The reason behind the low expansion and shrinkage of the plywood is ........
Dimensional Stability in Wood
Solid wood is hygroscopic, meaning it absorbs and releases moisture from the air. This causes it to expand when humid and shrink when dry. This movement is much greater across the grain than along the grain. Plywood is specifically engineered to counteract this natural tendency, resulting in a product with excellent dimensional stability.
📝 Detailed Analysis of the Options
(b), (c), and (d)
These options describe necessary steps in the manufacturing of plywood, but they are not the fundamental reason for its stability. Plywood is indeed made from veneers (d), held together with adhesives (c), and glued under high pressure (b). However, if all the veneers were glued with their grains aligned in the same direction, the resulting board would still shrink and swell significantly across the grain, just like solid wood.
(a) Plies are placed at right angles with each other.
This is the correct answer. The key to plywood's stability is cross-graining. By orienting the grain of each successive veneer at a 90° angle to the previous one, the panel's tendency to move is restricted. The longitudinal stability of one layer counteracts the transverse (cross-grain) instability of the layer next to it. This balanced, cross-laminated construction is the primary reason why plywood exhibits very low expansion and shrinkage compared to solid wood.
📊 Key Advantages of Plywood Construction
| Feature | Reason | Result |
|---|---|---|
| Cross-Graining | Plies are placed at 90° to each other. | High dimensional stability (low shrinkage/expansion). |
| Odd Number of Plies | Creates a balanced panel with outer grains parallel. | Prevents warping. |
| Adhesive Bonding | Veneers are glued under heat and pressure. | Creates a strong, solid panel. |
💡 Study Tips
- Cross-Graining = Stability: This is the most critical concept for plywood. The right-angle placement of veneers is what stops the wood from moving.
- Distinguish Cause from Process: While adhesives and pressure are part of the process, the *arrangement* of the plies is the scientific reason (the cause) for the stability.
- Think of a Net: The cross-graining creates a structure like a net, where fibres running in both directions provide strength and resist movement in all directions.