The close relationship between injection speed and product quality makes it a key parameter in injection molding. By defining the beginning, middle, and end of the filling speed segment and achieving a smooth transition from one set point to another, a stable melt surface velocity can be guaranteed to produce the desired molecular take-off and minimum internal stress.
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We recommend the following velocity segmentation principle.
The velocity of the fluid surface should be constant.
Rapid injection should be used to prevent melt freezing during the injection process.
The injection speed setting should take into account the fast filling in the critical area (e.g. runners) while slowing down at the inlet level.
The injection speed should be such that the cavity is filled and stopped immediately to prevent overfilling flying edges, and residual stresses.
The basis for setting speed segments must take into account the mold geometry, other flow constraints, and instabilities.
The speed must be set with a clear understanding of the injection molding process and material knowledge, otherwise, the quality of the product will be difficult to control. Because the melt flow rate is difficult to measure directly, it can be indirectly deduced by measuring the screw forward speed, or cavity pressure (to make sure there is no leakage of the check valve).
Material properties are very important because polymers may degrade due to different stresses. Increasing the molding temperature may lead to intense oxidation and degradation of the chemical structure, but at the same time the degradation caused by shear becomes less because the high temperature reduces the viscosity of the material and decreases the shear stress. Undoubtedly, multi-stage injection speeds are useful for molding heat-sensitive materials such as PC, POM, UPVC and for their blending.
The geometry of the mold is also a determining factor.
Thin-walled parts require maximum injection speed.
Thick-walled parts require slow-fast-slow type speed profiles to avoid defects.
To ensure that the part quality is up to standard, the injection speed should be set so that the melt front flow rate is constant. melt flow rate is very important because it affects the molecular alignment direction and surface condition in the part.
(b) The melt front should be slowed down when it reaches the cross area structure.
For complex molds with radial diffusion, a balanced increase in melt throughput should be ensured
Long runners must be filled quickly to reduce cooling of the melt front, with the exception of injecting high viscosity materials, such as PC, where too fast a speed can bring cold material into the cavity through the inlet.
Adjusting the injection speed can help eliminate defects caused by slowing flow at the inlet level. As the melt passes through the nozzle and runner to the inlet, the surface of the melt front may have cooled and solidified, or the melt may stall due to a sudden narrowing of the runner until enough pressure is built up to push the melt through the inlet, which can cause the pressure through the inlet to peak.
The high pressure will damage the material and cause surface defects such as flow marks and scorching at the inlet, which can be overcome by decelerating just before the inlet. This deceleration prevents excessive shearing at the entry point and then increases the shot rate to the original value. Since it is very difficult to precisely control the injection speed to slow down at the inlet level, deceleration at the end of the runner is a better solution.
We can avoid or reduce defects such as fluttering, scorching, and air trapping by controlling the injection speed at the end of the runner. Deceleration at the end of filling can prevent overfilling of the cavity, avoid flying edges and reduce residual stress. Air trapping caused by poor venting or filling problems at the end of the mold flow path can also be solved by reducing the venting speed, especially at the end of the injection stage.
Short shot is caused by slow speed at the inlet or local blockage of flow caused by melt solidification. This problem can be solved by speeding up the injection speed just after the water inlet or when the local flow is obstructed. Defects that occur in heat-sensitive materials such as flow marks, inlet scorch, molecular breakage, delamination, and flaking are caused by excessive shear when passing through the inlet.
Smooth parts depend on injection speed, and glass-filled materials are particularly sensitive especially nylon. Dark spots (wavy patterns) are caused by flow instability due to changes in viscosity. Distorted flow can lead to wavy patterns or uneven haze, and the exact defect produced depends on the degree of flow instability.
High-speed injection as the melt passes through the inlet can result in high shear, and thermosensitive plastics will experience scorching, and this scorched material will travel through the cavity to the flow front and present itself on the part surface.
To prevent shot ripples, the injection speed setting must ensure that the runner area is filled quickly and then passed through the inlet slowly. Finding this speed transition point is the essence of the problem. If it is too early, the fill time will increase excessively and if it is too late, excessive flow inertia will lead to the appearance of shot lines. The lower the melt viscosity and the higher the barrel temperature the more pronounced the tendency for this shot pattern to appear. Small inlets are also an important factor in flow defects because they require high-speed and high-pressure injection.
Shrinkage can be improved by more efficient pressure transfer and smaller pressure drop. Low mold temperature and slow screw advancement greatly shorten the flow length and must be compensated for by high injection velocities. High flow speeds reduce heat loss and can cause an increase in melt temperature due to high shear heat-generating frictional heat, slowing the thickening of the outer layer of the part. The cavity crossover must be thick enough to avoid too large a pressure drop, otherwise shrinkage will occur.
In short, most injection 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 segmentation in a reasonable way.