Was ist sperrholz?
How is plywood made?
So what exactly is plywood?
Plywood is a product obtained from thin layers of wood veneer glued together. Each layer is being placed perpendicular to one another. This is done in order to maximize the strength of the product. You see, wood has its strength against its grain. That means that if the force is applied perpendicular to the grain of the wood, then the resistance is at its maximum power. But if however the force is applied parallel to the grain of the wood, then you will have almost no resistance whatsoever. That is why, it is very important to make sure that the plywood has its plies placed perpendicular rather than parallel.
A simple way of checking is to simply look at the plies of the product. If the plies are perpendicular, than you will notice that the plies have 2 distinct colours ( this is caused by the grain of the wood). However, if the plies have virtually the same colour or if there is very little distinction between them, the plies may have been placed parallel. This can significantly decrease the resistance of the product. Also, besides increased strength, cross granulation can also reduce shrinkage, expansion and curvatures.
How exactly is plywood made?
As mentioned previously, plywood is made from thin layers of veneer, which can have anywhere between 0.5-0.6 mm up to 2.6-2.8 mm. These plies are usually obtained by using a veneer peeling machine. The log, after the bark was removed, is placed between 2 rollers. They start rotating and closing, thus applying pressure on the log against the knife, which effectively starts ‘peeling’ the log at the desired thickness. In some cases, when working with a very hard wood like beech, the logs have to previously be boiled for 48h at an average temperature of 75-80 degrees Celsius in order for the peeling process to be possible.
After the plies have resulted from the peeling process, the material will enter the dryer in order to reduce the humidity to about 8%. Then, depending on the quality of the material, some will go directly into the production. However, some will go to the cutting sections, where all defects will be eliminated in order to obtain a higher quality material. The sheets resulted will then have to be jointed together in order to form the layer in its final form.
Once the material reaches the ‘packaging stage’, the plywood is starting to take form. All the plies are glued and placed perpendicular to one another. Then, the material enters a cold press followed by a hot press, where all the plies are effectively pressed and the glue is dried. Now, the only remaining thing is to sand and cut the product according to its dimensions. It is also worth mentioning that in the case of film plywood the process, is the same, but the film is applied in a different special hot press.
What types of plywood are there?
Generally speaking, depending on the applications, it can be split into 3 main categories: hardwood, softwood and tropical plywood.
1. Hardwood plywood like birch or beech plywood is characterised by having a high density/ cubic meter. Higher than other types of plywood such poplar, pine or okoume. For reference, beech is arguably one of the most, if not the most dense and resistant plywood of all. Its density is around 750-800 kg/m3. Its competitor in this category birch, has a density of around 600-670 kg/ m3. These types of hardwood products can be used in various fields such as construction, furniture and as container floors.
2. Softwood plywood like polar or pine comes in at a much lower density around 500-520 kg/m3. Also, has much lower strength than hardwood products like beech plywood. Due to its different characteristics, it is not suitable for structural usage or in fields like construction. Thus, it is mainly used for furniture or packaging and boxes.
3. Tropical plywood may also be categorized as softwood. An okoume plywood has a density of around only 460 kg/m3. But due to its different characteristics deserves a different category. This type of plywood can be very malleable at lower thicknesses such as 3 or 5 mm. Thus it can be used in the manufacturing of boats and other marine vehicles. Tropical wood is often times very clean, without big knots or differences in coloration. Because it is also very light, this of plywood can also be easily used in the furniture industry.
How is plywood graded?
When you search on the internet, you may find different types of gradings, as they tend to differ according to the region where the plywood is produced or sold, but generally speaking there are 4 main grades as listed below:
I (B) – The face is practically flawless, with no defects, knots, splits, or differences in colorations.
II (BB) – The face has minor discolorations and may have very few very small knots.
III (C) – The face is allowed to have knots, splits, holes and discolorations. This type of quality is used mostly for its structural characteristics, not for aesthetic purposes.
IV (D)- The face is allowed to have discolorations, knots, splits, holes and even hard waviness and harm wholes.
Based on those quality classifications you may find different face combinations such as plywood with both faces Grade I ( I/I; B/B), or with one Grade 1 face and another Grade II face on reverse ( I/II; B/BB) and so on and so forth.
The important thing to keep in mind from all of this is that plywood can although has many variations, has also many usages and it is important to differentiate beforehand what each product can be used for, in order to buy from the beginning the right product for your needs.