Molecular orientation in injection molding

Fig. 13.13 Shrinkage distributions of injection molded amorphous PS (a) at two injection rates longitudinal direction, (b) solid curves, longitudinal direction broken curve, transverse direction. [Reprinted with permission from G. Menges and G. Wiibken, Influence of Processing Conditions on Molecular Orientation in Injection Molds, Soc. Plastics Eng., 31st Annual Technical Conference, Montreal, Canada, 1973.]...

Fig. 13.15 Schematic representation of the flow pattern in the central portion of the advancing front between two parallel plates. The coordinate system moves in the x direction with the front velocity. Black rectangles denote the stretching deformation the fluid particles experience. [Reprinted by permission from Z. Tadmor, Molecular Orientation in Injection Molding, J. Appl. Polym. Sci., 18, 1753 (1974).]...

G. Menges and G. Wiibken, Influence of Processing Conditions on Molecular Orientation in Injection Molds, SPE ANTEC Tech. Papers, 19, 519 (1973). 

Fujiyama, M. Wakino, T. Molecular orientation in injection-molded polypropylene copolymers with ethylene. Inter. Polym. Proc. 1992, VII (2), 159. 

Hierarchical structure and molecular orientation in injection molded PLCs... 

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) ... 

Mavridis H, Hrymak AN, Vlachopoulos J (1988) The effect of fountain flow on molecular orientation in injection molding. J Rheol 32 639-663 McLeish TCB, Larson RG (1998) Molecular constitutive equations for a class of branched polymers the Pom-Pom polymer. J Rheol 42 81-110... 

Mendoza R, R6giiier G, Seiler W and Lebrun J L (2003) Spatial distribution of molecular orientation in injected molded iPP Influence of processing conditions. Polymer 44 3363-3373. 

Menges, G. and W. Wiibken. 1973. Influence of Processing Con-dihons on Molecular Orientation in Injection Molds. Society of Plastics Engineers Technical Papers, 31, 519. 

The orientation functions and birefringence are widespreadly used as measures of molecular orientation of injection-molded PPs. In addition, the crystalline orientation fraction, skin thickness, and annealing shrinkage are determined in the practice. 

Studies of the orientation phenomena in injection molding have shown that irrespective of the molding conditions and type of thermoplastic, molecular orientation is higher near the walls and decreases toward the interior. " " Low crystallinity and flow-induced structures have been observed near the walls for injection molded isotactic polypropylene parts. " High modulus materials have also been obtained with some injection molding techniques. " ... 

The molecular orientation in an injection-molded part depends on [84] ... 

Molecular orientations resulting from shearing forces applied during processing vary widely depending on the process. In injection molding of thermoplastics and TPEs, molecular and fiber orientations have a dominant influence on the structural component properties. 

In injection molding of semicrystalline thermoplastics, for example polyamide, polyoxymethylene, ultrahigh molecular polyethylene (very high melt viscosity), linear polyesters (PET, PBT), local variations in molecular orientation result in dimensional variations in the part (warpage, memory effect), especially if the toothed gear is exposed to a thermal stress load. 

The strength of weld lines in injection-molded articles is generally weaker than that of the bulk. This is due to the incomplete entanglement of polymer chains at the two impinging fountain flow fronts in the mold cavity. When the two flow fronts meet in the cold cavity, the molecular orientation at the interface remains parallel to the weld line as evidenced by the frozen-in molecular orientation found in the molded article under proper injection-molding conditions... 

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