The proportional control of injection speed has been widely used by injection mechanism manufacturers. In this paper, the advantages of multi-stage speed injection molding are described systematically, and its application in eliminating the defects of short shot, trapped gas, shrinkage and other products is summarized.
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 velocity segment, and achieving a smooth transition from one set point to another, a stable melt surface velocity can be ensured to produce the desired molecular minimum internal stress.
We recommend the following speed segmentation principle:
1) The velocity of the fluid surface should be constant.
2) Rapid glue injection shall be used to prevent the melt from freezing during glue injection.
3) The setting of glue injection speed should take into account the rapid filling of critical area (such as flow channel) and the slowing down of speed at the water inlet.
4) The injection speed shall ensure that the mold cavity stops immediately after filling to prevent overfilling, flash and residual stress.
The basis for setting the speed segment must take into account the die 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 velocity is difficult to measure directly, it can be calculated indirectly by measuring the forward speed of the screw or the cavity pressure (make sure that the check valve has no leakage).
Material properties 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 and shear stress of materials. There is no doubt that the multi-stage glue injection speed is very helpful for the molding of heat sensitive materials such as PC, POM, UPVC and their blending materials.
The geometric shape of the mold is also a decisive factor: the maximum injection speed is required at the thin wall; the slow fast slow speed curve is required for thick wall parts to avoid defects; in order to ensure that the quality of the parts meets the standards, the injection speed setting should ensure that the melt front flow rate is constant.
The melt flow rate is very important because it will affect the molecular arrangement direction and surface state in the parts; when the front of the melt reaches the structure of the cross area, it should slow down; for the complex mold with radial diffusion, it should ensure that the melt throughput increases evenly; the long flow path must be filled quickly to reduce the cooling of the melt front, but the injection of high viscosity materials, such as PC, is an example External situation, because too fast speed will bring the cold material into the cavity through the water inlet.
Adjusting the injection rate can help to eliminate defects caused by slow flow at the inlet. When the melt passes through the nozzle and flow channel to the water inlet, the surface of the melt front may have cooled and solidified, or the melt may be stopped due to the sudden narrowing of the flow 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 water inlet scorching, which can be overcome by decelerating just before the water inlet. This deceleration prevents excessive shear at the intake and then increases the jet speed to the original value. Because it is very difficult to precisely control the deceleration at the inlet, it is a better scheme to decelerate at the end of the channel.
We can avoid or reduce the defects such as flash, scorch, trapped gas and so on by controlling the injection speed of the end section. The deceleration of the filling end can prevent the cavity from over filling, avoid flash and reduce the residual stress. The problem of air entrapment caused by poor exhaust or filling at the end of die flow path can also be solved by reducing the exhaust velocity, especially the exhaust velocity at the end of injection.
The short shot is caused by the slow speed at the inlet or the local flow obstruction caused by the melt solidification. This problem can be solved by speeding up the injection speed when just passing through the water inlet or local flow obstruction. The defects of heat sensitive materials, such as flow mark, entrance scorch, molecular fracture, delamination, peeling, etc., are caused by excessive shear when passing through the entrance.
The smooth part depends on the injection speed, glass fiber filling material is particularly sensitive, especially nylon. Dark spots (wavy lines) are caused by flow instability caused by viscosity changes. The distorted flow can lead to wavy lines or uneven fog shape. The defect 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 thermosensitive plastic will appear scorching. The scorched material will pass through the cavity, reach the flow front, and present on the part surface.
In order to prevent spray marks, the setting of spray speed must ensure that the flow passage area is filled quickly and then passes 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, the excessive flow inertia will lead to the emergence of striation. The lower the melt viscosity is, the higher the barrel temperature is, the more obvious the tendency of this pattern is. Because the small water inlet needs high-speed and high-pressure injection, it is also an important factor leading to flow defects.
