“Nothing is lost, nothing is created, everything is transformed” – (Lavoisier)
Cosa si intende per “Impiallacciatura” o “Nobilitazione”?
Wood veneering is the operation that allows the coating of a core wood panel with a thin layer of noble wood (walnut wood, olive wood, mahogany wood…) in order to give a natural effect to the panel at both sight and touch.
Veneering techniques date as far back as ancient Egypt and also during the Italian renaissance wooden furniture used to be coated with noble wood to embellish it.
How is veneering done?
Wood veneering consists in laminating a veneer (thin slice of wood) onto a core wood panel trough the use of membrane presses, these presses press and heat evenly the surface of the object to be laminated/veneered. Membrane presses use various technologies depending on the constructor and the type of requested finishing.
Membrane presses can be divided in:
- Fluid membrane presses: these presses have a tank filled up with liquid, which is then covered with a rubber membrane fixed on the perimeter of the press in order to seal and contain the liquid. Various liquids are used but mainly its either oil or water with the addition of ethylene glycol, depending on the final working temperature. The core wooden panel is then placed on top of the rubber or silicone membrane with the side to be veneered facing down, at this point the top part of the press comes down and pushes the wooden panel onto our ODZ rubber membrane, our membrane follows perfectly all of the shapes of the veneered piece thanks to the backpressure of the liquid which is evenly distributed onto the whole piece.
This type of presses is used to produce various articles amongst which ashlar panels or shutters, or more in general in all applications that require high pressures (up to 20 Kg/cm2) and maximum temperatures of 130°C.
- Air press / Vacuum press: This kind of membrane press is used for applications that require vacuum pressures of max. 0,9 ATM or in which the pressure of compressed air is sufficient to veneer the pieces. These presses use the pressure created with vacuum in the lower chamber or with compressed air blown from the top chamber, instead of using liquid. Heat is obtained either with the use of heated plates or with the use of electrical resistances that heat the air in the top part of the press. These presses can be fitted with either silicone or rubber membranes depending on working temperatures. Their main use is for the veneering of plastic materials such as PVC, polypropylene or other plastic materials that don’t require high pressures. Soft NR rubber or silicone membranes are used because they require little effort to deform and follow all shapes in the best of ways.
These presses can fit both our natural rubber membranes or EPDM rubber membranes. Fig. 1-2 (fluid membrane press outline), Fig. 3-4 (Double fluid membrane press outline).
Which are the characteristics that rubber or silicone membranes must have?
• High physical and mechanical properties at required working temperatures;
• Resistance to thermal aging, physical and mechanical characteristics decay in time when in contact with areas at high temperatures;
• High elasticity, membranes must not deform also after many work cycles;
• Low superficial coefficient of friction;
• Mechanical resistance to perforation, laceration, tear, elongation and compression;
• Even distribution of heat;
• Uniformity of pattern and even thickness of membranes, each superficial defect on the membrane will be transferred on the veneered panels.
The membranes/sheet are generally produced with two main elastomers:
• Natural rubber: used for temperatures up to 100/100°C because it’s the only elastomer that maintains excellent physical mechanical characteristics at these working temperatures and represents a good compromise of all of the above characteristics;
• Silicone rubber: used when temperatures ≥ 110/120°C; very resistant to thermal aging, but with lower physical and mechanical characteristics when compared to natural rubber, for these reasons the compound recipe is fundamental.
If the heat source is a heated plate its fundamental to keep the fine pattern of our membrane in contact with the heated plate.
Fig. 5-6 vacuum press + air heated plate; Fig. 7-8 vacuum press heated with electrical resistance.
In which sectors are rubber and silicone membranes used?
Its important to know that membranes are not only used in the veneering sector but can be used also in other sectors amongst which photovoltaic panel lamination, glass lamination, sublimation of colour on aluminium and many more.
Nowadays there are always more new application possibilities, for example silicone membranes are used also for the lamination of composite materials such as resin reinforced or carbon fibre articles.