There are many types of films produced by blow molding, such as low density polyethylene (LDPE), polypropylene (PP), high density polyethylene (HDPE), nylon (PA), ethylene-vinyl acetate copolymer (EVA), etc., here We briefly introduce the blow molding production process of commonly used low density polyethylene (LDPE) film.
The choice of polyethylene blown film material
1. The selected raw materials should be blown film grade polyethylene resin particles, containing an appropriate amount of slip agent to ensure the opening of the film.
2. The melt index (MI) of the resin particles should not be too large. If the melt index (MI) is too large, the viscosity of the molten resin is too small, the processing range is narrow, the processing conditions are difficult to control, the resin has poor film forming properties, and it is not easy to process into a film In addition, the melt index (MI) is too large, the relative molecular weight distribution of the polymer is too narrow, and the strength of the film is poor. Therefore, resin materials with a small melt index (MI) and a relatively wide molecular weight distribution should be selected, which can not only meet the performance requirements of the film, but also ensure the processing characteristics of the resin. The blown polyethylene film generally uses polyethylene raw materials with a melt index (MI) in the range of 2-6g/10min.
Key points of blow molding process control
The process flow of blown film is roughly as follows: hopper feeding→material plasticization and extrusion→blowing traction→air ring cooling→herringbone splint→traction roller traction→corona treatment→film winding. However, it is worth pointing out that the performance of blown film has a great relationship with the production process parameters. Therefore, in the process of blowing film, it is necessary to strengthen the control of process parameters, standardize process operations, and ensure the smooth progress of production. Obtain high-quality film products. In the production process of polyethylene blown film, the following process parameters are mainly controlled:
1. Extruder temperature
When blowing low-density polyethylene (LDPE) film, the extrusion temperature is generally controlled between 160°C and 170°C, and the temperature of the die must be uniform. The extrusion temperature is too high, the resin is easy to decompose, and the film is brittle, especially Make the longitudinal tensile strength drop significantly; if the temperature is too low, the resin will not be plasticized and can not be expanded and stretched smoothly. The tensile strength of the film is low, and the surface gloss and transparency are poor, and even appear like wood growth rings. The pattern and the unmelted crystal nucleus (fisheye).
2. Blow up ratio
The blow-up 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 blow-up ratio is the transverse expansion multiple of the film. In fact, the film is stretched in the transverse direction. Stretching will have a certain degree of orientation effect on the plastic molecules, and the blow-up ratio will increase, thereby increasing the transverse strength of the film. However, the blow-up ratio should not be too large, otherwise the bubble will become unstable and the film will be prone to wrinkles. Therefore, the inflation ratio should be properly matched with the traction ratio. Generally speaking, the inflation ratio of low-density polyethylene (LDPE) film should be controlled at 2.5-3.0.
3. Traction ratio
The traction ratio refers to the ratio between the traction speed of the film and the extrusion speed of the tube ring. The traction ratio is the longitudinal stretching multiple, which makes the film have a directional effect in the drawing direction. If 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 broken, causing film breakage. The traction ratio of low-density polyethylene (LDPE) film is generally controlled between 4-6.
4. Dew point
The dew point is also called the frost line, which refers to the dividing line of the plastic from the viscous flow state to the high elastic state. During the film blowing process, low-density polyethylene (LDPE) is in a molten state when extruded from the die, and has good transparency. After leaving the die, the blowing zone of the film bubble should be cooled by the cooling air ring. When the cooling air is blown to the plastic film bubble just extruded from the die at a certain angle and speed, the high temperature film bubble and cooling air In contact, the heat of the 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 will cool and solidify and become blurred. On the blown film bubble we can see a dividing line between transparency and blur, which is the dew point (or frost line).
During the film blowing process, the level of the dew point has a certain effect on the film performance. If the dew point is high and located above the inflated bubble, the film is inflated in a liquid state. Inflation only makes the film thinner, and the molecules are not stretched orientated. At this time, the performance of the inflated film is close to cast film. On the contrary, if the dew point is relatively low, the inflation is carried out in the solid state. At this time, the plastic is in a high elastic state, and the inflation is like the transverse stretching, which makes the molecules orientate, so that the performance of the inflation film is close to Orientation film.