The fibreboard market is dominated by MDF-type products. Such products are normally made via the ‘dry process’, which means that the fibres are dried prior to being formed into a mattress that is subsequently hot-pressed. MDF is, however, a relative newcomer to the market compared to other fibreboards as the very first fibreboard products were made using the ‘wet process’, which is similar to paper making, more than a century ago.
Consequently, I thought that it would be interesting to have a quick look at wet process fibreboards as they are an important part of the wood based panel sector’s heritage.
The initial manufacturing step is breaking the wood down to fibres. Most products today are made using a disc refining thermo-mechanical process.
In the US, some factories use the Masonite gun to steam explode hot wood chips into coarse fibres which are then refined using disc refiners. This explains why hardboard is often known as Masonite in the US. Both defibration processes generate fibres with lignin-rich surfaces. This is essential as the lignin has an important role to play in bonding the fibres together in the finished product.
A lot of water is added to the fibres to create dilute mixtures. Other additives such as glues, water repellents and various forming agents are also added. The mixture is then drained on wire screens to form webs. Drainage alone does not reduce the moisture content sufficiently and so the webs are then pressed between rollers to squeeze out more water.
If the factory is making low density (<400kg/m3) fibreboard, often known as softboard, or insulation board, then the next step is to dry the webs in ovens.
The softboard market has been experiencing something of a revival in recent years thanks to the fashion for floating floors which require an underlay; and to renewed interest in insulation materials from renewable sources.
If, on the other hand, the factory is making high density (>880kg/m3) hardboard-type products then the webs are pressed under high pressure and temperature. The webs have a high moisture content of 65% or more and so hot pressing generates a lot of steam. To help both liquid water and steam to escape, the webs are placed on screen meshes prior to pressing. This explains why one face is smooth and the other has an impression of the mesh. Such panels are often designated S1S, meaning smooth one side. S2S panels, ie smooth two sides, are possible, but the web must be dried before it is hot-pressed.
The press cycle of an S1S panel is designed to maximise the removal of water in its liquid form, as drying the panel requires a lot more energy. Therefore, a typical press cycle has three stages: initial and rapid high pressure to squeeze out water as a liquid; a low pressure drying stage; and finally a medium-to-high pressure consolidation phase which defines panel thickness.
The panels are then heat-treated by placing them in an oven for two to four hours at around 165oC to improve their water resistance. Unsaturated oil can be applied to panels prior to heat treating to make tempered panels, which are even more resistant to moisture.
After such a long press cycle and subsequent heat treatment, the panels are extremely dry and must be rehumidified before they are sent to the customer. Rehumidification is achieved by passing the panels through a chamber containing a high humidity. The panels are normally stood on their edge in order to expose both faces and maximise absorption.
The final manufacturing steps are trimming and packaging.
With the advent of MDF, it was thought that hardboard manufacture would disappear, but hardboards survive because of their superior surface quality – at least on one face – and their low formaldehyde emission characteristics.
