CAD Simulation for Pallet Film Stretch Tension

Simulating pallet wrapping film stretch tension in industrial Computer-Aided Design (CAD) and Finite Element Analysis (FEA) environments allows packaging engineers to predict containment forces, optimize film consumption, and prevent transit failures. By modeling the non-linear, viscoelastic behavior of thin polymer films around complex geometric loads, CAD-integrated simulation tools calculate the precise tension applied during wrapping, accounting for pre-stretch ratios, corner stress concentrations, and long-term tension decay.

The Physics of Film Stretch in Simulation

Pallet stretch wrap—typically manufactured from Linear Low-Density Polyethylene (LLDPE)—does not exhibit standard linear elastic behavior. In simulation software, tension calculations rely on non-linear material formulations:

  • Hyperelasticity: Models such as Ogden or Mooney-Rivlin are employed to capture large strain deformations as the film stretches between 150% and 350% during the mechanical pre-stretch phase.
  • Viscoelasticity: Time-dependent models (such as Prony series expansions) simulate stress relaxation, which determines how much holding force remains hours or days after the pallet is wrapped.
  • Plasticity: Yield criteria define the threshold where the film undergoes permanent deformation without snapping.

Essential Input Parameters

To compute accurate tension fields across a wrapped unit load, CAD and FEA environments require specific operational and material variables:

  1. Material Gauge and Profile: The nominal thickness (typically 12 to 30 microns) and width of the unstretched web.
  2. Pre-Stretch Percentage: The primary stretch imparted by dual rollers in the carriage before the film leaves the dispenser.
  3. Tension-to-Load (F-Force): The secondary stretch generated by the speed differential between the film delivery system and the rotating pallet.
  4. Pallet Geometry and Surface Friction: Sharp edges, irregular box boundaries, and surface friction coefficients that influence contact pressure and localized puncture risk.
  5. Kinematic Wrap Pattern: Carriage elevation speed, turntable or rotary arm RPM, overlap percentage, and bottom/top revolution counts.

Tension Calculation Methodology

The simulation resolves contact mechanics and dynamic equilibrium through several steps:

1. Shell or Membrane Element Meshing

The film is modeled using explicit membrane or thin-shell finite elements capable of handling zero-bending stiffness and large displacements without computational instability.

2. Pre-Stretch and Trajectory Simulation

The simulation tracks the film path from the delivery carriage to the pallet face. Kinematic boundary conditions replicate the rotational and vertical movement of the wrapping sequence. As the film spans the distance between the dispenser and the pallet, tension (\(T\)) is calculated based on instantaneous strain (\(\epsilon\)):

\[T = \sigma(\epsilon, \dot{\epsilon}) \cdot A\]

Where \(\sigma\) is the true stress as a function of strain and strain rate (\(\dot{\epsilon}\)), and \(A\) is the instantaneous, reduced cross-sectional area of the stretched film.

3. Corner Peak Stresses and Load Distribution

As the film contacts the 90-degree corners of a pallet, friction and geometry cause localized stress amplification. CAD simulations map these corner stress concentrations to verify that tension does not exceed the film's ultimate tensile strength, which would lead to web tears.

4. Containment Force Evaluation

The primary output of the calculation is the net containment force exerted on the cargo layers. This force is derived by integrating the normal vector components of film tension across all wrapping layers pressing against the product:

\[F_{\text{containment}} = \sum_{i=1}^{n} 2 \cdot T_i \cdot \sin\left(\frac{\theta_i}{2}\right)\]

Where \(n\) is the number of active layers, \(T_i\) is the layer tension, and \(\theta_i\) represents the contact angle around the periphery.

Industrial Applications and Advantages

Integrating tension calculations into CAD reduces the need for trial-and-error physical testing on end-of-line packaging systems:

  • Load Crushing Prevention: High tension on fragile or deformable products (such as empty bottles or lightweight cartons) can cause edge collapse. Simulations identify over-tensioned zones before physical production begins.
  • Resin Optimization: Engineers can systematically reduce film gauge or increase the pre-stretch ratio to discover the minimum material required to maintain load stability during transport standards (e.g., ASTM D4169 or EUMOS 40509).
  • Containment Profile Customization: Complex loads with varying mass distributions can receive targeted wrapping patterns—varying tension dynamically at different pallet heights to match structural requirements.