Crystallization during extrusion

Polyethylene can crystallize during extrusion if the conditions are favorable. This phenomenon, known as shear-induced crystallization, occurs when highly oriented melts are extruded at temperatures at or below their melting points (93). 

Crystallization from an oriented melt may occur during extrusion and injection molding processes of crystallizable polymers. The crystalline microstructure which results is... 

Tsebrenko et al. [1976] reported on fibrillation of POM in a copolyamide (CPA) matrix, as a result of flow through a capillary. Fine fibrils with diameters of about 20 j,m and length 3.2 mm were obtained during extrusion at T = T (POM) H- 6°C. The low extmsion temperature facilitated stress-induced crystallization of the POM fibers, preserving the morphology engendered at the entrance to the spinneret. 

Finally, in addition to the consumer properties dictated by the phase structure of soap, they are also affected by the orientation and compactness of the crystals imparted by the physical work processes. When soap is induced to flow as during extrusion, the crystallites tend to align themselves so as to offer the least resistance (Mills, 1942). This orientation affects the penetration of water into the soap bar which therefore influences the solubility and related features such as slush in the finished product. 

Figure 13.1.3a shows the low thermal expansion scheme of extruded cordierite due to the orientation of the crystals. The c-axes of flat-shaped kaoli-nite crystals are oriented vertical to the honeycomb wall by the shearing force generated during extrusion through the narrow slit of the die. The c-axis of... 

Figure 3. Data taken during extrusion of iPP with a low wind-up speed using a 2-D wire chamber to collect the SAXS data and an image plate to collect the WAXS data, (a) Close to the die head tear shaped scattering features are observed in 2-D SAXS with only an amorphous halo in 2-D WAXS. (b) At the furthest position from the die head, when crystallization starts, isotropic rings from crystals can be seen in SAXS and in the WAXS.

SPS sheet can be thermoformed only in its amorphous state by rapid cooUng from the melt during sheet extrusion to prevent crystallization. During ther-moforming, the amorphous sheet is heated to a temperature that allows... 

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