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Plastic “blown film technology”, is so magical!

Selection of polyethylene blown film materials
The selected raw material should be blown polyethylene resin particles with appropriate amount of smoothing agent to ensure the opening of the film.

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, and the film-forming property of the resin is poor;

In addition, the melt index (MI) is too large, the molecular weight distribution of the polymer is too narrow, and the strength of the film is poor. Therefore, it is necessary to select the resin raw materials with low melt index (MI) and wide relative molecular weight distribution, which can not only meet the performance requirements of the film, but also ensure the processing characteristics of the resin.

Polyethylene materials with melt index (MI) in the range of 2 ~ 6G / 10min are generally used for blown polyethylene film.

Key points of blow molding process control

The process flow of blown film is as follows:

Hopper feeding – material plasticizing extrusion → blowing traction → air ring cooling → herringbone splint → traction by traction roller → corona treatment → 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 process of blown film, it is necessary to strengthen the control of process parameters, standardize the process operation, ensure the smooth production and obtain high-quality film products.

In the production process of polyethylene blown film, the following process parameters should be controlled

01 extruder temperature
When blowing low density polyethylene (LDPE) film, the extrusion temperature is generally controlled between 160 ℃ and 170 ℃, and the die 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 decreases significantly;

If the temperature is too low, the resin will not plasticize smoothly, and the tensile strength of the film will be low, and the gloss and transparency of the surface will be poor, and even the pattern like wood rings and unmelted crystal nuclei (fish eyes) will appear.

02 blow up ratio
Blowing ratio is one of the key points in the production process of blown film, which refers to the ratio between the diameter of blown film bubble and the diameter of non blown tube ring.

The blowing ratio is the transverse expansion multiple of the film, which is actually the transverse stretching of the film. The stretching will produce a certain degree of orientation effect on the plastic molecules, and the blowing ratio will increase, so as to improve the transverse strength of the film.

However, the blow up ratio should not be too large, otherwise it is easy to cause instability of the film bubble and wrinkle of the film. In general, the blowing ratio of LDPE film should be controlled between 2.5 and 3.0.

03 traction ratio
Traction ratio refers to the ratio between film traction speed and pipe ring extrusion speed. The draw ratio is the longitudinal draw ratio, which makes the film oriented in the drawing direction.

If the traction ratio is too large, the thickness of the film is difficult to control, and it may even break the film, resulting in film breakage.

The traction ratio of low density polyethylene (LDPE) film is generally controlled between 4 and 6.

04 dew point
Dew point, also known as frost line, refers to the boundary of plastic from viscous flow state to high elastic 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 should be cooled by the cooling air ring. When the cooling air blows to the plastic film bubble just extruded from the die head at a certain angle and speed, the high temperature film bubble contacts with 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 viscous 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 boundary between transparent and fuzzy, which is dew point (or frost line).

In the process of film blowing, the dew point has a certain influence on the film properties.

If the dew point is high and located above the inflated bubble, the film is inflated in liquid state. The film is only thinned and the molecules are not oriented by stretching. The performance of the inflated film is close to that of the cast film.

On the contrary, if the dew point is low, the blow up is carried out in the solid state. At this time, the plastic is in the high elastic state, and the blow up is just like the transverse stretching, which makes the molecular orientation action, so that the performance of the blow up film is close to that of the oriented film.

Technical requirements for basic performance
01 specification and deviation
The width and thickness of polyethylene film should meet the requirements, the film thickness should be uniform, the thickness deviation of transverse and longitudinal is small, and the deviation distribution is relatively uniform.

02 appearance
It is required that the polyethylene film should be plasticized well without obvious “water mark” and “cloud”; The surface of the film should be flat and smooth, without wrinkles or only a small amount of wrinkles; No bubble, perforation or rupture is allowed; There is no obvious black spot, impurity, crystal point and rigid block; Serious hanging line and silk lines are not allowed.

03 physical and mechanical properties
As the blown polyethylene film is used in printing or composite processing, it is subject to the effect of mechanical force. Therefore, the physical and mechanical properties of polyethylene film should be excellent, mainly including tensile strength, elongation at break, tear strength and other indicators should meet the standard.

04 surface tension
In order to make the printing ink and composite adhesive have good wettability and adhesion on the surface of polyethylene film, the surface tension of 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 polyethylene film should be at least 38 dyne, and 40 dyne is better.

Common faults and solutions of LDPE blown film
01 film is too sticky, poor opening
Cause of failure:
① The type of resin raw material is wrong, it is not LDPE resin particles of blown film grade, which does not contain opening agent or the content of opening agent is low;
② The temperature of molten resin is too high and the fluidity is too high;
③ When the blowing ratio is too large, the opening of the film becomes poor;
④ The cooling speed is too slow, the film is not cooled enough, and the film is bonded with each other under the pressure of traction roller;
⑤ The traction speed is too fast.
terms of settlement:
① Change the resin raw materials, or add a certain amount of opening agent to the kedou;
② Reduce the extrusion temperature and resin temperature properly;
③ The blowing ratio should be reduced appropriately;

④ Increase the air volume, improve the cooling effect and speed up the film cooling speed;
⑤ Reduce the traction speed appropriately.
02 poor transparency of film
Cause of failure:
① Low extrusion temperature and poor plasticization of the resin result in poor transparency of the blown film;
② The blowing ratio is too small;
③ The cooling effect is not good, which affects the transparency of the film;
④ The moisture content in resin raw material is too high;
⑤ Traction speed is too fast, film cooling is insufficient.
terms of settlement:
① Increase the extrusion temperature properly to make the resin plasticize evenly;
② Increase the blowing ratio properly;
③ Increase the air volume to improve the cooling effect;
④ Drying the raw materials;
⑤ Reduce the traction speed appropriately.
03 film is wrinkled
Cause of failure:
① The film thickness is not uniform;
② The cooling effect is not enough;
③ If the blowing ratio is too large, the bubble will be unstable, swing back and forth, and easy to wrinkle;
④ If the angle of the herringbone splint is too large, the membrane bubble will be flattened in a short distance, so the membrane is prone to wrinkle;
⑤ The pressure on both sides of the traction roller is not consistent, one side is high and the other side is low;
⑥ The axis between the guide rollers is not parallel, which affects the stability and flat property of the film, resulting in wrinkles.
terms of settlement:
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 The blowing ratio should be reduced appropriately;
④ The angle of the herringbone splint should be reduced properly;
⑤ Adjust the pressure of the traction roller to ensure the uniform force on the film;
⑥ Check the axis of each guide shaft and make it parallel to each other.
04 film has foggy water mark
Cause of failure:
① The extrusion temperature is low and the resin plasticization is poor;
② Moisture content of resin is too high.
terms of settlement:
① Adjust the temperature setting of extruder and increase the extrusion temperature appropriately.

② When drying the resin raw material, the moisture content of the resin should not exceed 0.3%.
05 uneven film thickness
Cause of failure:
① The uniformity of die gap directly affects the uniformity of film thickness. If the die gap is not uniform, some parts of the gap is larger, some parts of the gap is smaller, resulting in more or less extrusion. Therefore, the film thickness formed is not consistent, some parts are thin, some parts are thick;
② The temperature distribution of the die is uneven, which makes the film thickness uneven;
③ The air supply around the cooling air ring is not consistent, resulting in uneven cooling effect and uneven film thickness;
④ It is difficult to control the bubble thickness because of the unsuitable blowing ratio and traction ratio;
⑤ Traction speed is not constant, constantly changing, which of course will affect the thickness of the film.
terms of settlement:
① Adjust the die gap of the die head to ensure the uniformity of all parts;
② Adjust the die temperature to make the die temperature uniform;
③ Adjust the cooling device to ensure the uniform air volume of the air outlet;
④ Adjust the blowing ratio and traction ratio;
⑤ Check the mechanical transmission to keep the traction speed constant.
The thickness of 06 film is too thick
Cause of failure:
① The die gap and extrusion amount are too large, so the film thickness is too thick;
② The air volume of cooling air ring is too large and the film cooling is too fast;
③ The towing speed is too slow.
terms of settlement:
① Adjust the die gap;
② The thickness of the film becomes thinner by reducing the air volume of the air ring;
③ Increase the traction speed appropriately.
07 thin film thickness is too thin
Cause of failure:
① The die gap is too small and the resistance is too large, so the film thickness is too thin;
② The air volume of cooling air ring is too small and the film cooling is too slow;
③ If the traction speed is too fast, the film will stretch too much, which will make the thickness thinner.
terms of settlement:
① Adjust the die gap;
② Increase the air volume of the air ring appropriately to speed up the cooling of the film;
③ Reduce the traction speed appropriately.
08 film has poor heat sealing property
Cause of failure:
① If the dew point is too low, the polymer molecules are oriented, which makes the performance of the film close to that of the oriented film, resulting in the decrease of the heat sealing performance;
② When the blowing ratio and drawing ratio are not appropriate (too large), the tensile orientation of the film occurs, which affects the heat sealing performance of the film.
terms of settlement:
① Adjust the air volume in the wind ring to make the dew point higher, and expand and pull at the melting point of the plastic as far as possible, so as to reduce the molecular stretching orientation caused by expansion and pull;
② If the blowing ratio is too large, the pulling speed is too fast, and the transverse and longitudinal stretching of the film is too large, the film performance will tend to biaxial stretching, and the heat sealing property of the film will become worse.

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