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Brief introduction of blow molding process of blow molding machine


Most thermoplastics can be produced with blow molding machines. Blown film is a cylindrical film product obtained by extruding plastic into a thin tube, then blowing the plastic with compressed air while hot, and then cooling and shaping. Such as blown water horse, the performance of this film is between the oriented film and the cast film: the strength is better than that of the cast film, and the heat sealability is worse than that of the cast film. Let the editor explain the control points of the blow molding process in detail.


The control points of the blow molding process, the selected raw materials should be blown film grade polyethylene resin particles, containing an appropriate amount of slip agent to ensure the openness of the film. When blowing low density polyethylene (LDPE) film, the extrusion temperature is generally controlled between 160 ℃ ~ 170 ℃, and the temperature of the head must be uniform, the extrusion temperature is too high, the resin is easily decomposed, and the film is brittle, especially The longitudinal tensile strength is significantly reduced; if the temperature is too low, the resin will be poorly plasticized, and the expansion and stretching cannot be performed smoothly. The tensile strength of the film is low, and the surface gloss and transparency are poor, and even appear like wood rings. pattern and unmelted nuclei (fish eyes).

The inflation ratio is one of the control points of the blown film production process. It refers to the ratio between the diameter of the film bubble after inflation and the diameter of the uninflated tube ring. The inflation ratio is the transverse expansion multiple of the film. In fact, the film is stretched in the transverse direction. The stretching will produce a certain degree of orientation of the plastic molecules, and the inflation ratio will increase, thereby increasing the transverse strength of the film. However, the blow-up ratio should not be too large, otherwise the film bubble will be unstable and the film will be prone to wrinkling. Therefore, the inflation ratio should be properly matched with the traction ratio. Generally speaking, the inflation ratio of low-density polyethylene (LDPE) films should be controlled at 2.5 to 3.0.

The draw ratio refers to the ratio between the draw speed of the film and the extrusion speed of the tube ring. The draw ratio is the stretching factor in the longitudinal direction, which makes the film have an orientation effect in the direction of extraction. When the traction ratio increases, the longitudinal strength will also increase, and the thickness of the film will become thinner. However, if the traction ratio is too large, the thickness of the film will be difficult to control, and the film may even be pulled off, resulting in film breakage. The traction ratio of low density polyethylene (LDPE) film is generally controlled between 4 and 6.

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