How to choose polyethylene-blown film material? The selected raw materials should be made of blown film grade polyethylene resin particles, containing an appropriate amount of slip agent to ensure thinness
The opening of the membrane.
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
Too small, the processing range is narrow, the processing conditions are difficult 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 poor. therefore,
The resin raw materials with a small melt index (MI) and a wide relative molecular weight distribution should be used, which can satisfy
The performance requirements of the film can also ensure the processing characteristics of the resin. The melt index is generally used for blown polyethylene film
(MI) Polyethylene raw material in the range of 2~6g/10min.
Key points of blow molding process control
The process flow of the blown film is rough as follows:
Material plasticizing and extruding on the hopper → inflating traction → air ring cooling → chevron 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 process of film blowing, it is necessary to strengthen the control of process parameters, standardize process operations, and ensure smooth production
And get high-quality film products. In the production process of polyethylene blown film, it is mainly
Control of the following process parameters:
When blowing low-density polyethylene (LDPE) film, the extrusion temperature is generally controlled between 160°C and 170°C.
And it must be ensured that the temperature of the die is uniform, the extrusion temperature is too high, the resin is easy to decompose, and the film is brittle, especially
The longitudinal tensile strength drops significantly; if the temperature is too low, the resin will be poorly plasticized, and it will not be able to expand and stretch smoothly.
The tensile strength of the film is low, and the surface gloss and transparency are poor, and even the appearance of wood growth rings.
Pattern and unmelted crystal nucleus (fisheye).
The blow-up ratio is one of the control points of the blown film production process. It refers to the diameter of the film bubble
The ratio between the diameters of the expanded pipe rings. The blow-up ratio is the transverse expansion multiple of the film, which is actually the film’s
Horizontal stretching, stretching will produce a certain degree of orientation effect on plastic molecules, increase the blow-up ratio, thus making the thin
The transverse strength of the film is improved. However, the blow-up ratio should not be too large, otherwise, it will easily cause bubble instability, and
The film is prone to wrinkles. Therefore, the inflation ratio should be properly matched with the traction ratio. Generally speaking, low density
The expansion ratio of high-density polyethylene (LDPE) film should be controlled within 2.5~3.0.
The traction ratio refers to the ratio between the traction speed of the film and the extrusion speed of the tube ring. The traction ratio is longitudinal
The extension factor makes the film have a directional effect in the drawing direction. If the traction ratio increases, the longitudinal strength will follow
Increase, and the thickness of the film becomes thinner, but if the traction ratio is too large, the thickness of the film is difficult to control, and it may even be difficult to control.
It can break the film, causing film breakage. The traction ratio of low-density polyethylene (LDPE) film is generally controlled at
4~6 is suitable.
The dew point is also called the frost line, which refers to the dividing line of the plastic from the viscous flow state to the high elastic state. During the film blowing process, low density
Polyethylene (LDPE) is in a molten state when extruded from the die, and has good transparency. After leaving the die,
The expansion zone of the film bubble should be cooled by the cooling air ring, and the cooling air is blown towards the frigid air at a certain angle and speed.
When the plastic film bubble is extruded from the die, the high-temperature film bubble is in contact with the cooling air, and the heat of the film bubble will be dissipated by the cold air.
If the air is taken away, its temperature will obviously drop below the viscous flow temperature of low-density polyethylene (LDPE), making it cooler.
But it solidified and became blurred. On the blown film bubble, we can see a line between transparent and fuzzy
The boundary is the dew point (or frost line).
During the film blowing process, the level of the dew point has a certain impact on the film performance. If the dew point is high,
Above the expanding bubble, the film is inflated in the liquid state, and the inflation only makes the film thinner, but
The film is not stretched orientated, and the performance of the blown film is close to that of the cast film. Conversely, if the dew point is relatively low,
The inflation is carried out in a solid-state. At this time, the plastic is in a highly elastic state, and inflation is like horizontal stretching.
Orient the molecules, so that the performance of the inflated film is close to that of the oriented film.
Technical requirements for basic performance
Specifications and deviations
The width and thickness of the polyethylene film should meet the requirements, the film thickness is uniform, and the horizontal and vertical thicknesses are uneven.
The difference is small, and the deviation distribution is relatively uniform.
The polyethylene film is required to be plasticized well, without obvious "watermarks" and "clouds"; the surface of the film should be flat
Smooth, no wrinkles or only a few live folds; no bubbles, perforations, and breaks are allowed; no obvious black
Spots, impurities, crystal spots, and rigid blocks; serious hanging lines and silk patterns are not allowed.
Physical and mechanical properties
Since the blown polyethylene film is used for printing or composite processing, it is subject to mechanical force.
Therefore, it is required that the physical and mechanical properties of the polyethylene film should be excellent, mainly including tensile strength, fracture
Several indicators such as elongation and tear strength should meet the standards.
In order to make printing inks and composite adhesives have good wettability and adhesion on the surface of the polyethylene film
Efforts should be made to require that the surface tension of the polyethylene film should reach a certain standard, otherwise it will affect the printing and reproduction
Co-production is proceeding smoothly. Generally speaking, the surface tension of polyethylene film should be at least 38 dynes or more.
It is better to reach 40 dynes or more.