Since the performance of blown film has a great relationship with the production process parameters, in the process of film blowing, it is necessary to strengthen the control of process parameters, standard process operations, to ensure the smooth production of production, and to obtain high-quality film products . In the production process of polyethylene blown film, it is mainly to control the following process parameters:
When blowing low density polyethylene (LDPE) film, the extrusion temperature is generally controlled between 160 °C and 170 °C, and the temperature of the machine head must be uniform, the extrusion temperature is too high, the resin is easy to synthesize, and the film is brittle, In particular, the longitudinal tensile strength is significantly reduced; if the temperature is too low, the plasticization of the resin will be poor, and the shrinkage and stretching cannot be stopped in a sophisticated way. Wheel-like patterns and uncondensed nuclei (fish eyes).
2. inflation ratio
The blow-up ratio is one of the control points of the blow-molded film production process. The inflation ratio is the transverse shrinkage multiple of the film. In fact, the transverse stretching of the film is performed. The stretching will produce a certain level of orientation effect on the plastic molecules, and the inflation ratio will increase, thereby improving the transverse strength of the film. However, the blow-up ratio should not be too large, otherwise it is easy to form bubbles that are unstable and the film is prone to wrinkling. Therefore, the inflation ratio should be appropriately matched with the traction ratio. Generally speaking, the inflation ratio of the low density polyethylene (LDPE) film should be controlled at 2.5~3.0.
The draw ratio refers to the ratio between the pulling 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 be pulled and broken, resulting in film breakage. The traction ratio of low density polyethylene (LDPE) film is generally controlled between 4 and 6.
4. Dew point
The dew point, also known as the frost line, refers to the point at which the plastic changes from a viscous flow state to a high elastic state. In the film blowing process, low density polyethylene (LDPE) is in a molten state when extruded from the die and has good transparency. After the die is separated, the cooling air ring must stop the cooling of the film bubble, and the cooling air blows at a certain angle and speed to the plastic film bubble just extruded from the machine head. In contact with the cooling air, the heat of the film bubble will be taken away by the cold air, and its temperature will drop significantly below the viscous flow temperature of low density polyethylene (LDPE), which will cool and solidify and become ambiguity. On the blown film bubble we can see a line between transparency and ambiguity, which is the dew point (or frost line).
In the film blowing process, the upper and lower dew point has a certain influence on the film performance. If the dew point is high and is located above the inflated film bubble, the inflation of the film is stopped in the liquid state, and the inflation only makes the film thinner, and the molecules are not stretched. close to cast film. On the contrary, if the dew point is relatively low, the inflation stops in the solid state. At this time, the blow-molded plastic is in a highly elastic state, and the inflation is like transverse stretching, causing the molecules to orientate, thereby making the blown film. The performance is close to that of an oriented film.
English
