Analysis of the main points of blown film process control

Selection of polyethylene blown film materials

1. The raw materials selected 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 selected, so that the performance requirements of the film can be satisfied, and the processing characteristics of the resin can be ensured. For blown polyethylene films, polyethylene raw materials having a melt index (MI) of between 2 and 6 g/10 min are generally used.

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

However, the performance of the blown film has a great relationship with the production process parameters. Therefore, in the blown film process, it is necessary to strengthen the control of the process parameters, standardize the process operation, ensure the smooth production, and obtain a high quality film. product.

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 blow up 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 uninflated tube ring. The inflation ratio is the transverse expansion factor of the film. In fact, the film is stretched transversely. The stretching exerts a certain degree of orientation on the plastic molecules, and the inflation ratio is increased, so that the transverse strength of the film is improved. However, the inflation ratio should not be too large, otherwise the bubble is unstable and the film is prone to wrinkles. Therefore, the inflation ratio should be properly matched with the traction ratio. Generally, the inflation ratio of the low density polyethylene (LDPE) film should be controlled to 2.5 to 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 pipe ring. The draw ratio is a longitudinal draw ratio that imparts an orientation in the direction of the draw. When the traction ratio is increased, the longitudinal strength is also increased, and the thickness of the film is thinned. However, 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.

Dew point

The dew point, also known as the frost line, refers to the dividing line of plastic from the viscous flow state to the high elastic state. In the film blowing process, low density polyethylene (LDPE) is in a molten state when extruded from a die, and the transparency is good. After leaving the die, the inflated zone of the bubble is cooled by the cooling air ring. When the cooling air is blown at a certain angle and speed to the plastic bubble just extruded from the handpiece, the high temperature bubble and cooling air Upon contact, the heat of the bubble is carried away by the cold air, and its temperature drops significantly below the viscous temperature of the low-density polyethylene (LDPE), causing it to cool and solidify and become blurred. On the blown bubble we can see a dividing line between transparency and blur, which is the dew point (or frost line).

In the film blowing process, the height of the dew point has a certain influence on the film properties. If the dew point is high and is located above the inflated bubble, the inflation of the film is carried out in a liquid state, and the inflation only thins the film, and the molecules are not subjected to the stretching orientation, and the performance of the inflation film is close to Cast film. Conversely, 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, causing the molecules to be oriented, so that the performance of the inflation film is close to Oriented film.

Technical requirements for basic performance:

1. Specifications and deviations

The width and thickness of the polyethylene film should meet the requirements, the film thickness is uniform, the thickness deviation in the horizontal and vertical directions is small, and the deviation distribution is relatively uniform.

2. Appearance

It is required that the polyethylene film is plasticized well without obvious "water grain" and "cloud"; the surface of the film should be smooth and smooth, without wrinkles or only a small amount of active pleats; no bubbles, perforations and cracks are allowed; no obvious Black spots, impurities, crystal points and stiff spots; no serious hanging lines and silk lines are allowed.

3. Physical and mechanical properties

Since the blown polyethylene film is subjected to mechanical force during printing or composite processing, the physical and mechanical properties of the polyethylene film are required to be excellent, mainly including tensile strength, elongation at break, tearing. Several indicators such as strength should meet the standards.

4. The size of the surface tension

In order to make the printing ink and the composite adhesive have good wettability and adhesion on the surface of the polyethylene film, it is required that the surface tension of the polyethylene film should reach a certain standard, otherwise the smooth progress of printing and composite production will be affected. In general, the surface tension of the polyethylene film should be at least 38 dynes or more, preferably 40 dynes or more.