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The relationship between injection speed segmentation and injection molded product quality

The close relationship between injection speed and product quality makes it a key parameter for injection molding.

By determining the beginning, middle, and end of the filling speed segments, and achieving a smooth transition from one set point to another, a stable melt surface velocity can be ensured to create the desired molecular extraction and minimum internal stress.

It is recommended to adopt the following speed segmentation principle:

The velocity of the fluid surface should be constant.
Rapid injection should be used to prevent melt freezing during injection.
The injection speed setting should take into account rapid filling of critical areas (such as flow channels) while slowing down the speed at the water inlet.
The injection speed should ensure that the mold cavity is filled and stopped immediately to prevent overfilling, flash and residual stress.https://solidcomould.com/product-category/injection-mould/storage-product-mould/crate-mould/

The basis for setting velocity segments must take into account the mold geometry, other flow restrictions and instability 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 measure directly, it can be calculated indirectly by measuring the forward speed of the screw or the cavity pressure (to make sure there is no leakage in the check valve).
Material properties are very important because polymers may degrade due to differences in stress. Increasing the molding temperature may lead to severe oxidation and degradation of the chemical structure, but at the same time degradation due to shear becomes smaller because high temperatures reduce the viscosity of the material. Shear stress is reduced. Undoubtedly, multi-stage injection speed is very helpful for molding heat-sensitive materials such as PC, POM, UPVC and their ingredients. The geometry of the mold is also a determining factor: thin-walled parts require the maximum injection speed; thick-walled parts require a slow-fast-slow speed profile to avoid defects; in order to ensure that part quality meets standards, the injection speed setting should ensure the melt front flow rate constant.https://www.sciencedirect.com/science/article/pii/S2238785423020549

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 intersection area structure, it should slow down; for complex molds with radial diffusion, the melt throughput should be guaranteed Increase evenly; long flow channels must be filled quickly to reduce the cooling of the melt front, but injecting high-viscosity materials, such as PC, is an exception because too fast a speed will bring cold material into the cavity through the water inlet. Adjusting the injection speed can help eliminate defects caused by flow slowdowns at the water inlet. When the melt reaches the water inlet through the nozzle and flow channel, the surface of the melt front may have cooled and solidified, or the melt may be stagnant due to the sudden narrowing of the flow channel until sufficient pressure is established to push the melt through the inlet. water inlet, which will cause a peak shape in the pressure through the water inlet.

High pressure will damage the material and cause surface defects such as flow marks and water inlet scorch, which can be overcome by slowing down just before the water inlet. This deceleration prevents excessive shearing at the water inlet level and then increases the rate of fire back to its original value. Because it is very difficult to accurately control the firing rate to slow down at the water inlet, slowing down at the end of the flow channel is a better solution. We can avoid or reduce defects such as flash, scorching, and trapped air by controlling the final injection speed. Deceleration at the end of filling can prevent overfilling of the cavity, avoid flash and reduce residual stress. Trapped air caused by poor exhaust at the end of the mold flow path or filling problems can also be solved by reducing the exhaust speed, especially at the end of the injection period.
Short shots are caused by slow speed at the water inlet or local flow obstruction caused by solidification of the melt. Increasing the injection speed just after passing through the water inlet or local flow obstruction can solve this problem. Defects such as flow marks, water inlet scorch, molecular cracking, delamination, and spalling that occur in heat-sensitive materials are caused by excessive shear when passing through the water inlet.
Smooth parts depend on injection speed, and fiberglass-filled materials are particularly sensitive, especially nylon. Dark spots (wavy patterns) are caused by flow instability caused by viscosity changes. Twisted flow can lead to ripples or uneven mist, depending on the degree of flow instability. High-speed injection will cause high shear when the melt passes through the water inlet, and the heat-sensitive plastic will be charred. This charred material will pass through the mold cavity, reach the flow front, and appear on the surface of the part. In order to prevent jet marks, the injection speed must be set to quickly fill the flow channel area and then slowly pass 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, and if it is too late, excessive flow inertia will lead to the appearance of jet streaks. The lower the melt viscosity and the higher the barrel temperature, the more obvious the tendency of such jet lines will appear. Since the small water inlet requires high-speed and high-pressure injection, it is also an important factor causing flow defects. Shrinkage can be improved with more efficient pressure transfer and smaller pressure drops. Low mold temperature and too slow screw advancement greatly shorten the flow length and must be compensated by high shot rates. High-speed flow will reduce heat loss, and the friction heat generated by high shear heat will increase the melt temperature and slow down the thickening rate of the outer layer of the part. The cavity intersection must be thick enough to avoid too much pressure drop, otherwise shrinkage will occur. In short, most injection molding defects can be solved by adjusting the injection molding speed, so the trick to adjusting the injection molding process is to reasonably set the injection speed and its segmentation.

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