The heating method affects the temperature selection, process control, and quality of processed products when the hot press is used. The following explains the heating system of the hot press in detail from the aspects of heating method, hot pressing plate, connection and sealing, and pipeline insulation.
(1) Comparison of heating methods
Traditional heating methods are steam heating and electric heating. Steam heating heats up quickly, but requires a pressure boiler, high pipeline pressure, and steam easily condenses into water, causing uneven temperature on the board surface; electric heating has the characteristics of fast heating, high heating temperature, and easy control, but it consumes a lot of electricity, has high operating costs, and has the disadvantages of being difficult to replace when the resistance wire fails during use.
The advantages of thermal oil heating are: high heat capacity, uniform temperature, can be heated to a very high temperature under normal pressure, low heat loss, and can reduce production costs. Its disadvantages are slow heating speed and difficulty in temperature control. In the furniture industry, when the hot pressing temperature requirements are generally not high, a hot water circulation furnace can also achieve satisfactory results, and the fixed investment and operating costs are very low.
The characteristics of the new gas thermal oil furnace: no pollution, high energy conversion rate, no need for coal ash storage, which can reduce the floor space, gas thermal oil furnace when fuel is transported, other auxiliary machines are less, power is small, and continuous operation time is short, which can greatly save the electricity used in the production of thermal oil boilers. In addition, since the coal-fired heat transfer oil circuit will produce a lot of smoke and dust when it is running, it will pollute the environment. As natural gas is a clean energy, the gas heat transfer oil furnace will not emit polluting air when it is running, so it can achieve the maximum environmental protection needs.
Thirdly, the gas heat transfer oil furnace can save labor intensity when it is running. Since the coal-fired heat transfer oil circuit requires high-intensity physical labor in the transportation and slag removal of coal, the gas heat transfer oil furnace avoids this high-intensity physical labor, which can save labor costs for enterprises.
(2) Hot press plate performance
The performance of the hot press plate can be evaluated from several aspects, first of all, the use of the plate material, then the processing accuracy, and finally the arrangement of the medium channels in the hot press plate.
The hot press plate is generally made of a whole piece of pressed steel plate through deep hole drilling, which is mainly used for oil heating and steam heating; the other is to use aluminum plate on the surface of the welded pipeline, which is mainly used for electric heating. Using a whole steel plate can avoid oil leakage caused by poor sealing, and it has high rigidity and strength, but the hot press plate is thicker and heavier; the latter usually adopts the form of thin plates, and stress will be generated during welding and processing, so high processing precision is required.
The hot press plate is thin and easy to preheat and control the temperature, but it has high requirements for the workpiece. The uneven thickness of the workpiece will cause the plate to deform, affecting the subsequent processing quality; the hot press plate is thicker, which is conducive to increasing the heat capacity and improving the strength and rigidity, but it will increase the size of the press and consume more steel.
During manufacturing and processing, the flatness of the hot press plate and the parallelism of the upper and lower surfaces should be ensured to be sufficient. The flatness tolerance of the hot press plate should be within the range of 0.1-0.18mm, the parallelism tolerance should be within the range of 0.15-027mm, and the surface roughness of the plate should be 3.2μm. The temperature difference in the entire width of the hot press plate should not exceed 2-3℃. If the temperature difference is too large, the moisture content will be uneven, the product will be easy to warp, and the strength and appearance quality of the product will be affected. The heating channels for the flow of heat medium in the hot press plate are generally not taken seriously by people in the wood-based panel industry. The correct circuit design should be based on the principle of facilitating the flow of heat medium, facilitating uniform heating of the board surface, and reducing the leakage of heat medium. For small press hot press plates used in the furniture industry, a single-channel dual-circuit design is generally adopted to meet the use requirements.
(3) Connection and sealing
The use of appropriate connection methods and sealing elements is of great significance for heat transfer under high temperature and high pressure.
Currently, the connection pipelines of hot press plates are basically based on hoses, which makes the connection very convenient. Domestically, metal hoses are mainly used, and some manufacturers also use imported Teflon hoses. Both methods can be connected by flanges. The fatigue resistance of Teflon hoses is better than that of metal hoses. Asbestos rubber sheets are traditionally used as sealing materials. New materials such as polytetrafluoroethylene and winding graphite have more ideal sealing effects.
(4) Pipeline insulation
Choosing seamless grade advanced insulation technology can save more than 20% energy compared to the insulation method of rock wool wrapped with asphalt cloth. Cold bridge phenomenon and moisture absorption phenomenon are the fatal shortcomings of the joint insulation process. The insulation measures using composite insulation technology can maximize energy saving and isolate the occurrence of moisture absorption. At the same time, it can ensure that the temperature of the heating system is constant, the temperature rises quickly, and the temperature drops slowly.
