Injection molding is a high-temperature and high-pressure process, which has a complex impact mechanism on the structural stability and service life of standard sprue bushings, as follows:
During injection molding, standard sprue bushings will experience rapid temperature changes. When the high-temperature plastic melt is injected, the inner wall of the bushing heats up rapidly, while the outer wall, due to contact with the mold body, rises relatively slowly. This temperature gradient will produce thermal stress. Thermal stress may cause the standard sprue bushing to deform. Over a long period of time, it will cause the dimensional accuracy of the bushing to decrease, affecting its fit with other parts of the mold, and thus affecting the quality of injection molding. For example, bushing deformation may cause the gate position to shift, so that the plastic melt cannot evenly fill the mold cavity.
High temperature environment will change the material properties of standard sprue bushings. Generally speaking, as the temperature increases, the strength and hardness of the material will decrease. For commonly used mold steel materials, their yield strength decreases at high temperatures, making plastic deformation more likely to occur. At the same time, the fatigue performance of the material will also be affected. Under repeated high temperature, the microstructure inside the material will change, such as grain growth and tissue coarsening, which will reduce the material's fatigue resistance and make the standard sprue bushing more prone to cracks and damage under cyclic loads, thereby shortening its service life.
The high pressure during the injection molding process will cause the plastic melt to flow through the standard sprue bushing at a high speed, which will have a strong scouring effect on the inner wall of the bushing. Especially when the plastic melt contains reinforcing fillers such as glass fiber, the wear will be more serious. The high temperature environment will reduce the hardness of the material surface, further aggravating the wear. Wear will cause the inner diameter of the standard sprue bushing to increase, change the size and shape of the gate, affect the flow characteristics of the plastic melt, and reduce the precision of injection molding. Moreover, the debris produced by wear may be mixed into the plastic melt, affecting the quality of the plastic product.
In some injection molding processes, the plastic melt may decompose to produce some corrosive gases. For example, when processing chlorine- or fluorine-containing plastics, acidic gases such as hydrogen chloride or hydrogen fluoride will be produced. Under high temperature and high pressure environment, these corrosive gases are more likely to react chemically with the materials of standard sprue bushing, causing corrosion. Corrosion will weaken the structural strength of standard sprue bushing, form surface defects, and accelerate its damage. In addition, the mold may be exposed to media such as coolant during use. If the seal is not good, the coolant will enter the surrounding of standard sprue bushing, which may also cause corrosion problems.
During the installation and use of standard sprue bushing, due to the structural characteristics and stress conditions, there may be areas of stress concentration, such as the shoulder and root of the bushing. Under the cyclic action of high temperature and high pressure, these stress concentration areas are more likely to produce micro cracks. With the increase of the number of injection molding, the cracks will gradually expand. When the cracks expand to a certain extent, the standard sprue bushing will break and fail, seriously affecting its service life.
The injection molding process is a cyclic process, and the standard sprue bushing continuously undergoes heating and cooling cycles. This thermal cycle will cause thermal fatigue stress inside the material. Under the repeated action of thermal fatigue stress, fatigue cracks will appear on the surface of the material, and then develop into damage forms such as peeling and falling off. Thermal fatigue damage will gradually weaken the structural integrity of the standard sprue bushing, reduce its service life, and may affect the stability of injection molding and product quality.
High temperature and high pressure environment will also affect the sealing performance of the standard sprue bushing. The seal between the bushing and the mold body is usually achieved by sealing rings or interference fits. At high temperatures, the material of the sealing ring may age, harden, and lose elasticity, resulting in a decrease in sealing performance. The interference fit part may also change the fit clearance due to thermal expansion, affecting the sealing effect. Once the seal fails, the plastic melt may leak into the mold, which will not only damage the mold, but also affect the normal injection molding, and will also have an adverse effect on the service life of the standard sprue bushing.
