The position of the upper and lower cavities of the hot-cured plastic injection mold has a great influence on the temperature distribution of each cavity or different parts of the same cavity, due to the natural convection of hot air moving upward from the lower part. The measured results show that the heat absorbed by the upper part can be twice as much as the heat absorbed by the lower part. Therefore, in order to improve this situation, when arranging multiple cavities, the distance between the upper and lower cavities should be shortened as much as possible. Thermoset injection molding parts need to be made of materials with good wear resistance due to the impact of the plastic filling, and the surface roughness should be low. The molded part is best plated to prevent corrosion.
Due to the low melting temperature of hot-curing plastics compared to the solidification temperature, under certain molding conditions, the material has good fluidity and can flow into small gaps to form flash. Therefore, the mold accuracy should be improved during manufacturing to avoid using ejector plates and minimize the use of insert parts. Because the injection pressure of hot-curing plastics is high, when the mold is subjected to unbalanced force, there will be more overflow and flash between the mold parting surfaces. Therefore, when arranging the cavity position, the center of the projected area on the parting surface should be coincident with the center of the clamping force of the injection molding machine.
In addition, there is also holding pressure time and mold opening and taking out time. The injection time for thermoset injection molding materials is 2-10s, the holding pressure time needs to be 5-20s, and the curing time is selected within 15-100s. The molding cycle requires a total of 45-120s. The curing time of hot-curing plastics should not only consider the structure, complexity and thickness of the plastic part, but also pay attention to the quality of the material. When determining the curing time based on the maximum wall thickness of the plastic part, this issue should be given more attention. Generally, the curing time after filling of the injection molded material can be calculated according to the hardening speed of 8-12s/mm based on the maximum wall thickness of the plastic part.
During the solidification reaction of hot-curing materials, contraction water and low molecular weight gases will be produced. The cavity must have a good exhaust structure, otherwise bubbles and flow marks will be left on the surface of the plastic part. For thick-walled plastic parts, measures such as depressurization and gas release should be taken during injection molding. The structure of the hot-curing plastic injection mold is similar to that of the thermoplastic injection mold, including the gating system, cavity, core, guiding mechanism, ejecting mechanism, and side core pulling mechanism. The installation method on the molding machine is also the same.
Because the thermoset injection molding material is not heated to a sufficient temperature inside the barrel, the cross-sectional area of the main runner should be smaller to increase the friction, generally with a cone angle of 2°-4°. In order to increase the surface area of the sub-runner for better heat transfer, sub-runners with circular or trapezoidal cross-sections are generally used. The depth of the sub-runner can be appropriately reduced under the same cross-sectional area. The selection principle for gate type and position is similar to that of the thermoplastic injection mold. The size of the point gate should not be too small, generally not less than 1.2mm, and the depth of the side gate should be selected within 0.8-3mm to prevent the melt temperature from increasing too much, which accelerates the chemical cross-linking reaction and increases the viscosity, making filling difficult.