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. 

Sheet forming processes, such as vacuum forming, do have effects on the product. The designer should be aware that these will affect the performance of one s product and one should learn how to modify the design to minimize any deleterious effects. Probably the most serious problem encountered in formed film or sheet products results from the fact that the materials are made from film or sheet at temperatures well below the melt softening point of the plastic, usually near the heat distortion temperature for the material. Forming under these condition when the draw down ratio is exceeded for a specific plastic can result in over stretched orientation of the material, the production of frozen-in stresses, poor product reproducibil-... 

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 proper draw down ratio depends on the polymer being extruded. 

Table 5.12. Effect of extrusion temperature and draw-down ratio on mechanical properties of as-spun fibers of PET/60HBA and PET/80HBA. ...

Extrusion temp. °C PET/60 HBA Draw-down ratio (Vo = 1.02 m/min) Modulus GPa Strength MPa PET/80 HBA Draw-down ratio (Vo = 1.06 m/min) Modulus GPa Strength MPa... 

Wang and Cakmak (52) studied the development of structure hierarchy in tubular film blown dynamically vulcanized PP/EPDM blend. The blown films were found to exhibit an unusual asymmetric structure. The PP phase was found to fibriUate at all the outside surface, while the inner surface remained relatively featureless. This was attributed to disproportionally rapid cooling of the outside surface by the air steam blown externally onto the film being extruded. This, in turn, resulted in sohdification of very thin PP surface layers that caused their fibrillation under the heavy stress they had to endure. Increase in the blow-up ratio was found to expand this web-like surface texture. As a result of this fibrillation mechanism, the increase of both the blow-up ratio and draw-down ratios was found to reduce the mechanical properties. 

DRAW RESONANCE - HIGH DRAW DOWN RATIO - INSUFFICIENT COOLING Sj. T. V /Vyg... 

CVC chemical vapor curing DDR draw down ratio... 

The principal drawback of water-bath quenching is that very thin films cannot be made from crystalline polymers such as polyethylene or polypropylene. Above a certain draw-down ratio there are thickness variations in the longitudinal direction. This phenomena is called draw resonance and will be discussed in the next section. 

The thickness variations due to draw resonance have a different character. The thickness may vary by a factor of approx. 2, but there is no surface roughness. The transition from thick to thin r ons is very gradual and occurs at regular distances there is a certain wave-length , which may be as large as one metre or more. This defect has been called draw resonance by Bergonzoni and Dicresce because it is only observed when the draw-down ratio exceeds a certain value, usually approx. 1 4. Draw resonance also plays a role in paper coating and in the manufacture of fibres and monofilaments. It is therefore remarkable that so far this technically important subject has only received scant attention in scientific and technical literature. 

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. 

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