Forest Utilisation Chapter 13: Plywood, Composite Wood, Improved Wood and Sawmills
Forest Utilisation Notes · Chapter 13
Chapter 13: Composite Wood, Improved Wood and Sawmills
Six forms of composite wood, plywood manufacture, laminated wood and glulam, core, sandwich, fibre and particle boards, impreg, compreg, heat stabilized and compressed wood, and sawmill sites, saws and machines.
1. Composite wood
Six forms: 1. plywood, 2. laminated wood, 3. core boards, 4. sandwich boards, 5. fibre boards, 6. particle boards. They make large pieces from small ones and use waste and inferior species.
2. Plywood
This is cross-banded construction. Outer plies are the face and back, the centre is the core, and layers between core and faces are cross bands. Plywood always has an odd number of plies, the simplest being three. It has dimensional stability, strength in both directions and little tendency to split.
| Veneer | Details |
|---|---|
| Rotary peeled | Comes off like a roll of paper; logs up to 255 to 280 cm long; common thickness 1.25, 1.5 and 2.5 mm (minimum 0.05 mm) |
| Sliced | Cut like bread, 0.6 to 1.25 mm; for decorative veneers |
Old glues were animal, starch, blood albumen and protein glues. Today synthetic resins are used: urea-formaldehyde (UF) for interior and phenol-formaldehyde (PF) for waterproof plywood.
3. Other composites
| Product | Key point |
|---|---|
| Glulam | Glued laminated structural timber; used for halls, hangars, boats, sports goods |
| Core board | Core of glued wood strips between veneers with grain at right angles: batten-board (strips up to 7.5 cm wide), block-board (up to 2.5 cm), lamin-board (strips up to 7 mm thick) |
| Sandwich board | Light core faced on both sides with thin strong skins; skins carry load, core carries shear; flush doors, aircraft, boats |
| Fibre board | Wood defibrated into pulp, felted into a mat and consolidated by pressure and heat; hard, semi-hard, medium and soft boards |
| Particle board | Chips, flakes or shavings bonded with resin; chipboard, flake board, shaving board. For interior use only |
4. Improved (modified) wood
Improved wood is treated by resin impregnation, compression or heat to reduce 'working' (shrinking and swelling) or to increase strength.
| Type | How made | Result |
|---|---|---|
| Impregnated wood (impreg) | Veneers soaked in phenol-formaldehyde resin, cured without compression, then bonded | Dimensional stability |
| Compregnated wood (compreg) | Resin-impregnated veneers compressed and cured | 10 to 20 times the hardness; propeller blades, gears, bearings, shuttles |
| Heat stabilized wood | Under molten metal or fused salt at 260 to 315°C for a few minutes | Lower hygroscopicity, decay resistance |
| Compressed wood | Compressed at 90 to 140 kg/cm² without resin | Specific gravity 1.2 to 1.3 |
| Heat stabilized compressed wood (staypak) | Compression that makes lignin flow, removing internal stresses | Less brittle than impreg and compreg |
| Chemically modified wood | Acetic anhydride (with pyridine) and other chemicals replace hydroxyl groups | Less hygroscopic |
5. Sawmills
A sawmill is a plant with machinery for sawing logs into planks, baulks and scantlings. Sawmills are permanent (near cities, breaking down logs for allied trades) or portable or semi-portable (moved from place to place in the forest).
Points in choosing a mill site
- Plentiful and regular supply of logs.
- Delivery of logs at reasonable cost.
- Good lines of export to the market.
- Sufficient demand for sawn timber and a reasonable profit.
- Cheap motive power.
| Machine | Use |
|---|---|
| Log band saw | Endless steel band over two pulleys; log on a travelling carriage; main breakdown saw |
| Frame (gang) saw | Several blades in a reciprocating frame cut many boards at once; now uncommon |
| Circular saw | Toothed steel disc; common for smaller logs and re-sawing |
| Resaws | Re-sawing flitches into boards |
| Woodworking machines | Surfacer, thicknesser, over-and-under planer, band scroll saw, spindle moulder, tenoning, mortising and boring machines |
6. Quick revision
- Composite wood = built-up bonded products; six forms.
- Plywood: grain of each veneer at right angles to the next; odd number of plies.
- Rotary veneers 1.25 to 2.5 mm; sliced 0.6 to 1.25 mm; press 7 to 18 kg/cm²; condition to 12%.
- Laminated wood: all grain parallel; glulam.
- Batten-board up to 7.5 cm strips; block-board up to 2.5 cm; lamin-board up to 7 mm.
- Compreg: 10 to 20 times harder. Heat stabilized: 260 to 315°C. Compressed: SG 1.2 to 1.3.
- Main breakdown saw: log band saw.
7. Interactive MCQ Quiz
16 questions. Tap an option to see the answer and explanation, then go to the next question.
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