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Name：Rebecca Dong
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The ratio of injection speed to injection molding is important. |
The ratio control of injection speed has been widely adopted by injection molding machine manufacturers. Although the computer controlled injection speed subsection control system already exists, but due to the limited information, the advantage of this machine setting is seldom used. This paper introduces the advantages of multi-speed injection molding system and introduces its use in eliminating the defects of short shot, trapping gas and shrinkage.
The close relationship between injection speed and product quality makes it a key parameter in injection molding. By determining the filling speed of segmentation, the end of the beginning, middle, and implement a set point to the other a smooth transition of the set point, can guarantee the stability of the melt surface velocity to create the desired molecules get asked about the smallest internal stress. We recommend the following speed segmentation principle: 1) the velocity of the fluid surface should be constant. 2) it should be used to prevent the melt freezing in the injection process. 3) the injection speed setting should take into account the rapid filling of the critical area (such as the runner) and the deceleration rate at the inlet level. 4) the injection speed should ensure that the cavity is filled immediately to stop the filling, flanging and residual stress.
The basis for setting the velocity segmentation must take into account the geometry of the mold, other flow constraints and instability factors. The speed setting must have a clear understanding of the injection molding process and material knowledge, otherwise, the quality of the products will be difficult to control. Because the melt flow rate is difficult to measure directly, it can be calculated by measuring the advance speed of the screw, or indirectly by the cavity pressure (to determine the non-leakage of the check valve).
Material properties is very important, because different degradation of polymers may be due to stress, increase the mold temperature can lead to severe oxidation and chemical structure of degradation, but at the same time caused by shear degradation of smaller, because of high temperature reduces the viscosity of material, reduces the shear stress. There is no doubt that the speed of multi-segment injection is very helpful for forming hot sensitive materials such as PC, POM, UPVC, and their mixing ingredients.
The geometry of the mold is also the determining factor: the maximum injection speed is required in the thin wall; Thick wall parts need to be slow - fast - slow speed curve to avoid defects; In order to ensure the quality of the parts to meet the standard, the injection speed setting should ensure that the melt forward velocity is constant. Melt flow velocity is very important because it affects the direction and surface state of the molecules in the parts. When the melt front reaches the cross region structure, it should decelerate; For the complex mould of radial diffusion, the melt should be increased by a balanced amount. The long runner must be quickly filled to reduce the cooling of the melt forward, but the injection of high viscosity material, such as the PC, is an exception, because the speed of the cold will pass the cold material through the inlet into the cavity.
Adjusting the injection speed can help eliminate the defects caused by the slow flow at the inlet level. When the melt through the nozzle and port when you arrive at the shuikou, melt the surface of the striker may have cooling solidification, or caused by port suddenly Narrows the stagnation of the melt, until establish enough pressure to push melt through the water entry, this will make the water entry through the pressure peak shape. The high pressure will damage the material and cause surface defects such as flow marks and the burning of the nozzle, which can be overcome by slowing down just before the inlet. This kind of deceleration can prevent the excessive shear of the entry level, and then increase the speed to the original value. Because it is very difficult to accurately control the speed of fire at the inlet level, it is a better solution to decelerate at the end of the flow passage.
We can avoid or reduce defects such as flanging, burning, trapping and so on by controlling the speed of the end shot. The filling terminal deceleration can prevent the cavity from overfilling, avoid the flying edge and reduce the residual stress. The exhaust velocity can be reduced by reducing the exhaust velocity, especially at the end of the injection.
The short shoot is caused by the slow speed of the inlet and the obstruction of local flow caused by the solidification of the melt. This problem can be solved by speeding up the injection speed when the water inlet or local flow is blocked.
The defects that occur in the heat sensitive material are caused by the excessive shear of the inlet.
The smooth molding depends on the injection speed, and the fiberglass filler is especially sensitive, especially nylon. Dark spots (wavy lines) are caused by unstable flows due to changes in viscosity. The distortion of the flow can lead to wavy or uneven fog, and the defect depends on the degree of fluid instability.
When the melt passes through the inlet, the high speed injection will cause high shear, and the heat-sensitive plastic will be charred. The charred material will pass through the cavity and reach the moving forward, which will appear on the part surface.
In order to prevent the ejection lines, the injection speed setting must ensure fast filling of the runner area and then slowly pass through the inlet. It's the nature of the problem to figure out this velocity. If too early, the filling time will be excessive, if too late, too large flow inertia will lead to the appearance of the pattern. The lower the melt viscosity, the higher the temperature of the cylinder, the more obvious the trend of this pattern. Due to the high speed and high pressure injection, it is also an important factor that causes the flow defect.
Shrinkage can be transmitted through more effective pressure, and smaller pressure drops can be improved. The speed of the low - mode mild screw has greatly reduced the flow length and must be compensated by the high speed. High speed flow reduces the loss of heat, and due to high shear heat, it can increase the melt temperature and slow the thickness of the parts. The cavity must be thick enough to avoid too much pressure, or it will shrink.
In a word, most of the injection defects can be solved by adjusting the injection speed, so the technique of adjusting the injection molding process is a reasonable setting of the injection speed and its segmentation.
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Up：How to improve the gloss and color difference of plastic products in injection molding? |
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