How to adjust injection molding speed with sectional injection speed?
2018-06-12 07:12:46
The relationship between ejection speed and product quality makes it a key parameter in injection molding. By determining the beginning, middle, end of the filling speed segment, and the smooth transition from a setting point to the other, the stable melt surface speed can be guaranteed to make the desired molecules to be asked about the minimum internal stress. It is suggested to use the following principle of velocity segmentation: the velocity of the surface of the fluid should be constant, and the rapid gel should be used to prevent the melt freezing during the injection process. The setting of the ejection speed should take into account the speed of slowing down at the entry point while the critical area (such as the flow channel) is filled quickly, and the ejection speed should be stopped immediately after the filling of the mold cavity. To prevent filling, flare and residual stresses. The geometry of the die, other flow restrictions and instability factors must be taken into account in setting the speed subsection. Speed setting must have a clear understanding of injection molding process and material knowledge, otherwise the quality of products will be difficult to control. Because the flow rate of the melt is difficult to measure directly, it can be indirectly calculated by measuring the screw advance speed or the cavity pressure. Material properties are very important because the polymer may be degraded due to different stresses, increasing molding temperature may lead to severe oxidation and chemical structure degradation, but the degradation caused by shear decreases as high temperature reduces the viscosity of the material and reduces shear stress. No doubt, the multi-stage injection speed is very helpful for forming thermal sensitive materials such as PC, POM, UPVC and their blending. The geometric shape of the die is also a decisive factor: the maximum injection speed is required in the thin wall; the thick wall parts need slow fast slow speed curve to avoid the defects; in order to ensure the quality of the parts to meet the standard, the injection speed setting should ensure the constant flow velocity of the melt The velocity of melt flow is very important because it affects the orientation and surface state of the molecules in the part; it should slow down when the melt reaches the cross region structure in front of the melt; for the complex mold of radiation like diffusion, the melt pass should be guaranteed to increase in equilibrium; the long flow path must be quickly filled to reduce the melt forward. Cooling, but injection of high viscosity materials, such as PC, is an exception, because too fast speed will bring cold material into the cavity through the inlet. Adjusting the injection speed can help eliminate the defects caused by the slow flow at the inlet. When the melt reaches the inlet through the nozzle and channel, the surface of the melt forward may be cooled and solidified, or the melt is stagnant due to the sudden narrowing of the flow channel, until sufficient pressure is established to push the melt through the inlet, which makes the pressure out of the inlet at its peak. High pressure will damage the material and cause surface defects such as the flow mark and the burning of the inlet, which can be overcome by slowing down just before the inlet. This deceleration can prevent excessive shear at the inlet and then increase the firing rate to the original value. Because it is very difficult to precisely control the speed of the injection at the intake port, it is a good solution to slow down the flow path at the end. We can avoid or reduce defects such as flash, scorch and trapped gas by controlling the speed of the end ejection. The final deceleration can prevent excessive cavity filling, avoid flash and reduce residual stress. It can also be solved by reducing the exhaust velocity, especially the exhaust velocity at the end of the ejection section, due to the poor gas exhaust or filling problem at the end of the die. Short shot is caused by slow speed at inlet or blocked local flow caused by melt solidification. This problem can be solved by speeding up the injection speed just after the entry or partial flow obstruction. Defects, such as flow marks, burning at the inlet, breakage of molecules, delamination and spalling, occur on thermosensitive materials due to excessive shearing at the inlet. Smooth parts depend on the injection speed, especially glass fiber filler materials, especially nylon. Dark spots (wavy stripe) are caused by flow instability caused by viscosity changes. Distorted flow can lead to ripple or uneven fog. What kind of defect is produced depends on the degree of flow instability. High speed injection can lead to high shear when the melt passes through the inlet. The thermal sensitive plastic will be burnt, and the scorched material will pass through the cavity to reach the flow forward and appear on the surface of the part. In order to prevent stripe, the injection speed setting must ensure fast filling area and slowly pass through the inlet. Finding this speed transition point is the essence of the problem. If too early, the filling time will increase too much. If too late, excessive flow inertia will lead to the appearance of stripe. The lower the melt viscosity and the higher the barrel temperature, the more obvious the trend is. Because of the need for high speed and high pressure injection at small inlet, it is also an important factor leading to flow defects. Shrinkage can be improved by more effective pressure transfer and smaller pressure drop. Low mode and slow screw speed greatly shorten the flow length, and must be compensated by high firing speed. High speed flow will reduce heat loss, and the friction heat caused by high shear heat will increase the melt temperature and slow down the thickening speed of the outer part of the parts. The cross section of the cavity must be sufficiently thick to avoid too much pressure drop, otherwise shrinkage will occur. In summary, most injection defects can be solved by adjusting the injection speed, so the technique of adjusting the injection process is a reasonable setting of the ejection speed and its segmentation.
