Newbie Discussion - PLY/MDF properties

Originally posted by Tony Tapp - Senior lecturer - Uni of Plymouth

Tony Tapp - Senior lecturer - Uni of Plymouth wrote:

bit1 - structural properties of timber itself

 If a branch is cut off close to the bole (trunk) of the tree, or if the branch is shed naturally, the scar left behind heals rapidly and the knot generally becomes "live", "sound" or "tight".

 If a branch stub is left after cutting then the scar is left open for some time and this generally produces a "dead" or "loose" knot.

Knots will decrease the strength of timber because they disturb the grain pattern in the timber; consider them as being a "hole" in the structural analysis of the timber.

part2 - moisture content

"Green" (or unseasoned) timber as felled may be saturated and could contain up to 250% of water or sap (as a % of the timber’s oven dry weight); a figure of around 100% is more common.

Most of this water is held inside the pores (tracheids or vessels) of the timber and as the wood dries out this water is lost; when the pores are empty the typical moisture content of timber is down to around 28%, which is known as the fibre saturation point (fsp).

Little dimensional change has taken place whilst getting timber to this moisture content.

As the wood is dried out further the water bound up in the cell walls begins to be removed, and shrinkage starts to occur.

 Water will continue to be lost (or gained) until there is equilibrium between the wood and its surroundings.

Timber with a moisture content of below 20% will be sufficiently dry to prevent fungal decay (such as mould or rot) from occurring, and most structural timber is strength graded, supplied and installed at this level of moisture content. Internal joinery (doors, windows) is typically produced at a moisture content of 12%.

Part3 - Plywood

Plywood is an assembly of layers of timber glued together, with the direction of grain in alternate layers usually being at right angles to one another.

Plywood is produced by:

 Conditioning logs by soaking or steaming.

 Peeling the logs to produce a long sheet of veneer.

 Clipping or cutting the veneer to size.

 Drying the veneer to a moisture content of between 4 and 8%.

 Joining and repairing the veneers to remove defects.

 Grading (which is usually done visually).

 Applying adhesive to the veneers by spray or curtain coating.

 Assembly of the veneers into a "lay-up".

 Pressing and heat curing of the adhesive.

 Cutting the finished plywood into sheets.

Plywood can be produced with glues which range from those suitable only for interior use (example: urea formaldehyde glue) to those which will withstand external exposure (example: phenol formaldehyde glue).

Different grades of plywood are available, including Marine plywood (to BS 1088), Structural plywood, Utility plywood (which often has a particular surface veneer - for joinery) and "Specials" which can be laid up to perform a specific function (for use as concrete formwork, where a smooth, impermeable and durable surface finish is required).

Blockboard is a form of plywood known as a core plywood; the core of the board is made up from many strips of wood no more than 30 mm wide.

Ply can be so strong, because the likelyhood of knots lining up throughout the sheet is minimal, sharing the weakness into the other layers. Imagine it like a sandwhich of steel cable like used in suspension bridges, if one steel core snaps internally, the other cores share the load.

Part4 - fibre boards (MDF)

These are wood-based panels made from fine fibres of wood and includes products such as Hardboard, Softboard and Medium Density Fibreboard (MDF).

Boards can be made in one of two ways:

 Using a wet process, where the fibres are naturally bonded together by the inherent adhesive properties of the felted fibres (this process works in the same way as making paper).

 Using a dry process, where the wood fibres are bonded together by an adhesive.