Views: 359 Author: Site Editor Publish Time: 2024-08-01 Origin: Site
A crucial stage in the production of wood-based composites, hot pressing has a direct impact on the characteristics and caliber of the finished goods. Interactions between the processes of heat and mass transport during hot pressing couple with the mechanical deformation process of wood-based composites under high temperatures. Furthermore, even though resin curing is controlled by the rules of chemical reactions, it may cause heat and water to be released or absorbed, which could have an impact on the process of heat and mass transfer. Many scientists have carried out lengthy, multifaceted, and multilevel studies to gain a better understanding of hot pressing after realizing its complicated and coupled nature.
The Hydraulic Plywood Press Machine plays a crucial role in the plywood machinery industry. Hydraulic Plywood press machines are utilized in the plywood manufacturing process to create various products such as plywood, ply boards, and industrial laminate plywood. Various Hydraulic Plywood Press models are produced in India based on the specific use of the hydraulic plywood press machine.
A hydraulic press is a machine that uses pressure on a fluid to generate a force following Pascal's principle. Pascal's Law explains that when force (F1) is applied to a confined fluid over an area (A1), the pressure (P) is transmitted without any reduction, resulting in a force (F2) over area (A2). This rule can be utilized to increase a tiny force by the proportion of the areas in order to produce a greater force – F2 = F1 (A2/A1)
Multi daylight hydraulic presses are hot presses. It basically includes a foundation that encloses one or multiple hydraulic cylinders that have the ability to ascend and descend. The top part of the printing machine is linked to the foundation using columns, frameworks, and sturdy metal panels. Typically, the lower bed is adjustable in height while the upper bed stays fixed. The typical thickness of the platens is between 40 and 50 mm, with holes measuring 15 to 20 mm drilled to allow hot oil or steam to pass through. They are constructed from steel and cast as a single unit. The platens are polished and coated with chromium to prevent the glue from adhering to them. Adding a layer of plating enhances the appearance of the panels and acts as a barrier against corrosion. The number of platens can range from 3 to 21 or higher, but typically 12 daylight which is equal to 13 platens are utilized. Platen dimensions commonly found in plywood factories are 270cm x 144cm, with larger platens available for specific needs. Presses can range in capacity from 100 tons to 5000 tons. The necessary pressure for pressing is generated using a mix of rotary and piston pumps. The rotary pump is first used to shut the press, while the piston pump is utilized for creating and sustaining high pressure. The pumps need to have enough capacity to generate a pressure of at least 18kg/cm2 (specific pressure) or higher.
The pillars holding up the press can be positioned at the corners, sides, or ends based on the press's layout and ease of loading. These must be strong enough to avoid any bending of the platen and to soak up all diagonal pressures. If there is only a slight deflection in the hot platens, the assembly being pressed can handle it. However, if a significant deflection occurs, the platen may apply uneven pressure on the assembly, leading to inadequate bonding in certain areas of the panel or crushing of wood fibers in specific spots.
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
Controlling temperature precisely is crucial during hot pressing operations, as any fluctuations can lead to significant defects. Among the various methods discussed earlier, the following trio of methods are commonly employed for heating plywood presses commercially.
Electric heating is expensive and is only appropriate for very small presses. Steam heating of the press is effective but comes with a costly installation process. In India, the use of steam heating for hot presses is commonly favored and widely utilized despite the increasing popularity of oil heating.
The drilling of heating platens and flow directions are planned accurately to minimize any temperature drop at the highest condensing rate, ensuring that saturated steam will condense uniformly in the channels at a consistent temperature across the entire platen area.
In a 10-daylight press with 270cm x 144cm platen, steam consumption typically varies from 180 to 275 kg/hr. However, when loading a heavy charge of 19mm hardwood veneer assemblies, the steam requirement can quickly exceed 450kg/hr to maintain working temperatures. The even temperature of the hot press platen can be kept steady with appropriate direct temperature control devices, or less precisely by regulating the steam pressure using a sample reducing valve that can be manually or automatically operated.
There are numerous issues concerning the heating and temperature controls of hot press platens. One of the primary causes is the compression of water vapor in the platen channels it travels through. Once a condensing layer forms, it acts as an insulator, hindering heat transfer from condensing steam to the platen surface by causing water accumulation in the platen channels. This could result in a significant decrease in temperature, particularly on the bottom side of the platens.
The solution is to ensure there is a well-designed platen channeling system to prevent significant pressure drop during maximum condensation and maintain uniform steam condensing at a consistent temperature throughout the platen. Air frequently disrupts the even distribution of temperature across the platen surface when mixed with steam. This air originates from the fresh water introduced into the boiler and is extremely challenging to completely remove. Consequently, it travels with steam and tends to gather where water is condensing and flow is slow. The air cannot condense and a layer of enriched air tends to build up near the condensing surfaces of channels, reducing heat transfer efficiency in those areas.
Air in steam can be removed by placing purge valves or air bleeding devices on inlet and outlet headers of the platens when a temperature drop is detected, indicating potential air buildup.
Temperature regulation in many hot presses (10 daylights) in India is typically carried out by manual means. A usual practice is to attach a temperature gauge to a platen and consider the recorded temperature as the average platen temperature. The control valve in the steam line is being opened or closed manually based on the temperature change in the meter. The location of the temperature meter's bulb may cause uncertainty, as placing it near the steam inlet or outlet could show varying temperatures. Not all platens will necessarily match the temperature shown by the connected platen. If water accumulates on the platen where the bulb is located, the temperature meter will display inaccurate readings.
In addition, taking a temperature reading like this can be a lengthy process. The steam channels transfer heat through the metal of the platens to reach the thermometer pocket and bulb, a process that can frequently take a significant amount of time. The bulb needs to be heated first, which also requires time for the thermometer bulb's contents to react to the display meter. In addition to human mistakes and hesitance to swiftly open and close the steam line valve, further delays may be caused.
Serious temperature fluctuations occur due to the accumulation of condensed water trapped within the platens. Condensate drainage and air binding issues differ significantly based on platen, header, and heater channel designs, as well as the quality of steam. Efficient steam trapping and a by-pass are necessary due to horizontal platens and slow drainage of condensate. It is important to install a water separator and steam trap at the press inlet to prevent water from entering the platens, which can greatly worsen drainage issues, and avoid air in the steam as much as possible under operational conditions.
The hot press is a crucial piece of equipment in plywood industry factories. The primary role of the hot press is to heat and bake the assembled panels of face, veneer, and core. The steam pressure from the boiler is powering the operational hot press. If the panels are not pressed with the correct steam pressure, high-quality results will not be achieved. The technical specifications of the hot press actually rely on the annual production needs of the industries. The hot press typically utilized in small scale plywood industries has a capacity of 640 tons and includes heating plates, bolt case, pressing table, electrical panels, and cylindrical ram unit as per technical specifications.