Proportional control of injection speed has been widely adopted by injection molding machine manufacturers. Although computer controlled injection speed segmentation control system already exists, but due to limited information, the advantages of this machine setting are rarely brought into play. The advantages of multi-stage speed injection molding, and a general introduction to its use in eliminating defects such as short shots, trapped air, and shrinkage.
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 filling speed segment, and achieving a smooth transition from one set point to another, a stable melting can be ensured. Body surface velocity to create the desired molecular and minimum internal stress.
We recommend the following speed segmentation principle:
1) The velocity of the fluid surface should be constant.
2) Fast injection should be used to prevent the melt from freezing during the injection process.
3) The setting of the injection speed should take into account the rapid filling in the critical area (such as the flow channel) while slowing down the water inlet.
4) The injection speed should ensure 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 factors. The speed setting must have a clear understanding of the injection molding process and material knowledge. Otherwise, the quality of the product will be difficult to control. The body flow rate is difficult to measure directly, and can be indirectly calculated 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, 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 for thin walls; the slow-fast-slow speed curve is required for thick-walled parts to avoid defects; 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 direction 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, it should be guaranteed The volume of the body is increased evenly; the long runner must be filled quickly to reduce the cooling of the melt front, but the injection of highly viscous materials, such as PC, is an exception, because too fast a rate will bring the cold through the inlet into the cavity .
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 runner to the water inlet, the surface of the melt front may have cooled or solidified, or the flow path suddenly Narrowing causes the melt to stagnate until sufficient pressure is established to push the melt through the water inlet, which causes the pressure through the water inlet to peak.
High pressure will damage the material and cause surface defects such as flow marks and charring of the water inlet. This situation can be overcome by decelerating just before the water inlet. This deceleration prevents excessive shearing of the water inlet, and then Increase the rate of fire to the original value. Because it is very difficult to accurately control the rate of fire at the water inlet, deceleration at the end of the runner is a better solution.
We can avoid or reduce defects such as flash, burnt, and trapped air by controlling the final shot speed. Filling the final stage of deceleration can prevent overfilling of the cavity, avoiding flash and reducing residual stress. Due to the end of the die flow path The trapped air caused by poor exhaust or filling problems can also be solved by reducing the exhaust speed, especially the exhaust velocity of the end of the spray.
The short shot is caused by the slow speed at the water inlet or the local flow caused by the solidification of the melt. This problem can be solved by speeding up the injection speed just after the water inlet or local flow obstruction. Flow marks, water inlet burning Coke, molecular rupture, delamination, spalling, etc., which occur on heat sensitive materials, are caused by excessive shearing through the water inlet.
Smooth parts depend on the speed of injection, glass fiber filling materials are particularly sensitive, especially nylon. Dark spots (wave lines) are caused by flow instability caused by viscosity changes. Distorted flow can cause wave patterns or uneven fog. 1. What kind of defects are generated depends on the degree of instability of the flow.
High-speed injection will 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 glue speed setting must ensure that the runner area is quickly filled and then slowly passes 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 too late, too large The flow inertia will lead to the appearance of the ray. The lower the melt viscosity, the higher the temperature of the barrel, the more obvious the trend of this ray. Because 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 smaller pressure drop is improved. Low mold temperature and screw advance speed are too slow to greatly shorten the flow length, which must be compensated by high rate of fire. High-speed flow reduces heat loss, and due to high shear Cutting heat to generate friction heat will cause the melt temperature to rise, slowing the thickening speed of the outer layer of the part. The cavity intersection must have sufficient thickness to avoid too much pressure drop, otherwise it will shrink.
In short, most injection 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.