Draw ratio balance

As explained in Chapter 17 die configurations meet certain melt flow requirements such a drawdown ratio and draw ratio balance. Draw-down ratio (DDR) in a circular die, such as a wire die, is the ratio of the cross sectional area of the die orifice/opening to the final extruded shape. Another guide for setting uniformity and best repeatable references is the draw ratio balance (DRB) that aids in determining the minimum and maximum values that can be used for different plastics. 

Target is to set uniformity and balance in the plastic coating. This draw ratio balance (DRB) aids in determining the minimum and maximum values that can be used for different plastics (Figure 17.13). 

Figure 17.13 (a) Schematic for determining wire coated draw ratio balance in dies. 

An important consideration in designing dies and tips for wire coating is draw-ratio balance (DRB) defined by Eq. (6.4). The ideal value of DRB is 1, which means that the interior and exterior surfaces of the tube are drawn to the same extent. For FEP and PFA, the DRB range is 0.9-1.15. Concentricity suffers at DRB <0.9 while a DRB >1.1 can result in tears. ETFE has a more narrow range, 1.04-1.07. Drawdown ratios for large diameter cables may range from 2 1 to 3 1. 

Fig. 3-18. Drawdown ratio (DDR) and draw ratio balance (DRB) applicable to different products, as shown here for wire coating. Plastics have different DDRs and DRBs, which can be used as guides to processability and help establish limits for the various melt characteristics.

A fourth measure of drawdown is the draw ratio balance or DRB. This is the diameter ratio of the die and tip divided by the diameter ratio of the tubing. 

The draw ratio balance should be equal to or larger than one DRB 1. Yet another parameter that is used is the sizing ratio or SR. This is the wall draw ratio divided by the diameter draw ratio. 

Die and tip size can be calculated from the drawdown ratio (DDR) and draw-ratio balance (DRB). By combining Eqs 8.1 through 8.4, Eqs. 8.5 and 8.6 can be obtained for calculation of tip and annular die diameters. DRB is assumed to have a value of 1 in these equations. Naturally, one can select a different value for DRB and recalculate these diameters. To prevent drool at the die, the tip must have a face radius roughly equal to 10% of the tip diameter (Dj). 

A small tube is usually one that has an outside diameter of 5 mmf l or less and a wall thickness of less than about 1 mm. A free extrusion technique, similar to the wire coating process, is the method by which small tubing is manufactured. In this case, the desirable drawdown ratio is much smaller than that of a wire insulation process. The draw-ratio balance should be equal to 1. Outside diameter can be controlled by placing a sizing die at the entrance to the quench bath. Air pressure can be employed to expand the tube against the die. The air pressure should be slight due to the low melt strength of the melt cone. 

The film tube is collapsed within a V-shaped frame of rollers and is nipped at the end of the frame to trap the air within the bubble. The nip roUs also draw the film away from the die. The draw rate is controlled to balance the physical properties with the transverse properties achieved by the blow draw ratio. The tube may be wound as such or may be sHt and wound as a single-film layer onto one or more roUs. The tube may also be direcdy processed into bags. The blown film method is used principally to produce polyethylene film. It has occasionally been used for polypropylene, poly(ethylene terephthalate), vinyls, nylon, and other polymers. 

Thus the combination of pre-orientation to suppress crazing and to reduce the natural draw ratio and rubber block introduction to bias the deformation response toward yielding has converted a brittle polymer to a moderately ductile one. Although the BPFC domain Tg has been reduced by roughly 30 °C, the improved balance of mechanical properties makes this sacrifice worthwhile. 

By carrying out reversion experiments he determined the actual orientations in both directions, which are, of course, lower than the corresponding draw ratios. His results are given in Figs. 7 and 8, which show that the impact strength decreases with increasing anisotropy, and increases with increasing orientation if equally balanced films are compared. 

Sanger, M. J. (2005). Evaluating students conceptual understanding of balanced equations and stoichiometric ratios using a particulate drawing. Jownua/ of Chemical Education, 52(1), 131-134. 

Multiproduct fractionator controls, where, after dynamic correction, the boil-up, side-draw and distillate flows are ratioed to the feed flow (left). On the right, the true boiling points are controlled by throttling the product flows, while heat balance is controlled by manipulating the reflux flows. 

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