Stretch Film

What is Stretch Film

Stretch film is a thin, elastic thermoplastic film whose job is unitization: turning many loose cartons, bags, or parts into one stable load that can be moved, stacked, and shipped without shifting. The film is wrapped under tension, and because polyethylene recovers elastically toward its original length, that tension translates into a continuous inward squeeze that holds the load together. The film also blocks dust, light moisture, and tampering during transit. It is one of the highest-volume packaging consumables in any warehouse, sitting alongside corrugated cartons, strapping, and pallets as a basic shipping material.

The dominant raw material is linear low-density polyethylene (LLDPE), often blended with low-density polyethylene (LDPE) and metallocene LLDPE (mLLDPE). LLDPE is preferred because its short-chain branched molecular structure gives high elongation, good puncture resistance, and the ability to be cold-stretched far beyond its as-extruded length. A cling additive, most commonly polyisobutylene (PIB) or a metallocene cling resin, is added to one or both surfaces so successive film layers adhere to each other and the wrap stays sealed without tape or heat. The combination of stretchability, memory, and cling is what separates a stretch film from an ordinary polyethylene sheet.

Stretch film differs fundamentally from shrink film. Shrink film is loosely draped, then heated so it contracts around the product, requiring a heat tunnel or heat gun and consuming energy. Stretch film is applied cold using mechanical tension only, which is faster, lower energy, and better suited to mixed pallet loads. Stretch film also differs from stretch hood film, which is a heavy tubular film pulled down over a full pallet by a dedicated machine; stretch hood is a separate category with its own equipment and resin grades.

Modern stretch film is the product of decades of downgauging. In 1985 a typical pallet was wrapped with roughly 90-gauge film, and a single pallet could consume around 12 ounces (about 340 g) of film. By the year 2000, advances in resin chemistry and multilayer co-extrusion had cut typical thickness to the 65 to 80 gauge range and film consumption per pallet to roughly 4 ounces (about 113 g). That threefold improvement in material efficiency is why thickness alone is a poor proxy for performance: a thin, well-engineered film correctly pre-stretched can out-perform a thick film applied loosely.

Four engineering properties define stretch film quality: load containment force (how hard it holds the load), puncture and tear resistance (whether it survives sharp corners and rough handling), stretch capability (how far it can be elongated to save material), and cling balance (whether layers bond to each other without sticking to adjacent pallets). The rest of this guide explains how each of these is created in manufacturing and read off a datasheet.

It is worth being precise about why polyethylene works for this duty. LLDPE molecules have a linear backbone with controlled short-chain branching, which lets the chains slide and reorient under tension without immediately rupturing. That cold-drawing behaviour is what allows the film to be stretched 150 percent or more and still recover most of the way back, converting stored elastic strain into the continuous holding force the load needs. The same property is why stretch film does not behave like rigid packaging: it is a tension member, and an under-tensioned wrap is a failed wrap no matter how thick the film is. Engineers who treat stretch film as a tensioned structural element, rather than as a wrapping consumable, make better selection decisions.

Cast, Blown, and Specialty Types

The first fork in any stretch film decision is the manufacturing process, because it sets the film's clarity, strength, noise, and price before any other choice is made. Cast film is extruded through a flat slot die onto chilled rolls; blown film is extruded through a circular die and inflated into a bubble cooled by an air ring. Roughly 70 to 80 percent of stretch wrap produced today is cast, mainly because the cast process is less expensive to run. The table below summarizes the practical differences.

Cast stretch film is made by feeding granular resin into a heated barrel, melting it, pushing the melt through a flat narrow slot die, and quenching the sheet on internally water-cooled chill rolls. The rapid, even cooling gives cast film its hallmark optical clarity and gloss, which matters when barcodes and RFID labels must be scanned through the wrap. Cast film unwinds quietly, has high memory that resists load creep, and is the lower-cost option. Its limitation is that it is oriented only in the machine direction, so its tear and puncture resistance, while good, trail blown film on jagged loads.

Blown stretch film is made by blowing the melt through a circular die into a vertical bubble, with the volume of air both setting the width and controlling the wall thickness as the bubble is cooled by an external air ring. Because the bubble is drawn in two directions, blown film is biaxially oriented, giving it markedly higher puncture and tear strength and better load retention at high stretch. The penalties are haze (poor for scanning), a loud crackling unwind, and higher cost. Blown film is the right call for sharp-cornered loads, heavy machine parts, building materials, and any load that punishes the film mechanically.

Beyond the cast/blown split, several specialty types address specific duties. Pre-stretched film is a film (often blown) that has been elongated near its breaking point at the factory and rewound, so it applies easily by hand at very low effort and yields more wrap per kilogram. Colored and opaque (black) film hides load contents for security or sorts loads by route. VCI stretch film carries a volatile corrosion inhibitor to protect bare metal parts. UV-resistant film adds stabilizers for outdoor storage, and vented (netting) film allows airflow for produce or hot-filled loads. Each specialty trades some base mechanical property for its added function, so it should be specified only when the duty requires it.

A practical note on application format complements the cast/blown choice. The same base film is supplied as hand grade on light rolls for manual dispensers and as machine grade on heavier high-yield rolls for powered wrappers. The distinction is not just roll weight: machine film is formulated to survive the violent elongation of a powered pre-stretch carriage, while hand film is formulated for a wide, forgiving stretch range applied at modest tension. Substituting one for the other is a common and costly mistake. A third format, bundling film, is a narrow stretch film (often 75 to 125 mm wide) used to consolidate small items such as pipe, lumber, or extrusions without a pallet, and a fourth, extended-core film, leaves the cardboard core protruding as a handle so light loads can be wrapped without a dispenser tool at all.

Multilayer Structure and Resins

Modern stretch film is not a single homogeneous sheet; it is a co-extruded sandwich of many thin layers, each tuned for a different job. Co-extrusion lets a manufacturer place a cling resin on one surface, a slip resin on the other, and high-strength resins in the core, all in one pass. Stretch film has evolved from 1 to 3 layers in the early years, to 5 layers becoming dominant in the mid-1990s, to 7 layers introduced commercially in 1996, to 9-layer lines first installed in 2000, and on to today's so-called nano-layer films built from dozens of ultrathin plies. The table below compares the common structures.

The strategic reason for more layers is the placement of metallocene LLDPE (mLLDPE). Metallocene resin, made with single-site catalysts, has a narrow molecular-weight distribution that delivers superior puncture resistance, cling control, and stretch when extruded in thin alternating layers. Metallocene grew from essentially zero use in 1995 to a meaningful share of stretch film resin within a decade. By splitting one resin stream into many thin slices, a 7-layer film with three mLLDPE plies can carry roughly 33 percent metallocene content while matching or beating a 5-layer film that uses around 40 percent metallocene over two thicker layers. More layers therefore let a converter use less of the expensive resin to reach the same strength, which is the economic engine behind downgauging.

The base resins blended into the structure each play a role. LLDPE (typically C6 hexene or C8 octene comonomer grades) is the workhorse that provides stretch and toughness. LDPE improves processability, optics, and bubble or web stability and lowers cost. mLLDPE is concentrated where puncture resistance and cling precision are needed. Major resin suppliers for stretch film include SABIC (SUPEER and COHERE metallocene grades), INEOS (Eltex metallocene LLDPE), and Qenos, among others. Converters select and ratio these resins per layer to hit a target balance of stretch, holding force, and cost.

Two functional additives complete the recipe. The cling layer uses polyisobutylene (PIB) tackifier or a metallocene cling resin so the film bonds to itself; PIB blooms to the surface over about 2 to 12 days after extrusion, which is why freshly made film can gain cling as it ages, and why excessive PIB in a skin layer can cause dripping if it exceeds roughly 4 to 5 percent. The opposite face usually carries a slip (release) layer so the outer surface of one wrapped pallet does not stick to the next. Films are therefore offered as one-side cling (tack inside, slip outside) or two-side cling, and rolls are commonly marked with an arrow indicating the cling side.

Gauge, Micron, and Standards

Thickness is the most quoted and most misunderstood stretch film number. Three units coexist: gauge (a US trade unit), micron (the SI unit, abbreviated um), and mil (one thousandth of an inch). The exact conversions are: 1 mil equals 25.4 microns, and 1 gauge equals 0.254 microns. To convert mils to gauge, multiply by 100, so 0.5 mil equals 50 gauge. The table below maps the common values an engineer will see on rolls and datasheets.

A critical trap is the difference between true gauge and equivalent (pre-stretched) gauge. A film marketed as 37 gauge or 47 gauge is usually an 80-gauge or higher resin web that has been pre-stretched at the factory; the small number describes the applied thickness after stretching, not the as-extruded thickness. As a rough field guide for true-gauge cast film, 37-gauge equivalent handles uniform loads up to roughly 800 lb (about 360 kg), 60 to 70 gauge up to roughly 1,800 to 2,200 lb (about 820 to 1,000 kg), 80 gauge up to roughly 2,400 lb (about 1,090 kg), and 100 to 115 gauge up to 3,000 lb (about 1,360 kg) or more. These figures depend heavily on load shape and applied tension, so treat them as a starting point, not a guarantee.

Performance is governed by published test methods, which is what lets two films be compared honestly. ASTM D882 measures tensile strength and elongation at break of thin plastic films, the basis for stretch and break figures. ISO 527-3 is the international equivalent for tensile properties of films. Reputable datasheets report tensile and elongation separately in the machine direction (MD) and transverse direction (TD), because cast and blown films differ sharply between the two axes.

Several methods target the abuse stretch film actually sees. ASTM D1709 is the free-falling dart impact test, where the energy that fails 50 percent of specimens (the F-50 value) correlates well with end-use puncture resistance. ASTM D5748 is the protrusion puncture resistance test written specifically for stretch wrap film, applying a biaxial stress through a probe and reporting maximum force, force at break, penetration distance, and energy to break. ASTM D1922 (Elmendorf) measures tear propagation resistance. Internationally, EN 14477 (also ASTM F1306) measures static puncture by a pointed probe with a 0.8 mm tip, sometimes called the Parker ball-point test. Finally, ASTM D4649 is the general standard practice for selecting and evaluating stretch wrap film, including cling, and ASTM D5459 covers load and stress retention of the wrapped film over time.

Key Specification Parameters

Reading a stretch film datasheet means looking past gauge to the parameters that actually determine whether a load arrives intact and at what cost. Seven properties drive selection: pre-stretch ratio, load containment (wrap) force, cling balance, puncture resistance, tear resistance, tensile and elongation, and roll dimensions. Each is explained below.

Pre-stretch ratio is the percentage a film is elongated before it contacts the load. A powered pre-stretch carriage uses two geared rollers turning at different speeds to elongate the film between them, and a 250 percent setting turns one metre of film into 3.5 metres applied, slashing resin use per pallet. Cast machine films commonly run 150 to 300 percent on a powered carriage, and high-performance multilayer films can reach an ultimate stretch of 300 percent or more, although real field application often averages only 165 to 180 percent. Higher pre-stretch lowers cost per load but leaves less residual force, so it must be tuned to the load.

Load containment force (wrap force or holding force) is the inward pressure the wrap exerts to keep the load stable, built from film tension multiplied by the number of revolutions at each height. It is measured in the field with a wrap-force plate reading in pounds or newtons, and characterized in the lab via ASTM D5459 stress retention. Typical pallet targets range from about 4 to 11 lb of wrap force depending on fragility and transport mode. Containment force, not thickness, is the property that prevents loads from collapsing in transit.

Cling balance describes which surfaces tack. One-side cling places tack on the inner (core) face so adjacent wrapped pallets do not bond to each other, with a slip outer surface; two-side cling tacks both faces. Cling is created by PIB tackifier or metallocene cling resin and is evaluated under ASTM D4649. Too little cling lets the wrap unravel; too much makes pallets stick together and tears film during de-palletizing.

Puncture and tear resistance determine survival on sharp loads. Puncture is characterized by ASTM D1709 dart impact and ASTM D5748 protrusion puncture; tear is characterized by ASTM D1922 Elmendorf. Blown and high-metallocene multilayer films lead here. Tensile strength and elongation at break, per ASTM D882 / ISO 527-3, define how hard the film can be pulled and how far it stretches before failure; these are reported separately in MD and TD.

Roll dimensions and yield are the commercial parameters. Common machine-film width is 500 mm (about 20 in), with cores typically 76 mm (3 in) inside diameter for machine rolls and 50 to 76 mm for hand rolls. Hand rolls are kept under about 6 kg so an operator can wrap comfortably. Roll length (often quoted in metres or feet) combined with width and gauge gives total area, and dividing film cost by the number of pallets a roll wraps at the target pre-stretch yields the true cost-per-load metric that should drive purchasing, not price per kilogram.

Two secondary properties round out a complete specification. Memory, the film's tendency to snap back toward its original length after stretching, is what sustains holding force over a multi-day transit; cast film generally has higher memory than blown. Optical clarity and gloss matter wherever the wrap must not obstruct barcode scanners, RFID readers, or visual inspection, and they favour cast film; where clarity is irrelevant, opaque or colored film can be specified for security or load identification. Both properties trade against others, so they belong in the requirement list explicitly rather than being assumed. Finally, a film's neck-down behaviour, how much the web narrows as it is stretched, affects how much vertical coverage each revolution provides on a machine, and reinforced (cabled) edges are sometimes added to resist edge tearing on automated lines.

Selection Decision Factors

To turn the preceding chapters into a purchase order, follow the decision sequence below. Most film waste and load failures come not from one wrong parameter but from skipping a step, for example specifying gauge before deciding cast versus blown, or chasing low price per kilogram instead of low cost per load. These eight steps work as a fixed RFQ template.

1. Application method: Decide hand wrap versus machine wrap first. Hand grade (9 to 20 micron, lightweight rolls) for low volume or odd loads; machine grade on powered turntable, rotary arm, or orbital wrappers for volume. Never run hand film on a powered pre-stretch carriage.
2. Cast or blown: Choose cast for clarity (barcode and RFID scanning), quiet unwind, high memory, and lower cost. Choose blown for sharp or jagged loads needing maximum puncture and tear strength and load retention.
3. Load profile and weight: Match film strength and gauge to load weight, shape, and corner sharpness. Use the load-capacity guide in Chapter 4 as a starting point, then validate with a wrap-force plate.
4. Containment force target: Set a wrap force target (commonly 4 to 11 lb) from load fragility and transport mode, then choose film and pre-stretch to hit it, rather than over-specifying thickness.
5. Pre-stretch and yield: For machine film, specify the carriage pre-stretch (150 to 300 percent) and confirm the film reaches it without breaking. Calculate cost per pallet at that setting, not price per kilogram.
6. Cling configuration: Specify one-side or two-side cling per storage and de-palletizing needs, and note the marked cling side so operators wrap with tack inward.
7. Special functions: Add VCI, UV stabilization, color/opacity, or venting only when the duty requires it, accepting that each trades some base mechanical property.
8. Standards and documentation: Require datasheets reporting ASTM D882 / ISO 527-3 tensile and elongation (MD and TD), ASTM D1709 / D5748 puncture, ASTM D1922 tear, and ASTM D4649 / D5459 stretch wrap and retention data, plus any food-contact or recycled-content (PCR) declarations.

A frequently overlooked dimension is supplier serviceability and consistency: roll-to-roll thickness consistency (gauge control technology reduces giveaway), reliable cling that does not fade or over-bloom, and supply continuity for a fast-moving consumable. Established makers such as Berry Global, Sigma Stretch Film (known for Rite-Gauging thickness control and 3, 5, 7, and 9-layer films), Paragon Films (including its Power Edge PCR recycled-content line), and Thong Guan offer documented multilayer films and technical support. For high-volume operations, a slightly higher unit price from a consistent supplier usually wins on total cost once film giveaway, load failures, and machine downtime are counted.