Electric vaporizers for plasticization of wood.
Wood, an extremely valuable raw material, has particular structural characteristics that derive from its biological nature.
Unlike other materials, such as metals, the deformation of wood elements often represents an uncertainty deriving from the uneven distribution of fibres along the section.
With its range of vaporizers, ISVE is able to “plasticize” wood by subjecting it to the action of vacuum and water vapour. In this condition, the element can be easily bent or shaped.
Vaporizers are available with single or double autoclave, and their length may vary depending on the customer’s needs and requirements.
Bending of wood
Wood, a very previous raw material, has some particular structural characteristics that derive from its biological nature.
Bending of wood, unlike other materials, for example metals, is often an unknown factor due to the non homogenous distribution of the fibres along the section.
In semi-finished products which undergo bending two types of forces are present:
- tensile stress in the convex peripheral zone;
- compression in the internal concave zone of the arc.
If subjected to overstrain, wood breaks, beginning with the surface layers on the external part of the bending arc.
Remaining in the field of elastic deformation, bending under normal temperatures returns to its primary state on termination of strain.
To make a deformation permanent it is necessary to increase the “plasticity” of the wood, that is its capacity to assume a certain shape.
Plasticization of wooden material
Production of solid curved wood
The plasticization of wooden material increases with heating from a temperature of 70°C up to the optimum temperatures of between 145-150°C. As regards to humidity, the most convenient percentages are between 14 to 16%, but can be up to 25%.
From the physical point of view, the maximum plasticity is reached due to compression, minimum in the strain zone (elongation). During the treatment of the neutral line, that is the area where the two forces are balanced, it tends to move towards the external area of the arc thus increasing the plastic section.
In order to facilitate this movement further, devices are used (shaped forms) which prevent elongation during bending. Furthermore, if the hot curved wood is allowed to cool and dry in the form, the curve stabilizes and the wood re-acquires its original strength.
The production of solid curved wood has therefore three distinct phases:
- plasticization due to the action of the heat;
- stabilizing of the bending by drying the wood at temperatures above 65°C.
In order to achieve satisfactory results of the wood without knots and defects such as fibre deviations and internal stress must be used. The most suitable type of wood is the Beech, followed by Ash, Maple, Birch, Oak, Chestnut, Robinia, Cherry, Bagolaro, etc. In the same way conifers and tropical wood are inappropriate.
Insight: 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;
- dielectric heating.
Heating bath of water
When using a hot water bath the temperature is kept between 70-80°C for the immersion period, which guarantees uniform heat distribution over 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.
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 anti-rust and protected against corrosion. During treatment the items become wet externally up to 25% and if the condensate remains in the cylinder even up to 40%. It has been proved that with condensate present the plasticization is better and faster.
Regarding treatment times, it is important to prevent the wood from plasticising too much, so that it does not deform in the cross section during bending. A steaming time from 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 helps further the drying processes.
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 certain fibre defects,while one of the disadvantages is the high operating cost of the application, which limits the sectors in which it can be used.