In actual production, the control and setting temperature of the ton bag fabric is slightly lower than the drawing temperature by 5~15 degrees. At this temperature, the shrinking and setting, in fact, not all shrinking, and it is not a real fixed shrinkage size. There is a slight retraction in the subsequent process, and the retraction amount changes with the length of time and the room temperature. This kind of "aging" return may be slightly retracted within 15 days after the fabric leaves the factory. Heat setting retraction and "aging" web editing are two concepts. Heat setting is mainly to eliminate the internal stress of the flat wire and retract under heating. "Aging" retraction is the relaxation and retraction over time at room temperature. Without heat setting treatment, the "aging" shrinkage is large, and after heat setting, the "aging" shrinking is small.
During heat setting, with the different setting temperature, its natural shrinkage rate is also different. The so-called natural retraction rate is the percentage of the ratio of the retraction amount of the flat yarn to the original length when the flat yarn is given a heating temperature after drawing and no tension is applied to the two ends of the flat yarn. For PP, the natural shrinkage rate can reach about 8%, and for HDPE, it can reach about 10%. In actual production, the shrinkage rate of PP only needs to be less than 8%, but it should be satisfied that the flat wire does not loosen it after the second drawing and before the third drawing. In fact, it does not matter that the heat setting shrinkage rate is zero. Because the produced flat yarn has a certain elastic tension, even though the tension between the two-draw and the three-draw is high when the shrinkage rate is low, the flat yarn stretches elastically, but after the flat yarn passes through the three-draw, the flat yarn is very small. Under the action of tension, it will naturally retract again. However, the amount of retraction may be slightly less.
The shrinkage rate in actual production is best controlled within 1%~3% (PP) or 2%~4% (HDPE).
