01
Background of PE Stretch Film
Before the 1990s, the domestic stretch film market was dominated by polyvinyl chloride (PVC). PVC stretch film, produced with PVC as the base material and DOA as both plasticizer and self-adhesive, had fatal flaws such as being non-environmentally friendly, high in cost, and poor in stretchability, so it was gradually replaced by PE stretch film.
Initially, PE stretch film used EVA (ethylene-vinyl acetate copolymer) as the self-adhesive material. However, its high cost and odor prompted the industry to seek better alternatives. Technological innovations drove material upgrades: self-adhesive materials gradually evolved into polyisobutylene (PIB) and very low-density polyethylene (VLDPE), while the base material became dominated by linear low-density polyethylene (LLDPE), including C4, C6, C8, and metallocene polyethylene (MPE).
The production process also shifted from blown film to cast film. Early LLDPE stretch film production mostly adopted the blown film method, evolving from single-layer to two-layer and three-layer structures. Today, the cast film method has become the mainstream production technology due to its obvious advantages, such as uniform thickness and high transparency of the produced stretch film.
02
Characteristics of Stretch Film
(1) Five Excellent Performances
High transparency: Facilitates cargo identification.
High longitudinal elongation: Conducive to pre-stretching, reducing material consumption.
High yield point: Makes packaged goods more secure.
High transverse tear strength: Resists accidental tearing.
Excellent puncture resistance: Not easy to break when encountering sharp edges and corners.
(2) Four Functional Values
Unitization capability: Integrates scattered items into transport units by virtue of the film’s strong wrapping force and retractability.
Primary protection: Forms a dust-proof, oil-proof, moisture-proof, and waterproof protective layer, ensuring items are evenly stressed to avoid damage.
Compression and fixation: Tightly wraps products through retractive force, preventing displacement during transportation.
Cost-saving: Compared with traditional box packaging, it can reduce costs by up to 85% while improving packaging efficiency.
(3) Wide Applications
From manual packaging to fully automatic mechanical packaging, from fixing palletized goods to protecting precision electronic components, PE stretch film has an extremely wide range of applications: wherever there is spatial transfer of objects, stretch film is present. It is not only widely used in papermaking, logistics, chemical industry, building materials, food and pharmaceutical fields but also extends to foreign trade export, hardware, plastic raw materials and other industries.
03
Production Formulation and Process
(1) Formulation
The formulation of PE stretch film is a complex scientific system. The three-layer co-extrusion structure is the most common because it allows for a wider range of material selection and better formulation cost. According to relevant patented technologies, a typical formulation includes three main parts: base resin, reinforcing materials, and functional additives. The resins are generally LLDPE (C4, C6, C8) and metallocene PE (MPE); reinforcing materials are bimodal linear low-density polyethylene (to improve tensile strength); functional additives include viscosity control agents (PIB or VLDPE), nano-reinforcing materials (such as nano-silica, activated calcium carbonate), antioxidants (such as 1076, 1010), and stabilizers (calcium-zinc composite stabilizers).
An example of a patented formulation for high-tensile-strength PE stretch film: 45-65 parts of PE resin, 30-40 parts of bimodal linear low-density polyethylene, 3-4 parts of methyl cellulose, 4-8 parts of propylene glycol alginate, 4-8 parts of acrylic acid, 6-10 parts of polyvinyl alcohol, 2-4 parts of activated calcium carbonate, 2-3 parts of antioxidant, 1-2 parts of nano-silica.
(2) Process
The cast film production process mainly includes: raw material pretreatment → multi-layer co-extrusion → cast forming → cooling and setting → thickness measurement and adjustment → corona treatment → winding → curing.
a. Raw material pretreatment includes two steps: raw material selection and raw material processing (drying and mixing).
Raw material selection: Three-layer or five-layer structures are usually adopted:
Surface layer: LLDPE or metallocene PE with high transparency, high gloss, and good opening property (anti-adhesion).
Adhesive layer: LLDPE containing adhesive resin (PIB masterbatch or VLDPE) to provide self-adhesion of the stretch film.
Core layer: High-strength LLDPE (such as C8 or mPE) to provide tensile strength, puncture resistance, and toughness. Reinforcing resins (such as bimodal LLDPE) and functional additives (such as antioxidants, slip agents) can be added.
Drying and mixing: Resin particles need to be dehydrated in a dryer (to prevent bubbles) and mixed in multiple loss-in-weight scales according to precise formulation proportions.
b. Multi-layer co-extrusion (mainly including extruders and die heads)
Extruders: Usually equipped with 3 or 5 independent single-screw extruders (corresponding to the number of layers). Each extruder is responsible for melting one formulation of resin.
Temperature control: Precise zoning temperature control (feeding section → compression section → metering section → die head connection section), with a temperature range usually between 180℃ – 250℃. Melt temperature is crucial, affecting fluidity, plasticization, and final performance (e.g., 280℃±5℃ is a common melt temperature for high-quality films).
Co-extrusion die head: Core equipment! It combines molten resins from different extruders to form a stable, multi-layer melt curtain with clear interfaces in the die flow channel.
Die head design: Mainly hanger-type (T-type) die heads, with a width of up to 3 meters or more. The die lip gap is precisely adjustable (determining the initial thickness).
Interlayer distribution: Controls the thickness ratio of each layer through precise flow channel design (e.g., the core layer is the thickest, the adhesive layer is moderate, and the surface layer is thinner).
c. Cast forming and cooling setting
The multi-layer melt is extruded from the die slit to form a molten film curtain, which is evenly attached to the high-speed rotating cooling steel roller under the action of an air knife and vacuum adsorption system (preventing air entrapment that causes streaks or uneven thickness).
d. On-line thickness detection and feedback control
Real-time scanning: The thickness gauge scans back and forth at high speed along the transverse direction (TD) of the film, measuring the thickness of hundreds of points in real-time.
Closed-loop control: Measurement data is fed back to the die’s automatic die lip adjustment system (thermal expansion bolts or piezoelectric ceramics). The system automatically fine-tunes the local opening of the die lip (with precision up to the micron level), achieving full-width thickness tolerance control within ±1-3%.
e. Corona treatment
Purpose: Improve the film’s surface tension (usually reaching 38-42 dynes/cm) to enhance subsequent printing, coating, or composite performance with other materials (such as label pasting).
f. Winding
Center/surface winding: Large production lines mostly use center winding (with contact rollers to assist in tension control).
Tension control: A precise segmented 递减 tension control system is crucial! Excessive winding tension can cause the film roll to be too tight, affecting the migration of adhesives, requiring curing treatment to restore adhesiveness; moreover, it can increase internal stress in the film, which may lead to deformation during storage and use.
On-line slitting: Large mother rolls are slit into customer-required specifications after winding or directly on-line.
g. Curing
When the adhesive layer is PIB: The 刚下线 film has low adhesiveness. It needs to be stored in a constant temperature (15℃-25℃) warehouse for 48-72 hours to allow PIB molecules to slowly migrate to the film surface, achieving stable adhesiveness.
When the adhesive layer is VLDPE: The curing time can be shortened.
04
Common Problems and Challenges
Wrinkle Problem
Wrinkles in PE stretch film are the most common issue, mainly caused by:
Deviation of the base material position: Leading to wrinkles in the composite film, which worsen as the deviation accumulates.
Uneven thickness: Excessive differences in thickness.
Improper tension setting: Uncoordinated tension in various parts.
Equipment issues: Misalignment of the axes of the silicone rubber pressure roller and cooling steel roller; dirty or uneven guide roller surfaces.
Solutions: Precisely calibrate equipment to ensure correct base material position; strictly control raw material quality; optimize the tension control system; regularly clean and maintain guide roller surfaces.
Film Breaking Problem
Film breaking during production may result from:
Equipment failure: Damaged chain clamps or problems with the pre-stretching motor.
Raw material issues: Containing impurity molecules with significantly different properties.
Uneven thickness: Especially thinner stretch films are more prone to breaking.
Solutions: Regularly inspect key equipment components; strengthen raw material quality control; optimize the casting process to ensure uniform thickness.
Unstable Adhesiveness
The adhesiveness of stretch film is significantly affected by temperature and time:
When the adhesive layer is PIB: Adhesiveness takes about 3 days to form, being stronger at high temperatures and weaker at low temperatures.
When the adhesive layer is VLDPE: Adhesiveness is relatively stable, but it is more adhesive when the temperature is above 30℃ and decreases when below 15℃.
Post time: Jul-19-2025