Extrusion Blow Molding of Polyethylene Film

 

Polyethylene (PE) film is the most extensively produced and widely used variety of plastic film. It is non‑toxic and odorless, and features high strength and toughness together with excellent moisture and water resistance, good openability (antiblocking), and low‑temperature performance, while retaining a certain level of transparency. Its heat‑sealability is outstanding, with a relatively low sealing temperature of about 100 °C, which enables convenient bag making by electrical heating. PE film is produced directly by extrusion blow molding from polyethylene resin pellets and generally requires no additives; coloration can be achieved by incorporating masterbatch. Polyethylene resins for blown film include low‑density polyethylene (LDPE), linear low‑density polyethylene (LLDPE), and high‑density polyethylene (HDPE).

 

 

1) Agricultural Greenhouse Film

Agricultural greenhouse film is a wide‑width film blown from LDPE or blends of LDPE with LLDPE, typically by the upward blown‑film process with flat lay‑flat take‑off. Layflat width spans a broad range (≤1000 mm or ≥5000 mm). The typical thickness is 0.03–0.14 mm, tensile strength ≥12 MPa, and elongation at break ≥300%. These films are rain‑resistant, light‑transmissive, and heat‑retentive, enabling year‑round cultivation of vegetables and fruits within greenhouse structures. According to agronomic requirements, the main functional sub‑types include:

• Reinforced greenhouse film: Prepared by adding more than 25 wt% LLDPE into LDPE. Tensile strength can be increased to approximately 20 MPa, and elongation at break ≥400%. If the film is blown entirely from LLDPE, strength is further enhanced, with improved puncture resistance and a smoother surface; film thickness can be reduced by about 20–30%.

• Long‑life greenhouse film: Ordinary greenhouse film typically lasts 0.5–1 year. Long‑life grades extend service life by at least twofold to 2–3 years. These are obtained by adding 0.3–0.5 parts by mass of a light stabilizer and 0.1–0.3 parts of an antioxidant to LDPE; the LDPE surface is first wetted with white oil, then mixed and blown into film.

• Anti‑drip greenhouse film: Designed so that no fog or droplets form on the film surface, thus improving light transmission and promoting crop growth. By adding an appropriate surfactant or anti‑drip masterbatch, the film's affinity for water is increased. Numerous small droplets coalesce into larger ones that flow down along the film wall to the ground, thereby eliminating internal fogging, improving transparency, and preventing droplets from falling directly onto crops.

 

2) Agricultural Mulch Film

Agricultural mulch film refers to ground‑cover film. It is a very thin LDPE film made by blow molding, with a thickness of 0.008–0.014 mm and a width of 600–1000 mm. Transparent, colorless mulch is mainly used as seedling‑raising and crop‑cover film to promote early maturity, increase yields, and control pests; it is generally manufactured by the upward blown‑film method. Functional mulch film is produced by incorporating 3–5 wt% colorants into PE to obtain red, yellow, blue, gray, black, and other colors. Different crops thus receive the specific spectral bands they need, achieving earlier maturity and higher yield, pest suppression, and weed inhibition.

 

3) Packaging Film

PE packaging film is divided into light‑duty and heavy‑duty types.

• Light‑duty packaging film: Manufactured from LDPE and mainly used to package foods, pastries, candies, vegetables, fruits, pharmaceuticals, textiles, daily necessities, apparel, and similar products. To suit automated packaging lines, small amounts of slip agents or antistatic agents are typically added. Production most commonly uses the upward blowing or flat blowing process. Layflat width spans a wide range (≤70 mm or ≥1000 mm). The thickness is 0.02–0.20 mm, and the tensile strength ≥10 MPa.

• HDPE T‑shirt (shopping) bags and garment bags: Widely used, and predominantly produced by the upward blowing process. Layflat width is 70–1000 mm, thickness 0.007–0.10 mm, and tensile strength ≥25 MPa.

• Heavy‑duty packaging film: Made by the upward blowing process from higher‑molecular‑weight LDPE. Typical thickness is 0.2–0.35 mm, with each bag capable of holding 20–30 kg of contents. Applications include packaging of agricultural products, chemical raw materials, fertilizers, pesticides, foods, sugar, salt, pharmaceuticals, and textiles.

LDPE resins for blown film exhibit a relatively broad melt flow rate (MFR) range of 0.5–7 g/10 min and are selected according to the film's intended use (see Table 1‑1). LLDPE resins for blown film typically have an MFR of 1–2 g/10 min. HDPE blown‑film grades have an MFR of 0.2–1 g/10 min. HDPE has a higher melting temperature and, within the range of shear‑rate changes, a high viscosity; owing to its linear molecular structure, it exhibits strong flow‑induced orientation.

Table 1‑1 Selection of LDPE Resins for Film Grades

Film grade	Resin MFR (g/10 min)
Light‑duty packaging film	2–4
High‑transparency film	5–7
Agricultural mulch film	4–7
Greenhouse film	1.5–4.5
Heat‑shrinkable film	2–5
Heavy‑duty packaging film	0.25–0.5

 

 

 

1) Extruder

Conventional LDPE film with layflat width ≥300 mm is typically manufactured by the upward blown‑film process with flat lay‑flat take‑off. Recommended screw diameter ranges from 45 to 150 mm. For layflat widths greater than 1 m, two single‑screw extruders may be used for co‑extrusion of wide‑width film. Screw L/D is 20–30 with a compression ratio of 3–3.5, and a progressive (tapered‑channel) screw is preferred.

For HDPE film, smaller‑size extruders are suitable, with screw L/D of 16–25. Screw designs typically incorporate a dedicated shearing section and a mixing section.

 

2) Die Head

LDPE film commonly employs spiral‑mandrel or spider‑type annular dies, with die diameters of 100–1000 mm. When superior thickness uniformity is required, a rotating die is recommended. Both upward and flat blowing processes are used.

HDPE film typically uses a spiral‑mandrel core die. Care must be taken in the design of the flow channel, spiral geometry, and die‑lip dimensions to avoid excessive shear rates, which can cause melt fracture. Because of these forming characteristics, HDPE film dies are relatively small, generally 30–200 mm in diameter.

3) Cooling

Air‑ring cooling is predominantly used. The typical blower pressure is 4000–8000 Pa with an air‑flow rate of 15–75 m³/min.

 

 

1) Extrusion Temperature

Extrusion temperature is determined primarily by the melt flow rate of the PE resin-the higher the MFR, the lower the required temperature. From the feed port to the screen pack (breaker plate), barrel temperatures are increased progressively so that the die‑head temperature is approximately equal to or 10–20 °C lower than the temperature at the screw tip. This promotes a stable thick parison and better film transparency. Typical extrusion temperatures for PE films are shown in Table 2‑1.

Table 2‑1 Extrusion Temperatures for Polyethylene Films (°C)

Film grade	Barrel Zone 1	Barrel Zone 2	Barrel Zone 3	Adapter	Die
LDPE – Light‑duty packaging film	120–140	130–140	140–160	150–160	160–170
LDPE – Greenhouse film	150–170	160–180	170–180	180–190	170–180
LDPE – Heavy‑duty packaging film	180–200	170–180	180–190	190–200	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 Gap

The die‑lip gap varies with resin type; representative ranges are listed in Table 2‑2. As resin MFR increases, a smaller die gap is selected; as target film thickness increases, a larger gap is used. For example, for LDPE heavy‑duty film (lower MFR and thicker gauge) a die gap of about 1.0 mm is selected; for LDPE light‑duty film (higher MFR and thinner gauge) the die gap is typically 0.5–0.6 mm. Because the melt viscosity of LLDPE is higher and relatively insensitive to changes in shear rate, its extrusion temperature is higher and the die gap significantly larger than for LDPE; otherwise a rough "sharkskin" surface may appear. HDPE blown‑film resins have relatively low MFR, so the die gap is generally slightly larger than for LDPE.

Table 2‑2 Typical Die‑Lip Gap Ranges (mm)

Film type	Die gap (mm)
LDPE	0.5–1.0
LLDPE	1.5–2.5
HDPE	1.2–1.5

 

3) Blow‑Up Ratio (BUR)

After exiting the annular die, the thick parison is inflated with compressed air, increasing its diameter, reducing its thickness, and imparting transverse draw. A typical blow‑up ratio is 2–5. If the BUR is too high, the bubble will flutter, bubble shape becomes unstable, thickness uniformity deteriorates, and the film is prone to breakage. As BUR increases, the film's transverse tensile strength, tear resistance, and impact strength increase, but transparency and gloss decrease and longitudinal tensile strength tends to decrease. Representative BURs for PE films are listed in Table 2‑3.

Table 2‑3 Representative Blow‑Up Ratios for PE Films

Film type	Typical BUR
LDPE	1.5–3.5
LLDPE	1.5–2.0
HDPE	3.2–6.0

 

4) Draw Ratio (Machine Direction)

After the parison leaves the die, it is guided to the haul‑off (nip) rolls and subjected to longitudinal drawing. The haul‑off line speed is generally 3–5 times the melt extrusion (output) speed, yielding 3–5× machine‑direction stretch and a marked increase in longitudinal tensile strength. This benefits bag applications where longitudinal load bearing predominates.