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Most thermoplastics can be used to produce blow-molded films through the blow molding process. In this method, plastic is extruded into a thin tube, which is then inflated with compressed air while still hot; after cooling and setting, a tubular film is obtained. The properties of such films lie between those of oriented films and cast films: they have better strength than cast films, but poorer heat-sealability. 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 copolymer (EVA), etc. Here, a brief introduction is given to the blow molding production process for the commonly used low-density polyethylene (LDPE) film. I. Selection of Polyethylene Blow-Molded Film Materials 1. The raw material to be used should be polyethylene resin particles of blow-molding grade, containing an appropriate amount of slip agent to ensure the film’s openness. 2. The melt index (MI) of the resin particles should not be too high; if the melt index is too high, the viscosity of the molten resin becomes too low, the range of processability is narrow, it is difficult to control the processing conditions, and the resin has poor film-forming properties, making it hard to form a film from it ； Furthermore, if the melt index (MI) is too high and the molecular weight distribution of the polymer is too narrow, the strength of the film is poor. Therefore, resin raw materials with a low melt index (MI) and a wide molecular weight distribution should be selected, as this will meet the performance requirements of the film while also ensuring the processing properties of the resin. For blow-molded polyethylene films, polyethylene raw materials with a melt index (MI) in the range of 2–6 g/10 min are generally used. II. Key Points in Blow Molding Process Control The process flow for blow molding films is roughly as follows: feeding material from a hopper → plasticizing and extruding the material → inflating and pulling the film → cooling with air rings → using clevis plates → pulling with traction rollers → corona treatment → winding up the film. It should be noted, however, that the performance of blow-molded films is highly dependent on the process parameters; therefore, it is essential to strictly control these parameters during the molding process, ensure proper operation of the process, and thus achieve high-quality film products. In the production process of polyethylene blow-molded films, it is essential to properly control the following process parameters: 1. Extruder temperature. When blowing molding low-density polyethylene (LDPE) films, the extrusion temperature is generally maintained between 160°C and 170°C. It is also important to ensure uniform temperature at the die head; if the extrusion temperature is too high, the resin tends to decompose, resulting in a brittle film, with a significant decrease in its longitudinal tensile strength ； If the temperature is too low, the resin fails to be properly plasticized, preventing smooth expansion and stretching; as a result, the tensile strength of the film is low, and its surface has poor luster and transparency. In addition, patterns similar to tree rings as well as unmelted crystal nuclei (fish eyes) may appear. 2. Expansion ratio: The expansion ratio is one of the key control parameters in the blown film production process; it refers to the ratio between the diameter of the film bubble after expansion and the diameter of the unexpanded tube shape. The inflation ratio is the factor by which the film expands in the transverse direction; in effect, it involves stretching the film horizontally, and this stretching exerts a certain degree of orientation on the plastic molecules. An increase in the inflation ratio leads to an improvement in the film’s transverse strength. However, the inflation ratio should not be too high either, as this can easily lead to instability of the film bubbles and wrinkles in the film. Therefore, the inflation ratio should be appropriately matched with the drawing ratio; generally speaking, the inflation ratio for low-density polyethylene (LDPE) films should be kept between 2.5 and 3.0. 3. Pulling ratio: The pulling ratio refers to the ratio between the pulling speed of the film and the extrusion speed of the tubular ring. The drawing ratio is the longitudinal stretching factor that imparts orientation to the film in the drawing direction. As the drawing ratio increases, the longitudinal strength also rises and the thickness of the film decreases. However, if the drawing ratio is too high, it becomes difficult to control the film’s thickness, and the film may even break, resulting in film rupture. The drawing ratio for low-density polyethylene (LDPE) films is generally preferably kept between 4 and 6. 4. Dew point: The dew point, also known as the frost line, refers to the boundary at which plastic transitions from a viscous flow state to a highly elastic state. During the film blowing process, low-density polyethylene (LDPE) is in a molten state as it is extruded from the die, exhibiting good transparency. After leaving the die orifice, the inflation area of the film bubble must be cooled using a cooling air ring. When the cooling air is blown toward the plastic film bubble that has just been extruded from the die at a certain angle and speed, the hot film bubble comes into contact with the cooling air; the heat from the bubble is carried away by the cold air, causing its temperature to drop significantly below the viscous flow temperature of low-density polyethylene (LDPE). As a result, the film bubble cools and solidifies, becoming opaque. On the blown film bubble, we can see a boundary line between transparency and opacity; this is the dew point (or frost line). During film blowing, the level of dew point has a certain impact on the properties of the film. If the dew point is high and located above the blown film bubbles, the blowing of the film takes place in a liquid state; the blowing only thins the film without causing the molecules to be stretched and aligned. In this case, the properties of the blown film are similar to those of the cast film. Conversely, if the dew point is low, bulking occurs in the solid state; at this time the plastic is in a highly elastic state, and bulking acts similar to transverse stretching, causing the molecules to align, thereby making the properties of the bulked film similar to those of an oriented film. :III. Technical requirements for basic performance 1. Specifications and tolerances: The width and thickness of the polyethylene film must meet the specified requirements; the thickness of the film should be uniform, with small deviations in both the horizontal and vertical directions, and these deviations should be evenly distributed. 2. Appearance: The polyethylene film is required to be well-plasticized, with no obvious \"water marks\" or \"fogging\"" ； The surface of the film should be flat and smooth, without wrinkles or with only a few minor folds ； Bubbles, perforations, and cracks are not allowed ； No obvious black spots, impurities, crystals, or lumps ； Severe stringing and streaks are not allowed. 3. Physical and mechanical properties: Since polyethylene films after blow molding are used in printing or lamination processes and are subjected to mechanical forces, it is necessary that these films possess excellent physical and mechanical properties. Key parameters such as tensile strength, elongation at break, and tear strength must meet the specified standards. 4. Level of surface tension: In order for printing inks and adhesives used for lamination to exhibit good wetting and adhesion properties on the surface of polyethylene film, it is necessary for the surface tension of this film to meet certain standards; otherwise, it will affect the smooth progress of printing and lamination processes. Generally, the surface tension of polyethylene film should be at least 38 dynes, with 40 dynes or more being preferable.