The plasticization process

Plasticization through heating and vaporizing 

On an industrial level three hearing systems for bending are used:

  • hearing in a bath of water;
  • steaming;
  • dielectric heating.

The most suitable type of wood is Beech followed by Ash, Maple, Birch, Oak, Chestnut, Robinia, Cherry, Bagolaro, etc. In the same way conifers and tropical wood are inappropriate.

Heating bath of water
When using a hot water bath the temperature is maintained between 70-80°C for the immersion period which guarantees the uniform distribution of the heat along the whole length of the wood. This system, used mainly with Birch and Ash, has the drawback of wetting the wood too much which becomes soft and difficult to bend and stabilize. There is also the added problem of causing unpleasant variations in the colour of the wood. The water bath, compared to the other two methods, is useful in cases where bending is required only along a certain portion of the length of the piece, this is because immersion can be limited to the area in question.

Steaming
Generally used with Beech this system uses cylindrical metal steamers at atmospheric pressure with saturated vapour at 100°C. The steamer must be well insulated, the metal must be anti-rust and protected against corrosion. During treatment the items become wet externally as much as 25% and if the condensate remains in the cylinder even up to 40%. It has been proved that with condensate present plasticizing is better and faster.

As regards treatment times, it is important to avoid allowing the wood to plasticate too much so that it does not deform in the cross section during bending. A steaming time of between 45 to 60 minutes for each 25 mm of thickness according to the type of wood should be sufficient. At the end of the process there is a range of humidity going from the outside towards the inside which further helps the drying processes.

Dielectric Heating 
This process makes use of electric current with a frequency of 5-15 MHz which causes immediate hearing of all the wood mass at 100°C with any loss of humidity. Among the most important advantages must be remembered the possibility of plasticizing semi-finished wood with some defects in the fibre, whereas among the disadvantages is the high running cost of the application which limits the sectors it can be used in.

In this table minimum bending radii of some wood types, pieces with 25.4 mm thickness, wood seasoned and steam heated (by Stevens and Turner).