LLDPE Stretch Film: Pre-Stretch, Gauge and Load Retention Explained

Linear Low-Density Polyethylene (LLDPE) used in stretch wrap is based on ethylene copolymerized with a C6 hexene or C8 octene alpha-olefin. Typical film resins carry a density from 0.917 g/cm³ to 0.922 g/cm³ and a melt index from 0.7 g/10 min to 2.5 g/10 min at 190 °C/2.16 kg per ASTM D1238-20. Cast film extrusion at melt temperatures of 240–260 °C and chill-roll temperatures of 18–28 °C develops low crystallinity and a high machine-direction orientation response. The absence of long-chain branching in conventional Ziegler-Natta LLDPE lowers melt strength but increases drawability; metallocene-catalysed grades narrow the short-chain branching distribution, which raises the strain-hardening onset and reduces edge tear during pre-stretch. At a cast line speed of 350 m/min, web thickness is typically controlled to a transverse variation of ±1.0 µm using automatic die bolts and beta or infrared back-scatter sensors.

Commercial LLDPE stretch film is usually a three-layer or five-layer coextruded structure. The skin layers contain 2–4% low molecular weight polyisobutylene or a metallocene plastomer for cling, and the core layer carries the pre-stretch load. The distribution of the cling layer across the film surface is controlled by cast roll texturing; a smooth roll produces higher immediate cling but blocks more readily at temperatures above 30 °C. Blown-film LLDPE, with lower machine-direction orientation and higher transverse tear resistance, is used less often in high-speed automatic pallet wrapping because its thickness variation is generally higher than cast film; however, it can function on manual wrappers at low pre-stretch ratios.

Draw resonance is a process instability that limits the maximum cast line speed for a given LLDPE. It appears as a periodic gauge band in the machine direction and is governed by the melt tension and the cooling rate. A resin with a molecular weight distribution below 3.0 and a melt index above 2.0 g/10 min typically shows draw resonance at lower line speeds than a broader-distribution LLDPE. Cast film producers reduce the risk by using a dual-chill-roll stack, by increasing the melt temperature within the 240–260 °C window, and by selecting a resin with a higher melt strength. When the film is subsequently stretched on a pallet wrapper, a residual gauge band as small as 1 µm can concentrate pre-stretch deformation and reduce local containment force by 8–12% relative to the surrounding film. This is an experience-based failure mode observed on production lines with integrated gauge scanners; the scanner map shows a diagonal or chevron pattern before the film is wound into slit rolls.

In a pallet-wrapping environment, variations in film gauge and pre-stretch create two distinct stress fields: the machine-direction tension applied by the wrapper and the transverse stress generated at the pallet corner. These stress fields are coupled because neck-in during pre-stretch reduces the film width and increases the edge thickness relative to the centre. A shift in pre-stretch ratio from 100% to 200% typically lowers the nominal post-stretch gauge by 50%, but the actual corner gauge is higher because of folded or tacked film layers. This distinction matters when the wrapper’s load-cell reports a web tension of 25 N; the stress concentrates on a contact zone of less than 10 mm² at a sharp corner.

What Happens to the Film’s True Gauge After Powered Pre-Stretch?

In powered pre-stretch, a film carriage passes the LLDPE web through two adjacent rollers operating at different surface speeds. The pre-stretch ratio is defined as the difference between the fast roller speed and the slow roller speed divided by the slow roller speed, multiplied by 100. A ratio of 250% elongates the film to 3.5 times its original length and reduces nominal thickness by approximately the same factor when width loss is absent. In practice, neck-in reduces film width by 4–8% on conventional cast LLDPE at 200–250% pre-stretch, so the measured gauge is 4–8% greater than a simple length-based calculation would predict. Final gauge after pre-stretch should be verified by ASTM D6988-21 because unwinding tension and elastic recovery raise instantaneous web thickness. The residual stretch capacity is lower after pre-stretch; film pre-stretched to 250% generally retains less than 20% further elongation before yield, shifting later deformation to local thin spots.

The values in the table are arithmetic calculations only; they do not include local thickness bands caused by die lines, gauge-profile asymmetry, or cast film draw resonance.

Pre-stretch carriage maintenance affects the true gauge more than the resin specification in some production environments. Worn roller surfaces with a roughness below 0.1 µm Ra allow film slip, which reduces the effective pre-stretch ratio and increases the post-stretch gauge. Worn bearings can produce a speed variation of 0.5 m/min at the fast roller; this is sufficient to create a tension cycle that a dancer cannot fully absorb. On a geometric carriage, a ratio error of 2% shifts a 23 µm input film to a post-stretch gauge error of approximately 0.2 µm, but the containment force error can be higher because the film stress state changes. Published data on the relationship between roller roughness and film slip is limited because equipment manufacturers and film suppliers do not test under identical wrap speeds.

Gauge Inhomogeneity, Corner Stresses, and Tear Propagation

Transverse gauge profile is measured with a contacting stylus, capacitance sensor, or optical micrometer. ASTM D6988-21 defines specimen preparation and calibration but does not set a universal manufacturing tolerance. Cast LLDPE stretch film supplied at a nominal gauge of 20 µm commonly has a transverse profile range of 1.5–3.0 µm across a 500 mm web. In downstream pallet wrapping, the pallet corner applies concentrated stress to the film. Film at the corner is locally reduced further because the geometry changes from a cylindrical to planar contact. The gauge reduction at the corner under typical wrapping tension of 30–60 N is highly dependent on film modulus and recovery; published data for corner radii below 5 mm are not consistently available. Finite-element evaluations of stretch wrap indicate substantial stress localisation when the corner radius is below 5 mm, and the film edge can become the failure initiation site before the centre gauge fails.

Failure on a pallet wrapper rarely appears as a clean tensile break. The film usually splits in the machine direction at the corner, where the combination of transverse tension and surface friction induces a shear component. A high-speed camera recording at 2,000 fps can capture a split that propagates at 50–150 m/s through the web. The occurrence is more frequent when the film contains a transverse gauge band caused by a die-bolt response lag, or when the pre-stretch carriage releases tension transiently at the end of a wrap cycle. On cast lines with a die width of 3.0 m, an edge trim of 20–30 mm is normally removed before slitting, but the slit edge remains more susceptible to nicks than the film centre.

Powered pre-stretch can be geometric or variable. Geometric systems use fixed gear ratios and are simpler but cannot adjust the ratio during the wrap cycle. Variable pre-stretch systems use servo or AC drive rolls with dancer feedback; they hold the ratio to ±1% at delivery speeds up to 0.8 m/s. At 0.8 m/s and 250% pre-stretch, the slow roller is driven at 0.229 m/s and the fast roller at 0.8 m/s. If the fast roller speed is not synchronised with the wrapper carriage speed, the film sees a periodic tension spike. Using a pre-stretch carriage load-cell signal, operators observe a tension increase of 5–15 N when the film passes around a pallet corner. The tension spike is less severe with a higher pre-stretch ratio because the film has already exceeded its yield point and the tangent modulus is higher. Above 250%, however, the strain-hardening reserve is reduced, so an additional 2–3% elongation at the corner can exceed break elongation. This is one reason automatic wrappers reduce carriage speed at corners rather than increasing pre-stretch ratio.

Under simulated over-the-road vibration in ASTM D4169-22, a palletised load experiences vertical and rotational inputs that shift product layers and reduce film tension. Load retention is not the same as final measured gauge; it is the outward force the film exerts on the load after recovery. According to ASTM D5459-18, machine-direction stress retention is determined by stretching a film specimen to a specified elongation and measuring the retained force after a recovery interval. A cast LLDPE grade with a hexene comonomer content of 6–10% by weight typically retains 60–70% of the initial stretch force after 60 s recovery at 200% elongation. The retained force declines further over hours in a wrapped pallet because the load settles and the film relaxes. Field correlation is made with containment force meters that use a compression column or load-cell pallet; these instruments record the force required to displace the film a fixed distance, often 50 mm, at a defined test speed. For a pallet mass of 500 kg and a 23 µm nominal film wrapped at 200% pre-stretch, containment force readings commonly fall in the 20–35 N range on five-sided column tests, but film supplier data sheets should be consulted because cling additive type and migration rate alter the measured force by 10–20%.

When Pre-Stretch Exceeds the Local Strain-Hardening Reserve of the Film

At pre-stretch ratios above 300%, a conventional Ziegler-Natta LLDPE cast film approaches the upper limit of its machine-direction strain-hardening reserve. Local gauge defects, die lines, and transverse thickness variations then become fracture initiation sites. On a high-speed wrapper running at 0.5 m/s film delivery, a ±0.5 µm gauge deviation may produce a local cross-section area reduction of 5–8% when mean gauge is 6 µm. The concentrated stress at a rectangular pallet corner can exceed 25 MPa, which is close to the yield stress of the oriented film. Processing conflicts arise because raising cling agent concentration to improve layer adhesion reduces the coefficient of friction and can permit roll blocking during storage at ambient warehouse temperatures above 35 °C.

Increasing pre-stretch ratio from 200% to 300% does not produce a proportional reduction in film consumption. The width loss increases, and the film becomes more sensitive to damage at the pallet corner. On a rotary arm wrapper with a variable pre-stretch carriage, the optimum ratio is not a fixed value; it depends on film grade, pallet geometry, and load weight. Published data for full-scale packaging lines at pre-stretch ratios above 300% is limited because most suppliers report only drum tests measured by ASTM D5459-18 or vendor-specific containment force methods.

Published compliance limits for stretch film are often customer-specific; the above ranges are representative of commercial cast films but do not constitute a universal product specification.

On wrappers with pre-stretch ratios above 200%, the film’s post-stretch gauge is the primary quality parameter, not the as-cast gauge. Automatic film-delivery systems maintain a constant tension of 6–15 N per 500 mm film width by adjusting pre-stretch ratio in response to load-cell feedback. A 1 kN change in pallet corner force can reduce containment force by 10% within 10 s when the wrapper does not compensate for corner passage. These dynamic corner loads are a principal reason that load retention is specified as a wrapped-pallet measurement rather than as a film-drum property.