Problems and Solutions in the Production of Blown Polyethylene (LDPE) Films

Most thermoplastics can be used to produce blown film by blow molding. The blown film is a tubular film product obtained by squeezing plastic into a thin tube and then blowing the plastic with compressed air while hot, and then cooling and shaping. The performance of this film lies between the oriented film and the cast film: the strength is better than that of the cast film, and the heat sealability is inferior to 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 (EVA), etc. We briefly introduce the blow molding process and common failures of commonly used low-density polyethylene (LDPE) films.

The choice of polyethylene blown film material

1. The raw materials used 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 can not be too large, 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 of the resin is poor, and it is not easy to form 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 relatively low melt index (MI) and a relatively wide molecular weight distribution should be selected so as to satisfy the performance requirements of the film and ensure the processing characteristics of the resin. Blown polyethylene films typically use polyethylene materials with a melt index (MI) in the range of 2 to 6 g/10 min.

Blow process control points

The blown film process is roughly as follows:

Material hopper plastification extrusion blow traction air ring cooling spline traction roller pulling 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, during the blown film process, it is necessary to strengthen the control of process parameters, standardize the process operation, and ensure the smooth progress of production. Get high quality thin film products. In the production process of polyethylene blown film, the following process parameters are mainly controlled:

1. Extruder temperature

When low-density polyethylene (LDPE) film is blown, the extrusion temperature is generally controlled between 160° C. and 170° C., and the head temperature must be uniform, the extrusion temperature is too high, the resin is easily decomposed, and the film is brittle. The longitudinal tensile strength is significantly decreased; when the temperature is too low, the resin is poorly plasticized, and the film cannot be stretched and drawn in a smooth manner. The film has a low tensile strength, and the surface is poor in gloss and transparency, and even appears like a wood ring. Pattern and unmelted crystal nuclei (fish eyes).

2. Blow-up 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 uninflated ring diameter. The blow-up ratio is the lateral expansion ratio of the film. In fact, the film is stretched in the transverse direction. The 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 must not be too large. Otherwise, the bubble is liable to be unstable, and the film is prone to wrinkles. Therefore, the blow-up ratio should be matched with the traction ratio. In general, the blow-up ratio of the low-density polyethylene (LDPE) film should be controlled at 2.5 to 3.0.

3. Traction ratio

Traction ratio refers to the ratio between the traction speed of the film and the extrusion speed of the tube ring. Traction ratio is the longitudinal stretching ratio, so that the film has a directional effect in the direction of extraction. When the traction ratio increases, the longitudinal strength also increases, and the thickness of the thin film becomes thin. However, if the traction ratio is too large, the thickness of the thin film is difficult to control, and there is a possibility that the thin film will be broken, resulting in a broken film phenomenon. The traction ratio of low density polyethylene (LDPE) film is generally controlled between 4 and 6.

Dew point

Dew point, also known as frost line, refers to the dividing line between plastic and viscous flow state into high elastic state. In the process of blown film, low density polyethylene (LDPE) is in a molten state when extruded from a die, and transparency is good. After leaving the die, the inflation zone of the bubble is cooled by the cooling air ring. When the cooling air is blown at a certain angle and speed toward the plastic film bubble just extruded from the head, the high temperature bubble and cooling air When in contact, the heat of the bubble will be carried away by the cold air, and its temperature will obviously drop below the viscosity temperature of low density polyethylene (LDPE), so that it will be cooled and solidified and become blurred. On the blown film bubble we can see a dividing line between transparency and blurring, which is the dew point (or frost line).

During the film blowing process, the dew point has a certain influence on the film properties. If the dew point is high, located above the blown bubble, the film blows up in the liquid state. The blow-up only thins the film, and the molecules are not stretched. At this time, the blown film performance is close to In 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 highly elastic state and the inflation is the same as the transverse stretching, so that the molecules are oriented so that the performance of the blown film is close to Orientation film.

Technical requirements for basic performance

1. Specifications and deviations

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

2. Appearance

Requires good plasticization of the polyethylene film, no obvious “watermarks” and “clouds and fog”; the surface of the film should be smooth and smooth with no wrinkles or only a small amount of living pleats; no bubbles, perforations, and cracks are allowed; no Black spots, impurities, crystal spots and deadlocks; no serious hanging lines and threads are allowed.

3. Physical and mechanical properties

Since the blown polyethylene film is subjected to a mechanical force when it is used in a printing or compounding process, the physical and mechanical properties of the polyethylene film should be excellent, including tensile strength, elongation at break, and tearing. Several indicators such as strength should meet the standards.

4. Surface tension

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

Low-density polyethylene (LDPE) blown film common faults and solutions

1. Film is too viscous and poor opening

cause of issue:

1 The type of resin raw material is not correct, not blown film-grade low-density polyethylene resin particles, in which the content of the opening agent or opening agent is not low;

2 The temperature of the molten resin is too high and the fluidity is too high

3 Blowing ratio is too large, resulting in poor opening of the film

4 The cooling rate is too slow, the film cooling is insufficient, and mutual adhesion occurs under the action of the pulling roller pressure;

5 Pulling speed too fast.

Solution:

1Replace the resin raw material, or add a certain amount of opening agent to the branch;

2 properly reduce the extrusion temperature and the temperature of the resin;

3 appropriately reduce the blow-up ratio;

4 Increase the air volume, improve the cooling effect, and accelerate the cooling rate of the film;

5 properly reduce the traction speed.

2. Thin film transparency

cause of issue:

1 The extrusion temperature is too low, and the plasticization of the resin is poor, resulting in poor transparency of the film after blow molding;

2 blow ratio is too small;

3 poor cooling effect, thus affecting the transparency of the film;

4 The moisture content in the resin material is too large;

5 The pulling speed is too fast and the film cooling is insufficient.

Solution:

1 Properly increase the extrusion temperature so that the resin can be uniformly plasticized;

2 properly increase the blow-up ratio;

3 increase the air volume, improve the cooling effect;

4 Drying of raw materials;

5 properly reduce the traction speed.

3. Film wrinkles

cause of issue:

1 film thickness is not uniform;

2 cooling effect is not enough;

3 The blow-up ratio is too large, causing the bubble to be unstable, swinging back and forth, and prone to creases.

The angle of the 4 character splint is too large, and the bubble is flattened in a short distance, so the film is also prone to wrinkles;

5 The pressure on both sides of the pulling roller is inconsistent, one side high and the other low;

6 The axes between the guide rollers are not parallel, affecting the stability and flatness of the film and causing wrinkles.

Solution:

1 Adjust the thickness of the film to ensure uniform thickness;

2 improve the cooling effect, to ensure that the film can be fully cooled;

3 appropriately reduce the blow-up ratio;

4 appropriately reduce the angle of the splint;

5 Adjust the pressure of the traction roller to ensure that the film is evenly loaded;

6 Check the axes of the guide shafts and make them parallel to each other.

4, film misty water lines

cause of issue:

1 extrusion temperature is low, poor resin plasticization;

2 The resin is damp and its moisture content is too high.

Solution:

1 Adjust the temperature setting of the extruder and increase the extrusion temperature appropriately.

2 The drying of resin raw materials generally requires that the moisture content of the resin should not exceed 0.3%.

5 uneven film thickness

cause of issue:

1 The uniformity of the die gap directly affects the uniformity of the film thickness. If the die gap is not uniform, some gaps are larger and some gaps are smaller, resulting in more or less extrusion. Therefore, the gap is formed. The thickness of the film is also inconsistent, some parts are thin, some parts are thick;

2 mold mouth temperature distribution is not uniform, there are high and low, so that the thin film thickness after blowing;

3 The amount of air supplied around the cooling air ring is inconsistent, resulting in non-uniform cooling effect, which results in uneven film thickness.

4 Inflatable ratio and traction ratio are not suitable, so that the bubble thickness is not easy to control;

5 The pulling speed is not constant and changes constantly, which of course affects the thickness of the film.

Solution:

1Adjust the head die gap to ensure uniformity in all parts;

(2) Adjust the temperature of the die mouth to make the die part temperature uniform;

3Adjust the cooling device to ensure the air output from the air outlet is even

4Adjust the blow-up ratio and traction ratio;

5 Check the mechanical transmission to keep the traction speed constant.

6. Thin film thickness

cause of issue:

1 mold gap and extrusion volume is too large, so the film thickness is thick;

2 The air volume of the cooling air ring is too large and the film is cooled too quickly

3 The pulling speed is too slow.

Solution:

1 Adjust the die gap;

2 appropriately reduce the air volume of the wind ring, so that the film further blow, so that its thickness becomes thinner;

3 properly increase the traction speed.

Thin film thickness

cause of issue:

1 die mouth gap is too small, too much resistance, so thin film thickness;

2 The air volume of the cooling air ring is too small, the film cooling is too slow

3 The traction speed is too fast and the film is stretched too much to make the thickness thinner.

Solution:

1 Adjust the die gap;

2 Appropriately increase the air volume of the air ring to speed up the cooling of the film;

3 properly reduce the traction speed.

8. The poor heat sealing property of the film

cause of issue:

1 The dew point is too low, the polymer molecules are oriented, so that the performance of the film is close to that of the oriented film, resulting in a decrease in the heat seal performance;

2 Insufficient blow-up ratio and traction ratio (excessively large), the stretching orientation of the film occurs, which affects the heat-sealing properties of the film.

Solution:

1 Adjust the amount of wind in the air ring to make the dew point a bit higher. As far as possible, inflate and pull under the melting point of the plastic so as to reduce the molecular stretching orientation caused by inflation and traction;

2 blow ratio and traction ratio should be properly smaller, if the blow-up ratio is too large, and the traction speed is too fast, the film is stretched horizontally and vertically, then it will make the performance of the film tends to biaxial stretching, the film's Heat sealability will deteriorate.

9. Film longitudinal tensile strength difference

cause of issue:

1 the temperature of the molten resin is too high, will make the longitudinal tensile strength of the film decreased;

2 The pulling speed is slower, and the orientation effect of the film in the longitudinal direction is not enough, so that the longitudinal tensile strength is deteriorated;

(3) The blow-up ratio is too large and does not match the traction ratio, so that the transverse orientation and tensile strength of the film are improved, and the longitudinal tensile strength is deteriorated;

4 The cooling rate of the film is too fast.

Solution:

1 properly reduce the temperature of the molten resin;

2 properly increase the traction speed;

3 Adjust the blow-up ratio so that it is compatible with the traction ratio;

4 Reduce the cooling rate appropriately.

10. Film transverse tensile strength difference

cause of issue:

1 The traction speed is too fast, and the difference between the blow-up ratio and the blow-up ratio is too large, so that the longitudinal fiberization occurs and the lateral strength becomes worse;

2 The cooling speed of the cooling air ring is too slow.

Solution:

1Properly reduce the traction speed so that it matches the blow-up ratio;

2 Increase the air volume of the air ring to allow the blown film to cool quickly and avoid high

The high elasticity of the temperature is stretched orientation.

11. Bubble instability

cause of issue:

1 extrusion temperature is too high, the fluidity of the molten resin is too large, the viscosity is too small, easily fluctuate;

2 extrusion temperature is too low, the amount of discharge is small;

3 The air volume of the cooling air ring is unstable, and the bubble cooling is not uniform;

4 interfered with and affected by strong external air flow.

Solution:

1Adjust the extrusion temperature;

2 adjust the extrusion temperature;

3 Check the cooling air ring to ensure that the amount of air around is uniform;

4 Prevent and reduce the interference of outside air flow.

12, the film surface is rough, uneven

cause of issue:

1 extrusion temperature is too low, poor resin plasticization;

2 extrusion speed is too fast.

Solution:

1Adjust the extrusion temperature setting, and increase the extrusion temperature properly to ensure good resin plasticization;

2 properly reduce the extrusion speed.

13. Film has odor

cause of issue:

1 resin raw material itself has odor;

2 Extrusion temperature of the molten resin is too high, causing the resin to decompose, resulting in odor;

3 Insufficient bubble cooling, hot air in the bubble was not removed.

Solution:

1Replacement of resin raw materials;

2 adjust the extrusion temperature;

3 Improve the cooling efficiency of the cooling air ring and allow the bubble to cool sufficiently.