Extrudate roughness

Surface analysis includes observing extrudate roughness associated with flow instability and viewing fracture surfaces. Stereomicroscopes are especially suited to this type of analysis. Incident lighting at a shallow angle serves to enhance topographic contrast. 

Output, which depends on the extruded compound. For example, for given compounds extruded with a given extruder, the linked outputs (base 100 for PVC) can roughly be... 

Werner (1980) has studied devolatilization in corotating twin-screw extruders when the volatile component was stripped from the polymeric solution by applying a vacuum to the system. Rough estimates of the equilibrium partial pressure of the volatile component in the feedstream for each of the systems studied by Werner indicate that this pressure was less than the applied pressure, which means that bubbles could have been formed. Figure 17 shows the influence of the externally applied pressure on the exit concentration for a methyl methacrylate-poly(methyl methacrylate) system of fixed concentration. Note that the exit concentration decreases as the pressure is decreased, but seems to approach an asymptotic value at the lowest pressures studied. Werner also reported that at a fixed flow rate and feed concentration the exit concentration did not vary with screw speed (over the range 150-300 min" ), which also suggests that ky alay, is independent of screw speed. Figure 18 is a plot of data obtained by Werner on an ethylene-low-density poly(ethylene) system and also shows that decreases in the applied pressure result in decreases in the exit concentration, but here a lower asymptote is not observed. 

A study was made of the effects of processing conditions and of blowing and nucleating agents and external lubricants on the apparent density, mechanical properties, linear thermal expansion coefficient and surface roughness of extruded cellular MDPE. Based on the results, recommendations were formulated for the properties of cellular MDPE pipes for the protection of light conducting cables. 16 refs. (Translation of Polimery, Tworzywa Wielkoczasteczkowe, No. 10, 1996, p.580). 

Addition of oxidizing agent to soy flour resulted in a less-puffed or undertexturized extrudates. The extrudate showed rough, discontinuous surface and small dieimeter. This result further proves that the change of disulfide linkages is an essential reaction for texture formation. 

Since MMBS additives are typically used in resin matrix formulations in small amounts of 0.1-15% and these additives are manufactured as aqueous emulsions containing roughly the same amount of water. The frictional heating of mixers, kneaders, and extruders can be used to remove efficiently this small amount of water. Therefore, elaborate and expensive isolation procedures are not needed, because mixing and drying can be combined into one step (13). 

Fig. 13. Effect of core rotation according to the pattern given in Fig. 9 a upon critical shear velocities in polypropylene D r — the first critical velocity at which minor roughness occurs on the surface of the extrudate D" is the second critical velocity at which the extrudate features large-scale distortions, corrugation, roughness...

To evaluate extensively the processability of PVC/EPR and to compare it with that of PVC having roughly the same molecular weight, we carried out a set of measurements by a capillary extrusion rheometer and Brabender plastograph. Other runs were carried out by an industrial size extruder. 

A second consequence of the elastic behaviour of the melt is the occurrence of melt fracture. At high rates of extrusion the elastic deformation of the melt may become so high that it locally breaks, resulting in an irregularly shaped extrudate or in surface roughness. 

Destructuring the native organization of the raw agricultural products is possible with a combination of thermal, mechanical and chemical effects in an extruder (Figure 5.21). In the first constraint zone, the matter is roughly crushed. In the second, a compression (up to 20 bars) in the reverse screw induces a non-degradative break... 

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