HOT PRESSING: - Theory and Mechanism during Manufacturing of Wood Based Panels

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The hot-pressing process is a vital step in the manufacture of wood-based composites and directly affects the properties and quality of the final products. During hot pressing, the heat and mass transfer process interact with each other, coupling with the mechanical deformation process of wood-based composites under the high-temperature interaction. In addition, the curing of resin, although governed by the laws of chemical reactions, can result in the release or absorption of heat and water, which may, in turn, affect the heat and mass transfer process. Realizing its complex and coupling nature, many scientists have conducted long-term, multisided and multilevel studies to better understand hot pressing.

Hydraulic Plywood Press Machine performs a major role among all the plywood machinery. Hydraulic Plywood press machines are used in plywood manufacturing for making different products like, plywood, ply boards, industrial laminate plywood. Different types of Hydraulic Plywood Press are manufactured in India depending up on the application of hydraulic plywood press machine.

  • Hydraulic Plywood Press,
  • Hydraulic Laminate Press
  •  Hydraulic Bonding Press
  • Hydraulic Door Press
  • Hydraulic Densified Plywood Press
  • Hydraulic Special Purpose Plywood Press.

A hydraulic press is a compression device that makes use of an exerted force placed upon a fluid, to create a resulting force according to Pascal's Law. Pascal's Law is a theory which states that the pressure (P) in a confined fluid, caused by a force (F1), over an area (A1), is transmitted undiminished, causing a force (F2), over the area (A2). This law can be applied to magnify a small force by the ratio of the areas to give a larger force – F2 = F1 (A2/A1)

Hot presses are multi daylight hydraulic presses. It essentially consists of a base which houses one or several hydraulic cylinders which can move up and down. The head of the press is connected to the base by means of column, frames and solid steels sheets. Usually the bottom bed can move up and down and the top head remains stationary. The platens are usually 40 to 50 mm thick through which 15 to 20 mm holes are drilled for the passage of the hot oil or steam. They are made of steel and cast in one piece. The platens are ground and chromium plated for preventing the glue from sticking on to the platens. Plating also improves the surface finish of the panels and prevents corrosion. The no of platens may vary from 3 to 21 or more but usually 12 daylight i.e 13 platens are used. Platen size used in plywood factory is usually 270cm x 144cm, although platens of oversize are there for specially purposes. The capacity of press varies from 100 tons to 5000 tons .The pressure required for pressing is applied by a combination of rotary and piston pump. The rotary pump is used initially for closing the press and the piston pump for applying and maintaining high pressure. The capacity of the pumps should be sufficient enough to develop a pressure of up to 18kg/cm2 or more (specific pressure).

The columns which support the press may be placed at the corners, sides or end according to the design of the press and convenience of loading. These should be sufficiently strong to prevent any deflection of the platen and to absorb all diagonal stresses. A smaller deflection in the hot platens may be absorbed by the assembly which is being pressed but if serious deflection occurs, the platen may exert uneven pressure on the assembly which may result in formation of poor bond in some spot of the panel or result in crushing of the wood fiber in some part.

Accurate control of temperature during hot press operations is most important as temperature variations can cause very serious faults to develop. Out of several methods mentioned above the following three methods are used commercially for heating of plywood press

1. Electrical, 2. Oil heating , 3. Steam

Electrical heating is costly and is suitable only for very small press. Heating of the press by steam is satisfactory, but requires high installation cost. In India, steam heating of hot press is very popular and is being widely used although oil heating also getting popular.

Drilling of heating platens and flow directions are designed correctly so that very little drop occurs at maximum condensing rate and saturated steam will condensed in the channels at a uniform temperature over the hole of the platen area.

In a 10 daylight press with 270 cm x 144cm platen, the average steam consumption usually ranges between 180 to 275 kg/hr, but within seconds of loading a heavy charge of say 19mm hardwood veneer assemblies, the steam requirement to maintain working temperatures can rise to well over 450kg/hr. The uniform temperature of the hot press platen can be maintained by suitable direct temperature control equipment or less accurately by controlling the steam pressure with a sample reducing valve operated manually or mechanically (automatic ).

The problems related to heating and controls of temperature in hot press platens are many. One of the major reasons is the condensation of steam within channel of the platens through which it passes. As soon as a condensing layer is formed, then this layer of water act as an insulator so that water accumulation in the platen channels can upset efficient transfer of heat from the condensing steam to the platen surface. This would cause serious fall in temperature specially on the underside of platens.

The remedial measure is to have good platen channeling design so that very little pressure drop occurs at maximum condensing rate and saturated steam will condense in the channels at a uniform temperature over the whole of the platen. The presence of air in the steam very often disturbs uniform temperature distribution over the platen surface. This air comes from the fresh water added to the boiler and it is very difficult to eliminate completely. It, therefore, passes along with steam and tend to accumulate at points where water is condensing and flow is sluggish. The air, of course, cannot condense and an air enriched layer tend to form near the condensing surfaces of channels and to destroy the efficiency of heat transfer at these points.

Air present in steam can be eliminated by installing special purge valves or air bleeding devices on inlet and outlet headers to the platen s and these are used if any sign of temperature fall due to possible air accumulation is observed.

In India, temperature control in most hot presses (10 daylights) is done manually .Normally a temperature meter is connected to a platen and the recorded temperature is taken as the average platen temperature. The opening or closing of the control valve in this steam line is being done manually depending on the temperature fall or rise in the meter. Firstly, the placement of bulb of the temperature meter may be doubtful –the placement near the steam in let or out let will indicate different temperature .All the platens may not have temperature equal to that shown by the platen connected to temperature meter .In the worst case, if there is water accumulation on the platen where the bulb is placed, the temperature meter will show entirely faulty record.

Secondly, such a temperature recording is time consuming. The heat from the steam channels pass through the metal of the platens to reach the thermometer pocket and bulb, and this takes often an appreciable time. The bulb must then be heated and this again takes time before the contents of the thermometer bulb can respond to the display meter. Over and above human error and lack of sincerity to act quickly in opening and closing the steam line valve may be further cause of delay

Accumulation of condensed water air bound to it within the platens cause serious temperature fluctuations. The problem of condensate drainage and air binding varies considerably with different platens, header and heater channel designs and also with quality of steam. Since the platens are horizontal and drainage of condensate sluggish, steam trapping must be efficient and by-pass provided. A water separator and steam trap should be fitted to the inlet of the press to ensure that water is not fed into the platens as this can increase drainage problems enormously and as far as operating conditions permit, air in the stem should be avoided.

Hot press is the important machinery among all machines used inside the factory of plywood industries. The basic function of the hot press is to heat and bake the prepared panels of face, veneer and core. The hot press is fully functioning with a steam pressure provided by the boiler. It should be noted that if proper steam pressure is not provided at the time of pressing the panels then quality output cannot be found. In fact the technical specification of the hot press is depending on annual production requirement of the industries. The technical specification on the hot press normally used in small scale plywood industries is of 640 tones capacity complete with heating plates, bolt case, pressing table, electrical panels, and cylindrical ram unit.



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