Extrusion Blow Molding Of Polyethylene Films

 

Polyethylene (PE) film is the most produced and widely used type of plastic film. It is non-toxic, odorless, and possesses high strength and toughness, along with excellent moisture resistance, water resistance, openability, low-temperature resistance, and a certain degree of transparency. It has outstanding heat-sealing properties with a relatively low heat-sealing temperature (around 100°C), which allows heat-sealing via electric heating and convenient bag-making. Polyethylene film is directly produced through extrusion blow molding using polyethylene resin pellets, generally without the need for additives; color masterbatches can be mixed in for dyeing. The polyethylene resin pellets used for blow-molded films include low-density polyethylene (LDPE), linear low-density polyethylene (LLDPE), and high-density polyethylene (HDPE).

 

 

1. Agricultural Greenhouse Films

Agricultural greenhouse films are wide-width films blow-molded from LDPE or a blend of LDPE and LLDPE resins, using the horizontal extrusion upward-blowing method. The folded diameter of the film ranges widely (≤1000mm or ≥5000mm), with a thickness of 0.03–0.14mm, a tensile strength of ≥12MPa, and an elongation at break of ≥300%.

 

Agricultural greenhouse films are rainproof, light-transmitting, and heat-insulating, enabling year-round cultivation of various vegetables, fruits, and other crops in greenhouse shelters. Based on agricultural needs, there are currently several types of functional agricultural greenhouse films as follows:
(1) Reinforced Greenhouse Films: These films are made by adding more than 25% (mass fraction) of LLDPE resin to LDPE, which increases the tensile strength to 20MPa and the elongation at break to ≥400%. If LLDPE resin is used entirely for film blowing, the strength will be even higher, with additional advantages such as puncture resistance and a smooth film surface, and the film thickness can be reduced by 20%–30%.
(2) Long-Life Greenhouse Films: The service life of ordinary greenhouse films is 0.5–1 year, while that of long-life greenhouse films can be doubled or more, reaching 2–3 years. They are produced by adding 0.3–0.5 parts by mass of light stabilizers and 0.1–0.3 parts by mass of antioxidants to LDPE resin. First, white oil is used to wet the surface of LDPE, then mixed with it, and finally extruded and blow-molded into films.
(3) Anti-Dripping Greenhouse Films: These films have no water mist or water droplets on their surface, which can improve light transmittance and promote better crop growth. An appropriate amount of surfactants or anti-dripping masterbatches is added to polyethylene resin to enhance the film's affinity for water, allowing numerous small water droplets on the film surface to coalesce into large ones that flow down the film wall to the ground. This eliminates water mist on the inner surface of the film, improves film transparency, and prevents water droplets from directly dripping onto crops.

 

2. Agricultural Mulch Films

Agricultural mulch films are abbreviated as "ground-covering films" for agricultural use. They are ultra-thin films blow-molded from LDPE, with a thickness of 0.008–0.014mm and a width of 600–1000mm. Ordinary colorless and transparent mulch films are mainly used as seedling-raising films and crop-covering films, which can promote early maturity and increase yield of crops, as well as prevent diseases and pests. They are generally produced using the upward-blowing method.

 

Functional mulch films are dyed by adding 3%–5% (mass fraction) of colorants to polyethylene resin, producing films of various colors such as red, yellow, blue, gray, and black. These colored films allow different vegetables and fruits to obtain the required light spectrum, achieving the goals of early crop maturity, yield increase, pest control, and weed growth inhibition.

 

3. Packaging Films

Polyethylene packaging films are divided into light packaging films and heavy-duty packaging films.
(1) Light Packaging Films: Made from LDPE resin, they are mainly used for packaging various products such as food, pastries, candies, vegetables, fruits, pharmaceuticals, textiles, daily necessities, and clothing. To facilitate production on automatic packaging lines, a small amount of slip agents or antistatic agents needs to be added. They are mainly produced using the upward-blowing method or horizontal-blowing method. The folded diameter of the film ranges widely (≤70mm or ≥1000mm), with a thickness of 0.02–0.20mm and a tensile strength of ≥10MPa.

 

Vest bags, garment bags, and other products made from HDPE resin are shopping bags for various commodities and have a considerable usage volume. They are mainly produced using the upward-blowing method, with a folded diameter of 70–1000mm, a thickness of 0.007–0.10mm, and a tensile strength of ≥25MPa.

 

(2) Heavy-Duty Packaging Films: These films are produced from LDPE resin with a relatively high molecular weight using the upward-blowing method, with a thickness of 0.2–0.35mm. Each bag can hold items weighing 20–30kg. They are mainly used for packaging various products such as agricultural products, chemical raw materials, chemical fertilizers, pesticides, food, sugar, salt, pharmaceuticals, and textiles.

 

 

The melt flow rate (MFR) of LDPE resin used for blow-molded films has a wide range of 0.5–7g/10min, and the selection is mainly based on the different uses of the film (see Table 6-8).

 

 

Table 6-8 Selection of LDPE Resin for Film Production

Types of Films	Lightweight Packaging Film	High-Transparency Film	Agricultural Plastic Mulch Film	Greenhouse Film	Heat Shrink Film	Heavy-Duty Packaging Film
Resin MFR / ( g/10min )	2~4	5~7	4~7	1.5~4.5	2~5	0.25~0.5

 

The MFR of resin used for LLDPE films is 1–2g/10min, and that for HDPE films is 0.2–1g/10min. HDPE resin has a relatively high melting temperature and high viscosity within the range of shear rate changes. Due to its linear molecular structure, it has strong flow orientation.

 

 

 

1. Extruders

Ordinary LDPE films with a folded diameter of more than 300mm are formed using the horizontal extrusion upward-blowing method, with a screw diameter of 45–150mm. If the folded diameter is larger than 1m, two single-screw extruders can be used for co-extrusion to produce wide-width films. The length-diameter ratio of the screw is 20–30, and the compression ratio is 3–3.5. The screw structure is a gradual-type screw.

 

For the production of HDPE films, small-sized extruders are suitable, with a screw length-diameter ratio of 16–25. The screw structure is equipped with a shear section and a mixing section.

 

2. Die Heads

LDPE films mostly use spiral die heads or spider die heads, with a die orifice diameter of 100–1000mm. If good thickness uniformity is required, a rotating die head can be selected. The main processes are the upward-blowing method and horizontal-blowing method.

 

HDPE films use spiral mandrel die heads. It should be noted that excessive shear rates should be avoided in the structure and dimensions of the flow channel, spiral lines, and die orifice; otherwise, melt fracture is likely to occur. Due to the above molding characteristics of HDPE, the die orifice size of HDPE films is relatively small, generally 30–200mm.

 

3. Cooling

The air ring cooling method is mainly used. The blower pressure is 4000–8000Pa, and the flow rate is 15–75m³/min.

 

 

1. Extrusion Temperature

The extrusion temperature is mainly determined by the melt flow rate of polyethylene. The higher the melt flow rate, the lower the extrusion temperature. The temperature of the extruder gradually increases from the feed port to the filter plate, and the temperature at the die head is basically equal to the temperature at the screw head or slightly lower by 10–20°C. This ensures stable thick film blanks and good film transparency. The extrusion temperature of polyethylene films is shown in Table 6-9.

 

Table 6-9 Extrusion Temperature of Polyethylene Films

Types of Films		Screw & Barrel Temperature/℃			Temperature/℃
		Zone 1	Zone 2	Zone 3	Connector	Mold
LDPE Film	Lightweight Packaging Film Greenhouse Film Heavy-Duty Packaging Film	120~140 130~140 140~160	150~160 150~170 160~180	170~180 180~190 180~200	170~180 180~190 190~200	160~170 170~180 180~190
LLDPE Film		160~180	180~200	200~220	200~220	220~230
HDPE Film		200~220	220~240	240~260	250~260	240~250

 

2. Die Orifice Gap

The size of the die orifice gap varies with different resins, and its range is shown in Table 6-10.

 

Table 6-10 Range of Die Orifice Gaps

Film Type	LDPE	LLDPE	HDPE
Die Gap/mm	0.5~1.0	1.5~2.5	1.2~1.5

 

 

When the melt flow rate of the resin increases, a smaller die orifice gap is selected; when the film thickness increases, a larger gap is selected. For example, for LDPE heavy-duty packaging films, the MFR is relatively small and the film is thicker, so a die orifice gap of 1mm is selected; for LDPE light packaging films, the MFR is relatively large and the film is thinner, so the die orifice gap is 0.5–0.6mm.

 

LLDPE resin has high melt viscosity, and its melt viscosity is not sensitive to changes in shear rate. Therefore, the extrusion temperature of LLDPE is higher than that of LDPE, and the die orifice gap is also much larger than that of LDPE. If the gap is not increased, the film surface will appear rough "sharkskin" patterns.

 

HDPE resin has a relatively small melt flow rate, so its die orifice gap is slightly larger than that of LDPE.

 

3. Blow-Up Ratio

After the film blank exits the die head, compressed air blows up the thick film blank, increasing its diameter and reducing its thickness (transverse stretching). The blow-up ratio is generally 2–5. The blow-up ratio cannot be too large; otherwise, the bubble tube will shake, the bubble shape will be unstable, the film thickness will be uneven, and the film will easily break. As the blow-up ratio increases, the transverse tensile strength, tear strength, and impact strength of the film all increase, but the transparency and gloss decrease, and the longitudinal tensile strength decreases. The blow-up ratios of polyethylene films are shown in Table 6-11.

 

Table 6-11 Blow-Up Ratios of Polyethylene Films

 

Film Type	LDPE	LLDPE	HDPE
Blow-Up Ratio	1.5–3.5	1.5–2.0	3.2–6.0

 

4. Draw Ratio

After the polyethylene film exits the extrusion die head and is guided to the pull rolls, the film undergoes longitudinal stretching. The linear speed of the pull rolls is generally 3–5 times faster than the extrusion linear speed, so the film is stretched 3–5 times in the longitudinal direction, and its longitudinal tensile strength is significantly improved. This is beneficial for the use of film packaging bags, as they mainly bear longitudinal tensile force.