The woodworking industry never fails to amaze me with its ambiguous choice of words. For example, when someone says 'Ply Board' does the person mean plywood or block boards? Here's my understanding of plyboards and its relation to block boards and plywood sheets.
So, let's start with, What are plyboards?
The term 'Plyboard' is just a naming convention used by companies in India, to mark their plywood sheets and block boards, which are two distinctly separate things.
What this essentially means is that when you as a customer, see a wooden board that contains some marking such as 'XYZ Company Name - Ply Board', it means that the XYZ company mentioned on the board, is in the business of manufacturing and selling both plywood and block boards.
Here are the different ways in which some of the Indian manufacturers mark their wooden sheets or boards.
First is the name of the company, that is followed by either of these words.
Here are a couple of sample photos.
And here's another board from a separate manufacturer that is marked differently.
While the 3rd example listed above gives quite a clear picture, the same cannot be said about the first two. So, when you see a wooden sheet with a 'Ply Board' mark, it means that it could be either a 'plywood sheet' or a 'block board', and you either need to look at the edges to confirm, or you need to probe the shopkeeper further about it.
From the outside, there is no noticeable difference between the two things (except if you look at the edges), and in general the awareness amongst end-customers about the availability and use of block boards is quite low, so the impression that many people have is that 'Ply Board' always means 'Plywood'. That may not always be the case.
So, what are block boards?
Block boards are made up of rectangular blocks of wooden strips at the core, that are covered by a layer of ply on either of the front and back faces of the board. These solid wooden blocks that make up the core of the blockboard are usually, but not always, made from wood obtained from softwood trees such as Pine or Mango wood.
Technically speaking, softwood is lighter, less denser, weaker and less durable compared to hardwood, and hence block boards are not considered to be as strong or as long-lasting as plywood, which is usually made from hardwood plies (veneers) obtained from trees such as Gurjan, teak, rosewood or other hardwoods.
Having said that, the composition of the block boards does offer it a peculiar characteristic that makes it better than plywood in certain scenarios. The advantage is that block boards are less prone to bend or sag in the middle, compared to plywood sheets. Due to this reason, block boards are better suited in cases where lengthy pieces of wood are required, for example, wardrobe cabinet doors, single and double beds, and for making doors (called solid core flush doors).
And, what is plywood?
Plywood is made by glueing several layers of plies over one another. These plies are called wood veneers, and are obtained by peeling thin layers from logs of wood using a wood lathe machine. It does not contain blocks of wood.
The plies (veneers) are usually obtained from hardwood trees, and the layers are stacked over each other such that the grain runs perpendicular in each adjacent layers. The number of plies in a plywood sheet is almost always an odd number.
Ply board grades.
Ply boards (both plywood and block boards) are available in 'Commercial' as well as 'Water Proof' grade.
Commercial grade (known as MR - Moisture resistant grade) is made using Urea Formaldehyde resins for bonding. It is not of a completely waterproof type, and is suited for interior use in homes and offices.
The Waterproof variety (known as BWR - 'Boiling Water Resistant' or BWP -'Boiling Water Proof' grade) is made using Phenolic resins (plastic resins) for forming the bonds. It is adequately water-proof and suited for interior as well as exterior outdoor use.
Ply Boards sizes and Thickness
The most commonly available plywood as well as blockboard size in India is 8' x 4' (8 feet by 4 feet). Other sizes such as 8'x3', 6'x4' etc. are also available.
Here's more detailed information about Plyboards sizes and thickness.
Ply Board prices
Plywood is costlier compared to Block boards, since it is usually made from hardwood and is much stronger. The prices vary a lot based on whether the wood you are buying is of a well-known reputed brand or one that is made by a local manufacturer.
In general, customers can assume that the best quality plyboards will be those that have a genuine ISI mark on them (the Indian Standards quality mark) since only the products that meet the standard and stringent quality benchmarks are allowed to have this mark on them.