Drawdown ratio

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. 

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)]. 

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. 

Table 6.5. Drawdown Ratio of Fluoropolymers for Wire Insulation Extrusion ...

Table 6.6. A Comparison of Drawdown Ratios for Various Polymers ...

Resin Drawdown Ratio Maximum Drawdown Ratio... 

Table 6.7. Drawdown Ratio for Medium Size Fluoropolymer Tube ...

Extrusion temperature of the PVDF/PMMA was in the range of 220°C-240°C. The preferred drawdown ratio of the first bubble was in the range of 10 1 to 15 1, and 2 1 to 3.75 1 forthe second bubble. The desirable blowup ratio ranged from 1.5 1 to 1.75 1. A cooling air temperature of 10°C-20°C was required while the hot air temperature, before the second bubble, varied from 120°C-140°C. The film can be dimensionally stabilized by an annealing process at 120°C for 5 minutes. 

Melt fracture has been a very perplexing but fascinating problem ever since it was discovered. Another problem that seems to have the same degree of perplexity and fascination is draw resonance. Both are instabilities in polymer flows. (Draw resonance may also occur in Newtonian fluids.) Draw resonance is a periodic variation in the diameter of a spinning thread line above a critical drawdown ratio. Polypropylene and high-density polyethylene are both particularly susceptible to draw resonance. Petrie and Denn have presented a comprehensive review of the numerous theoretical and experimental studies of draw resonance conducted prior to 1976 [99]. 

The theoretical studies of draw resonance were initiated by Kase and Yoshimoto [102] and Pearson and Matovich [103]. These authors found that the critical drawdown ratio for isothermal Newtonian fluids is about 20. Fisher and Denn analyzed both isothermal and nonisothermal flows of spinning of viscoelastic fluids [104,105]. Figure 3.17 gives the results for the isothermal case, where... 

Petrie and Denn [99] give a table with critical drawdown ratios for a number of polymers. Various authors have found the critical ratio of polypropylene ranges from 2.7 to 33. Higher values are tabulated, but the experimental conditions for these results are uncertain. 

TABLE 3.8 Critical Drawdown Ratios for Polypropylene Isothermal critical Nonisothermal critical... 

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.

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