1. Inspection Purpose
This is a simple non-destructive sampling inspection method widely used in factory quality control (QC) and third-party inspection, aiming to quickly screen internal defects of plywood, including interlayer debonding, hollow spots, blisters, lack of adhesive and delamination. It features high efficiency and easy operation, making it an essential QC technique for batch plywood inspection.
2. Core Principles (Popular & Professional Interpretation)
2.1 Acoustic Resonance / Hollow Echo Principle
- Well-bonded & compact plywood: The tapping energy is transmitted uniformly through the panel, and the overall structural rigidity is consistent. As a result, the tapping sound is crisp, short and solid without obvious echo.
- Interlayer debonding / blistering: Air interlayers or weak debonded interfaces form between the surface layer and core layer, which changes the reflection path of sound waves and reduces acoustic damping. This leads to a muffled, hollow sound with a long echo, just like tapping a drum.
2.2 Local Stiffness Difference Principle
- Intact plywood (no debonding): The surface layer and core layer are firmly bonded and bear force together, featuring high deformation resistance and stable rigidity.
- Debonded & blistered plywood: The local surface layer is “suspended” without effective bonding with the core layer, resulting in a sharp drop in local rigidity. When tapped lightly, the panel shows obvious vibration and a soft hand feeling.
2.3 Why Use Rubber Mallet/Wooden Mallet Instead of Steel Hammer?
- Avoid damaging the panel surface and leaving dents or indentations, ensuring the appearance integrity of plywood.
- Generate a soft and stable tapping frequency spectrum, which makes it easier to distinguish the sound difference between compact and hollow areas.
- Suitable for large-area rapid scanning inspection, improving the efficiency of batch sampling inspection.
3. Standard Operating Procedure (Commonly Used in Inspection)
- Tapping Force: Apply light and uniform tapping force; no heavy hitting is allowed to prevent panel damage and misjudgment.
- Tapping Points: Arrange tapping points in a uniform grid pattern on the entire panel surface. Focus on and increase the density of tapping at key positions such as panel edges, corners, splicing seams and core board joints, where defects are prone to occur.
- Reference Comparison: Prepare a normal compact plywood sample from the same batch as a sound reference to accurately identify abnormal sounds during inspection.
4. Result Judgment (Practical QC Criteria)
4.1 Qualified (No Delamination / No Hollow)
- Sound Feature: Crisp, crisp and sharp, no dragging echo or hollow tone
- Hand Feeling: Short vibration duration, firm and hard without obvious shaking
- Conclusion: Good bonding performance, no hollow spots, blisters or delamination defects
4.2 Hollow Spot / Blister (Minor Defect)
- Sound Feature: Muffled, hollow and drum-like sound with a long lingering echo
- Hand Feeling: Local softness, long residual vibration and obvious shaking
- Common Causes: Insufficient hot pressing pressure, improper moisture content of veneers, local lack of adhesive, or impurities blocking the bonding interface
4.3 Obvious Delamination / Unbond (Unqualified)
- Sound Feature: Large-area continuous hollow sound with clear defect boundaries
- Hand Feeling & Phenomenon: Obvious micro-movement can be felt when pressing the defective area lightly
- Judgment: Unqualified product. This defect seriously affects the structural strength, moisture resistance of plywood, and may cause cracking during subsequent processing.
5. Subsequent Risks Caused by Defects (For Customer Inspection Report)
- Edge chipping, surface cracking or depression during sanding, cutting and blanking processes
- Rapid expansion of delamination defects when exposed to moisture or humid environments
- Reduced bearing strength and decreased service life/circulation times of plywood
- Rejection due to failed appearance acceptance, and deduction points in LWI/QC inspection
