The packaging of the stretch film must be stretched, and the stretching form of the pallet mechanical packaging has direct stretching and pre-stretching. Pre-drawing is divided into two kinds, one is roller pre-drawing, and the other is electric drawing.
Direct stretching is a stretch between the tray and the film. This method has a low tensile ratio (about 15% to 20%), if the tensile ratio exceeds 55% to 60%, exceeding the original yield point of the film, the film width is reduced, the puncture performance is also lost, and the film is easy to break. And at 60% tensile rate, the tension is still very large, for light goods, it is likely to deform the goods.
The pre-drawing is done by two rollers. The two rollers of the roll pre-stretched are connected by the gear unit, the stretch ratio can be different according to the gear ratio, the tension generated by the turntable, because the stretch is generated in a short distance, the friction between the roll and the film is large, so the film is wide and not shrinking, the original piercing performance of the film is also maintained. No stretching occurs during actual winding, reducing breakage due to sharp edges or angles, and this pre-stretching can increase the stretch ratio to 110%.
The tensile mechanism of electric predrawing is the same as that of roller predrawing, but the difference is that the two rollers are driven by electricity, and the stretching has nothing to do with the rotation of the tray. Therefore, the adaptability is stronger, light, heavy, irregular goods are applicable, because the packaging tension is low, so this method of pre-stretching ratio of up to 300%, greatly saving materials and reducing costs. Suitable for film thickness 15 ~ 24μm.
