Cooling 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. 

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... 

During the cooling process the fiber is subjected to a draw-down force, which introduces the orientation. Additional orientation may be introduced later by stretching the fiber to a higher draw ratio. 

From the dried components, various PET/PP/C blends (in wt. ratios40/60/0 40/59/1,40/57/3,40/54/6, and 40/51/9) were prepared and processed on a Brabender twin-screw extruder. The extrudate was cooled down to a temperature of75-80 °C before it was continuously drawn to a draw ratio of about 10-12. Part of the drawn bristles (diameter of 0.6-0.7 mm) was pelletized for injection molding experiments. 

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