Most thermoplastics can be blown to produce blown film. Blow blown film is a tubular film product obtained by extruding plastic into a thin tube, then inflating the plastic with compressed air, and then cooling and setting. 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. 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 are the blow molding process for commonly used low density polyethylene (LDPE) films. A brief introduction to its common faults.
Selection of polyethylene blown film materials
1. The raw materials used should be blown film grade polyethylene resin particles, containing appropriate amount of slip agent to ensure the opening of the film.
2. The melt index (MI) of the resin particles is not too large, and 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 film formation property of the resin is poor, 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, a resin material having a small melt index (MI) and a relatively large molecular weight distribution should be used. To meet the performance requirements of the film, and to ensure the processing characteristics of the resin. Blow-molded polyethylene film generally uses a polyethylene material with a melt index (MI) between 2 and 6g/10min.
Blowing process control points
The blown film process is roughly as follows:
Hopper loading material plasticizing extrusion→inflating traction→wind ring cooling→lipring plate→traction roller traction→corona treatment→film winding
Hopper loading
Material plasticizing extrusion→inflating traction
Wind ring cooling → herringbone plate → traction roller traction
Corona treatment → film winding
Process parameter control
The performance of blown film has a great relationship with the production process parameters. Therefore, in the film blowing process, it is necessary to strengthen the control of the process parameters, standardize the process operation, ensure the smooth production, and obtain high quality film products. In the production process of polyethylene blown film, it is mainly to control the following process parameters:
Extruder temperature
When blowing low density polyethylene (LDPE) film, the extrusion temperature is generally controlled between 160 ° C ~ 170 ° C, and the temperature of the head must be uniform, the extrusion temperature is too high, the resin is easy to decompose, and the film is brittle, especially The longitudinal tensile strength is significantly lowered; if the temperature is too low, the resin is poorly plasticized, and the expansion and stretching cannot be carried out smoothly, the tensile strength of the film is low, and the gloss and transparency of the surface are poor, and even the wood rings are formed. Pattern and unmelted crystal nucleus (fisheye).
2. Blowing ratio
The inflation ratio is one of the control points of the blown film production process, and refers to the ratio between the diameter of the bubble after inflation and the diameter of the unexpanded tube. The inflation ratio is the transverse expansion factor of the film, which is actually The film is stretched in the transverse direction. The stretching will exert a certain degree of orientation on the plastic molecules, and the inflation ratio will increase, so that the transverse strength of the film is increased. However, the inflation ratio cannot be too large, otherwise the bubble will not be easily formed. Stable, and the film is prone to wrinkles. Therefore, the inflation ratio should be properly matched with the traction ratio. Generally, 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 pulling speed of the film and the extrusion speed of the tube. The pulling ratio is the stretching ratio in the longitudinal direction, which makes the film have an orientation effect in the direction of the drawing. When the traction ratio is increased, the longitudinal strength will also follow. Increase, and the thickness of the film becomes thinner, but if the traction ratio is too large, the thickness of the film is difficult to control, and even the film may be broken, causing a film breakage. The traction ratio of the low density polyethylene (LDPE) film is generally controlled. Between 4 and 6 is appropriate.
Dew point
Dew point, also known as frost line, refers to the boundary line of plastic from a viscous flow state to a high elastic state. During the film blowing process, low density polyethylene (LDPE) is molten when extruded from the die, and the transparency is good. After the die, the inflation zone of the bubble is cooled by the cooling air ring. When the cooling air is blown to the plastic bubble just extruded from the machine head at a certain angle and speed, the high temperature bubble contacts the cooling air. The heat of the bubble is taken away by the cold air, and its temperature drops significantly below the viscous temperature of the low-density polyethylene (LDPE), so that it cools and solidifies and becomes blurred. On the blown 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 height of the dew point has a certain effect on the film performance. If the dew point is high, it is located after inflation. Above the bubble, the inflation of the film is carried out in a liquid state. The inflation only thins the film, and the molecules are not subjected to the stretching orientation. At this time, the performance of the inflation film is close to that of the cast film. Conversely, if the dew point At a lower level, the inflation is carried out in the solid state, at which time the plastic is In the high elastic state, the inflation is like the transverse stretching, which causes the molecules to be oriented, so that the performance of the inflation film is close to that of the oriented film.