Extruder-blown-film frost line

A typical blown film extrusion device is shown in Figure 1.7. The molten polymer from the extruder enters the die and is forced around a mandrel and emerges from a ring shaped die. The extruded film tube is then expanded to a specific diameter by air pressure from the center of the mandrel. As the bubble expands, film thickness decreases. The "frost line" shown in Figure 1.7 is the line where solidification of the extrudate commences. 

When extruding a tube through an annular die there are two kinds of orientation processes available, blown film process and tubular film process. Blown film conducts the orientation in the melt state. The tube is rapidly pulled away from the die by a nip at the top of a tower. Air is pumped through the annular die to infiate the tube and to provide additional cooling. The molecular orientation produced in blown film is quite low compared to solid-state orientation. The molecules are above their melt and have very fast relaxation times. One often refers to the frost line in a blown film process. This is the point where the melt crystallizes. The polymeric web goes through a clear to hazy transition at this point. Further molecular orientation in the web in this stage of the process typically does not occur. 

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