Knowledge of injection molding process adjustment

2022-08-11
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Injection molding process adjustment knowledge of plastic processing

release date: Source: find plastic editor: number of views of package: 1900 copyright and disclaimer

core tip: proportional control of injection molding speed has been widely used by injection molding machine manufacturers. Although the computer-controlled injection molding speed segmented control system has long existed, due to the limited relevant information, the advantages of this machine setting have rarely been brought into play

[China Packaging News] the proportion control of injection molding speed has been widely used by injection molding machine manufacturers. Although the computer-controlled injection molding speed segmented control system has long existed, due to the limited relevant information, the advantages of this machine setting have rarely been brought into play. This paper will systematically explain the advantages of applying multi-stage speed injection molding, and briefly introduce its application in eliminating product defects such as short shot, trapped gas, shrinkage and so on

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

we suggest the following velocity segmentation principle:

1) the velocity of the fluid surface should be constant

2) rapid glue injection should be used to prevent the melt from freezing during the glue injection process

3) the setting of glue injection speed should take into account the rapid filling of critical areas (such as flow channels) while slowing down at the water inlet

4) the glue injection speed should ensure that it stops immediately after the mold cavity is filled to prevent overfilling, flash 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 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 (it is confirmed that the check valve has no leakage)

material properties are very important, because the polymer 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 by shearing 5. Limit protection: it has two-level limit protection functions of mechanical and program control; The degradation is reduced because high temperature reduces the viscosity of the material and reduces the shear stress. Undoubtedly, the multi-stage glue 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 maximum injection speed is required at the thin wall; Thick wall parts need slow fast slow type speed, try to calculate 1 degree curve to avoid defects; In order to ensure that the quality of parts meets the standard, the injection speed should be set to ensure that the flow rate of melt front remains unchanged. Melt flow rate is very important, because it will affect the direction of molecular arrangement and surface state in parts; When the front of the melt reaches the cross region structure, it should slow down; For complex dies with radial diffusion, the melt throughput should be increased in a balanced manner; The long runner must be filled quickly to reduce the cooling of the melt front, but the injection of high viscosity materials, such as PC, is an exception, because too fast will bring the cold material into the cavity through the water inlet

adjusting the injection speed can help eliminate the defects caused by the slow flow at the water inlet. When the melt reaches the water inlet through the nozzle and channel, the surface of the melt front may have cooled and solidified, or the melt may stagnate due to the sudden narrowing of the channel, until sufficient pressure is established to push the melt through the water inlet, which will make the pressure through the water inlet peak

high pressure will damage the material and cause surface defects such as flow marks and scorched water inlet, which can be overcome by slowing down just in front of the water inlet. This deceleration can prevent excessive shear at the water inlet, and then increase the fire speed to the original value. Because it is very difficult to accurately control the rate of fire to slow down at the water inlet, it is a better scheme to slow down at the end of the channel

we can avoid or reduce defects such as flash, scorch, gas trap and so on by controlling the final injection speed. Deceleration at the end of filling can prevent over filling of the cavity, avoid flash and reduce residual stress. The gas trap caused by poor exhaust or filling at the end of the mold flow path will reach 2trillion yuan in the whole industry by 2015, which can also be solved by reducing the exhaust speed, especially at the end of glue injection

short shot is caused by the slow speed at the water inlet or the local flow obstruction caused by melt solidification. This problem can be solved by accelerating the glue injection speed when it just passes through the water inlet or local flow obstruction. Flow marks, water inlet scorching, molecular cracking, delamination, spalling and other defects on heat sensitive materials are caused by excessive shear when passing through the water inlet

smooth parts depend on the injection speed, and glass fiber filling materials are particularly sensitive, especially nylon. Dark spots (wavy lines) are caused by flow instability caused by viscosity changes of scientific testing instruments. Distorted flow can lead to wavy lines or uneven fog. What kind of defects are produced depends on the degree of flow instability

when the melt passes through the water inlet, high-speed injection will lead to high shear, and the heat sensitive plastic will be scorched. This scorched material will pass through the cavity, reach the flow front, and appear on the surface of the part

in order to prevent injection lines, the glue injection speed must be set to ensure that the runner area is filled quickly and then passes through the water inlet slowly. Finding this speed transition point is the essence of the problem. If it is too early, the filling time will increase excessively. If it is too late, too much flow inertia will lead to the emergence of projectiles. The lower the melt viscosity is, the higher the barrel temperature is, the more obvious the trend of this kind of projection is. Because the small water inlet needs 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 advance speed greatly shorten the flow length, which must be compensated by high injection speed. High speed flow will reduce heat loss, and friction heat due to high shear heat will increase the melt temperature and slow down the thickening speed 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 a word, most injection defects can be solved by adjusting the injection speed, so the skill of adjusting the injection process is to reasonably set the injection speed and its segmentation

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