Varieties or Types of Industrial Timber | Materials | Engineering

The timber which is prepared scientifically in a factory is termed as the industrial timber and such timber possesses desired shape, appearance, strength, etc. Following are the varieties od types of industrial timber: (1) Veneers (2) Plywoods (3) Fibre Boards (4) Impreg Timbers(5) Compreg Timbers (6) Block Board and Lamin Board (7) Glulam (8) Flushdoor Shutters (9) Particle Board or Chip Board (10) Hardboard.

(1) Veneers (IS- 303-1989):

These are thin sheets or slices of wood of superior quality. The thickness of veneers varies from 0.40 mm to 6 mm or more. They are obtained by rotating a log of wood against a sharp knife of rotary cutter or saw as shown in fig. 9-21. The veneers after being removed are dried in kilns to remove moisture.

Following facts should be noted:

(i) The edges of veneers are joined and sheets of decorative designs are prepared.

(ii) The Indian timbers which are suitable for veneers are mahogany, oak, rosewood, sissoo, teak, etc.

(iii) The process of preparing a sheet of veneers is known as the veneering.

(iv) The veneers are used to produce plywoods, batten boards and lamin boards.

(v) The veneers may be fixed on corners or bent portions. It creates an impression that the whole piece is made of expensive timber.

(vi) The veneers may be glued with suitable adhesives on the surface of inferior wood. The appearance of inferior wood is then considerably improved.

(2) Plywoods:

The meaning of term ply is a thin layer. The plywoods are boards which are prepared from thin layers of wood or veneers. The three or more veneers in odd numbers are placed one above the other with the direction of grains of successive layers at right angles to each other. They are held in position by application of suitable adhesives. The placing of veneers normal to each other increases the longitudinal and transverse strengths of plywoods.

While being glued, the pressure may be applied on veneers. The pressure may either be applied hot or cold. For hot pressure, the hydraulic press is employed to press plywoods. The temperature varies from 150°C to 260°C. For cold pressure, the plywoods are pressed at room temperature only. The pressure applied on plywoods varies from 0.70 to 1.40 N/mm².

The plywoods are used for various purposes such as ceilings, doors, furniture, partitions, panelling walls, packing cases, railway coaches, formwork for concrete, etc. The plywoods however are not suitable in situations subjected to direct shocks or impacts. The use of plywood and its products has become so common at present that it has totally changed the design and complex of various structures such as buildings, offices, theatres, restaurants, churches, temples, hospitals, etc.

The plywoods are available in different commercial forms such as batten board, lamin board, metal faced plywood, multiply, three-ply, veneered plywood, etc.

The batten board is a solid block with core of sawn thin wood as shown in fig. 9-22. The thickness of core is about 20 mm to 25 mm and total thickness of board is about 50 mm. The direction of the grains of core battens is at right angles to that of the adjacent outer ply sheets. These boards are light and strong. They do not crack or split easily. They are widely used for making partition walls, packing cases, furniture pieces, ceilings, shutters of doors and windows, etc.

The lamin board is similar to the batten board except that the core is made of multiply veneers as shown in fig. 9-23.

The thickness of each veneer does not exceed 6 mm and total thickness of board is about 50 mm. The external plies are of thick veneers and they are firmly glued with core to form a solid block. The grains of core veneers are at right angles to those of outer plies. These boards have the same uses as those of batten boards.

In metal faced plywood, the core is covered by a thin sheet of aluminium, copper, bronze, steel, etc. This plywood is rigid and it is cleaned.

The plywoods prepared from more than three-ply are designated as the multiply. The number of veneers is odd. The thickness may vary from 6 mm to 25 mm or more.

The plywoods prepared from three plies only are known as the three-ply. Its thickness is upto 5 mm.

The thickness of different plywood board in mm are as under:

3 ply – 4 mm

7 ply – 12 mm, 15 mm, 16 mm

9 ply – 12 mm, 15 mm, 19 mm

11 ply – 19 mm, 22 mm, 25 mm

Following are the rules regarding the thickness of plywood:

(i) For 3 ply boards, the combined thickness of face veneers should not exceed twice the thickness of the centre ply.

(ii) In multiply boards, the thickness of veneer should not be more than thrice the thickness of any other ply.

(iii) The sum of thickness of wood with veneers in one direction shall approximately equal to the sum of thickness of veneers at right angles to them and in any case, is should not be greater than 1.5 times this sum except for 3 ply given above.

In veneered plywood, the facing veneer is of decorative appearance and it is used to develop an ornamental effect.

According to CPWD specifications plywood for general purposes are of following three grades:

(i) Boiling Water Resistant (BWR)

(ii) Warm Water Resistant (WWR)

(iii) Cold Water Resistant (CWR)

Advantages of Plywoods:

Following are the advantages of plywoods:

(i) As plies are placed at right angles to each other, the expansion and shrinkage are comparatively very low.

(ii) They are available in a variety of decorative appearance.

(iii) They are available in large sizes. The commercial sizes have widths upto 1.50 m and lengths upto 3 m.

(iv) They are elastic and hence they are not liable to split or crack due to changes in atmosphere.

(v) They are light in weight.

(vi) They are not easily affected by moisture.

(vii) They are stronger than solid boards. For instance, a three-ply board is nearly three times stronger than the solid board of the same thickness.

(viii) They are very easy to work and they can be made to suit any design.

(ix) They do not split in an axial direction.

(x) They do not split when nailed near edges because of their cross grained nature.

(xi) They make use of rare and valuable timbers in a quite economical way.

(xii) They possess uniform tensile strength in all directions.

(3) Fibre Boards:

These are rigid boards and they are also known as the pressed wood or reconstructed wood. The thickness varies from 3 mm to 12 mm. They are available in lengths varying from 3 m to 4.50 m and in widths varying from 1.20 m to 1.80 m. The weight of Fiber boards depends on the pressure applied during manufacture. The maximum and minimum limits of weight are respectively 9600 N/m³ and 500 to 600 N/m³.

Following is the procedure adopted in the manufacture of Fiber boards:

(i) The pieces of wood, cane or other vegetable fibres and chippings are collected and they are heated and boiled in a hot water boiler.

(ii) The wood fibres separated by heat are put in a vessel.

(iii) The steam under pressure is admitted in the vessel.

(iv) The pressure of steam is then suddenly increased to 7 N/mm². This increased pressure is maintained for few seconds only.

(v) The valve located at the bottom of vessel is opened and the steam is allowed to expand.

(vi) The sudden release of pressure makes the wood pieces to explode and in doing so, the natural adhesive contained in the wood fibres is separated out.

(vii) The wood fibres are then allowed to flow out.

(viii) These fibres are cleaned of all superfluous or extra gums.

(ix) Such cleaned fibres are spread on wire screens in the form of loose sheets or blankets of required thickness.

(x) Such loose sheets of wood fibres are prepared between steel plates and ultimately, the Fibre boards are obtained.

Depending upon their form and composition, the Fibre boards are classified as insulating boards, medium hard boards, hard boards, super hard boards and laminated boards. They are also available under various trade names such as Euraka, Indianite, Insulite, Masonite, Nordex, Treetex, etc.

The Fiber boards form an ideal base for practically all types of decorative finishes such as distemper, oil paint, etc. The hard boards are also suitable for polish and varnish. Several patterns of Fiber boards with pre-decorated surfaces are available in the market and thus the necessity of treating them after fixing in position is eliminated.

Following are the uses of Fiber boards:

(i) For internal finish of rooms such as wall panelling, suspended ceilings, etc.

(ii) To construct formwork for cement concrete i.e. to retain cement concrete in position when it is wet.

(iii) To construct partitions.

(iv) To prepare flush doors, tops of tables, etc.

(v) To provide an insulating material of heat and sound.

(vi) To work as paving or flooring material.

(4) Impreg Timbers:

The timber which is fully or partly covered with resin is known as the impreg timber. The usual resin employed is phenol formaldehyde which is soluble in water. The veneers or thin strips of woods are taken and they are immersed in resin. The resin fills the space between wood cells and by chemical reaction, a consolidated mass develops.

It is then cured at a temperature of about 150°C to 160°C. The impreg timber is available under trade names such as Formica, Sungloss, Sunmica, etc. and it is used for moulds, furniture, decorative articles, etc.

The advantages of impreg timbers are as follows:

(i) It is not affected by moisture and weather conditions.

(ii) It is strong and durable.

(iii) It possesses more electrical insulation.

(iv) It presents a decent appearance.

(v) It resists the acidic effects.

(vi) The contraction and expansion of impreg timbers are about 25 to 40 per cent less than ordinary timber.

(5) Compreg Timbers:

The process of preparing compreg timbers is same as that of impreg timbers except that curing is carried out under pressure. The strength and durability of compreg timbers are more as compared to the impreg timbers. The sp. gravity of compreg timbers is about 1.30 to 1.35.

(6) Block Boards and Lamin Boards:

Block boards are boards having a core made up of strips of wood, each not exceeding 25 mm in width. The edges are glued together to form a solid sheet, which is then finished with one or two cross bonded veneers on each face. The wooden strips may be obtained from small round wooden logs, leftovers from solid timber conversion or peeler cores remaining from veneer production.

Grain direction of the core pieces is at right angles to that of the facing veneers. These boards are also known as batten boards or solid core plywood, when the core strips consists of a core made up of large pieces of 80 mm width.

These boards are extensively used for railway carriages, bus bodies, marine and river crafts, for furniture, partitions, panelling, prefabricated houses, etc.

When the thickness of core strips does not exceed 7 mm, such boards are known as lamin boards.

(7) Glulam:

It means glued and laminated wood. It is not made of veneers but with solid wood. Solid wood is glued to large sections. Glulam sheets are mainly used for supporting long span roofs in sports stadium, indoor swimming pools, sheds for chemical factories, etc. where other materials like steel cannot last long. They can be used as beams of many shapes including curved members. The timber sections are first dried in the kiln.

Then they are machined to form interlocking V-cuts across the width and the end sections of each piece. Suitable glues are then applied and the pieces are forced together under longitudinal pressure, while the glue cures. However for such type of fabrication, good shop facilities for woodworking and gluing are necessary and hence they are not very popular in India.

(8) Flush Door Shutters:

In modern trends, factory-made flush door shutters have become more popular for interior work. They are available in thickness of 25 mm, 30 mm or 35 mm.

Following different types of flush door shutters are available in the market:

(i) Cellular core type

(ii) Hollow core type

(iii) Block board core type; and

(iv) Particle or MDF board core type.

(9) Particle Boards or Chip Boards:

These boards are made of wood particles or rise husk or bagasse (remains of sugarcane after crushing), embedded in resins and subjected to heat with pressure. These boards are manufactured by extrusion pressing or by pressing in parallel plates.

The extrusion pressing process orientates the wood particles in the direction at right angle to the plane of the board, while pressing in parallel plates orientates the particles parallel to the plane of the board.

Following are the four types of particle boards:

(i) Flat pressed single layer board – FP SL

(ii) Flat pressed three layer board – FP TH

(iii) Extrusion pressed solid board – XP SO

(iv) Extrusion pressed tubular core – XP TU

Particle boards are heavier than solid wood plywood. As they provide broad and stable panels of reasonable strength, they can be sawn like wood and are mainly used for furniture making.

(10) Hard Boards:

Hardboard is made from wood pulp which is compressed to make sheet usually of 3 mm thick. Its face surface is made smooth and hard while back surface is made rough with pattern or cross lines. Its width is usually 1.2 m and length varies from 1.2 m to 5.5 m.

Table 9-7 shows classification and thickness of hardboard as per CPWD specifications.