Injection molding process adjustment tips
Proportional control of injection molding speed has been widely adopted by injection molding machine manufacturers. Although computer-controlled injection molding speed segmentation control systems have long existed, due to limited relevant information, the advantages of this machine setup have rarely been brought into play. This article will systematically explain the advantages of applying multi-stage speed injection molding, and briefly introduce its use in eliminating product defects such as short shots, trapped air, and shrinkage. picture
The close relationship between injection speed and product quality makes it a key parameter for injection molding. By determining the start, middle, and end of the filling velocity segment, and achieving a smooth transition from one setpoint to another, a stable melt surface velocity can be ensured to produce the desired molecule and minimize internal stress.
We recommend the following principles for speed divisions:
1) The velocity of the fluid surface should be constant.
2) Rapid 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 of 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 overfilling, flash and residual stress.
The basis for setting the velocity segment must take into account the geometry of the mold, other flow restrictions and instabilities. 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 screw advance speed, or the cavity pressure (to ensure that the check valve is not leaking).
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 the chemical structure, but at the same time the degradation caused by shear becomes smaller because high temperature reduces the viscosity of the material, reducing the shear stress. Undoubtedly, multi-stage injection speed is very helpful for molding heat-sensitive materials such as PC, POM, UPVC and their blending materials.
The geometry of the mold is also a determining factor: thin-walled parts require maximum injection speed; thick-walled parts need a slow-fast-slow speed curve to avoid defects; in order to ensure part quality meets the standard, the injection speed should be set to ensure the melt front flow rate constant.
The melt flow rate is very important because it will affect the molecular arrangement direction and surface state in the part; when the melt front reaches the cross region 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 the 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 injection speed can help eliminate defects caused by slowed 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 stagnates due to the sudden narrowing of the runner until sufficient pressure is established to push the melt through the inlet. The water inlet, which causes the pressure peaking through the water inlet.
High pressure will damage the material and cause surface defects such as flow marks and charred inlets, which can be overcome by decelerating just before the inlet. This deceleration prevents excessive shear at the inlet level before increasing the rate of fire to its original value. Because it is very difficult to precisely control the rate of fire to slow down at the water inlet, it is a better solution to slow down at the end of the runner.
We can avoid or reduce defects such as flash, burnt, trapped air, etc. by controlling the final injection speed. Deceleration at the end of filling prevents overfilling of the cavity, avoids flashing and reduces residual stress. Trapped air caused by poor exhaust at the end of the mold flow path or filling problems can also be resolved by reducing the exhaust velocity, especially at the end of the injection.
The short shot is caused by the slow speed at the water inlet or the partial flow obstruction caused by the solidification of the melt. Speeding up the injection speed just past the water inlet or local flow obstruction can solve this problem.
Defects such as flow marks, scorched water inlets, molecular breakage, delamination, and peeling that occur on heat-sensitive materials are caused by excessive shear when passing through the water inlets.
Smooth parts depend on injection speed, and glass-fiber-filled materials are particularly sensitive, especially nylon. Dark spots (wavy lines) are caused by flow instabilities due to viscosity changes. Distorted flow can result in wavy or non-uniform haze, depending on the degree of flow instability.
When the melt passes through the water inlet, high-speed injection will cause high shear, and the heat-sensitive plastic will be charred. This charred material will pass through the cavity, reach the flow front, and appear on the surface of the part.
To prevent shot streaking, the shot speed must be set so that the runner area is filled quickly and then passed slowly through the inlet. Finding this speed transition point is the essence of the problem. If it is too early, the fill time will increase excessively, if it is too late, the excessive flow inertia will cause the appearance of jet streaks. The lower the melt viscosity and the higher the barrel temperature, the more obvious the tendency of this shot 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 with more efficient pressure transfer, less pressure drop. Low mold temperature and slow screw advance speed greatly shorten the flow length, which must be compensated by high firing rate. High-speed flow reduces heat loss, and frictional heat due to high shear heat can cause an increase in melt temperature and slow down the thickening 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 defects can be solved by adjusting the injection speed, so the trick to adjust the injection molding process is to set the injection speed and its segments reasonably.
