Semicrystalline polymers molding

Processes such as film extrusion, fiber spinning, injection molding, and drawing tend to impart orientation to products made from semicrystalline polymers. Mechanical, dielectric, and optical properties, to mention only three, are often strongly influenced by orientation. X-ray diffraction offers a direct approach to studying crystallite orientation because the Intensity that is diffracted into a detector placed at an appropriate position is directly proportional to the number of crystal lattice planes that are in the correct orientation for diffraction. The principles of such measurements are well described in textbooks 0,2). 

Adiabatic Compression Heating Melting of polymers by adiabatic compression has been shown to be feasible for processes such as injection molding (2). Discuss this method, in principle, in terms of an order-of-magnitude analysis of the terms of the thermal energy balance for an amorphous (PS) and a semicrystalline polymer (LDPE). Use the data in Appendix A. 

AH/R = 8.525 K 1 for PB H-300, is known. Finally, we note that the nonisothermal temperature profiles here exist only in the absence of a phase transition, which occurs in the gas-assisted injection-molding process, at least with semicrystalline polymers. 

Li et al. (26) reported a polymer crystallization method to modify CNTs and nanofibers with semicrystalline polymers in a periodic manner, leading to a novel nanohybrid shish—kebab superstructure. Zou et al. (27) proposed a "roping and wrapping" method to disperse MWCNTs by non-covalently wrapping with linear polymers. Liang et al. (28) used molding processing technique to modify... 

Weld lines (also known as knit lines) are a potential source of weakness in molded and extruded plastic products. These occur when separate polymer melt flows meet and weld more or less into each other. Knit lines arise from flows around barriers, as in double or multigating and use of inserts in injection molding. The primary source of weld lines in extrusion is flow around spiders (multiarmed devices that hold the extrusion die). The melt temperature and melt elasticity (which is mentioned in the next section of this chapter) have major influences on the mechanical properties of weld lines. The tensile and impact strength of plastics that fail without appreciable yielding may be reduced considerably by in doublegated moldings, compared to that of samples without weld lines. Polystryrene and SAN copolymers are typical of such materials. The effects of weld lines is relatively minor with ductile amorphous plastics like ABS and polycarbonate and with semicrystalline polymers such as polyoxymethylene. Tliis is because these materials can reduce stress concentrations by yielding [22]. 

Fig. 3.4 Possible effects of processing (injection molding) on the microstructure of a semicrystalline polymer. In contact with the mold wall (which is assumed to be perpendicular to the plane of the scheme) a surface skin morphology is formed. An isotropic microstructure can be observed in the center core interior. Adopted with permission from [15]...

The coalescence of polymers is driven by the work of surface tension, which counteracts the viscous dissipation associated with the molecular diffusion within the coalescing domain. This phenomenon is often referred to in the literature as polymer sintering. In the rotational molding process, coalescence occurs at temperatures above that of the material melting point when dealing with semicrystalline polymers, or above the glass transition temperature for amorphous resins. The first analytical model describing the coalescence process was proposed by Frenkel ... 

The optical properties of semicrystalline polymers are often anisotropic. On the other hand, amorphous polymers are normally isotropic unless directional stresses are frozen in a glassy specimen during fabrication by a process such as injection molding. Anisotropy can often be induced in an amorphous polymer by imposing an electric field (Kerr effect), a magnetic field (Cotton-Mouton effect), or a mechanical deformation. Such external perturbations can also increase the anisotropy of a polymer that is anisotropic even in the absence of the perturbation. 

The effect of biaxial strain on the transport properties of semicrystalline polymers has been demonstrated for several systems (42) However, particular attention has been paid to poly(ethylene terephthalate)(47-49) because of its wide use in blow molding applications. 

Jang et al. (24-27) studied extensively craze formation in semicrystalline polymers, namely in virgin and rubber-modified polypropylene. They studied the effects of injection-molding conditions on PP morphology and its relation to crazing at low temperatures and high strain rates. Their investigation characterized... 

Mendoza et al. (2003) studied experimentally the influence of processing conditions on the spatial distribution of the molecular orientation in injection molded isotactic polypropylene (iPP) plates. They found that the anisotropy of injection molded semi-crystalline polymers is governed by the orientation of the crystalline phase, and the distribution of the orientation strongly depends on the shear rate. Doufas et al. (2000), followed by Zheng and Kennedy (2001), have applied a rigid dumbbell model to simulate crystalline orientation in injection molded semicrystalline polymers. The model reads, in the form used by Zheng and Kennedy (2001, 2004) ... 

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