Particleboard Plant production Manufacturing
1.4.1 Particle preparation
1.4.2 Particle drying and screening
1.4.3 Blending and mat forming
1.4.5 Board finishing
In most cases, particle production involves a certain number of operations as described below (see Figure 3) which require different amounts and types of energy.
Figure 3. Particleboard production – A simplified process flow
Particleboard furnish is derived from a multiple of sources and as the competition for solid wood and solid wood residues increases, manufacturers are having to resort to the use of low grade residues, such as hogged mill waste, sawdust, planer shavings, etc., as well as wood species not previously considered.
In view of the wide assortment of furnish delivered to the mill-yard, segregation as to size, and if possible, species, must be carried out prior to the reduction process. Bark is removed from logs, if not already done in the forests, so as to avoid blunting chipper knives, and the provision of stone-traps and magnetic separators safeguard other reduction equipment from damage which would otherwise be caused if contraries were introduced with the fibre furnish.
The particle size and geometry, as required for the core and surface layers of the particleboard, are produced by a diverse range of reduction equipment which is matched to the variety and size of wood and wood residues used. Chippers, knife-ring-flakers, hammer mills, disc refiners, etc., each operating on a different principle, using either knives, hammer bars, grooved disc plates, etc., are but some in common use in the industry.
The greater part of the furnish delivered to the mill needs to be dried so that the overall moisture level of the particles is in the order of three to eight percent for the purpose of bonding with liquid resins.
Particle drying is a continuous process with the particles moving along the length of rotating horizontal dryers whilst being suspended and exposed to hot gases or heat emitted from tube bundles which convey hot water, steam or thermic oil. Heat is produced by the combustion of oil, gas or process residues. Flash drying is now being considered an acceptable alternative to rotary dryers and requires somewhat lower drying temperatures.
Directly after drying, the particles are screened for size in vibrating or gyrating screens, or by way of air classification. Screening normally takes place after the dryers as moist particles tend to stick together, plugging screen plates and lowering the overall efficiency of the screening process.
Particles are separated according to size, for the purpose of grading the furnish for the board face and core layers. It is essential that the oversized particles be recycled for further reduction and that the fines are screened out, so as to avoid consuming a disproportionate amount of resin binder, and to provide a valued source of fuel.
Adhesives in the form of urea, phenol and melamine formaldehyde are generally used to bind together the particle mix, with the former being the most favoured resin in use. Between three and ten percent by weight of resin, together with other additives used to impart such properties as fire resistance, etc., are blended under controlled conditions in batches or as a continous operation. Blending may either take place in large vats at slow speed, or in small blenders with rapid mixing and shorter blending times.
In the more modern particleboard plants mat forming is a wholly mechanical process, whereas the older formers require manual equalizing. In spite of the wide variety of formers currently available, the underlying principles of mat formation are generally similar, in that a uniform flow of particles are fed to the former from a surge bin, which in turn meters an evenly distributed layer of particles into a frame on a moving belt or caul.
The formers may be fitted with single or multiple forming heads, which are either stationary or moving, and are so designed that the finest particles are delivered to form the surface layers of the mat and the coarser materials to form the core. In all cases it is paramount that an evenly distributed mat of the desired weight be formed. Mats that do not conform to standard are rejected and recycled.
Transportation of the mats to the pre-press and hot press is undertaken by either forming the mat on metal plates, called cauls, which are then either manually or mechanically wheeled to the presses, or in the case of caulless systems, by using flexible metal webs, plastic belts and trays that transport the mats through to the hot-press.
Pre-pressing of the mats prior to the introduction in the multi-platen hot presses, is now becoming a common feature in the pressing operation, due to the consolidation and reduction in mat width. This allows for ease of handling and the use of narrower openings in the hot-press, thereby considerably reducing pressing time.
Whereas the pre-presses may be of the hot or cold type, the main press is always heated, by passing hot water, steam or oil through the platens to attain temperatures in the order of 140-200°C, depending on the resins in use and the type of press.
Single or multiple opening hot presses may be used with the loading and unloading undertaken manually or mechanically by cable, chain lifts or hydraulics, depending on the age and sophistication of the plant. Although in the larger modern installations both pressing time and pressures are automatically regulated, hand control is still preferred in many plants as it permits adjustments to be made for the different mat qualities.
On leaving the hot press the boards are either separated from the cauls by hand, or mechanically by means of chains or turning devices. The cauls are stacked, allowed to cool and then returned to the forming station on push carts or mechanically transported on a fixed return line. The boards in turn, are cooled and conditioned so as to avoid degradation of the urea resins.
Trimming saws are used to cut the boards to size, with the edge trimmings being either recycled or used for fuel. In order to meet set standards as to thickness and surface quality, a combination of knife planers and belt or drum sanders may be used.
Once the boards have been surface finished they are cut to size along their length and widths with a combination of saws, according to the dictates of the market. Particleboard is normally produced as 1220 x 2440 mm panels with thicknesses ranging from 3-35 mm, 19 mm being the most common. Generally boards are manufactured in the medium-density range of 400-800 kg per cubic metre, although high-density board of 800-1120 kg per cubic metre is used as core stock.
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