Die draw-down ratio

In the tubular process a thin tube is extruded (usually in a vertically upward direction) and by blowing air through the die head the tube is inflated into a thin bubble. This is cooled, flattened out and wound up. The ratio of bubble diameter to die diameter is known as the blow-up ratio, the ratio of the haul-off rate to the natural extrusion rate is referred to as the draw-down ratio and the distance between the die and the frost line (when the extrudate becomes solidified and which can often be seen by the appearance of haziness), the freeze-line distance. 

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. 

Monofilament Extruder-melt pump-spinneret-quench-stretch- heat-set -wind. For Newtonian fluids the critical draw-down ratio is about 20 (take up velocity/ die exit velocity). Extrusion speeds 1 to 8 m/s. Feed must be dry. 20 to 200 filaments per spinneret. Operating limit is linear speed filaments can be drawn. 

Because TUR requires some calculating to obtain, some workers prefer to use the draw down ratio (DDR) to indicate the total degree of film stretching. DDR is determined from three easily obtained measurements, the die gap, final film thickness, and BUR ... 

The draw down ratio is the ratio of the thickness of the extruder die opening to the final thickness of the product. 

The two main parameters of this process are the blow ratio (or blowup ratio), Br (or BUR), and the machine-direction draw (or draw-down) ratio. Dr. The blow ratio is defined as the ratio of the final tube radius, R, to the initial tube outside radius just downstream of the annular die, Ro (see also Figs. 9.20 and 9.21, and Section 1.2) ... 

Elongation stress and hoop stress have been used as main stress components in the tooling draw down ratio investigation because the cable extrudate was mainly undo-elongational stretch beyond the die exit However, shear stress and radial stress plots were also ealeulated and presented in Figures 7 and 8 for the convenienee of further analysis. The reduction on the shear stress was 67 %, and the reduction on radial stress was 63 %. 

Die extmsion, draw down ratio, viscoelastic fluid model, PVDF, post-extrusion shrinkage, tensile yam buckling... 

This example illustrates the simplified approach to film blowing. Unfortunately in practice the situation is more complex in that the film thickness is influenced by draw-down, relaxation of induced stresses/strains and melt flow phenomena such as die swell. In fact the situation is similar to that described for blow moulding (see below) and the type of analysis outlined in that section could be used to allow for the effects of die swell. However, since the most practical problems in film blowing require iterative type solutions involving melt flow characteristics, volume flow rates, swell ratios, etc the study of these is delayed until Chapter 5 where a more rigorous approach to polymer flow has been adopted. 

A plastic film, 0.1 mm thick, is required to have its orientation in the transverse direction twice that in the machine direction. If the film blowing die has an outer diameter of 100 mm and an inner diameter of 98 mm estimate the blow-up ratio which will be required and the lay flat film width. Neglect extrusion induced effects and assume there is no draw-down. 

Design a die which will produce plastic film 0.52 mm thick at a linear velocity of 20 mm/s. The lay-flat widfli of the film is to be 450 mm and it is known that a blow-up ratio of 1.91 will give the necessary orientation in the film. Assume that there is no draw-down. 

---