Most thermoplastics can be blown to produce the blown film. Blown film is a cylindrical film product obtained by extruding plastic into a thin tube, then blowing the plastic with compressed air while it is hot, and then cooling and shaping it.

The performance of this film is between the oriented movie and the cast film: the strength is better than the cast film, and the heat sealability is worse than that of the cast film.

There are many types of films produced by blow moulding, such as low-density polyethylene (LDPE), polypropylene (PP), high-density polyethylene (HDPE), nylon (PA), ethylene-vinyl acetate copolymer (EVA) and so on.

This article briefly introduces the commonly used low-density polyethylene (LDPE) film blow moulding production process and its common faults.

Selection of LDPE roll polyethylene blown film material

1. The selected raw materials should be blown film grade polyethylene resin particles containing an appropriate amount of slip agent to ensure the film's opening.

2. The melt index (MI) of the resin particles should not be too large. If 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 challenging to control, the resin has poor film-forming properties, 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 lacking.

Therefore, resin materials with a small melt index (MI) and a relatively wide molecular weight distribution should be selected, which can meet the film's performance requirements and ensure the processing characteristics of the resin. For example, the blown polyethylene film generally sets polyethylene raw materials with a melt index (MI) in the range of 2-6g/10min.

LDPE roll blow molding process control points

The process flow of the blown film is rough as follows:
Material plasticizing and extruding on the hopper→blowing traction→air ring cooling→herringbone splint→traction roller traction→corona treatment→film winding

However, it is worth pointing out that the performance of the blown film has a great relationship with the production process parameters. Therefore, in the blowing film process, it is necessary to strengthen the control of process parameters, standardize process operations, and ensure the smooth progress of production. Obtain high-quality film products.

Technical requirements for the basic performance of LDPE roll

1. Specifications and deviations
The width and thickness of the polyethylene film should meet the requirements, the film thickness should be uniform, the horizontal and vertical thickness deviations should be minor, and the deviation distribution should be relatively uniform.

2. Appearance
The polyethylene film is required to be well plasticized, without noticeable "water lines" and "clouds"; the surface of the film should be flat and smooth, without wrinkles or only a few live folds; no bubbles, perforations and ruptures are allowed; no prominent Black spots, impurities, crystal spots and rigid blocks; no severe hanging lines and silk patterns are permitted.

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

4. Surface tension
To make the printing ink and composite adhesive have good wettability and adhesion on the polyethylene film surface, the surface tension of the polyethylene film should reach a certain standard. Otherwise, it will affect the smooth progress of printing and composite production.

Generally speaking, the surface tension of the polyethylene film should be at least 38 dynes or more, preferably more than 40 dynes.

Common faults and solutions of low-density polyethylene (LDPE) blown film

1.LDPE roll is too sticky and poor opening
cause of the issue:
①The type of resin raw material is wrong, it is not blown film grade low-density polyethylene resin particles, which does not contain anti-blocking agent or the content of anti-blocking agent is too low;
②The temperature of the molten resin is too high, and the fluidity is too large;
③The blow-up ratio is too large, resulting in the inferior opening of the film;
④The cooling rate is too slow, the film is insufficiently cooled, and mutual adhesion occurs under the pressure of the traction roller;
⑤ Towing speed is too fast.

Solution:
①Replace resin raw materials, or add a certain amount of anti-blocking agent to the bucket;
②Appropriately reduce the extrusion temperature and the temperature of the resin;
③Appropriately reduce the inflation ratio;
④Increase the air volume, improve the cooling effect, and speed up the film cooling speed;
⑤Appropriately reduce the traction speed.

2.LDPE roll has poor transparency
cause of the issue:
① Extrusion temperature is low, resin plasticization is poor, resulting in lacking clarity of the movie after blow moulding;
②The inflation ratio is too small;
③The cooling effect is poor, which affects the clarity of the film;
④ The moisture content in the resin raw material is too large;
⑤The traction speed is too fast, and the film cooling is insufficient.

Solution:
① Increase the extrusion temperature appropriately so that the resin can be plasticized uniformly;
②Appropriately increase the inflation ratio;
③Increase the air volume and improve the cooling effect;
④ Dry the raw materials;
⑤Appropriately reduce the traction speed.

3.The LDPE roll has wrinkles
cause of the issue:
①Uneven film thickness;
②The cooling effect is not enough;
③ Too large blow-up ratio will cause the film bubble to be unstable, swinging back and forth, prone to wrinkles;
④ The angle of the herringbone splint is too large, and the film bubble is squashed in a short distance, so the film is also prone to wrinkles;
⑤The pressure on both sides of the traction roller is inconsistent, one side is high, and the other is low;
⑥ The axis between the guide rollers is not parallel, which affects the stability and flatness of the film, and causes wrinkles.

Solution:
①Adjust the thickness of the film to ensure uniform thickness;
②Improve the cooling effect to ensure that the film can be thoroughly cooled;
③Appropriately reduce the inflation ratio;
④Appropriately lessen the angle of the herringbone splint;
⑤Adjust the pressure of the traction roller to ensure uniform force on the film;
⑥Check the axis of each guide shaft and make them parallel to each other.

4.LDPE roll has misty water lines
cause of the issue:
①Extrusion temperature is low; resin plasticization is poor;
②The resin is damp, and the moisture content is too high.

Solution:
①Adjust the temperature setting of the extruder and increase the extrusion temperature appropriately.
② Drying the resin raw materials generally requires that the water content of the resin should not exceed 0.3%.

5.The film thickness of LDPE roll is uneven
cause of the issue:
①The uniformity of the die gap directly affects the uniformity of the film thickness. If the die gap is not uniform, the gap in some parts will be more significant, and the hole in some features will be smaller, resulting in more or less extrusion volume. The thickness of the film is also inconsistent; some parts are thin, some features are thick;
②The temperature distribution of the die mouth is uneven, with high and low temperatures, resulting in an uneven thickness of the film after blow moulding;
③The air supply volume around the cooling air ring is inconsistent, resulting in variable cooling effect, resulting in varying film thickness;
④The inflation ratio and traction ratio are inappropriate, making the thickness of the film bubble challenging to control;
⑤The traction speed is not constant and constantly changes, which will affect the film's thickness.

Solution:
①Adjust the gap between the die orifice of the machine head to ensure uniformity throughout;
②Adjust the temperature of the die mouth of the machine head to make the temperature of the die mouth part uniform;
③Adjust the cooling device to ensure uniform air output from the air outlet;
④Adjust the inflation ratio and traction ratio;
⑤Check the mechanical transmission to keep the traction speed constant.

6.The thickness of the LDPE roll is too thick
cause of the issue:
①The gap between the die and the extrusion volume is too large, so the film thickness is too thick;
②The air volume of the cooling air ring is too large, and the film cooling is too fast;
③The traction speed is too slow.

Solution:
①Adjust the gap of the die;
②Appropriately reduce the air volume of the air ring to inflate the film further, thereby making its thickness thinner;
③ Increase the traction speed appropriately.

7.The thickness of the LDPE roll is too thin
cause of the issue:
①The die gap is too small and the resistance is too large, so the film thickness is too narrow;
②The air volume of the cooling air ring is too small, and the film cooling is too slow;
③The pulling speed is too fast, the film is stretched too much, and the thickness becomes thinner.

Solution:
①Adjust the gap of the die;
②Appropriately increase the air volume of the air ring to speed up the cooling of the film;
③Appropriately reduce the traction speed.

8.LDPE roll has poor heat sealability
cause of the issue:
①The dew point is too low; the polymer molecules will be oriented so that the performance of the film is close to that of the oriented film, resulting in the reduction of heat-sealing performance;
② The expansion ratio and traction ratio are inappropriate (too large), and the film is stretched orientated, which affects the heat-sealing performance of the film.

Solution:
①Adjust the air volume in the air ring to make the dew point higher, and inflate and pull at the melting point of the plastic as much as possible to reduce the molecular stretch orientation caused by the inflation and traction;
②The expansion ratio and traction ratio should be appropriately small. If the expansion ratio is too large and the traction speed is too fast, the transverse and longitudinal stretching of the film will be excessive. The performance of the film will tend to be biaxially stretched. The heat-sealability will deteriorate.

9.The longitudinal tensile strength of the LDPE roll is poor
cause of the issue:
① The temperature of the molten resin is too high, which will reduce the longitudinal tensile strength of the film;
②The traction speed is slow, and the longitudinal direction of the film is not enough, which makes the longitudinal tensile strength worse;
③The blow-up ratio is too large and does not match the traction ratio, which will increase the orientation and tensile strength of the film in the transverse direction, while the tensile strength in the longitudinal direction will become worse;
④ The cooling rate of the film is too fast.

Solution:
①Appropriately reduce the temperature of the molten resin;
②Appropriately increase the traction speed;
③Adjust the inflation ratio to make it compatible with the traction ratio;
④ Reduce the cooling rate appropriately.

10.LDPE roll has poor transverse tensile strength
cause of the issue:
① The traction speed is too fast, and the difference is too significant with the inflation ratio, which will cause the longitudinal fibre to occur, and the transverse strength will deteriorate;
② The cooling speed of the cooling air ring is too slow.

Solution:
①Appropriately reduce the traction speed to match the inflation ratio;
②Increase the air volume of the air ring to cool the inflated film quickly, avoiding the
It is stretched orientated under the high elastic state of temperature.

11.The bubble of LDPE roll is unstable
cause of the issue:
①Extrusion temperature is too high, the fluidity of the molten resin is too large, the viscosity is too small, and it is easy to produce fluctuations;
②The extrusion temperature is too low, and the output is small;
③The air volume of the cooling air ring is unstable, and the film bubble is not uniformly cooled;
④ Interfered and affected by external solid air currents.

Solution:
①Adjust the extrusion temperature;
②Adjust the extrusion temperature;
③Check the cooling air ring to ensure that the air supply around it is even and consistent;
④ Prevent and reduce the interference of external airflow.

12.The surface of the LDPE roll is rough and uneven
cause of the issue:
①Extrusion temperature is too low; resin plasticization is poor;
②Extrusion speed is too fast.

Solution:
①Adjust the temperature setting of extrusion, and appropriately increase the extrusion temperature to ensure good resin plasticization;
②Appropriately reduce the extrusion speed.

13.LDPE roll has peculiar smell
cause of the issue:
①The resin material itself has a peculiar smell;
②Extrusion temperature of molten resin is too high, causing the resin to decompose, resulting in a peculiar smell;
③The film bubble is insufficiently cooled, and the hot air in the film bubble is not exhausted cleanly.

Solution:
①Replace resin raw materials;
②Adjust the extrusion temperature;
③Improve the cooling efficiency of the cooling air ring to fully cool the film bubble.