Proportional control of injection speed has been widely used by injection molding machine manufacturers. Although the computer-controlled injection speed segmented control system has existed for a long time, the advantages of this kind of machine setting are rarely given full play due to the limited relevant information. This article will systematically explain the advantages of applying multi-speed injection molding, and briefly introduce its use in eliminating short shots, trapped air, shrinkage and other product defects.
Injection Speed Usage & Advantage – Injection Speed Segment
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 filling speed segments, and realizing the smooth transition from one set point to another, a stable melt surface velocity can be ensured to produce the desired molecules and obtain the minimum internal stress.
We suggest the following speed segmentation principle:
1) The velocity of the fluid surface should be constant.
2) Rapid injection should be used to prevent melt freezing during injection.
3) The setting of injection speed should take into account the rapid filling in the critical area (such as the flow channel) and the slow speed at the inlet.
4) The injection speed of injection molding should ensure that the mold cavity is stopped immediately after filling to prevent overfilling, flash and residual stress.
Application of multi-stage speed for injection molding:
● The basis for setting the speed segment must take into account the geometry of the die, other flow limitations and instability factors. Speed setting must have a clear understanding of injection molding process and material knowledge, otherwise, the product quality will be difficult to control. Because the melt velocity is difficult to be measured directly, it can be calculated indirectly by measuring screw speed or cavity pressure.
● 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, the degradation caused by shear becomes smaller, because high temperature reduces the viscosity and shear stress of the material. There is no doubt that the multi-stage injection rate is very helpful to the molding of heat sensitive materials such as PC, POM, UPVC and their blending materials.
● The geometry of the die is also a determining factor. The maximum injection speed is required at the thin wall, and the slow fast slow curve is required for the thick wall parts to avoid defects. In order to ensure that the quality of the parts meets the standard, the injection speed setting should ensure that the melt front velocity remains unchanged.
● It is very important that the direction of the flow of the molecules in the surface of the part is affected. When the melt reaches the cross zone structure in front of it, it should slow down. For the complex mold with radial diffusion, the melt throughput should be increased evenly. Long runner must be filled quickly to reduce melt front cooling, but inject high viscosity material.
The Use Of Injection Speed In Product Defects
1.Adjusting the injection speed can help eliminate defects caused by slow flow at the inlet. When the melt reaches the water inlet through the nozzle and the flow channel, the surface of the melt front may have cooled and solidified, or the melt may stagnate due to the sudden narrowing of the flow channel until enough pressure is established to push the melt through the water inlet, which will make the pressure through the water inlet peak.
2.High pressure will damage the material and cause surface defects such as flow marks and scorched inlets, which can be overcome by slowing down just in front of the water inlet. This deceleration can prevent excessive shear at the inlet and then increase the firing rate to the original value.
3.We can avoid or reduce the defects such as flash, scorch, trapped gas and so on by controlling the end injection speed. The deceleration at the end of filling can prevent cavity overfilling, avoid flash and reduce residual stress. The air trapping caused by poor exhaust or filling at the end of the die flow path can also be solved by reducing the exhaust speed, especially at the end of the injection.
4.The defects of heat sensitive materials, such as flow marks, scorch at the inlet, molecular fracture, delamination and spalling, are caused by excessive shear when passing through the inlet.
5.Smooth parts depend on injection speed, and fiberglass fillers are particularly sensitive, especially nylon. Dark spots are caused by flow instability caused by viscosity changes. Distorted flow can cause wavy or uneven fog. The defect depends on the degree of flow instability.
6.When the melt passes through the water inlet, high-speed injection will lead to high shear, and the thermosensitive plastic will be scorched. This kind of scorched material will pass through the cavity and reach the flow front and appear on the surface of the part.
7.In order to prevent shot marks, the injection speed must be set to ensure that the flow channel area is filled quickly and then passed slowly through the water inlet. It is the essence of the problem to find out the velocity conversion point. If it is too early, the filling time will increase too much. If it is too late, too much flow inertia will lead to the appearance of projectile.
8.The lower the melt viscosity is, the higher the barrel temperature is, and the more obvious the tendency of this kind of projectile is. Because the small water inlet needs high speed and high pressure injection, it is also an important factor leading to flow defects.
9.Shrinkage can be improved by more effective pressure transfer and smaller pressure drop. The low mold temperature and the slow screw speed greatly shorten the flow length, which must be compensated by high injection speed. High speed flow can reduce heat loss, and friction heat generated by high shear heat will increase melt temperature and slow down the thickening speed of outer layer of parts. The cross position of the cavity must be thick enough to avoid too much pressure drop, otherwise shrinkage will occur.
In short, most of the injection molding defects can be solved by adjusting the injection speed, so the skill of adjusting the injection process is to set the injection speed and its segments reasonably.
Precautions For Adjusting Injection Mold Machine
The general injection molding machine can be adjusted according to the following procedures:
● According to the temperature range provided by the raw material supplier, adjust the barrel temperature to the middle of the range, and adjust the mold temperature.
● Estimate the required injection volume and adjust the injection molding machine to two thirds of the estimated maximum injection volume. Adjust the stroke of inverted cable (rubber drawing). Estimate and adjust the secondary injection time and adjust the secondary injection pressure to zero.
● Adjust the primary injection pressure to half (50%) of the injection molding machine limit; adjust the injection speed to the maximum. Estimate and adjust the cooling time required. Adjust the back pressure to 3.5 bar. Remove the degraded resin from the barrel. The semi-automatic injection mode was adopted; the injection procedure was started and the action of screw was observed.
● It is necessary to adjust the injection speed and pressure properly. In order to shorten the filling time, the injection pressure can be increased. Since there is a process before full mold filling, the final mold filling pressure can be adjusted to 100% of the primary injection pressure. Finally, the pressure must be adjusted high enough that the maximum speed that can be reached is not limited by the set pressure. If there is overflow, the speed can be reduced.
● After each observation cycle, the injection volume and the conversion point are adjusted. Set the program so that 95-98% of the injection weight can be obtained in the first stage injection molding.
● When the injection volume, transfer point, injection speed and pressure of the first injection molding are adjusted properly, the adjustment procedure of the holding pressure of the second stage can be carried out.
● Adjust the packing pressure properly as required, but do not overfill the mold cavity.
● Adjust the screw speed to ensure that the melt glue is finished just before the cycle is completed, and the injection cycle is not limited.
