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The relationship between injection quality and injection speed

Update:01-12-2020
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The close relationship between injection speed and prod […]

The close relationship between injection speed and product quality makes it a key parameter of injection molding.
By determining the start, middle, and end of the filling speed segment, and realizing a smooth transition from one set point to another set point, a stable melt surface speed can be ensured to produce the desired molecule and the smallest internal stress.

The following principle of speed segmentation is recommended:
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 injection speed setting should take into account the rapid filling in the critical area (such as the runner) while slowing down the speed at the water inlet.
4. The injection speed should be guaranteed to stop immediately after the cavity is filled to prevent over-filling, flashing and residual stress.
The basis for setting the speed segmentation must take into account the geometry of the mold, other flow restrictions and unstable factors. 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. Because the melt flow rate is difficult to directly measure, it can be calculated indirectly by measuring the screw forward speed or cavity pressure (make sure that the check valve is not leaking).
Material characteristics are very important because polymers may degrade due to different stresses. Increasing the molding temperature may lead to severe oxidation and degradation of chemical structure, but at the same time the degradation caused by shear becomes smaller, because 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 blending ingredients. The geometry of the mold is also a decisive factor: the thin-walled part needs the maximum injection speed; the thick-walled parts need a slow-fast-slow speed curve to avoid defects; in order to ensure that the quality of the parts meets the standard, the injection speed setting should ensure the melt front flow rate constant.
The melt flow speed is very important, because it will affect the molecular arrangement direction and surface state of the part; when the front of the melt reaches the intersecting area structure, it should slow down; for complex molds with radial diffusion, the melt throughput should be guaranteed Increase evenly; long runners must be filled quickly to reduce the cooling of the melt front, 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 water inlet.
Adjusting the injection speed can help eliminate defects caused by the slowing of the flow at the water inlet. When the melt reaches the water inlet through the nozzle and the runner, the surface of the melt front may have cooled and solidified, or the melt will stagnate due to the sudden narrowing of the runner, until sufficient pressure is established to push the melt through the inlet. Nozzle, which will cause a peak in the pressure passing through the inlet.


High pressure will damage the material and cause surface defects such as flow marks and scorching of the water inlet. This situation can be overcome by decelerating just before the water inlet. This deceleration can prevent excessive shear at the water inlet, and then increase the rate of fire to the original value. Because it is very difficult to precisely control the rate of fire to slow down at the inlet, it is a better plan to slow down at the end of the runner.
We can avoid or reduce defects such as flashing, scorching, trapped air, etc. by controlling the injection speed at the end. The deceleration at the end of filling can prevent the cavity from being overfilled, avoid flashing and reduce residual stress. Air traps caused by poor exhaust or filling problems at the end of the mold flow path can also be solved by reducing the exhaust speed, especially the exhaust speed at the end of the injection.
The short shot is caused by the slow speed at the water inlet or the local flow obstruction caused by the solidification of the melt. This problem can be solved by increasing the injection speed just after passing the water inlet or local flow obstruction.
Flow marks, scorching of the water inlet, molecular rupture, delamination, peeling and other defects that occur on the heat-sensitive material are caused by excessive shear when passing through the water inlet.
Smooth parts depend on injection speed, glass fiber filling materials are particularly sensitive, especially nylon. Dark spots (wavy lines) are caused by unstable flow caused by viscosity changes. Distorted flow can cause wavy lines or uneven fog. What kind of defects are produced depends on the degree of instability of the flow.
When the melt passes through the water inlet, high-speed injection will cause high shear, and the heat-sensitive plastic will be burnt. This burnt material will pass through the cavity and reach the flow front, appearing on the surface of the part. In order to prevent injection patterns, the injection speed must be set to fill the runner area quickly and then pass through the water inlet slowly. Finding this speed conversion point is the essence of the problem. If it is too early, the filling time will increase excessively, and if it is too late, excessive flow inertia will cause the appearance of jetting. The lower the melt viscosity and the higher the barrel temperature, the more obvious the tendency of this kind of pattern to appear. Since the small water inlet requires high-speed and high-pressure injection, it is also an important factor leading to flow defects.
Shrinkage can be improved by more effective pressure transmission and smaller pressure drop. Low mold temperature and too slow screw advancement speed greatly shorten the flow length, which must be compensated by high shooting speed. High-speed flow will reduce heat loss, and due to frictional heat generated by high shear heat, it will increase the melt temperature and slow down the thickening speed of the outer layer of the part. The cavity intersection must have sufficient thickness to avoid too much pressure drop, otherwise shrinkage will occur.
In short, most injection molding defects can be solved by adjusting the injection speed, so the skill to adjust the injection process is to set the injection speed and its segmentation reasonably.

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