Plastic processing injection molding process adjustment knowledge
Proportional control of injection speed has been widely adopted by injection molding machine manufacturers. Although computer-controlled injection speed segmentation control systems have long existed, the advantages of such machine settings have rarely been exploited due to limited data. In this paper, the description of the system is applied to the advantages of multi-stage speed injection molding, and its application in eliminating defects such as short shots, trapped air, and shrinkage is generally introduced. The close relationship between the speed of injection and the quality of the product makes it a key parameter for injection molding. By determining the beginning, middle, and end of the fill velocity segment and achieving a smooth transition from one set point to another, a stable melt surface velocity can be ensured to create the desired molecular challenge and minimal internal stress. We recommend the following speed segmentation principle: 1) The velocity of the fluid surface should be constant. 2) Fast shot rubber should be used to prevent the melt from freezing during the shot. 3) The setting of the injection speed should take into account the rapid filling of the critical area ( such as the flow path ) while slowing down the water inlet. 4) The injection speed should be such that the cavity is stopped immediately after filling to prevent overfilling, flashing and residual stress. The basis for setting the speed segment must take into account the geometry of the mold, other flow restrictions, and instability. The speed setting must have a clear understanding of the injection molding process and material knowledge. Otherwise, the product quality will be difficult to control. Since the melt flow rate is difficult to measure directly , it can be estimated by measuring the advance speed of the screw or the cavity pressure ( determining that the check valve is not leaking ) . Material properties are very important because the polymer may degrade due to different stresses. Increasing the molding temperature may result in severe oxidation and degradation of the chemical structure, but at the same time the degradation caused by shearing becomes smaller because the high temperature reduces the viscosity of the material. Reduced shear stress. Undoubtedly, the multi-stage injection speed is very helpful for forming heat sensitive materials such as PC , POM , UPVC and their ingredients. The geometry of the mold is also a decisive factor: the maximum injection speed is required at the thin wall ; the slow - fast - slow speed curve is required for thick-walled parts to avoid defects ; in order to ensure that the part quality meets the standard, the injection speed setting should ensure the melt front flow rate. constant. The melt flow rate is very important because it affects the orientation of the molecules in the part and the surface state ; when the front of the melt reaches the cross-sectional structure, it should be decelerated ; for complex molds with radial diffusion, the melt throughput should be guaranteed. balanced increase; long flow channel must be cooled to reduce the melt filled fast forward, but the injection of high viscosity material, such as PC are the exception, because the speed is too fast will feed into the cavity through the cold water inlet. Adjusting the injection speed can help eliminate defects caused by slowing of the flow at the water inlet. When the melt passes through the nozzle and the flow path to the water inlet, the surface of the melt front may have cooled or solidified, or the melt is stagnant due to the sudden narrowing of the flow path until sufficient pressure is established to push the melt through. The nozzle, which causes the peak pressure to appear through the water inlet. High pressure will damage the material and cause surface defects such as flow marks and charring of the water inlet , which can be overcome by decelerating just before the water inlet. This deceleration prevents excessive shearing of the water inlet and then increases the rate of fire to its original value. Because it is very difficult to accurately control the rate of fire at the water inlet, deceleration at the end of the flow path is a better solution. We can avoid or reduce defects such as flash, burnt, and trapped air by controlling the end shot speed. The final stage deceleration of the fill prevents overfilling of the cavity, avoiding flash and reducing residual stress. Due to the exhaustion caused by poor exhaust or filling problems at the end of the die flow path, it can also be solved by reducing the exhaust velocity, especially the exhaust velocity at the end of the injection. Short shots are caused by slow speed at the water inlet or local flow obstruction caused by melt solidification. This problem can be solved by speeding up the shot speed just after the water inlet or local flow obstruction. Flow marks, burn into the water inlet, molecular cracking, delamination, flaking, and the like which occur on the heat sensitive material are caused by excessive shearing through the water inlet. Smooth parts depend on the speed of injection and glass fiber filling materials are particularly sensitive, especially nylon. Dark spots ( wavy lines ) are caused by flow instability caused by viscosity changes. Twisted flow can cause wavy lines or uneven haze, and what kind of defects occur depends on the degree of flow instability. High-speed injection can cause high shear when the melt passes through the water inlet, and the heat-sensitive plastic will burn out. This charred material will pass through the cavity and reach the flow front, appearing on the surface of the part. In order to prevent the shot, the shot speed setting must ensure that the runner area is quickly filled and then slowly passed through the water inlet. Finding this speed transition point is the essence of the problem. If it is too early, the filling time will increase excessively. If it is too late, excessive flow inertia will lead to the appearance of ray. The lower the melt viscosity, the higher the barrel temperature and the more pronounced the tendency of such a ray. Since the small inlet requires high-speed high-pressure injection, it is also an important factor leading to flow defects. Shrinkage can be achieved by more efficient pressure transfer and less pressure drop. Low mold temperature and screw advance speed are too slow to greatly shorten the flow length and must be compensated by high rate of fire. High-speed flow reduces heat loss, and frictional heat due to high shear heat causes an increase in melt temperature and slows the thickening of the outer layer of the part. The cavity intersection must be thick enough to avoid too much pressure drop or shrinkage will occur. In short, most injection molding defects can be solved by adjusting the injection speed, so the skill of adjusting the injection molding process is to set the injection speed and its segmentation reasonably. 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