The Physics of Load Stabilization: Engineering the Pallet Interlayer

A pallet interlayer is a sheet-like material placed between unit loads on a pallet to improve load distribution, increase frictional resistance, and reduce load shift during handling and transport.

Overview

Pallet interlayers are thin sheets—made of corrugated fiberboard, plastic, kraft paper with anti-slip coatings, or other substrates—inserted between stacked layers of packaged goods on a pallet. Their primary functions are to improve load stabilization by evenly distributing vertical loads, increasing friction between layers to resist sliding, and protecting products from point loads or abrasion. Interlayers are a low-cost, widely used element of packaging and unitization strategies in warehousing and transportation.

How interlayers work — the basic physics

Two physical mechanisms are central to an interlayer’s effectiveness: load distribution and friction augmentation

• Load distribution. A stiff interlayer spreads point loads from uneven or heavy elements (such as rigid containers, drums, or pallet overhang) across a wider contact area. This reduces local stress concentrations that can deform packaging or cause densification of compressible layers below. Stiffness is a function of material modulus, thickness, and geometry (e.g., honeycomb or corrugated core). A stiffer interlayer resists bending under the applied load and maintains more uniform support.
• Friction augmentation. Interlayers increase the resistance to lateral movement by raising the coefficient of friction (COF) between adjacent layers. Frictional resistance depends on surface roughness, microtexture, materials in contact, and normal force. Interlayers with anti-slip surfaces, textured plastic, or high-friction paper coatings increase static friction and reduce the likelihood of slip during acceleration, braking, or cornering motions encountered in transport.

Materials and how they behave

Different substrates bring distinct mechanical and tribological properties

• Corrugated fiberboard. Common as disposable interlayers. Corrugated sheets (single-face, double-wall, or laminated) provide moderate stiffness and cushioning. They spread loads well when oriented correctly (flutes perpendicular to bending direction) and are economical and recyclable. Their effectiveness drops with moisture exposure—wet corrugated loses stiffness and can crush under high static loads.
• Plastic (HDPE, PP, composite sheets). Plastic interlayers offer high tensile strength, constant stiffness across moisture and temperature ranges, and durable textured surfaces to boost COF. Reusable plastic sheets are common in automotive and appliance supply chains where repeated use justifies higher upfront cost. Plastics are less compressible and resist creep better than paper under sustained loads.
• Anti-slip paper / coated kraft. These are paper-based sheets treated with a tacky or high-friction coating (e.g., micro-embossed coatings or polymer adhesives) that significantly increase COF while remaining flexible and cost-effective. They excel at preventing initial slip but may perform differently under dynamic vibration—selection should consider the expected motion profile.
• Specialty interlayers. Honeycomb, laminated composites, and edge-reinforced variants combine stiffness and energy absorption, useful for heavy machinery, brittle goods, or mixed pallets.

Dynamic considerations: transport forces and slip

During road or rail transport, pallets experience lateral accelerations (cornering/braking), vertical shocks (potholes), and sustained vibration. These induce shear forces between layers. Two key friction regimes matter: static friction (resistance to the start of motion) and kinetic friction (resistance while sliding). Interlayers that raise static COF reduce the chance of initial slip; those that maintain high kinetic COF help if slip starts. Additionally, interlayer stiffness reduces relative displacement by minimizing local deformation that can initiate sliding.

Design and selection best practices

• Match interlayer stiffness to load type: heavy, rigid loads benefit from stiffer plastic or honeycomb sheets; lightweight, uniform cartons often do well with coated paper interlayers.
• Consider moisture exposure: use moisture-resistant plastics or waxed/corrugated treated paper when humidity or condensation is likely.
• Choose surface texture/coating for friction needs: micro-embossed or tacky coatings increase COF; if residue or product contamination is a concern, select non-transfer coatings.
• Account for repeated handling: reusable plastic interlayers justify cost for multi-trip pallets; disposable paper is better for one-way logistics or export where recyclability is important.
• Combine methods: interlayers work best with proper pallet stretch-wrapping, corner boards, and strapping. Avoid relying solely on an interlayer to stabilize poorly stacked or incompatible items.

Testing and performance metrics

Key lab and field tests guide selection:

• Coefficient of friction tests (e.g., ASTM D1894 or other industry methods) measure static and kinetic COF between interlayer and product packaging.
• Compression and bending tests assess stiffness and load-bearing capacity, often using simple deflection under load or standardized edge crush and bending setups.
• Transport simulation via vibration tables, shock testing, and lateral acceleration rigs replicate expected handling scenarios to confirm slip resistance under dynamic conditions.
• Field trials remain essential: real-world pallet patterns, film application, and environmental conditions can change outcomes compared to lab tests.

Common mistakes and pitfalls

• Using an interlayer that is too thin or flexible for the load, which allows local bending and reduces stabilization benefits.
• Ignoring moisture effects—choosing corrugated interlayers in humid or wet environments without protection leads to loss of stiffness and early failure.
• Failing to test the interlayer with the actual packaging materials—the COF between the interlayer and the product packaging, not the interlayer alone, determines slip performance.
• Over-reliance on interlayers without adequate stretch-wrapping, strapping, or corner protection. Interlayers are one element of a system, not a standalone fix.

Practical examples

• A beverage bottler stacks shrink-wrapped multipacks on pallets. Adding corrugated interlayers every two layers improves load alignment and protects lower cases from case-edge compression during forklift handling.
• An electronics distributor ships boxes with smooth polyethylene surfaces; a micro-embossed plastic interlayer combined with full-wrap stretch film prevents slip during long-haul transport where sustained vibration is present.
• An exporter uses anti-slip coated kraft sheets between carton layers for a low-cost method to increase static COF and reduce rework at destination warehouses.

Sustainability and lifecycle

Consider lifecycle impact: recycled corrugated has a good sustainability profile for single-use. Reusable plastic interlayers reduce waste for high-cycle operations but require end-of-life recycling streams. Manufacturers increasingly offer recycled-content plastics and recyclable coatings to balance durability and environmental impact.

Summary

Pallet interlayers are a small but critical component of load stabilization. Selection should be driven by the mechanics of the load (weight, rigidity, surface), environmental exposure, handling profile, and whether the operation favors reusable or recyclable materials. Properly matched interlayers distribute loads, increase frictional resistance, and—when used as part of a complete palletization strategy—significantly reduce product damage and transport-related losses.