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Compression molding temperature is the temperature specified in the mold during compression molding and has a decisive influence on the rate of molten flow, mold filling and cross-linking reaction of the molded plastic in the mold cavity. In a sense, the choice of temperature is more important than for thermoplastics because thermoset plastics require a certain temperature range for curing reactions. Compression molding temperature should be determined by considering various factors, such as the type of material, preheating, structure of the product, curing reaction characteristics and production efficiency. The control of temperature is the key to the compression molding process, which mainly includes the determination and control of mold temperature, heating rate, molding temperature and holding time.
In the compression molding process, temperature, pressure and duration under pressure are important process parameters, and they have their own functions and mutual constraints. High temperature is conducive to shortening the molding cycle and improving the physical and mechanical properties of the product. However, if the temperature is too high, the fluidity of the melt will be reduced to fill the mold cavity, or the surface layer will solidify prematurely and affect the discharge of moisture and volatiles. Decrease the surface quality of the product, and undesirable phenomena such as product expansion and cracking may occur during mold opening. On the contrary, if the molding temperature is too low, the curing time is prolonged, and the cross-linking reaction is not perfect, which will also affect the quality of the product. It will also cause problems such as gray surface of the product, mold sticking and decreased mechanical properties.
Refers to the temperature of the compression mold when the material is put into the mold cavity. When it is difficult to install a mold for a complex or large molded product, the temperature of the mold should be lower, and the mold can be controlled at room temperature. Increasing the mold assembly temperature can shorten the production cycle and improve production efficiency. The temperature of mold loading depends on the type and quality index of the molded plastic, and is also related to the structure and production efficiency of the compression molded products. For example, the molding temperature of the magnesium phenolic premixed molding compound is the molding curing temperature, not at room temperature.
It also depends on the material variety. For example, the heating rate of 616# phenolic premixed molding material is 1-20℃/min. The speed of the temperature rise is closely related to the speed of the resin curing reaction. If the temperature rises too fast, it is easy to cause uneven curing of the resin and affect product quality. However, if the heating rate is too slow, the production efficiency is low. Usually the heating rate can be controlled at (1-2) degrees Celsius/minute, and generally at (10~30) degrees Celsius/hour for slow molding.
The resin will release or absorb a certain amount of heat during the curing process. The degree of resin polycondensation reaction can be judged according to the heat release to determine the curing temperature. Under normal circumstances, first determine a relatively large temperature range, and then select a reasonable curing temperature through testing. However, the compression molding temperature is too high, which will make the cross-linking rate of plastic too fast leading to a rapid increase in its melt viscosity and a decrease in fluidity, resulting in incomplete filling of the mold; on the other hand, because On the other hand, because plastic is a bad conductor of heat, too high mold temperature makes the outer layer of plastic harden quickly due to the high temperature of the mold first, and when the inner layer starts to harden, it is difficult to discharge the low molecular volatiles, resulting in a great internal stress in the product, making the surface of the product overheat or produce bulging phenomenon. If the temperature is too low, not only the material flowability, and slow curing speed, product insulation time is not enough, cross-linking reaction is difficult to fully carry out, will cause low strength products, poor surface gloss and other defects. After the preheating treatment of the material, because the temperature of the inner and outer layer of the material is more uniform, so the molding temperature can be higher than without preheating.
Molding heat preservation time refers to the time of heat preservation under the molding pressure and molding temperature. The main function of this stage is to completely cure the product and eliminate internal stress. The length of the molding holding time depends on the type of material, the level of molding temperature and the structural size and performance of the product. When pressing different parts with the same molded plastic, the holding time is not similar. In order to shorten the time of the product on the mold and hydraulic press, and to improve the production efficiency and equipment utilization, the post-treatment process can also be used.
After the compression molded products have been insulated, they are generally cooled down gradually under a certain pressure. There are two cooling methods: natural cooling and forced cooling. Although natural cooling can avoid the internal stress formed by uneven cooling and ensure the quality of products, but due to the low production efficiency, so in addition to special requirements are generally used to force cooling method. Forced cooling can be in the heating plate through the cold water, or fan cooling mold and products. In the rapid pressing process, the cooling operation can also be dispensed with, and the products can be demolded and removed at the molding temperature after the insulation is completed. In slow molding, after the heat preservation, the temperature must be gradually reduced under a certain pressure, and the mold can be demolded when the mold temperature drops below 60°C.
MDC Mould is a leading OEM compression mold manufacturer in china, knows many complex compression products involve a variety of tools, processes and materials. Our products include car bottom guard, truck front mask and tractor roof and so on.
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