s ratio Fiber radius [pm]... 

J. F. Villeneuve, R. Naslain, R. Fourmeaux and J. Sevely, Longitudinal/radial thermal expansion and Poisson ratio of some ceramic fibers as measured by TEM, Composites Sci. Technology, 49,89-103 (1993). 

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