Plywood is among one of the most versatile and common materials used for structural and decorative purposes. But what most don't know is what exactly it is, how it's made and why it is often preferred to timber sheets.
How It is Made
Plywood is a type of sheet stock material made from wood veneers, pressured and glued using either interior glue or exterior glue. Plywood veneers are glued together with adjacent layers having their wood grain rotated up to 90 degrees to one another.
This arrangement in plywood is called crossbanding.
Crossbanded layers and balanced construction mean that hardwood plywood won't shrink, swell, crack or warp as much as lumber. Its thin plies, lying at right angles to each other, as well as the various core materials available, produce uniform strength both with and across the grain. Baltic Birch, a widely distributed product from Europe, is among one of the strongest plywood types out in the market.
Production Process
The manufacturing of plywood has 22 steps:
1) Wood stock
Wood logs are harvested from plantations or forests, and immediately transported to plywood mills.
2) Hydrothermal treatment
Logs undergo hydrothermal treatment, being submerged in hot water pools of 40°C , left to soak for 1-2 days. This process gives the wood elasticity, softens its bark, increases its density and extracts the natural tar from the logs.
3) Debarking
After hydrothermal treatment, wood logs are placed on () and spun, while a peeling knife strips the logs of bark to leave unblemished wooden logs.
4) Peeling
The wood logs are "peeled" with a peeling blade, using Archimed's concept of a perfect spiral as the log keeps spinning, made into thin veneers of up to 16m in length.
5) Cutting of veneer sheet
Veneer sheets are cut to size, with slightly excess dimensions to account for cutting of the pressed plywood sheets later in the process.
6) Drying
Cut veneer sheets are dried to get rid of excess moisture, ready for grading and further processing.
7) Grading
Veneers are graded based on their quality and the amount of deformations on each surface.
8) Patching and splicing
The grading of veneers is based on the amount of deformations the cut veneer sheets have; if, for example, a veneer sheet has a knot, the veneer is sent to repairs (unless low quality plywood is made).
Graded veneers are patched (veneer defects are removed from a veneer sheet and replaced with patches that are made from high quality veneer), and the cut veneer "plugs" are joint at splicing lines.
9) Composing
Graded, patched and spliced veneers continue to the composing shop, where dry veneer sheets are interlaced at 90° with glue covered sheets.
This process involves making a sandwiched structure of raw and glue-covered veneers, which is why plywood sheets always have an odd number of plies and what makes plywood so structurally stable.
10) Cold pressing (pre-pressing)
Batches of glued veneers are cold-pressed together in preparation for the hot pressing, which will set in the glue and ensure the layers of veneer are static in the future plywood sheets.
11) Hot pressing
Cold pressed glued veneers are sent to the hot-press, with the veneer sandwiches being interlaced between the heating platforms of the hot press.
Under high pressure and temperature, the hot press compresses the plywood, setting the glue in and allowing the sheets to straighten out. After hot pressing, plywood sheets are left for 24 hours so the glue can go through polymerisation (aka hardening).
12) Cutting to size
The now settled plywood sheets have their uneven edges trimmed and then the sheets are cut to size (a standard plywood sheet's dimensions are 2440x1220x18 mm, with the last dimension changing depending on the thickness of the manufactured plywood).
13) Sanding
The plywood sheets are sanded to have a smooth, flawless surface to reduce the risk of splinters and other deformations that might affect using or processing the plywood further (eg. applying phenolic film on it).
14) Grading
The sanded plywood sheets are graded once again, this time by their structural integrity and other properties depending on what the plywood is made from.
The strength of plywood products (also known as the structural integrity) is graded using the F grade system. An F grade is a measure of the bending strength of a piece of wood, measured in megapascals (MPa), that the wood can withstand before it goes beyond the safe working stress zone.
So, if 18mm cross grain poplar plywood is graded F30 traversal, this means that this plywood can withstand up to 30 MPa (equivalent to 305.915 kilogram-force per square centimetre) and stay in the safe working stress zone without breaking. The grain of plywood can affect that value.
15) Manual repair
Manual repair involves hand-made changes such as additional sanding or putty.
16) Film-facing or Coating (if applicable)
Film facing/coating occurs if the plywood required is supposed to have some sort of film on one or both sides of the plywood. The overlays may be:
- Phenolic film
- Melamine film
- High-pressure laminate (HPL)
- Precious veneer (eg. oak, beech, walnut)
- Aluminium film
- Thermoplast film
- Fiberglass film
- Rubber film
- Lacquer
- UV-hardened acrylic paint
- Primer (white or clear)
Generally this happens before the plywood sheets are cut to size.
17) Cutting to size and edge profile trimming
18) Processing
Plywood can be further processed by being cut-to-size or CNC (Computerized Numerical Control) routed.
19) Grading
The completed plywood is sorted into its respective grades.
20) Edge Sealing
Depending on what sort of plywood it is, the edges of the plywood are sealed for waterproofing purposes. This mostly applies to plywood for concrete formwork (form ply)
21) Packaging
The plywood sheets are packaged into packs of multiple sheets, ready to be shipped out.
22) Warehousing and shipping
Why Choose Plywood
- The crossbanding and multi-layered structure of plywood makes it stronger than solid timber and metal.
- Plywood is tolerant to temperature fluctuations, bending and twisting which also makes it a more reliable and sturdy material to use for construction compared to timber and metal.
- Unlike timber sheets, plywood also has little to no risks of cracking. The cost of plywood production is significantly less than that of plastic or metal.
- As a composite material, plywood comes in all different dimensions, materials, variations and films, making it highly versatile for interior OR exterior application.
- Plywood is a sustainable material as it is made from renewable sources.
- Plywood has a pleasant, natural appearance which acts positively on the body and mind.
- Endless processing possibilities! CNC routing, laser-cutting, engraving, carving, bending and etc.
Ply Online offers a wide range of plywood products from birch and poplar plywood, as well as poplar OSB! Check out our range of plywood and OSB products on our website, or call us on (03) 8522 1508.