The blowing ratio is one of the key control points of film blowing machine production process, which refers to the ratio between the diameter of the bubble after blowing and the diameter of the non inflated pipe ring. The blowing ratio is the lateral expansion multiple of the film, which is actually the transverse stretching of the film. The stretching will have a certain degree of orientation effect on the plastic molecules, and the blowing ratio will increase, so as to improve the transverse strength of the film. However, the blowing ratio should not be too large, otherwise it is easy to cause the instability of the film bubble, and the film is prone to wrinkle. In general, the blowing ratio of LDPE film should be controlled at 2.5 ~ 3.0.
Film blowing machine
Traction ratio refers to the ratio between the traction speed of film and the extrusion speed of tube ring. The traction ratio is the longitudinal stretching ratio, which makes the film oriented in the drawing direction. With the increase of traction ratio, the longitudinal strength will also increase, and the film thickness will become thinner. Bubble film machine, air cushion film machine, bagging machine, tearing film machine, winding film machine, but if the traction ratio is too large, the film thickness is difficult to control, and it may even break the film, resulting in film breakage. Generally, the traction ratio of LDPE film should be controlled between 4 and 6.
Dew point, also known as frost line, refers to the boundary between plastic flow state and high elastic state. In the process of film blowing machine, low density polyethylene (LDPE) is in a molten state and has good transparency when extruded from the die. After leaving the die, the blowing area of the film bubble should be cooled through the cooling air ring. When the cooling air blows to the plastic film bubble just extruded from the die head at a certain angle and speed, the high temperature film bubble contacts with the cooling air, and the heat of the film bubble will be taken away by the cold air, and its temperature will obviously drop below the viscous flow temperature of low density polyethylene (LDPE), so that it can be cooled and solidified It's blurry. On the blown foam, we can see a line between transparency and blur, which is the dew point (or frost line).
In the process of blowing film, the dew point has a certain influence on the film properties. If the dew point is high and located above the inflated bubble, the film is inflated in liquid state. The blown film only thins the film and the molecules are not oriented. The performance of the blown film is close to that of the tape cast film. On the contrary, if the dew point is low, the blowing is carried out in the solid state. At this time, when the plastic is in the high elastic state, the blowing will be like the transverse stretching, and the molecules will be oriented, so that the performance of the blown film of the film blowing machine is close to that of the directional film.