The following is a detailed explanation of the heating system of the hot press from the heating method, hot plate, connection and sealing.
(1) Comparison of heating methods
Traditional heating methods are steam heating and electric heating. Steam heating rises quickly, but it needs to be equipped with a pressure boiler, the pressure of the pipeline is high, and the steam is easy to condense into water, which causes uneven surface temperature; electric heating has the characteristics of fast heating, high heating temperature, simple control, etc., but it consumes large power and works. The cost is high, and there are some disadvantages such as the resistance wire being difficult to replace when it is used.
The advantages of heat transfer oil heating are: high heat capacity, uniform temperature, can be heated to a very high temperature under normal pressure, small heat loss, can reduce production costs, its disadvantages are slow heating speed, difficult to control the temperature. In the furniture industry, when the hot pressing temperature is generally not high, satisfactory results can be achieved by using a hot water circulation furnace, and fixed investment and work costs are very low.
(2) The function of the hot platen
The function of the hot pressing plate can be evaluated in several aspects. The first is the application of the pressing plate material, then the processing accuracy, and the ending is the placement of the dielectric channels in the hot pressing plate.
The hot press plate is generally made of a whole constrained steel plate through deep hole drilling, which is mainly used for oil heating and steam heating; the other is the method of pasting aluminum plates on the surface of the welded pipe, which is mainly used for electric heating. The entire steel plate can prevent oil leakage caused by poor sealing. The rigidity and strength are high, but the hot-pressed plate is thick; the latter is generally used in the thin plate method, because the welding and processing will cause stress, requiring high processing accuracy.
The thin hot platen is easy to preheat and control the temperature, but the requirements on the processed parts are high. The uneven thickness of the workpiece will cause the deformation of the platen and affect the subsequent processing quality. The thicker platen is conducive to increasing the heat capacity and the forward strength and stiffness. But it will increase the size of the press and consume more steel.
When processing, the flatness of the hot-pressed plate and the upper and lower surfaces must be sufficiently parallel. The flatness tolerance of the hot-pressed plate should be within the range of 0.1-0.18mm, and the parallelism tolerance should be within the range of 0.15-027mm. The roughness was 3.2 μm. The temperature difference throughout the entire width of the hot-pressed plate should not exceed 2-3 ° C. If the temperature difference is too large, the moisture content will be uneven, the product will easily warp, and the strength and appearance of the product will be affected.
Generally, the heating holes for the hot medium in the hot-pressing plate are not valued by the furniture industry. The correct circuit planning should be based on the principles of facilitating the movement of the heat medium, facilitating the uniform heating of the board surface, and reducing the leakage of the heat medium. About the hot press of small presses used in the furniture industry, single-channel dual-circuit planning is generally used, which can satisfy the application requirements.
(3) coupling and sealing
The selection of a suitable connection method and sealing element has important implications for heat transfer at high temperatures and pressures.
At that time, the connecting pipe of the hot-pressing plate was basically a hose, which made the connection very convenient. The domestic metal hose was the main one, and some manufacturers chose imported Teflon hoses. Both methods can be used. With a blue connection, Teflon hose has better fatigue resistance than a metal hose. The gasket material traditionally uses asbestos rubber sheet. New materials such as polytetrafluoroethylene, coiled graphite, etc., have a more ambitious sealing effect.