LDPE blown film production process points, and analysis of common problems, 99% of the problems are solved!

Most thermoplastics can be blown to produce blown film, which is a thin tube of plastic squeezed into a tube, then blown with compressed air to expand the plastic while it is still hot, and then cooled and shaped to produce a cylindrical film product. The performance of this film is between oriented film and cast film: the strength is better than cast film and the heat seal is worse than cast film.

There are many varieties of film produced by the blow moulding method, such as low density polyethylene (LDPE), polypropylene (PP), high density polyethylene (HDPE), nylon (PA), ethylene a vinyl acetate copolymer (EVA), etc.

This article provides a brief introduction to the blow moulding production process of commonly used low density polyethylene (LDPE) films and their common failures.

01 Selection of blown polyethylene film materials

1. The raw material used should be blown film grade polyethylene resin particles containing the right amount of slip agent to ensure the openness of the film.

2. The melt index (MI) of resin particles should 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-forming properties of the resin is poor, not easy to process into film; in addition, the melt index (MI) is too large, the relative molecular weight distribution of the polymer is too narrow, the strength of the film is poor.

Therefore, a smaller melt index (MI) and a wider relative molecular weight distribution of the resin raw material should be used, so as to meet the performance requirements of the film, but also to ensure the processing characteristics of the resin. Blown polyethylene films are generally made from polyethylene with a melt index (MI) in the range of 2 to 6 g/10min.

02.Blow moulding processcontrol points

The blown film process is roughly as follows.
Hopper loading – plasticized material extrusion – blowing and hauling – air ring cooling – herringbone splinting – hauling by haul-off rollers – corona treatment – film winding

However, it is worth pointing out that the performance of blown film has a great deal to do with the parameters of the production process. Therefore, it is important to strengthen the control of the process parameters and standardize the process operation to ensure smooth production and to obtain high quality film products.

In the production of blown polyethylene film, the main focus is on good control of the following process parameters.

Extruder temperature

When blow moulding low density polyethylene (LDPE) film, the extrusion temperature is generally controlled between 160°C and 170°C, and must ensure that the head temperature is 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 reduced.

If the temperature is too low, the resin does not plasticize well, it does not expand and stretch smoothly, the tensile strength of the film is low, and the gloss and transparency of the surface is poor, even appearing like a wood yearning pattern and unmelted nuclei (fish eyes).

Blow-up ratio

The blow-up ratio is one of the main points of control in the blown film production process and refers to the ratio between the diameter of the blown film bubble and the diameter of the unblown tube ring.

The blow-up ratio is a multiple of the lateral expansion of the film, which actually stretches the film laterally. The stretching produces a certain degree of orientation of the plastic molecules and an increase in the blow-up ratio, which results in an increase in the lateral strength of the film.

However, the blowing ratio should not be too large, otherwise it is easy to cause instability in the film bubble and the film is prone to wrinkling. Therefore, the blowing ratio should be appropriate with the traction ratio. Generally speaking, the blowing ratio of low density polyethylene (LDPE) film should be controlled at 2.5 to 3.0.

Traction ratio

The traction ratio is the ratio between the traction speed of the film and the extrusion speed of the tube ring. The traction ratio is the longitudinal stretch multiplier that gives the film its orientation in the direction of draw.

If the traction ratio increases, the longitudinal strength increases and the thickness of the film become thinner, but if the traction ratio is too large, the thickness of the film is difficult to control and there is even a risk that the film will be pulled off, causing the phenomenon of broken film. The traction ratio of 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 where the plastic enters the high elastic state from the viscous flow state. In the blown film process, low density polyethylene (LDPE) is in a molten state when extruded from the die opening with good transparency.

When leaving the die opening, the blown area of the film bubble is to be cooled by the cooling air ring. When the cooling air is blown at a certain angle and speed to the plastic film bubble just extruded from the head, the hot film bubble comes into contact with the cooling air, the heat of the film bubble will be taken away by the cold air and its temperature will drop significantly below the viscous flow temperature of low density polyethylene (LDPE), thus making it cool and cured and blurred.

On the blown film bubble we can see a dividing line between clear and faint, this is the dew point (or frost line).

During the film blowing process, the level of dew point has a definite influence on the film properties.

If the dew point is high and located above the blown film bubble, the film is blown in a liquid state and the blowing only thins the film without the molecules being subjected to stretching orientation, at which point the performance of the blown film is close to that of a cast film.

Conversely, if the dew point is relatively low, the blowing takes place in the solid state, when the plastic is in a highly elastic state, and the blowing acts as a transverse stretch, orienting the molecules so that the performance of the blown film is close to that of an oriented film.

03. Technical requirements for basic performance

Specifications and deviations

The width and thickness of the polyethylene film should meet the requirements; the film should be of uniform thickness, with small thickness deviations in the horizontal and vertical directions and a relatively even distribution of deviations.

Appearance

Polyethylene films are required to be well plasticized, with no obvious “water lines” or “clouds”; the surface of the film should be flat and smooth, with no wrinkles or only a few live folds; no bubbles, perforations or ruptures; no obvious black spots, impurities, crystals or stiff blocks; no serious hanging lines or filaments.

Physical and mechanical properties

As the blown polyethylene film is subject to mechanical forces when used in printing or compounding processes, the physical and mechanical properties of the polyethylene film are required to be excellent, mainly including tensile strength, elongation at break, tear strength and several other indicators that should meet the standards.

The magnitude of surface tension

In order for printing inks and laminating adhesives to have good wetting and adhesion on the surface of polyethylene film, the surface tension of the polyethylene film should be required to meet certain standards, otherwise the smooth running of printing and laminating production will be affected.

As a general rule, polyethylene films should have a surface tension of at least 38 dynes, with 40 dynes or more being preferable.

04. Common faults and solutions

Film too sticky, poor open ability
1. Cause of failure.
a. incorrect type of resin raw material, not blown film grade LDPE resin particles, which do not contain openers or have a low content of openers.
b. The temperature of the molten resin is too high and the flow is too great.
c. Too large a blow-up ratio, resulting in poor openness of the film.
d. too slow cooling, insufficient cooling of the film and mutual bonding under the action of the traction roller pressure.
e.Traction speed is too fast.

2. Solutions.
a. Replace the resin raw material or add a quantity of opening agent to the cobbler.
b. Appropriate lowering of the extrusion temperature and the temperature of the resin.
c. Appropriate reduction of the blow-up ratio.
d. Increasing the air volume to improve the cooling effect and accelerate the film cooling rate.
e. Reduce the traction speed appropriately.

Poor film transparency

1. Cause of failure.
a. Low extrusion temperature and poor plasticization of the resin, resulting in poor transparency of the blown film.
b. The blow-up ratio is too small.
c. Poor cooling, this affects the transparency of the film.
d. Excessive moisture content in the resin raw material.
e. Traction speed is too fast and the film is not cooled enough.

2. Solutions.
a. Raising the extrusion temperature appropriately so that the resin can be plasticized uniformly.
b. An appropriate increase in the blow-up ratio.
c. Increasing the air volume to improve the cooling effect.
d. Drying of raw materials.
e. Reduce the traction speed appropriately.

Wrinkling of the film

1 Cause of failure.
a. Uneven film thickness.
b. Insufficient cooling effect.
c. The blowing ratio is too large, causing the film bubble to be unstable, swinging back and forth from side to side and prone to wrinkling.
d. The angle of the herringbone cleat is too large and the film bubble is flattened over a short distance, so the film is also prone to wrinkling.
e. inconsistent pressure on both sides of the traction roller, one side being high and the other low.
f. The axes between the guide rollers are not parallel, which affects the stability and spreading of the film, resulting in wrinkles.

2. Solutions.
a. Replace the resin raw material or add a quantity of opening agent to the cobbler.
b. Appropriate lowering of the extrusion temperature and the temperature of the resin.
c. Appropriate reduction of the blow-up ratio.
d. Increasing the air volume to improve the cooling effect and accelerate the film cooling rate.
e. Reduce the traction speed appropriately.

Foggy water pattern on film

1. Cause of failure.
a. Low extrusion temperature and poor plasticization of the resin.
b. Resin is damp and has too high a moisture content.

2. Solution.
a. Adjust the temperature setting of the extruder and increase the extrusion temperature appropriately.
b. Dry the resin raw material, generally requiring the moisture content of the resin not to exceed 0.3%.

Uneven film thickness

1. Cause of failure.
a. The uniformity of the die gap directly affects the uniformity of the film thickness, if the die gap is not uniform, some parts have a larger gap, some parts have a smaller gap, thus causing the extrusion volume to be more or less, therefore, the film thickness formed is not uniform, some parts are thin, some parts are thick.
b. Uneven temperature distribution at the mould mouth, with high and low temperatures, resulting in uneven film thickness after blow moulding.
c. Inconsistent air supply around the cooling air ring, resulting in an uneven cooling effect and thus an uneven film thickness.
d. inappropriate blowing and traction ratios, making it difficult to control the thickness of the film bubble.
e. The traction speed is not constant and constantly changes, which of course affects the film thickness.

2. Solution.
a. Adjusting the headstock die opening gap to ensure uniformity at all points.
b. adjust the temperature of the die opening of the head so that the temperature of the die opening part is uniform.
c. Adjusting the cooling device to ensure uniform airflow from the air outlet.
d. Adjustment of the blow-up and traction ratios.
e. Check the mechanical drive to keep the traction speed constant.

Thickness of film is on the thick side

1. Cause of failure.
a. Large die gap and extrusion volume, hence thick film thickness.
b. The air volume of the cooling air ring is too high and the film cools too quickly.
c. The traction speed is too slow.

2. Solution.
a. Adjustment of the die opening gap.
b. Reduce the air volume of the wind ring appropriately to blow the film further and thus make it a little thinner.
c. Increase the traction speed appropriately.

Thin film thickness

1. Cause of failure.
a. The die opening gap is too small and the resistance is too high, so the film thickness is thin.
b. the air volume of the cooling air ring is too small and the film cools too slowly.
c. Traction speed is too fast and the film stretches excessively, thus thinning the thickness.

2. Solution.
a. Adjustment of the die opening gap.
b. Increase the air volume of the air ring appropriately to speed up the cooling of the film.
c. Reduce the traction speed appropriately.