Reason of Cracking in WPC Decking

Five main causes of longitudinal cracking (through-cracking along the wood grain) in WPC decking

Yongte Plastic Machinery is a professional manufacturer of WPC decking machines, offering comprehensive solutions that transform waste plastic and wood sawdust into high-quality wood-plastic composite (WPC) profiles—enhancing dimensional stability while providing optimized formulations and production processes to reduce susceptibility to cracking.  

Below are the five main causes of longitudinal cracking in WPC decking:

1. Defects in raw material formulation (the most fundamental internal cause)

1. Excessive wood powder content: The proportion of wood powder exceeds 60%, with insufficient plastic matrix to retain the wood fibers; exposure to sunlight causes water absorption and swelling, while drying leads to shrinkage, resulting in repeated stretching of the panels and longitudinal tearing along the wood grain direction; excessive moisture content in the wood powder (>8%) causes internal vapor pressure buildup during high-temperature extrusion, followed by stress cracking due to subsequent environmental temperature fluctuations.

2. Adjuvant deficiency/low quality: absence of coupling agents → delamination and debonding at the wood powder-PE/PP plastic interface; lack of UV and antioxidant agents, leading to UV-induced degradation of the plastic matrix under outdoor sunlight and brittleness of the panels with cracking along molding lines; extensive use of low-cost recycled plastics resulting in damaged molecular structures and extremely poor toughness.

3. Substrate type issue: Non-coextruded solid wood-plastic composite (without an external weather-resistant protective layer) exhibits direct surface aging under sunlight exposure, leading to cracks extending inward from the surface grain; in contrast, coextruded variants rarely develop continuous longitudinal cracks.

II. Issues in the Extrusion Production Process

1. Extrusion temperature imbalance: excessive barrel temperature causes wood powder to coking and become brittle; insufficient temperature results in incomplete plasticization and poor mixing of wood-plastic blends, leading to latent layered stresses within the panels that eventually crack along the extrusion direction (wood grain direction) due to thermal expansion.

2. Abnormal cooling and setting: Rapid cooling after mold ejection results in inconsistent contraction rates between the inner and outer surfaces of the sheet, leading to significant internal stress accumulation. Upon exposure to outdoor temperature cycling, this stress is released, causing longitudinal cracks along the wood grain direction; excessive extrusion traction speed may induce longitudinal stretching of the sheet, posing a risk of pre-existing crack formation.

III. Natural aging of outdoor environments (caused by usage factors)

1. Thermal expansion and contraction stress: Due to significant temperature fluctuations between winter and summer outdoors, wood-plastic composite materials undergo thermal expansion and contraction; the longitudinal expansion of a single panel can reach 0.8–1.2 mm per meter, and if the panel lacks sufficient toughness, it may crack directly under tension.

2. Dry-wet freeze-thaw cycle: Rainwater penetrates the capillary pores of the board, freezes at low temperatures, and expands volumetrically, causing cracking of the board; repeated exposure to rain for water absorption and intense sunlight for dehydration leads to periodic expansion and contraction of wood fibers, resulting in cracking of the substrate.

3. Ultraviolet-induced aging: Prolonged exposure to outdoor sunlight causes the surface plastic layer to age and powder, losing its toughness. Stress concentrates in the wood grain grooves, leading to longitudinal cracking starting from these areas.

IV. Errors in on-site installation work

1. Insufficient expansion joint reservation: No adequate expansion gaps are provided at plate ends or sides, leaving no allowance for thermal expansion under high temperatures; this causes the plates to become interlocked, generating longitudinal tensile stress and leading to delamination along the grain direction. Excessive spacing between joists (>400 mm) results in suspended plates subjected to bending loads, with cracking occurring at the bottom due to tensile forces.

2. Overly tight fixation of the fastening nail: The self-tapping nail locks the panel, preventing free expansion and contraction; thermal expansion and contraction are restricted by the nail position, causing stress to concentrate at the nail site and extend toward both ends, resulting in longitudinal cracks.

V. Factors of External Forces in Subsequent Use

Long-term single-point compaction of heavy loads and impact from sharp or hard objects cause cracks to propagate continuously along the wood grain from the point of stress application; persistent water immersion accelerates moisture absorption and degradation of the substrate, further exacerbating crack propagation.