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Analysis of blown film related processes and common problems!

Time:2019-4-2 17:12:24
Abstract: In the process of film blowing, common problems such as film wrinkles, poor film transparency, fisheyes, uneven film thickness, winding during winding, etc., this paper will carry out some simple analysis and discussion on these issues, and propose The corresponding solution.
Key words: blown film, plastic film, film transparency, bubble
 
1 Introduction
The plastic film blowing machine heats and melts the plastic particles and then blows them into a film. There are many kinds of plastic film blowing machines, such as PE, POF and so on. The new material is blown out with new particles, the color is called, clean, and the bag is stretched well. Some people also use recycled plastic bags to make particles. These particles are generally called old materials. When they are made into particles, they are usually gray. When making bags, they usually add pigments, making the bags unevenly colored, brittle and fragile. The price is also lower. Although it is a recycled plastic bag, it is generally a waste bag in the bag making process and a garbage plastic in the usual sense is still very different. The plastic blown film machine produces plastic film for a variety of high-grade film packaging. The film is widely used for light and heavy packaging because of its good barrier property, preservation, moisture resistance, frost resistance, oxygen barrier and oil resistance. Such as a variety of fresh fruit, meat food, pickles, fresh milk, liquid beverages, medical supplies and so on.
 
2, the choice of polyethylene blown film materials
2.1 The selected raw materials should be blown film grade polyethylene resin particles, containing appropriate amount of slip agent to ensure the opening of the film.
2.2 If the melt index (MI) of the resin particles 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 film forming property is poor, and it is not easy to process into a film; further, the melt index (MI) is too large. The relative molecular weight distribution of the polymer is too narrow and the film strength is poor. Therefore, a numerical 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. Blow-molded polyethylene films are generally selected from polyethylene raw materials having a melt index (MI) in the range of 2-6 g/10 min.
 
3, blow molding process control points
3.1 The blown film process is roughly as follows:
Hopper loading→material plasticizing extrusion→inflating traction→wind ring cooling→lipring plate→tracting roller traction→film winding However, it is worth noting that 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 [5]. In the production process of polyethylene blown film, it is mainly to control the following process parameters:
3.1.1 Extruder temperature film blowing machine
When blowing low density polyethylene (LDPE) film, the extrusion temperature is controlled between 160 °C and 170 °C for half a year, and the temperature of the head must be uniform, the extrusion temperature is too high, the resin is easily decomposed, and the film is brittle. In particular, the longitudinal tensile strength is remarkably lowered; if the temperature is too low, the resin is poorly plasticized, the expansion and stretching cannot be performed smoothly, the tensile strength of the film is lowered, and the gloss of the surface is poorly transparent, and even appears like a wood ring. Patterns and unmelted fish eyes.
3.1.2 blow ratio
The inflation ratio is one of the control points of the blown film production work, and refers to the ratio between the diameter of the bubble after inflation and the diameter of the uninflated pipe. 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 cannot be too large, otherwise the bubble is easily 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 2.5-3.0.
3.1.3 traction ratio
The traction ratio refers to the ratio between the pulling speed of the film and the extrusion speed of the tube. The draw ratio is a longitudinal draw ratio that imparts a directional effect in the direction of the draw. As the traction ratio increases, the longitudinal strength also increases, and the thickness of the film becomes thinner. However, if the pulling ratio is too large, the thickness of the film is difficult to control, and it is even possible to break the film. The draw ratio of the low density polyethylene (LDPE) film is generally controlled between 4-6.
3.1.4 Dew point
The dew point is also called the frost white line, which refers to the boundary line between the plastic oil and the fluid state entering the high elastic state. In the blown film process, the low density polyethylene (LDPE) film is in a molten state when extruded from the die, and has good transparency. After leaving the die, the inflation zone of the bubble is cooled by the pit but the wind 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 and cooling When the air is in contact, the heat of the bubble is carried away by the cold air, and its temperature drops significantly below the viscous flow temperature of the low-density polyethylene (LDPE), so that it cools and solidifies and becomes blurred. On the blown bubble we can see the dividing line between the blur of a transparent box, which is the frost white line.
In the process of blowing film, 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 In the cast film, on the contrary, if the dew point is relatively low, the inflation is carried out in the solid state. At this time, in the high elastic state, the inflation is like the transverse stretching, causing the molecules to be oriented, thereby making the inflation film The performance is close to that of the oriented film.
 
4, technical requirements for film properties
4.1 Specifications and deviations
The width and thickness of the polyethylene film should meet the requirements, the thickness of the film is uniform, the thickness deviation in the longitudinal direction and the transverse direction is small, and the deviation distribution is relatively uniform.
4.2 Appearance
It is required that the polyethylene film is plasticized well, no obvious fish eyes, the film surface is smooth and smooth, no wrinkles, no bubbles, cracking phenomenon, no obvious black spots, impurities, and no hanging diagonal lines and silk lines are allowed.
4.3 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 meet the standards.
 
5 Analysis and solution of common problems in blown film
5.1 film wrinkles
The film is wrinkled. The fundamental reason is that the thickness of the film is not uniform, even if it is very small. After accumulation, it can cause obvious wrinkles, affecting the measurement data of the falling flaw test, film tear test, film friction coefficient [11], etc. . It can be solved from the following aspects: (1) Lowering the melt temperature can effectively reduce the melt fluidity, and is relatively firm when the die is extruded, and the film is not skewed due to the cooling wind, resulting in uneven film cooling. (2) Reduce the cooling air volume or mediate the uneven cooling air. (3) The angle between the herringbone plates is too large, so that the film is flattened in a short time, so that wrinkles occur, and the angle between the herringbone plates should be appropriately reduced. (4) The film extrusion is not uniform, and the die should be adjusted to make it in a vertical state. (5) The tension of the winding roller is too large to pleat the film. The winding roller tension should be appropriately reduced.
5.2 poor transparency of the film
Polyolefin is a crystalline polymer. The key to improving transparency is to reduce crystallinity and make the spherulites fine. The main influencing factor is the resin raw material. The blown film process can only improve the transparency of the film to some extent. (1) The extrusion temperature is low and the melt plasticization is poor, and the melt temperature should be appropriately increased. (2) The cooling effect is not good, which affects the transparency, and the cooling air volume should be appropriately increased. (3) The resin particles contain a large amount of water, and the particles should be sufficiently dried.
5.3 film fish eyes more
Fisheyes are mainly additives in raw materials, low-molecular-weight resins and dusts. They are condensed on the die during processing. After a certain amount is accumulated, the film is continuously taken away to form fisheyes on the film. Because it is caused by precipitates in the resin, it cannot be eliminated fundamentally. The main mediation methods in the process are: (1) After a certain period of time, increase the screw speed, increase the melt extrusion pressure, and take away the precipitates. (2) Clean the die regularly. (3) Properly increase the melt temperature and fully plasticize.
5.4 Uneven thickness of film transverse
The lateral thickness of the film is not uniform. The root cause is unevenness in the discharge of the die. The solution is as follows: (1) The lip gap is not properly adjusted, and partial adjustment or full re-adjustment is required. (2) There are impurities in the cavity, which causes logistics disorder and uneven discharge. The die should be cleaned regularly. (3) The melt pressure or temperature is too high or fluctuates too much, which causes the extrusion melt flow to be unstable, affecting the thickness unevenness, adjusting the temperature, and improving the extrusion speed.
5.5 Running the coil when winding
Running the coil during winding can cause a lot of waste and loss, and should be avoided as much as possible. (1) The tension of the winding roller is too small, and the film roll is too loose. (2) The winding tension is unstable. (3) The axes of the respective guide rolls are not parallel. (4) The lateral thickness of the film is not uniform.
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