Film blowing, also known as flat-fold film tube extrusion or blown film extrusion, is a major method for producing plastic film. Plastic is added to the extruder and melted, and then extruded from the annular gap of the front port die into a cylindrical (tube) shape. Compressed air is introduced from the center hole of the mandrel of the die head to blow the cylinder into a bubble tube shape (generally blown 2-3 times). At this time, it is elongated in both the vertical and horizontal directions to obtain a bubble tube with a certain blown multiple. It is cooled by the outer air ring (sometimes with additional internal cooling), and then enters the guide clamp and traction roller to flatten the bubble tube to prevent the air in the bubble tube from leaking out to maintain the required constant blowing pressure. The flattened bubble tube becomes a flat-folded double-layer film, and finally enters the winding device for winding.

Classification

In order to make the film thickness uniform, many production devices place the extruder on a rotating platform that swings back and forth slowly, or make the clamping plate and the traction roller part rotate or swing. According to the different traction methods, it can also be divided into:

(1) Flat extrusion blowing method, which is the most commonly used method and is suitable for blow molding PVC and PE films;

(2) Flat extrusion blowing method, which is suitable for plastics with low melt viscosity, such as PP, polyphthalamide, etc.;

(3) Flat extrusion blowing method, which is only used for small diameter products and heat shrinkable films.

Selection of polyethylene blown film materials

1. The raw materials selected should be blown film grade polyethylene resin particles, containing an appropriate amount of lubricant to ensure the opening of the film.

2. The melt index (MI) of the resin particles cannot be too large. If the melt index (MI) is too large, the viscosity of the molten resin will be too small, the processing range will be narrow, the processing conditions will be difficult to control, the film-forming property of the resin will be poor, and it will not be easy to process into a film. In addition, if the melt index (MI) is too large, the relative molecular weight distribution of the polymer will be too narrow, and the strength of the film will be poor. Therefore, a resin raw material with a smaller melt index (MI) and a wider relative 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. Blown polyethylene film generally uses polyethylene raw materials with a melt index (MI) in the range of 2 to 6 g/10min.

Key points of blow molding process control

The process flow of blown film is as follows:

Hopper loading ¡ú material plasticizing and extrusion ¡ú inflation and traction ¡ú air ring cooling ¡ú herringbone plywood ¡ú traction roller ¡ú corona treatment ¡ú 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, ensure the smooth progress of production, and obtain high-quality film products. In the production process of polyethylene blown film, the following process parameters should be controlled:

1. Extruder temperature

When blowing low-density polyethylene (LDPE) film, the extrusion temperature is generally controlled between 160¡æ and 170¡æ, and the head temperature must be uniform. If the extrusion temperature is too high, the resin is easy to decompose, and the film is brittle, especially the longitudinal tensile strength is significantly reduced; if the temperature is too low, the resin is poorly plasticized and cannot be expanded and stretched smoothly. The tensile strength of the film is low, and the surface gloss and transparency are poor, and even wood ring-like patterns and unmelted crystal nuclei (fish eyes) may appear.

2. Blowing ratio

The blowing 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 blowing and the diameter of the tube ring before blowing. The blowing ratio is the transverse expansion multiple of the film. In fact, the film is stretched transversely. Stretching will have a certain degree of orientation effect on the plastic molecules. The blowing ratio increases, thereby improving the transverse strength of the film. However, the blow-up ratio cannot be too large, otherwise it will easily cause the film bubble to be unstable and the film will be prone to wrinkles. Therefore, the blow-up ratio should be properly matched with the traction ratio. Generally speaking, the blow-up 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. As 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 is difficult to control, and it may even break the film, causing the film to break. 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 dividing line where the plastic enters the high elastic state from the viscous flow state. In the process of film blowing, low-density polyethylene (LDPE) is in a molten state when it is extruded from the die, and has good transparency. After leaving the die, the blowing area of the film bubble needs to be cooled by the cooling air ring. When the cooling air blows at a certain angle and speed to the plastic film bubble just extruded from the die head, the high-temperature film bubble contacts 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 viscosity flow temperature of low-density polyethylene (LDPE), so that it cools and solidifies and becomes 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 height of the dew point has a certain influence on the film performance. If the dew point is high and located above the film bubble after blowing, the film is blown in a liquid state, and the blowing only makes the film thinner, while the molecules are not stretched and oriented. At this time, the performance of the blown film is close to that of the cast film. On the contrary, if the dew point is relatively low, the blowing is carried out in a solid state. At this time, the plastic is in a highly elastic state, and the blowing is like lateral stretching, which orients the molecules, so that the performance of the blown film is close to that of the oriented film.

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