Mold temperature has a great influence on the molding quality and molding efficiency of plastic parts. In a mold with a higher temperature, the fluidity of the molten rubber is better, which is beneficial for the rubber to cling to the cavity and obtain a high-quality surface of the plastic part, but it will make the curing time of the rubber longer and easily deformed when ejected, for crystalline rubber, it is more conducive to the crystallization process, avoiding changes in the size of the plastic parts during storage and use; In a mold with a lower temperature, it is difficult for the molten rubber to adhere to the cavity, resulting in increased internal stress, dull surface, and defects such as silver streaks and weld marks.
Different rubber materials have different processing techniques, and the surface requirements and structures of various rubber parts are different. In order to produce rubber parts that meet the quality requirements in the most effective time, this requires the mold to maintain a certain temperature. The more stable the temperature, the more consistent the requirements of the produced plastic parts in terms of size, shape and appearance quality of the plastic parts. Therefore, in addition to mold manufacturing factors, mold temperature is an important factor to control the quality of plastic parts, and mold design should fully consider the mold temperature control method.
In order to ensure the production of plastic parts with high appearance quality requirements, stable dimensions and small deformation in the most effective time, the basic principles of mold temperature control should be clearly understood during design.
(1) Different rubber materials require different mold temperatures.
(2) Molds with different surface quality and different structures require different mold temperatures, which requires targeted design of the temperature control system.
(3) The temperature of the front mold is higher than that of the rear mold, and the temperature difference is generally about 20~30°C.
(4) The temperature of the front mold required by the fire pattern is higher than that required by the general smooth surface. When the front mold must pass through hot water or hot oil, the general temperature difference is about 40°C.
(5) When the actual mold temperature cannot reach the required mold temperature, the mold should be heated up. Therefore, when designing the mold, it should be fully considered whether the heat brought by the rubber into the mold can meet the mold temperature requirements.
(6) The heat brought into the mold by the rubber is consumed by heat radiation and heat conduction, and most of the heat needs to be taken out of the mold by the circulating heat transfer medium. The heat in easy-to-heat transfer parts such as copper plating is no exception.
(7) The mold temperature should be balanced, and there should be no local overheating or overcooling.
The mold temperature is generally controlled by adjusting the temperature of the heat transfer medium, adding heat shields and heating rods. The heat transfer medium generally uses water, oil, etc., and its channels are often called cooling water channels.
To reduce the mold temperature, it is generally achieved by passing "machine water" (about 20°C) through the front mold and "frozen water" (about 4°C) through the rear mold. When the channel of the heat transfer medium, that is, the cooling water channel, cannot pass through some parts, materials with high heat transfer efficiency (such as copper plating, etc.) should be used to transfer heat to the heat transfer medium, or "heat pipes" can be used for local cooling.
Raising the mold temperature is generally achieved by passing hot water or hot oil into the cooling water channel (heating by a hot water machine). When the mold temperature is required to be high, in order to prevent heat loss due to heat conduction, a heat shield should be added to the mold panel.
(1) The distance from the hole wall of the cooling water channel to the surface of the cavity should be as equal as possible, generally 15~25mm.
(2) The number of cooling water channels should be as many as possible, and it should be easy to process. The distance between two parallel water channels should be 40~60mm.
(3) All molded parts are required to pass through the cooling water channel, unless there is no place, and the part where the heat gathers is strengthened for cooling.
(4) Reduce the temperature difference between the water inlet and the water outlet. The temperature difference between entering water and exiting water will affect the uniformity of mold cooling, so the direction of entering water and exiting water should be marked when designing, and it is required to mark it on the mold blank when making the mold. The water transportation process should not be too long to prevent the temperature difference between the water entering and exiting from being too large.
(5) Minimize the existence of "dead water" (medium that does not participate in flow) in the cooling water channel.
(6) Cooling water channels should be avoided at the foreseeable welding marks of plastic parts.
(7) To ensure the minimum side distance of the cooling water channel (that is, the minimum steel level thickness around the water hole), it is required that when the length of the water channel is less than 150mm, the side spacing is greater than 3mm; when the water channel length is greater than 150mm, the side spacing is greater than 5mm.
(8) When the cooling water channel is connected, it should be sealed with "O" type glue, and the seal should be reliable without water leakage.
(9) Other cooling methods, such as copper plating, heat pipes, etc., should be adopted for the parts where the cooling water channel arrangement is difficult.
(10) Reasonably determine the position of the cooling water joint to avoid affecting the installation and fixation of the mold.