Drawdown Ratio DDR

The drawdown ratio, DDR, characterizes a tubular die and is defined as the ratio of the cross-sectional area of the annular die to the cross-sectional area of the finished insulation, as seen in Fig. 6.20 and Eqs. (6.2) and (6.3). Each resin has a characteristic range of DDR shown in Table 6.5. A comparison of drawdown ratios for a number of polymers is listed in Table 6.6. 

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. 

Resin Drawdown Ratio Maximum Drawdown Ratio 

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The drawdown ratio (DDR) in a wire die or a circular die, is the ratio of the cross sectional area of the die orifice/opening to the final extruded shape [Figure 17.13(b)]. 

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.

Orientation of multiwalled carbon nanotubes (MWNT) in melt-spun fibers produced from a melt-compounded polycarbonate composite with 2 wt% MWNT using different take-up velocities TEM photomicrographs of thin sections of the fibers prepared along the fiber axis (assigned by the arrows). In all images the cut direction is 35° to the fiber axis. Varied is the take-up velocity (drawdown ratio, ddr, in brackets). (A) 50 m/min (ddr = 7.8) (B) 200 m/min (ddr = 31.4) (C) 400 m/min (ddr = 62.8) and (D) 800 m/min (ddr = 126). (From P. Potschke, H. Brunig, A. Janke, D. Fischer, and D. Jehnichen, Polymer 46,10355-10363,2005. With permission.)... 

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). 

Figure 6.6 Bubble conditions at various drawdown ratios (DDR) and blow-up ratios (BUR) with frost-line height of 0.2 m for LDPE ( stable metastable x unstable), LLDPE (A stable A metastable, x imstable), and mPE(l) ( stable metastable x unstable)...

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. 

It should be noted that sometimes the area draw ratio is represented by the letters DDR as an acronym for drawdown ratio. Obviously, this can cause confusion with the diameter draw ratio. Often the area draw ratio is called simply the draw ratio. It is important therefore to check what exactly is meant when the term draw ratio is used. The ADR can be calculated from the WDR and the DDR as follows ... 

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