Pulling rate

The presence of defects and impurities is unavoidable. They are created during tire growtli or penetrate into tlie material during tlie processing. For example, in a crystal grown from tire melt, impurities come from tire cmcible and tire ambient, and are present in tire source material. Depending on factors such as tire pressure, tire pull rate and temperature gradients, tire crystal may be rich in vacancies or self-interstitials (and tlieir precipitates). 

The process of growing a pure crystal is sensitive to a host of process parameters that impact the iacorporation of impurities ia the crystal, the quality of the crystal stmcture, and the mechanical properties of the crystal rod. For example, the crystal-pulling mechanism controls the pull rate of the crystallisa tion, which affects the iacorporation of impurities ia the crystal, and the crystal rotation, which affects the crystal stmcture. 

The second justification for the angular condition is that this condition is necessary for the determination of the radius of the crystal at the trijunction as a function of heat-transfer conditions and pull rate. This argument is simple. The dimensionless Young-Laplace equation of capillary statics gives the shape of an axisymmetric melt-ambient meniscus as... 

Equation 37 describes the relationship between the rate of change of the crystal radius at the trijunction and the deviation of the local angle from the equilibrium value < >o. In this expression, < )(t) is the dynamic angle formed between the local tangents to the melt-ambient and crystal-ambient surfaces, and Vg(T) is the dimensionless pull rate of the crystal. For steady-state growth, equation 37 simply sets the angle with what must be a solid cylinder of constant radius. The importance of the dynamical form equation 37 is brought out in the next section. 

Figure 19 shows sample isotherms and interface shapes predicted by the QSSM for calculations with decreasing melt volume in the crucible, as occurs in the batchwise process. Because the crystal pull rate and the heater temperature are maintained at constant values for this sequence, the crystal radius varies with the varying heat transfer in the system. Two effects are noticeable. First, decreasing the volume exposes the hot crucible wall to the crystal. The crucible wall heats the crystal and causes the decrease in... 

Process Stability and Control. Operationally, automatic control of the crystal radius by varying either the input power to the heater or the crystal pull rate has been necessary for the reproducible growth of crystals with constant radius. Techniques for automatic diameter control have been used since the establishment of Czochralski growth. Optical imaging of the crystal or direct measurement of the crystal weight has been used to determine the instantaneous radius. Hurle (156) reviewed the techniques currently used for sensing the radius. Bardsley et al. (157,158) described control based on the measurement of the crystal weight. 

The polymers used and some of their physical properties are listed in Table I. Polymers were mixed and blended on a two-roll mill at 450 K. Samples were compression molded at 450 K for 7 min and cooled in the press with tap water for 5 min. ASTM D412 6.35-mm (Va in.) dumbbells were cut parallel to the mill grain from sheets having 1.9-mm (75 mils) thickness. Instron tensile tests were carried out at least 48 hr after molding. Pull rate was 50.8 cm/min (20 in./min). 

The blown products, such as upward blown film, are basically natural for providing orientation (Figure 5.19). The blow-up ratio determines the degree of circumferential orientation, and the pull rate of the bubble determines the longitudinal orientation. 

The optimum stretching heat for amorphous plastics (PVC, etc.) is usually just above its glass transition temperature (Tg Chapter 1). Generally the orientation temperature is 60 to 75% between the Tg and Tm (melt temperature). For crystalline plastics (PE, PET, etc.) generally it is below the Tg. Stretching can take place in-line or off-line with or without tenter frames using the appropriate temperature-pull rates as the plastic travels first through a series of heated rolls. For unidirectional orientation just the rolls are used. 

The corresponding force-extension profiles for these calculations are shown in Fig. 27. The data presented for SMD and SMD-NH correspond to an individual trajectory. In order to arrive at a meaningful basis for comparisons between different schemes, the PMF for SMD and SMD-NH simulations is time averaged over 0.1 A(the bin width in EXEDOS), thereby reducing some of the statistical noise. For higher pulling rates (rates comparable to those employed in the literature), the forces and the PMF obtained from steered MD without a cantilever (SMD-NH) exhibit less noise than those obtained from... 

Conversion to full oxy/fuel also provides an opportunity for production increase. The change in pull rate achieved with an oxy/fuel furnace, in comparison with an air/fuel furnace, varies depending on furnace type. Pull rate increases of up to 60% have been observed for unit melters. Cross-fired regenerative furnaces have seen increases as little as 10%. End-fired regenerative furnaces converted to oxy/fuel increase pull capacity by 20%. Recuperative melters typically achieve a 30% pull rate increase. 

The determinations where made at 25°C using a Model 1122 Instron tester with a 500 gram load cell. Pull rate was two inches per minute. Apparent tensile strengths averaging 144 to 420 grams per square inch were found with elongations of 85 to 408 percent. 

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